Independent Submission M. Fox
Request for Comments: 7609 C. Kassimis
Category: Informational J. Stevens
ISSN: 2070-1721 IBM
August 2015
IBM's Shared Memory Communications over RDMA (SMC-R) Protocol
Abstract
This document describes IBM's Shared Memory Communications over RDMA
(SMC-R) protocol. This protocol provides Remote Direct Memory Access
(RDMA) communications to TCP endpoints in a manner that is
transparent to socket applications. It further provides for dynamic
discovery of partner RDMA capabilities and dynamic setup of RDMA
connections, as well as transparent high availability and load
balancing when redundant RDMA network paths are available. It
maintains many of the traditional TCP/IP qualities of service such as
filtering that enterprise users demand, as well as TCP socket
semantics such as urgent data.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7609.
Fox, et al. Informational [Page 1]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction ....................................................5
1.1. Protocol Overview ..........................................6
1.1.1. Hardware Requirements ...............................8
1.2. Definition of Common Terms .................................8
1.3. Conventions Used in This Document .........................11
2. Link Architecture ..............................................11
2.1. Remote Memory Buffers (RMBs) ..............................12
2.2. SMC-R Link Groups .........................................18
2.2.1. Link Group Types ...................................18
2.2.2. Maximum Number of Links in Link Group ..............21
2.2.3. Forming and Managing Link Groups ...................23
2.2.4. SMC-R Link Identifiers .............................24
2.3. SMC-R Resilience and Load Balancing .......................24
3. SMC-R Rendezvous Architecture ..................................26
3.1. TCP Options ...............................................26
3.2. Connection Layer Control (CLC) Messages ...................27
3.3. LLC Messages ..............................................27
3.4. CDC Messages ..............................................29
3.5. Rendezvous Flows ..........................................29
3.5.1. First Contact ......................................29
3.5.1.1. Pre-negotiation of TCP Options ............29
3.5.1.2. Client Proposal ...........................30
3.5.1.3. Server Acceptance .........................32
3.5.1.4. Client Confirmation .......................32
3.5.1.5. Link (QP) Confirmation ....................32
3.5.1.6. Second SMC-R Link Setup ...................35
3.5.1.6.1. Client Processing of ADD LINK
LLC Message from Server ........35
3.5.1.6.2. Server Processing of ADD LINK
Reply LLC Message from Client ..36
3.5.1.6.3. Exchange of RKeys on
Second SMC-R Link ..............38
3.5.1.6.4. Aborting SMC-R and
Falling Back to IP .............38
Fox, et al. Informational [Page 2]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.2. Subsequent Contact .................................38
3.5.2.1. SMC-R Proposal ............................39
3.5.2.2. SMC-R Acceptance ..........................40
3.5.2.3. SMC-R Confirmation ........................41
3.5.2.4. TCP Data Flow Race with SMC
Confirm CLC Message .......................41
3.5.3. First Contact Variation: Creating a
Parallel Link Group ................................42
3.5.4. Normal SMC-R Link Termination ......................43
3.5.5. Link Group Management Flows ........................44
3.5.5.1. Adding and Deleting Links in an
SMC-R Link Group ..........................44
3.5.5.1.1. Server-Initiated ADD
LINK Processing ................45
3.5.5.1.2. Client-Initiated ADD
LINK Processing ................45
3.5.5.1.3. Server-Initiated DELETE
LINK Processing ................46
3.5.5.1.4. Client-Initiated DELETE
LINK Request ...................48
3.5.5.2. Managing Multiple RKeys over
Multiple SMC-R Links in a Link Group ......49
3.5.5.2.1. Adding a New RMB to an
SMC-R Link Group ...............50
3.5.5.2.2. Deleting an RMB from an
SMC-R Link Group ...............53
3.5.5.2.3. Adding a New SMC-R Link to a
Link Group with Multiple RMBs ..54
3.5.5.3. Serialization of LLC Exchanges,
and Collisions ............................56
3.5.5.3.1. Collisions with ADD
LINK / CONFIRM LINK Exchange ...57
3.5.5.3.2. Collisions during
DELETE LINK Exchange ...........58
3.5.5.3.3. Collisions during
CONFIRM RKEY Exchange ..........59
4. SMC-R Memory-Sharing Architecture ..............................60
4.1. RMB Element Allocation Considerations .....................60
4.2. RMB and RMBE Format .......................................60
4.3. RMBE Control Information ..................................60
4.4. Use of RMBEs ..............................................61
4.4.1. Initializing and Accessing RMBEs ...................61
4.4.2. RMB Element Reuse and Conflict Resolution ..........62
4.5. SMC-R Protocol Considerations .............................63
4.5.1. SMC-R Protocol Optimized Window Size Updates .......63
4.5.2. Small Data Sends ...................................64
4.5.3. TCP Keepalive Processing ...........................65
Fox, et al. Informational [Page 3]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
4.6. TCP Connection Failover between SMC-R Links ...............67
4.6.1. Validating Data Integrity ..........................67
4.6.2. Resuming the TCP Connection on a New SMC-R Link ....68
4.7. RMB Data Flows ............................................69
4.7.1. Scenario 1: Send Flow, Window Size Unconstrained ...69
4.7.2. Scenario 2: Send/Receive Flow, Window Size
Unconstrained ......................................71
4.7.3. Scenario 3: Send Flow, Window Size Constrained .....72
4.7.4. Scenario 4: Large Send, Flow Control, Full
Window Size Writes .................................74
4.7.5. Scenario 5: Send Flow, Urgent Data, Window
Size Unconstrained .................................77
4.7.6. Scenario 6: Send Flow, Urgent Data, Window
Size Closed ........................................79
4.8. Connection Termination ....................................81
4.8.1. Normal SMC-R Connection Termination Flows ..........81
4.8.2. Abnormal SMC-R Connection Termination Flows ........86
4.8.3. Other SMC-R Connection Termination Conditions ......88
5. Security Considerations ........................................89
5.1. VLAN Considerations .......................................89
5.2. Firewall Considerations ...................................89
5.3. Host-Based IP Filters .....................................89
5.4. Intrusion Detection Services ..............................90
5.5. IP Security (IPsec) .......................................90
5.6. TLS/SSL ...................................................90
6. IANA Considerations ............................................90
7. Normative References ...........................................91
Appendix A. Formats ...............................................92
A.1. TCP Option .................................................92
A.2. CLC Messages ...............................................92
A.2.1. Peer ID Format ......................................93
A.2.2. SMC Proposal CLC Message Format .....................94
A.2.3. SMC Accept CLC Message Format .......................98
A.2.4. SMC Confirm CLC Message Format .....................102
A.2.5. SMC Decline CLC Message Format .....................105
A.3. LLC Messages ..............................................106
A.3.1. CONFIRM LINK LLC Message Format ....................107
A.3.2. ADD LINK LLC Message Format ........................109
A.3.3. ADD LINK CONTINUATION LLC Message Format ...........112
A.3.4. DELETE LINK LLC Message Format .....................115
A.3.5. CONFIRM RKEY LLC Message Format ....................117
A.3.6. CONFIRM RKEY CONTINUATION LLC Message Format .......120
A.3.7. DELETE RKEY LLC Message Format .....................122
A.3.8. TEST LINK LLC Message Format .......................124
A.4. Connection Data Control (CDC) Message Format ..............125
Fox, et al. Informational [Page 4]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Appendix B. Socket API Considerations ............................129
B.1. setsockopt() / getsockopt() Considerations ................130
Appendix C. Rendezvous Error Scenarios ...........................131
C.1. SMC Decline during CLC Negotiation ........................131
C.2. SMC Decline during LLC Negotiation ........................131
C.3. The SMC Decline Window ....................................133
C.4. Out-of-Sync Conditions during SMC-R Negotiation ...........133
C.5. Timeouts during CLC Negotiation ...........................134
C.6. Protocol Errors during CLC Negotiation ....................134
C.7. Timeouts during LLC Negotiation ...........................135
C.7.1. Recovery Actions for LLC Timeouts and Failures .....136
C.8. Failure to Add Second SMC-R Link to a Link Group ..........142
Authors' Addresses ...............................................143
1. Introduction
This document specifies IBM's Shared Memory Communications over RDMA
(SMC-R) protocol. SMC-R is a protocol for Remote Direct Memory
Access (RDMA) communication between TCP socket endpoints. SMC-R runs
over networks that support RDMA over Converged Ethernet (RoCE). It
is designed to permit existing TCP applications to benefit from RDMA
without requiring modifications to the applications or predefinition
of RDMA partners.
SMC-R provides dynamic discovery of the RDMA capabilities of TCP
peers and automatic setup of RDMA connections that those peers can
use. SMC-R also provides transparent high availability and
load-balancing capabilities that are demanded by enterprise
installations but are missing from current RDMA protocols. If
redundant RoCE-capable hardware such as RDMA-capable Network
Interface Cards (RNICs) and RoCE-capable switches is present, SMC-R
can load-balance over that redundant hardware and can also
non-disruptively move TCP traffic from failed paths to surviving
paths, all seamlessly to the application and the sockets layer.
Because SMC-R preserves socket semantics and the TCP three-way
handshake, many TCP qualities of service such as filtering, load
balancing, and Secure Socket Layer (SSL) encryption are preserved, as
are TCP features such as urgent data.
Because of the dynamic discovery and setup of SMC-R connectivity
between peers, no RDMA connection manager (RDMA-CM) is required.
This also means that support for Unreliable Datagram (UD) Queue Pairs
(QPs) is also not required.
Fox, et al. Informational [Page 5]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
It is recommended that the SMC-R services be implemented in kernel
space, which enables optimizations such as resource-sharing between
connections across multiple processes and also permits applications
using SMC-R to spawn multiple processes (e.g., fork) without losing
SMC-R functionality. A user-space implementation is compatible with
this architecture, but it may not support spawned processes (e.g.,
fork), which limits sharing and resource optimization to TCP
connections that originate from the same process. This might be an
appropriate design choice if the use case is a system that hosts a
large single process application that creates many TCP connections to
a peer host, or in implementations where a kernel-space
implementation is not possible or introduces excessive overhead for
"kernel space to user space" context switches.
1.1. Protocol Overview
SMC-R defines the concept of the SMC-R link, which is a logical
point-to-point link using reliably connected queue pairs between
TCP/IP stack peers over a RoCE fabric. An SMC-R link is bound to a
specific hardware path, meaning a specific RNIC on each peer. SMC-R
links are created and maintained by an SMC-R layer, which may reside
in kernel space or user space, depending upon operating system and
implementation requirements. The SMC-R layer resides below the
sockets layer and directs data traffic for TCP connections between
connected peers over the RoCE fabric using RDMA rather than over a
TCP connection. The TCP/IP stack, with its requirements for
fragmentation, packetization, etc., is bypassed, and the application
data is moved between peers using RDMA.
Multiple SMC-R links between the same two TCP/IP stack peers are also
supported. A set of SMC-R links called a link group can be logically
bonded together to provide redundant connectivity. If there is
redundant hardware -- for example, two RNICs on each peer -- separate
SMC-R links are created between the peers to exploit that redundant
hardware. The link group architecture with redundant links provides
load balancing and increased bandwidth, as well as seamless failover.
Each SMC-R link group is associated with an area of memory called
Remote Memory Buffers (RMBs), which are areas of memory that are
available for SMC-R peers to write into using RDMA writes. Multiple
TCP connections between peers may be multiplexed over a single SMC-R
link, in which case the SMC-R layer manages the partitioning of the
RMBs between the TCP connections. This multiplexing reduces the RDMA
resources, such as QPs and RMBs, that are required to support
multiple connections between peers, and it also reduces the
processing and delays related to setting up QPs, pinning memory, and
other RDMA setup tasks when new TCP connections are created. In a
kernel-space SMC-R implementation in which the RMBs reside in kernel
Fox, et al. Informational [Page 6]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
storage, this sharing and optimization works across multiple
processes executing on the same host. In a user-space SMC-R
implementation in which the RMBs reside in user space, this sharing
and optimization is limited to multiple TCP connections created by a
single process, as separate RMBs and QPs will be required for each
process.
SMC-R also introduces a rendezvous protocol that is used to
dynamically discover the RDMA capabilities of TCP connection partners
and exchange credentials necessary to exploit that capability if
present. TCP connections are set up using the normal TCP three-way
handshake [RFC793], with the addition of a new TCP option that
indicates SMC-R capability. If both partners indicate SMC-R
capability, then at the completion of the three-way TCP handshake the
SMC-R layers in each peer take control of the TCP connection and use
it to exchange additional Connection Layer Control (CLC) messages to
negotiate SMC-R credentials such as QP information; addressability
over the RoCE fabric; RMB buffer sizes; and keys and addresses for
accessing RMBs over RDMA. If at any time during this negotiation a
failure or decline occurs, the TCP connection falls back to using the
IP fabric.
If the SMC-R negotiation succeeds and either a new SMC-R link is set
up or an existing SMC-R link is chosen for the TCP connection, then
the SMC-R layers open the sockets to the applications and the
applications use the sockets as normal. The SMC-R layer intercepts
the socket reads and writes and moves the TCP connection data over
the SMC-R link, "out of band" to the TCP connection, which remains
open and idle over the IP fabric, except for termination flows and
possible keepalive flows. Regular TCP sequence numbering methods are
used for the TCP flows that do occur; data flowing over RDMA does not
use or affect TCP sequence numbers.
This architecture does not support fallback of active SMC-R
connections to IP. Once connection data has completed the switch to
RDMA, a TCP connection cannot be switched back to IP and will reset
if RDMA becomes unusable.
The SMC-R protocol defines the format of the RMBs that are used to
receive TCP connection data written over RDMA, as well as the
semantics for managing and writing to these buffers using Connection
Data Control (CDC) messages.
Fox, et al. Informational [Page 7]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Finally, SMC-R defines Link Layer Control (LLC) messages that are
exchanged over the RoCE fabric between peer SMC-R layers to manage
the SMC-R links and link groups. These include messages to test and
confirm connectivity over an SMC-R link, add and delete SMC-R links
to or from the link group, and exchange RMB addressability
information.
1.1.1. Hardware Requirements
SMC-R does not require full Converged Enhanced Ethernet switch
functionality. SMC-R functions over standard Ethernet fabrics,
provided that endpoint RNICs are provided and IEEE 802.3x Global
Pause Frame is supported and enabled in the switch fabric.
While SMC-R as specified in this document is designed to operate over
RoCE fabrics, adjustments to the rendezvous methods could enable it
to run over other RDMA fabrics, such as InfiniBand [RoCE] and iWARP.
1.2. Definition of Common Terms
This section provides definitions of terms that have a specific
meaning to the SMC-R protocol and are used throughout this document.
SMC-R Link
An SMC-R link is a logical point-to-point connection over the RoCE
fabric via specific physical adapters (Media Access Control /
Global Identifier (MAC/GID)). The link is formed during the
"first contact" sequence of the TCP/IP three-way handshake
sequence that occurs over the IP fabric. During this handshake,
an RDMA reliably connected queue pair (RC-QP) connection is formed
between the two peer SMC hosts and is defined as the SMC-R link.
The SMC-R link can then support multiple TCP connections between
the two peers. An SMC-R link is associated with a single LAN (or
VLAN) segment and is not routable.
SMC-R Link Group
An SMC-R link group is a group of SMC-R links between the same two
SMC-R peers, typically with each link over unique RoCE adapters.
Each link in the link group has equal characteristics, such as the
same VLAN ID (if VLANs are in use), access to the same RMB(s), and
access to the same TCP server/client.
Fox, et al. Informational [Page 8]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
SMC-R Peer
The SMC-R peer is the peer software stack within the peer
operating system with respect to the Shared Memory Communications
(messaging) protocol.
SMC-R Rendezvous
SMC-R Rendezvous is the SMC-R peer discovery and handshake
sequence that occurs transparently over the IP (Ethernet) fabric
during and immediately after the TCP connection three-way
handshake by exchanging the SMC-R capabilities and credentials
using experimental TCP option and CLC messages.
RoCE SendMsg
RoCE SendMsg is a send operation posted to a reliably connected
queue pair with inline data, for the purpose of transferring
control information between peers.
TCP Client
The TCP client is the TCP socket-based peer that initiates a TCP
connection.
TCP Server
The TCP server is the TCP socket-based peer that accepts a TCP
connection.
CLC Messages
The SMC-R protocol defines a set of Connection Layer Control
messages that flow over the TCP connection that are used to manage
SMC-R link rendezvous at TCP connection setup time. This
mechanism is analogous to SSL setup messages.
LLC Commands
The SMC-R protocol defines a set of RoCE Link Layer Control
commands that flow over the RoCE fabric using RoCE SendMsg, that
are used to manage SMC-R links, SMC-R link groups, and SMC-R
link group RMB expansion and contraction.
Fox, et al. Informational [Page 9]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
CDC Message
The SMC-R protocol defines a Connection Data Control message that
flows over the RoCE fabric using RoCE SendMsg that is used to
manage the SMC-R connection data. This message provides
information about data being transferred over the out-of-band RDMA
connection, such as data cursors, sequence numbers, and data flags
(for example, urgent data). The receipt of this message also
provides an interrupt to inform the receiver that it has received
RDMA data.
RMB
A Remote (RDMA) Memory Buffer is a fixed or pinned buffer
allocated in each of the peer hosts for a TCP (via SMC-R)
connection. The RMB is registered to the RNIC and allows remote
access by the remote peer using RDMA semantics. Each host is
passed the peer's RMB-specific access information (RMB Key (RKey)
and RMB element offset) during the SMC-R Rendezvous process. The
host stores socket application user data directly into the peer's
RMB using RDMA over RoCE.
RToken
The RToken is the combination of an RMB's RKey and RDMA virtual
address. An RToken provides RMB addressability information to an
RDMA peer.
RMBE
The Remote Memory Buffer Element (RMBE) is an area of an RMB that
is allocated to a specific TCP connection. The RMBE contains data
for the TCP connection. The RMBE represents the TCP receive
buffer, whereby the remote peer writes into the RMBE and the local
peer reads from the local RMBE. The alert token resolves to a
specific RMBE.
Alert Token
The SMC-R alert token is a 4-byte value that uniquely identifies
the TCP connection over an SMC-R connection. The alert token
allows the SMC peer to quickly identify the target TCP connection
that now has new work. The format of the token is defined by the
owning SMC-R endpoint and is considered opaque to the remote peer.
However, the token should not simply be an index to an RMBE; it
should reference a TCP connection and be able to be validated to
avoid reading data from stale connections.
Fox, et al. Informational [Page 10]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
RNIC
The RDMA-capable Network Interface Card (RNIC) is an Ethernet NIC
that supports RDMA semantics and verbs using RoCE.
First Contact
"First contact" describes an SMC-R negotiation to set up the first
link in a link group.
Subsequent Contact
"Subsequent contact" describes an SMC-R negotiation between peers
who are using an already-existing SMC-R link group.
1.3. Conventions Used in This Document
In the rendezvous flow diagrams, dashed lines (----) are used to
indicate flows over the TCP/IP fabric and dotted lines (....) are
used to indicate flows over the RoCE fabric.
In the data transfer ladder diagrams, dashed lines (----) are used to
indicate RDMA write operations and dotted lines (....) are used to
indicate CDC messages, which are RDMA messages with inline data that
contain control information for the connection.
2. Link Architecture
An SMC-R link is based on reliably connected queue pairs (QPs) that
form a "logical point-to-point link" between the two SMC-R peers over
a RoCE fabric. An SMC-R link extends from SMC-R peer to SMC-R peer,
where typically each peer would be a TCP/IP stack and would reside on
separate hosts.
,,.--..,_
+----+ _-`` `-, +-----+
|QP 8| - RoCE ', |QP 64|
| | / VLAN M . | |
+----+--------+/ \+-------+-----+
| RNIC 1 | SMC-R Link | RNIC 2 |
| |<--------------------->| |
+------------+ , /+------------+
MAC A (GID A) MAC B (GID B)
. .`
`', ,-`
``''--''``
Figure 1: SMC-R Link Overview
Fox, et al. Informational [Page 11]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Figure 1 illustrates an overview of the basic concepts of SMC-R peer-
to-peer connectivity; this is called the SMC-R link. The SMC-R link
forms a logical point-to-point connection between two SMC-R peers via
RoCE. The SMC-R link is defined and identified by the following
attributes:
SMC-R link = RC QPs
(source VMAC GID QP + target VMAC GID QP + VLAN ID)
The SMC-R link can optionally be associated with a VLAN ID. If VLANs
are in use for the associated IP (LAN) connection, then the VLAN
attribute is carried over on the SMC-R link. When VLANs are in use,
each SMC-R link group is associated with a single and specific VLAN.
The RoCE fabric is the same physical Ethernet LAN used for standard
TCP/IP-over-Ethernet communications, with switches as described in
Section 1.1.1.
An SMC-R link is designed to support multiple TCP connections between
the same two peers. An SMC-R link is intended to be long lived,
while the underlying TCP connections can dynamically come and go.
The associated RMBs can also be dynamically added and removed from
the link as needed. The first TCP connection between the peers
establishes the SMC-R link. Subsequent TCP connections then use the
previously established link. When the last TCP connection
terminates, the link can then be terminated, typically after an
implementation-defined idle timeout period has elapsed. The TCP
server is responsible for initiating and terminating the SMC-R link.
2.1. Remote Memory Buffers (RMBs)
Figure 2 shows the hosts -- Hosts X and Y -- and their associated
RMBs within each host. With the SMC-R link, and the associated RKeys
and RDMA virtual addresses, each SMC-R-enabled TCP/IP stack can
remotely access its peer's RMBs using RDMA. The RKeys and virtual
addresses are exchanged during the rendezvous processing when the
link is established. The combination of the RKey and the virtual
address is the RToken. Note that the SMC-R link ends at the QP
providing access to the RMB (via the link + RToken).
Fox, et al. Informational [Page 12]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Host X Host Y
+-------------------+ ,.--.,_ +-------------------+
| | .'` '. | |
| Protection | ,' `, | Protection |
| Domain X | / \ | Domain Y |
| +------+ / \ +------+ |
| QP 8 |RNIC 1| | SMC-R Link | |RNIC 2| QP 64 |
| | | |<-------------------->| | | |
| | | || || | | |
| | +------+| VLAN A |+------+ | |
| | || || | |
| | | | RoCE | | | |
| |RToken X | \ / |RToken Y | |
| | | \ / | | |
| V | `. ,' | V |
| +--------+ | '._ ,' | +--------+ |
| | | | `''-'`` | | | |
| | RMB | | | | RMB | |
| | | | | | | |
| +--------+ | | +--------+ |
+-------------------+ +-------------------+
Figure 2: SMC-R Link and RMBs
An SMC-R link can support multiple RMBs that are independently
managed by each peer. The number and the size of RMBs are managed by
the peers based on the host's unique memory management requirements;
however, the maximum number of RMBs that can be associated to a link
group on one peer is 255. The QP has a single protection domain, but
each RMB has a unique RToken. All RTokens must be exchanged with the
peer.
Each peer manages the RMBs in its local memory for its remote SMC-R
peer by sharing access to the RMBs via RTokens with its peers. The
remote peer writes into the RMBs via RDMA, and the local peer (RMB
owner) then reads from the RMBs.
When two peers decide to use SMC-R for a given TCP connection, they
each allocate a local RMB element for the TCP connection and
communicate the location of this local RMB element during rendezvous
processing. To that end, RMB elements are created in pairs, with one
RMB element allocated locally on each peer of the SMC-R link.
Fox, et al. Informational [Page 13]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
--- +------------+---------------+
/\ |Eye Catcher | |
| +------------+ |
| | |
RMB Element 1 | |
| | Receive Buffer |
| | |
| | |
\/ | |
--- +------------+---------------+
/\ |Eye Catcher | |
| +------------+ |
| | |
RMB Element 2 | |
| | Receive Buffer |
| | |
| | |
\/ | |
--- +----------------------------+
| . |
| . |
| . |
| . |
| (up to 255 elements) |
+----------------------------+
Figure 3: RMB Format
Figure 3 illustrates the basic format of an RMB. The RMB is a
virtual memory buffer whose backing real memory is pinned, which can
support up to 255 TCP connections to exactly one remote SMC-R peer.
Each RMB is therefore associated with the SMC-R links within a link
group for the two peers and a specific RoCE Protection Domain. Other
than the two peers identified by the SMC-R link, no other SMC-R peers
can have RDMA access to an RMB; this requires a unique Protection
Domain for every SMC-R link. This is critical to ensure integrity of
SMC-R communications.
RMBs are subdivided into multiple elements for efficiency, with each
RMB Element (RMBE) associated with a single TCP connection.
Therefore, multiple TCP connections across an SMC-R link group can
share the same memory for RDMA purposes, reducing the overhead of
having to register additional memory with the RNIC for every new TCP
connection. The number of elements in an RMB and the size of each
RMBE are entirely governed by the owning peer, subject to the SMC-R
architecture rules; however, all RMB elements within a given RMB must
be the same size. Each peer can decide the level of resource-sharing
that is desirable across TCP connections based on local constraints,
Fox, et al. Informational [Page 14]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
such as available system memory. An RMB element is identified to the
remote SMC-R peer via an RMB Element Token, which consists of the
following:
o RMB RToken: The combination of the RKey and virtual address
provided by the RNIC that identifies the start of the RMB for RDMA
operations.
o RMB Index: Identifies the RMB element index in the RMB. Used to
locate a specific RMB element within an RMB. Valid value range is
1-255.
o RMB Element Length: The length of the RMB element's eye catcher
plus the length of the receive buffer. This length is equal for
all RMB elements in a given RMB. This length can be variable
across different RMBs.
Multiple RMBs can be associated to an SMC-R link group, and each peer
in an SMC-R link group manages allocation of its RMBs. RMB
allocation can be asymmetric. For example, Server X can allocate two
RMBs to an SMC-R link group while Server Y allocates five. This
provides maximum implementation flexibility to allow hosts to
optimize RMB management for their own local requirements. The
maximum number of RMBs that can be allocated on one peer to a link
group is 255. If more RMBs are required, the peer may fall back to
IP for subsequent connections or, if the peer is the server, create a
parallel link group.
One use case for multiple RMBs is multiple receive buffer sizes.
Since every element in an RMB must be the same size, multiple RMBs
with different element sizes can be allocated if varying receive
buffer sizes are required.
Also, since the maximum number of TCP connections whose receive
buffers can be allocated to an RMB is 255, multiple RMBs may be
required to provide capacity for large numbers of TCP connections
between two peers.
Fox, et al. Informational [Page 15]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Separately from the RMB, the TCP/IP stack that owns each RMB
maintains control data for each RMB element within its local control
structures. The control data contains flags for maintaining the
state of the TCP data (for example, urgent data indicator) and, most
importantly, the following two cursors, which are illustrated below
in Figure 4:
o The peer producer cursor: This is a wrapping offset into the
RMB element's receive buffer that points to the next byte of data
to be written by the remote peer. This cursor is provided by the
remote peer in a Connection Data Control (CDC) message, which is
sent using RoCE SendMsg processing, and tells the local peer how
far it can consume data in the RMBE buffer.
o The peer consumer cursor: This is a wrapping offset into the
remote peer's RMB element's receive buffer that points to the next
byte of data to be consumed by the remote peer in its own RMBE.
The local peer cannot write into the remote peer's RMBE beyond
this point without causing data loss. This cursor is also
provided by the peer using a Connection Data Control message.
Each TCP connection peer maintains its cursors for a TCP connection's
RMBE in its local control structures. In other words, the peer who
writes into a remote peer's RMBE provides its producer cursor to the
peer whose RMBE it has written into. The peer who reads from its
RMBE provides its consumer cursor to the writing peer. In this
manner, the reads and writes between peers are kept coordinated.
For example, referring to Figure 4, Peer B writes the hashed data
into the receive buffer of Peer A's RMBE. After that write
completes, Peer B uses a CDC message to update its producer cursor to
Peer A, to indicate to Peer A how much data is available for Peer A
to consume. The CDC message that Peer B sends to Peer A wakes up
Peer A and notifies it that there is data to be consumed.
Similarly, when Peer A consumes data written by Peer B, it uses a CDC
message to update its consumer cursor to Peer B to let Peer B know
how much data it has consumed, so Peer B knows how much space is
available for further writes. If Peer B were to write enough data to
Peer A that it would wrap the RMBE receive buffer and exceed the
consumer cursor, data loss would result.
Note that this is a simplistic description of the control flows, and
they are optimized to minimize the number of CDC messages required,
as described in Section 4.7 ("RMB Data Flows").
Fox, et al. Informational [Page 16]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Peer A's RMBE Control Info Peer B's RMBE Control Info
+--------------------------+ +--------------------------+
| | | |
/----Peer producer cursor | +-----+-Peer consumer cursor |
/| | | | |
| +--------------------------+ | +--------------------------+
| Peer A's RMBE |
| +--------------------------+ |
| | +------------------+
| | | |
| | \/ |
| | +------------|
| |-------------+/////////// |
| |//RDMA data written by ///|
| |/// Peer B that is ////// |
| |/available to be consumed/|
| |///////////////////////// |
| |///////// +---------------|
| |----------+/\ |
| | | |
\| | |
\ / |
|\---------/ |
| |
| |
Figure 4: RMBE Cursors
Additional flags and indicators are communicated between peers. In
all cases, these flags and indicators are updated by the peer using
CDC messages, which are sent using RoCE SendMsg. More details on
these additional flags and indicators are described in Section 4.3
("RMBE Control Information").
Fox, et al. Informational [Page 17]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
2.2. SMC-R Link Groups
SMC-R links are logically grouped together to form an SMC-R link
group. The purpose of the link group is for supporting multiple
links between the same two peers to provide for:
o Resilience: Provides transparent and dynamic switching of the link
used by existing TCP connections during link failures, typically
hardware related. TCP traffic using the failing link can be
switched to an active link within the link group, thereby avoiding
disruptions to application workloads.
o Link utilization: Provides an active/active link usage model
allowing TCP traffic to be balanced across the links, which
increases bandwidth and also avoids hardware imbalances and
bottlenecks. Note that both adapter and switch utilization can
become potential resource constraint issues.
SMC-R link group support is required. Resilience is not optional.
However, the user can elect to provision a single RNIC (on one or
both hosts).
Multiple links that are formed between the same two peers fall into
two distinct categories:
1. Equal Links: Links providing equal access to the same RMB(s) at
both endpoints, whereby all TCP connections associated with the
links must have the same VLAN ID and have the same TCP server and
TCP client roles or relationship.
2. Unequal Links: Links providing access to unique, unrelated and
isolated RMB(s) (i.e., for unique VLANs or unique and isolated
application workloads, etc.) or having unique TCP server or client
roles.
Links that are logically grouped together forming an SMC-R link group
must be equal links.
2.2.1. Link Group Types
Equal links within a link group also have another "Link Group Type"
attribute based on the link's associated underlying physical path.
The following SMC-R link types are defined:
1. Single link: the only active link within a link group
2. Parallel link: not allowed -- SMC-R links having the same physical
RNIC at both hosts
Fox, et al. Informational [Page 18]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3. Asymmetric link: links that have unique RNIC adapters at one host
but share a single adapter at the peer host
4. Symmetric link: links that have unique RNIC adapters at both hosts
These link group types are further explained in the following figures
and descriptions.
Figure 2 above shows the single-link case. The single link
illustrated in Figure 2 also establishes the SMC-R link group. Link
groups are supposed to have multiple links, but when only one RNIC is
available at both hosts then only a single link can be created. This
is expected to be a transient case.
Figure 5 shows the symmetric-link case. Both hosts have unique and
redundant RNIC adapters. This configuration meets the objectives for
providing full RoCE redundancy required to provide the level of
resilience required for high availability for SMC-R. While this
configuration is not required, it is a strongly recommended "best
practice" for the exploitation of SMC-R. Single and asymmetric links
must be supported but are intended to provide for short-term
transient conditions -- for example, during a temporary outage or
recycle of an RNIC.
Host X Host Y
+-------------------+ +-------------------+
| | | |
| Protection | | Protection |
| Domain X | | Domain Y |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 64 |
|RToken X| | |<-------------------->| | | |
| | | | | | |RToken Y|
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| | | | | | |
|| RMB | | | | RMB | |
|| | | | | | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
|RToken Z| | | SMC-R Link 2 | | |RToken W|
| | |RNIC 3|<-------------------->|RNIC 4| | |
| QP 9 | | | | QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
Figure 5: Symmetric SMC-R Links
Fox, et al. Informational [Page 19]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Host X Host Y
+-------------------+ +-------------------+
| | | |
| Protection | | Protection |
| Domain X | | Domain Y |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 64 |
|RToken X| | |<-------------------->| | | |
| | | | .->| | |RToken Y|
| \/ +------+ .` +------+ \/ |
|+--------+ | .` | +--------+ |
|| | | .` | | | |
|| RMB | | .` | | RMB | |
|| | | .`SMC-R | | | |
|+--------+ | .` Link 2 | +--------+ |
| /\ +------+ .` +------+ |
|RToken Z| | | .` | |down or |
| | |RNIC 3|<-` |RNIC 4|unavailable |
| QP 9 | | | | |
| +------+ +------+ |
+-------------------+ +-------------------+
Figure 6: Asymmetric SMC-R Links
In the example provided by Figure 6, Host X has two RNICs but Host Y
only has one RNIC because RNIC 4 is not available. This
configuration allows for the creation of an asymmetric link. While
an asymmetric link will provide some resilience (for example, when
RNIC 1 fails), ideally each host should provide two redundant RNICs.
This should be a transient case, and when RNIC 4 becomes available,
this configuration must transition to a symmetric-link configuration.
This transition is accomplished by first creating the new symmetric
link and then deleting the asymmetric link with reason code
"Asymmetric link no longer needed" specified in the DELETE LINK LLC
message.
Fox, et al. Informational [Page 20]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Host X Host Y
+-------------------+ +-------------------+
| | | |
| Protection | | Protection |
| Domain X | | Domain Y |
| +------+ SMC-R Link 1 +------+ |
| QP 8 |RNIC 1|<-------------------->|RNIC 2| QP 64 |
|RToken X| | | | | | |
| | | |<-------------------->| | |RToken Y|
| \/ +------+ SMC-R Link 2 +------+ \/ |
|+--------+ QP 9 | | QP 65 +--------+ |
|| | | | | | | | |
|| RMB |<-- + | | +---->| RMB | |
|| | | | | | |
|+--------+ | | +--------+ |
| +------+ +------+ |
| down or| | | |down or |
| unavailable|RNIC 3| |RNIC 4|unavailable |
| | | | | |
| +------+ +------+ |
+-------------------+ +-------------------+
Figure 7: SMC-R Parallel Links (Not Supported)
Figure 7 shows parallel links, which are two links in the link group
that use the same hardware. This configuration is not permitted.
Because SMC-R multiplexes multiple TCP connections over an SMC-R link
and both links are using the exact same hardware, there is no
additional redundancy or capacity benefit obtained from this
configuration. In addition to providing no real benefit, this
configuration adds the unnecessary overhead of additional queue
pairs, generation of additional RKeys, etc.
2.2.2. Maximum Number of Links in Link Group
The SMC-R protocol defines a maximum of eight symmetric SMC-R links
within a single SMC-R link group. This allows for support for up to
eight unique physical paths between peer hosts. However, in terms of
meeting the basic requirements for redundancy, support for at least
two symmetric links must be implemented. Supporting more than two
links also simplifies implementation for practical matters relating
to dynamically adding and removing links -- for example, starting a
third SMC-R link prior to taking down one of the two existing links.
Recall that all links within a link group must have equal access to
all associated RMBs.
Fox, et al. Informational [Page 21]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The SMC-R protocol allows an implementation to assign an
implementation-specific and appropriate value for maximum symmetric
links. The implementation value must not exceed the architecture
limit of 8; also, the value must not be lower than 2, because the
SMC-R protocol requires redundancy. This does not mean that two
RNICs are physically required to enable SMC-R connectivity, but at
least two RNICs for redundancy are strongly recommended.
The SMC-R peers exchange their implementation maximum link values
during the link group establishment using the defined maximum link
value in the CONFIRM LINK LLC command. Once the initial exchange
completes, the value is set for the life of the link group. The
maximum link value can be provided by both the server and client.
The server must supply a value, whereas the client maximum link value
is optional. When the client does not supply a value, it indicates
that the client accepts the server-supplied maximum value. If the
client provides a value, it cannot exceed the server-supplied maximum
value. If the client passes a lower value, this lower value then
becomes the final negotiated maximum number of symmetric links for
this link group. Again, the minimum value is 2.
During run time, the client must never request that the server add a
symmetric link to a link group that would exceed the negotiated
maximum link value. Likewise, the server must never attempt to add a
symmetric link to a link group that would exceed the negotiated
maximum value.
In terms of counting the number of active links within a link group,
the initial link (or the only/last) link is always counted as 1.
Then, as additional links are added, they are either symmetric or
asymmetric links.
With regards to enforcing the maximum link rules, asymmetric links
are an exception having a unique set of rules:
o Asymmetric links are always limited to one asymmetric link allowed
per link group.
o Asymmetric links must not be counted in the maximum symmetric-link
count calculation. When tracking the current count or enforcing
the negotiated maximum number of links, an asymmetric link is not
to be counted.
Fox, et al. Informational [Page 22]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
2.2.3. Forming and Managing Link Groups
SMC-R link groups are self-defining. The first SMC-R link in a link
group is created using TCP option flows on the TCP three-way
handshake followed by CLC message flows over the TCP connection.
Subsequent SMC-R links in the link group are created by sending LLC
messages over an SMC-R link that already exists in the link group.
Once an SMC-R link group is created, no additional SMC-R links in
that group are created using TCP and CLC negotiation. Because
subsequent SMC-R links are created exclusively by sending LLC
messages over an existing SMC-R link in a link group, the membership
of SMC-R links in a link group is self-defining.
This architecture does not define a specific identifier for an SMC-R
link group. This identification may be useful for network management
and may be assigned in a platform-specific manner, or in an extension
to this architecture.
In each SMC-R link group, one peer is the server for all TCP
connections and the other peer is the client. If there are
additional TCP connections between the peers that use SMC-R and have
the client and server roles reversed, another SMC-R link group is set
up between them with the opposite client-server relationship.
This is required because there are specific responsibilities divided
between the client and server in the management of an SMC-R link
group.
In this architecture, the decision of whether to use an existing
SMC-R link group or create a new SMC-R link group for a TCP
connection is made exclusively by the server.
Management of the links in an SMC-R link group is also a server
responsibility. The server is responsible for adding and deleting
links in a link group. The client may request that the server take
certain actions, but the final responsibility is the server's.
Fox, et al. Informational [Page 23]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
2.2.4. SMC-R Link Identifiers
This architecture defines multiple identifiers to identify SMC-R
links and peers.
o Link number: This is a 1-byte value that identifies an SMC-R link
within a link group. Both the server and the client use this
number to distinguish an SMC-R link from other links within the
same link group. It is only unique within a link group. In order
to prevent timing windows that may occur when a server creates a
new link while the client is still cleaning up a previously
existing link, link numbers cannot be reused until the entire link
numbering space has been exhausted.
o Link user ID: This is an architecturally opaque 4-byte value that
a peer uses to uniquely define an SMC-R link within its own space.
This means that a link user ID is unique within one peer only.
Each peer defines its own link user ID for a link. The peers
exchange this information once during link setup, and it is never
used architecturally again. The purpose of this identifier is for
network management, display, and debugging. For example, an
operator on a client could provide the operator on the server with
the server's link user ID if he requires the server's operator to
check on the operation of a link that the client is having trouble
with.
o Peer ID: The SMC-R peer ID uniquely identifies a specific instance
of a specific TCP/IP stack. It is required because in clustered
and load-balancing environments, an IP address does not uniquely
identify a TCP/IP stack. An RNIC's MAC/GID also doesn't uniquely
or reliably identify a TCP/IP stack, because RNICs can go up and
down and even be redeployed to other TCP/IP stacks in a
multiple-partitioned or virtualized environment. The peer ID is
not only unique per TCP/IP stack but is also unique per instance
of a TCP/IP stack, meaning that if a TCP/IP stack is restarted,
its peer ID changes.
2.3. SMC-R Resilience and Load Balancing
The SMC-R multilink architecture provides resilience for network high
availability via failover capability to an alternate RoCE adapter.
The SMC-R multilink architecture does not define primary, secondary,
or alternate roles to the links. Instead, there are multiple active
links representing multiple redundant RoCE paths over the same LAN.
Fox, et al. Informational [Page 24]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Assignment of TCP connections to links is unidirectional and
asymmetric. This means that the client and server may each choose a
separate link for their RDMA writes associated with a specific TCP
connection.
If a hardware failure occurs or a QP failure associated with an
individual link occurs, then the TCP connections that were associated
with the failing link are dynamically and transparently switched to
use another available link. The server or the client can detect a
failure, immediately move their TCP connections, and then notify
their peer via the DELETE LINK LLC command. While the client can
notify the server of an apparent link failure with the DELETE LINK
LLC command, the server performs the actual link deletion.
The movement of TCP connections to another link can be accomplished
with minimal coordination between the peers. The TCP connection
movement is also transparent to, and non-disruptive to, the TCP
socket application workloads for most failure scenarios. After a
failure, the surviving links and all associated hardware must handle
the link group's workload.
As each SMC-R peer begins to move active TCP connections to another
link, all current RDMA write operations must be allowed to complete.
The moving peer then sends a signal to verify receipt of the last
successful write by its peer. If this verification fails, the TCP
connection must be reset. Once this verification is complete, all
writes that failed may then be retried, in order, over the new link.
Any data writes or CDC messages for which the sender did not receive
write completion must be replayed before any subsequent data or CDC
write operations are sent. LLC messages are not retried over the new
link, because they are dependent on a known link configuration, which
has just changed because of the failure. The initiator of an LLC
message exchange that fails will be responsible for retrying once the
link group configuration stabilizes.
When a new link becomes available and is re-added to the link group,
each peer is free to rebalance its current TCP connections as needed
or only assign new TCP connections to the newly added link. Both the
server and client are free to manage TCP connections across the link
group as needed. TCP connection movement does not have to be
stimulated by a link failure.
The SMC-R architecture also defines orderly versus disorderly
failover. The type of failover is communicated in the LLC
DELETE LINK command and is simply a means to indicate that the link
has terminated (disorderly) or link termination is imminent
(orderly). The orderly link deletion could be initiated via operator
command or programmatically to bring down an idle link. For example,
Fox, et al. Informational [Page 25]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
an operator command could initiate orderly shutdown of an adapter for
service. Implementation of the two types is based on implementation
requirements and is beyond the scope of the SMC-R architecture.
3. SMC-R Rendezvous Architecture
"Rendezvous" is the process that SMC-R-capable peers use to
dynamically discover each others' capabilities, negotiate SMC-R
connections, set up SMC-R links and link groups, and manage those
link groups. A key aspect of SMC-R Rendezvous is that it occurs
dynamically and automatically, without requiring SMC-R link
configuration to be defined by an administrator.
SMC-R Rendezvous starts with the TCP/IP three-way handshake, during
which connection peers use TCP options to announce their SMC-R
capabilities. If both endpoints are SMC-R capable, then Connection
Layer Control (CLC) messages are exchanged between the peers' SMC-R
layers over the newly established TCP connection to negotiate SMC-R
credentials. The CLC message mechanism is analogous to the messages
exchanged by SSL for its handshake processing.
If a new SMC-R link is being set up, Link Layer Control (LLC)
messages are used to confirm RDMA connectivity. LLC messages are
also used by the SMC-R layers at each peer to manage the links and
link groups.
Once an SMC-R link is set up or agreed to by the peers, the TCP
sockets are passed to the peer applications, which use them as
normal. The SMC-R layer, which resides under the sockets layer,
transmits the socket data between peers over RDMA using the SMC-R
protocol, bypassing the TCP/IP stack.
3.1. TCP Options
During the TCP/IP three-way handshake, the client and server indicate
their support for SMC-R by including experimental TCP option 254 on
the three-way handshake flows, in accordance with [RFC6994] ("Shared
Use of Experimental TCP Options"). The Experiment Identifier (ExID)
value used is the string "SMCR" in EBCDIC (IBM-1047) encoding
(0xE2D4C3D9). This ExID has been registered in the "TCP Experimental
Option Experiment Identifiers (TCP ExIDs)" registry maintained
by IANA.
Fox, et al. Informational [Page 26]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
After completion of the three-way TCP handshake, each peer queries
its peer's options. If both peers set the TCP option on the
three-way handshake, inline SMC-R negotiation occurs using CLC
messages. If neither peer, or only one peer, sets the TCP option,
SMC-R cannot be used for the TCP connection, and the TCP connection
completes the setup using the IP fabric.
3.2. Connection Layer Control (CLC) Messages
CLC messages are sent as data payload over the IP network using the
TCP connection between SMC-R layers at the peers. They are analogous
to the messages used to exchange parameters for SSL.
The use of CLC messages is detailed in the following sections. The
following list provides a summary of the defined CLC messages and
their purposes:
o SMC Proposal: Sent from the client to propose that this TCP
connection is eligible to be moved to SMC-R. The client
identifies itself and its subnet to the server and passes the
SMC-R elements for a suggested RoCE path via the MAC and GID.
o SMC Accept: Sent from the server to accept the client's TCP
connection SMC Proposal. The server responds to the client's
proposal by identifying itself to the client and passing the
elements of a RoCE path that the client can use to perform RDMA
writes to the server. This consists of such SMC-R link elements
as RoCE MAC, GID, and RMB information.
o SMC Confirm: Sent from the client to confirm the server's
acceptance of the SMC connection. The client responds to the
server's acceptance by passing the elements of a RoCE path that
the server can use to perform RDMA writes to the client. This
consists of such SMC-R link elements as RoCE MAC, GID, and RMB
information.
o SMC Decline: Sent from either the server or the client to reject
the SMC connection, indicating the reason the peer must decline
the SMC Proposal and allowing the TCP connection to revert back to
IP connectivity.
3.3. LLC Messages
Link Layer Control (LLC) messages are sent between peer SMC-R layers
over an SMC-R link to manage the link or the link group. LLC
messages are sent using RoCE SendMsg and are 44 bytes long. The
44-byte size is based on what can fit into a RoCE Work Queue Element
(WQE) without requiring the posting of receive buffers.
Fox, et al. Informational [Page 27]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
LLC messages generally follow a request-reply semantic. Each message
has a request flavor and a reply flavor, and each request must be
confirmed with a reply, except where otherwise noted. The use of LLC
messages is detailed in the following sections. The following list
provides a summary of the defined LLC messages and their purposes:
o ADD LINK: Used to add a new link to a link group. Sent from the
server to the client to initiate addition of a new link to the
link group, or from the client to the server to request that the
server initiate addition of a new link.
o ADD LINK CONTINUATION: A continuation of ADD LINK that allows the
ADD LINK to span multiple commands, because all of the link
information cannot be contained in a single ADD LINK message.
o CONFIRM LINK: Used to confirm that RoCE connectivity over a newly
created SMC-R link is working correctly. Initiated by the server.
Both this message and its reply must flow over the SMC-R link
being confirmed.
o DELETE LINK: When initiated by the server, deletes a specific link
from the link group or deletes the entire link group. When
initiated by the client, requests that the server delete a
specific link or the entire link group.
o CONFIRM RKEY: Informs the peer on the SMC-R link of the addition
of an RMB to the link group.
o CONFIRM RKEY CONTINUATION: A continuation of CONFIRM RKEY that
allows the CONFIRM RKEY to span multiple commands, in the event
that all of the information cannot be contained in a single
CONFIRM RKEY message.
o DELETE RKEY: Informs the peer on the SMC-R link of the deletion of
one or more RMBs from the link group.
o TEST LINK: Verifies that an already-active SMC-R link is active
and healthy.
o Optional LLC message: Any LLC message in which the two high-order
bits of the opcode are b'10'. This optional message must be
silently discarded by a receiving peer that does not support the
opcode. No such messages are defined in this version of the
architecture; however, the concept is defined to allow for
toleration of possible advanced, optional functions.
Fox, et al. Informational [Page 28]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
CONFIRM LINK and TEST LINK are sensitive to which link they flow on
and must flow on the link being confirmed or tested. The other flows
may flow over any active link in the link group. When there are
multiple links in a link group, a response to an LLC message must
flow over the same link that the original message flowed over, with
the following exceptions:
o ADD LINK request from a server in response to an ADD LINK from a
client.
o DELETE LINK request from a server in response to a DELETE LINK
from a client.
3.4. CDC Messages
Connection Data Control (CDC) messages are sent over the RoCE fabric
between peers using RoCE SendMsg and are 44 bytes long. The 44-byte
size is based on the size that can fit into a RoCE WQE without
requiring the posting of receive buffers. CDC messages are used to
describe the socket application data passed via RDMA write
operations, as well as TCP connection state information, including
producer cursors and consumer cursors, RMBE state information, and
failover data validation.
3.5. Rendezvous Flows
Rendezvous information for SMC-R is exchanged as TCP options on the
TCP three-way handshake flows to indicate capability, followed by
inline TCP negotiation messages to actually do the SMC-R setup.
Formats of all rendezvous options and messages discussed in this
section are detailed in Appendix A.
3.5.1. First Contact
First contact between RoCE peers occurs when a new SMC-R link group
is being set up. This could be because no SMC-R links already exist
between the peers, or the server decides to create a new SMC-R link
group in parallel with an existing one.
3.5.1.1. Pre-negotiation of TCP Options
The client and server indicate their SMC-R capability to each other
using TCP option 254 on the TCP three-way handshake flows.
A client who wishes to do SMC-R will include TCP option 254 using an
ExID equal to the EBCDIC (codepage IBM-1047) encoding of "SMCR" on
its SYN flow.
Fox, et al. Informational [Page 29]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A server that supports SMC-R will include TCP option 254 with the
ExID value of EBCDIC "SMCR" on its SYN-ACK flow. Because the server
is listening for connections and does not know where client
connections will come from, the server implementation may choose to
unconditionally include this TCP option if it supports SMC-R. This
may be required for server implementations where extensions to the
TCP stack are not practical. For server implementations that can add
code to examine and react to packets during the three-way handshake,
the server should only include the SMC-R TCP option on the SYN-ACK if
the client included it on its SYN packet.
A client who supports SMC-R and meets the three conditions outlined
above may optionally include the TCP option for SMC-R on its ACK
flow, regardless of whether or not the server included it on its
SYN-ACK flow. Some TCP/IP stacks may have to include it if the SMC-R
layer cannot modify the options on the socket until the three-way
handshake completes. Proprietary servers should not include this
option on the ACK flow, since including it on the SYN flow was
sufficient to indicate the client's capabilities.
Once the initial three-way TCP handshake is completed, each peer
examines the socket options. SMC-R implementations may do this by
examining what was actually provided on the SYN and SYN-ACK packets
or by performing a getsockopt() operation to determine the options
sent by the peer. If neither peer, or only one peer, specified the
TCP option for SMC-R, then SMC-R cannot be used on this connection
and it proceeds using normal IP flows and processing.
If both peers specified the TCP option for SMC-R, then the TCP
connection is not started yet and the peers proceed to SMC-R
negotiation using inline data flows. The socket is not yet turned
over to the applications; instead, the respective SMC layers exchange
CLC messages over the newly formed TCP connection.
3.5.1.2. Client Proposal
If SMC-R is supported by both peers, the client sends an SMC Proposal
CLC message to the server. It is not immediately apparent on this
flow from client to server whether this is a new or existing SMC-R
link, because in clustered environments a single IP address may
represent multiple hosts. This type of cluster virtual IP address
can be owned by a network-based or host-based Layer 4 load balancer
that distributes incoming TCP connections across a cluster of
servers/hosts. For purposes of high availability, other clustered
environments may also support the movement of a virtual IP address
dynamically from one host in the cluster to another. In summary, the
client cannot predetermine that a connection is targeting the same
host by simply matching the destination IP address for outgoing TCP
Fox, et al. Informational [Page 30]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
connections. Therefore, it cannot predetermine the SMC-R link that
will be used for a new TCP connection. This information will be
dynamically learned, and the appropriate actions will be taken as the
SMC-R negotiation handshake unfolds.
In the SMC-R proposal message, the initiator (client) proposes the
use of SMC-R by including its peer ID, GID, and MAC addresses, as
well as the IP subnet number of the outgoing interface (if IPv4) or
the IP prefix list for the network over which the proposal is sent
(if IPv6). At this point in the flow, the client makes no local
commitments of resources for SMC-R.
When the server receives the SMC Proposal CLC message, it uses the
peer ID provided by the client, plus subnet or prefix information
provided by the client, to determine if it already has a usable SMC-R
link with this SMC-R peer. If there are one or more existing SMC-R
links with this SMC-R peer, the server then decides which SMC-R link
it will use for this TCP connection. See Sections 3.5.2 and 3.5.3
for the cases of reusing an existing SMC-R link or creating a
parallel SMC-R link group between SMC-R peers.
If this is a first contact between SMC-R peers, the server must
validate that it is on the same LAN as the client before continuing.
For IPv4, the server does this by verifying that it has an interface
with an IP subnet number that matches the subnet number sent by the
client in the SMC Proposal. For IPv6, it does this by verifying that
it is directly attached to at least one IP prefix that was listed by
the client in its SMC Proposal message.
If the server agrees to use SMC-R, the server begins the setup of a
new SMC-R link by allocating local QP and RMB resources (setting its
QP state to INIT) and providing its full SMC-R information in an SMC
Accept CLC message to the client over the TCP connection, along with
a flag set indicating that this is a first contact flow. While the
SMC Accept message could flow over any IP route back to the client
depending upon Layer 3 IP routing, the SMC-R credentials provided
must be for the common subnet or prefix between the server and
client, as determined above. If the server cannot or does not want
to do SMC-R with the client, it sends an SMC Decline CLC message to
the client, and the connection data may begin flowing using normal
TCP/IP flows.
Fox, et al. Informational [Page 31]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.1.3. Server Acceptance
When the client receives the SMC Accept from the server, it
determines whether this is a new or existing SMC-R link, using the
combination of the following: the first contact flag, its MAC/GID and
the MAC/GID returned by the server, the VLAN over which the
connection is setting up, and the QP number provided by the server.
If it is an existing SMC-R link and the client agrees to use that
link for the TCP connection, see Section 3.5.2 ("Subsequent Contact")
below. If it is a new SMC-R link between peers that already have an
SMC-R link, then the server is starting a new SMC-R link group.
Assuming that either (1) this is a first contact between peers or
(2) the server is starting a new SMC-R link group, the client now
allocates local QP and RMB resources for the SMC-R link (setting the
QP state to RTR (ready to receive)), associates them with the server
QP as learned from the SMC Accept CLC message, and sends an SMC
Confirm CLC message to the server over the TCP connection with its
SMC-R link information included. The client also starts a timer to
wait for the server to confirm the reliably connected queue pair, as
described below.
3.5.1.4. Client Confirmation
Upon receipt of the client's SMC Confirm CLC message, the server
associates its QP for this SMC-R link with the client's QP as learned
from the SMC Confirm CLC message and sets its QP state to RTS (ready
to send). The client and the server now have reliably connected
queue pairs.
3.5.1.5. Link (QP) Confirmation
Since setting up the SMC-R link and its QPs did not require any
network flows on the RoCE fabric, the client and server must now
confirm connectivity over the RoCE fabric. To accomplish this, the
server will send a CONFIRM LINK Link Layer Control (LLC) message to
the client over the newly created SMC-R link, using the RoCE fabric.
The CONFIRM LINK LLC message will provide the server's MAC, GID, and
QP information for the connection, allow each partner to communicate
the maximum number of links it can tolerate in this link group (the
"link limit"), and will additionally provide two link IDs:
o a 1-byte server-assigned link number that is used by both peers to
identify the link within the link group and is only unique within
a link group.
Fox, et al. Informational [Page 32]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
o a 4-byte link user ID. This opaque value is assigned by the
server for the server's local use and is provided to the client
for management purposes -- for example, to use in network
management displays and products.
When the server sends this message, it will set a timer for receiving
confirmation from the client.
When the client receives the server's confirmation in the form of a
CONFIRM LINK LLC message, it will cancel the confirmation timer it
set when it sent the SMC Confirm message. The client will also
advance its QP state to RTS and respond over the RoCE fabric with a
CONFIRM LINK response LLC message that (1) provides its MAC, GID,
QP number, and link limit, (2) confirms the 1-byte link number sent
by the server, and (3) provides its own 4-byte link user ID to the
server.
Fox, et al. Informational [Page 33]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Host X -- Server Host Y -- Client
+-------------------+ +-------------------+
| Peer ID = PS1 | | Peer ID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| |RNIC 2| QP 64 |
|RToken X| |MAC MA| |MAC MB| | |
| | |GID GA| |GID GB| |RToken Y|
| \/ +------+ (Subnet S1) +------+ \/ |
|+--------+ | | +--------+ |
|| RMB | | | | RMB | |
|+--------+ | | +--------+ |
| +------+ +------+ |
| |RNIC 3| |RNIC 4| |
| |MAC MC| |MAC MD| |
| |GID GC| |GID GD| |
| +------+ +------+ |
+-------------------+ +-------------------+
SYN TCP options(254,"SMCR")
<---------------------------------------------------------
SYN-ACK TCP options(254,"SMCR")
--------------------------------------------------------->
ACK [TCP options(254,"SMCR")]
<--------------------------------------------------------
SMC Proposal(PC1,MB,GB,S1)
<--------------------------------------------------------
SMC Accept(PS1,first contact,MA,GA,MTU,QP8,RToken=X,RMB elem index)
--------------------------------------------------------->
SMC Confirm(PC1,MB,GB,MTU,QP64,RToken=Y,RMB element index)
<--------------------------------------------------------
CONFIRM LINK(MA,GA,QP8, link lim, server link user ID, linknum)
.........................................................>
CONFIRM LINK rsp(MB,GB,QP64, link lim, client link user ID, linknum)
<........................................................
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Square brackets ("[ ]") indicate optional information
Figure 8: First Contact Rendezvous Flows
Fox, et al. Informational [Page 34]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Technically, the data for the TCP connection could now flow over the
RoCE path. However, if this is a first contact, there is no
alternate for this recently established RoCE path. Since in the
current architecture there is no failover from RoCE to IP once
connection data starts flowing, this means that a failure of this
path would disrupt the TCP connection, meaning that the level of
redundancy and failover is less than that provided by IP. If the
network has alternate RoCE paths available, they would not be usable
at this point. This situation would be unacceptable.
3.5.1.6. Second SMC-R Link Setup
Because of the unacceptable situation described above, TCP data will
not be allowed to flow on the newly established SMC-R link until a
second path has been set up, or at least attempted.
If the server has a second RNIC available on the same LAN, it
attempts to set up the second SMC-R link over that second RNIC. If
it only has one RNIC available on the LAN, it will attempt to set up
the second SMC-R link over that one RNIC. In the latter case, the
server is attempting to set up an asymmetric link, in case the client
does have a second RNIC on the LAN.
In either case, the server allocates a new QP over the RNIC it is
attempting to use for the second link and assigns a link number to
the new link; the server also creates an RToken for the RMB over this
second QP (note that this means that the first and second QP each
have their own RToken to represent the same RMB). The server
provides this information, as well as the MAC and GID of the RNIC
over which it is attempting to set up the second link, in an ADD LINK
LLC message that it sends to the client over the SMC-R link that is
already set up.
3.5.1.6.1. Client Processing of ADD LINK LLC Message from Server
When the client receives the server's ADD LINK LLC message, it
examines the GID and MAC provided by the server to determine whether
the server is attempting to use the same server-side RNIC as the
existing SMC-R link or a different one.
If the server is attempting to use the same server-side RNIC as the
existing SMC-R link, then the client verifies that it has a second
RNIC on the same LAN. If it does not, the client rejects the
ADD LINK request from the server, because the resulting link would be
a parallel link, which is not supported within a link group. If the
client does have a second RNIC on the same LAN, it accepts the
request, and an asymmetric link will be set up.
Fox, et al. Informational [Page 35]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
If the server is using a different server-side RNIC from the existing
SMC-R link, then the client will accept the request and a second
SMC-R link will be set up in this SMC-R link group. If the client
has a second RNIC on the same LAN, that second RNIC will be used for
the second SMC-R link, creating symmetric links. If the client does
not have a second RNIC on the same LAN, it will use the same RNIC as
was used for the initial SMC-R link, resulting in the setup of an
asymmetric link in the SMC-R link group.
In either case, when the client accepts the server's ADD LINK
request, it allocates a new QP on the chosen RNIC and creates an RKey
over that new QP for the client-side RMB for the SMC-R link group,
then sends an ADD LINK reply LLC message to the server providing that
information as well as echoing the link number that was sent by the
server.
If the client rejects the server's ADD LINK request, it sends an ADD
LINK reply LLC message to the server with the reason code for the
rejection.
3.5.1.6.2. Server Processing of ADD LINK Reply LLC Message from Client
If the client sends a negative response to the server or no reply is
received, the server frees the RoCE resources it had allocated for
the new link. Having a single link in an SMC-R link group is
undesirable. The server's recovery is detailed in Appendix C.8
("Failure to Add Second SMC-R Link to a Link Group").
If the client sends a positive reply to the server with
MAC/GID/QP/RKey information, the server associates its QP for the new
SMC-R link to the QP that the client provided. Now, the new SMC-R
link is in the same situation that the first was in after the client
sent its ACK packet -- there is a reliably connected queue pair over
the new RoCE path, but there have been no RoCE flows to confirm that
it's actually usable. So, at this point, the client and server will
exchange CONFIRM LINK LLC messages just like they did on the first
SMC-R link.
If either peer receives a failure during this second CONFIRM LINK LLC
exchange (either an immediate failure -- which implies that the
message did not reach the partner -- or a timeout), it sends a DELETE
LINK LLC message to the partner over the first (and now only) link in
the link group. This DELETE LINK LLC message must be acknowledged
before data can flow on the single link in the link group.
Fox, et al. Informational [Page 36]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Host X -- Server Host Y -- Client
+-------------------+ +-------------------+
| Peer ID = PS1 | | Peer ID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 64 |
|RToken X| |MAC MA|<-------------------->|MAC MB| | |
| | |GID GA| |GID GB| |RToken Y|
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| | | | | | |
|| RMB | | | | RMB | |
|| | | | | | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
| | |RNIC 3| SMC-R Link 2 |RNIC 4| | |
|RToken Z| |MAC MC|<-------------------->|MAC MD| |RToken W |
| QP 9 |GID GC| (being added) |GID GD| QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
First SMC-R link setup as shown in Figure 8
<-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.->
ADD LINK request(QP9,MC,GC, link number = 2)
............................................>
ADD LINK response(QP65,MD,GD, link number = 2)
<............................................
ADD LINK CONTINUATION request(RToken=Z)
............................................>
ADD LINK CONTINUATION response(RToken=W)
<............................................
CONFIRM LINK(MC,GC,QP9, link number = 2, link user ID)
.............................................>
CONFIRM LINK response(MD,GD,QP65, link number = 2, link user ID)
<.............................................
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Figure 9: First Contact, Second Link Setup
Fox, et al. Informational [Page 37]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.1.6.3. Exchange of RKeys on Second SMC-R Link
Note that in the scenario described here -- first contact -- there is
only one RMB RKey to exchange on the second SMC-R link, and it is
exchanged in the ADD LINK CONTINUATION request and reply. In
scenarios other than first contact -- for example, adding a new SMC-R
link to a longstanding link group with multiple RMBs -- additional
flows will be required to exchange additional RMB RKeys. See
Section 3.5.5.2.3 ("Adding a New SMC-R Link to a Link Group with
Multiple RMBs") for more details on these flows.
3.5.1.6.4. Aborting SMC-R and Falling Back to IP
If both partners don't provide the SMC-R TCP option during the
three-way TCP handshake, the connection falls back to normal TCP/IP.
During the SMC-R negotiation that occurs after the three-way TCP
handshake, either partner may break off SMC-R by sending an SMC
Decline CLC message. The SMC Decline CLC message may be sent in
place of any expected message and may also be sent during the CONFIRM
LINK LLC exchange if there is a failure before any application data
has flowed over the RoCE fabric. For more details on exactly when an
SMC Decline can flow during link group setup, see Appendices C.1
("SMC Decline during CLC Negotiation") and C.2 ("SMC Decline during
LLC Negotiation").
If this fallback to IP happens while setting up a new SMC-R link
group, the RoCE resources allocated for this SMC-R link group
relationship are torn down, and it will be retried as a new SMC-R
link group next time a connection starts between these peers with
SMC-R proposed. Note that if this happens because one side doesn't
support SMC-R, there will be very little to tear down, as the TCP
option will have failed to flow on either the initial SYN or the
SYN-ACK before either side had reserved any local RoCE resources.
3.5.2. Subsequent Contact
"Subsequent contact" means setting up a new TCP connection between
two peers that already have an SMC-R link group between them and
reusing the existing SMC-R link group. In this case, it is not
necessary to allocate new QPs. However, it is possible that a new
RMB has been allocated for this TCP connection, if the previous TCP
connection used the last element available in the previously used
RMB, or for any other implementation-dependent reason. For this
reason, and for convenience and error checking, the same TCP
option 254, followed by the inline negotiation method described for
initial contact, will be used for subsequent contact, but the
processing differs in some ways. That processing is described below.
Fox, et al. Informational [Page 38]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.2.1. SMC-R Proposal
When the client begins the inline negotiation with the server, it
does not know if this is a first contact or a subsequent contact.
The client cannot know this information until it sees the server's
peer ID, to determine whether or not it already has an SMC-R link
with this peer that it can use. There are several reasons why it is
not sufficient to use the partner IP address, subnet, VLAN, or other
IP information to make this determination. The most obvious reason
is distributed systems: if the server IP address is actually a
virtual IP address representing a distributed cluster, the actual
host serving this TCP connection may not be the same as the host that
served the last TCP connection to this same IP address.
After the TCP three-way handshake, assuming that both partners
indicate SMC-R capability, the client builds and sends the
SMC Proposal CLC message to the server in exactly the same manner as
it does in the "first contact" case, and in fact at this point
doesn't know if it's a first contact or a subsequent contact. As in
the "first contact" case, the client sends its peer ID value,
suggested RNIC MAC/GID, and IP subnet or prefix information.
Upon receiving the client's proposal, the server looks up the
provided peer ID to determine if it already has a usable SMC-R
link group with this peer. If it does already have a usable SMC-R
link group, the server then needs to decide whether it will use the
existing SMC-R link group or create a new link group. For the case
of the new link group, see Section 3.5.3 ("First Contact Variation:
Creating a Parallel Link Group") below.
For this discussion, assume that the server decides to use the
existing SMC-R link group for the TCP connection, which is expected
to be the most common case. The server is responsible for making
this decision. The server then needs to communicate that information
to the client, but it is not necessary to allocate, associate, and
confirm QPs for the chosen SMC-R link. All that remains to be done
is to set up RMB space for this TCP connection.
If one of the RMBs already in use for this SMC-R link group has an
available element that uses the appropriate buffer size, the server
merely chooses one for this TCP connection and then sends an SMC
Accept CLC message providing the full RoCE information for the chosen
SMC-R link to the client, using the same format as the SMC Accept CLC
message described in Section 3.5.1 ("First Contact") above.
Fox, et al. Informational [Page 39]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The server may choose to use the SMC-R link that matches the
suggested MAC/GID provided by the client in the SMC Proposal for its
RDMA writes but is not obligated to do so. The final decision on
which specific SMC-R link to assign a TCP connection to is an
independent server and client decision.
It may be necessary for the server to allocate a new RMB for this
connection. The reasons for this are implementation dependent and
could include the following:
o no available space in existing RMB or RMBs, or
o desire to allocate a new RMB that uses a different buffer size
from the ones already created, or
o any other implementation-dependent reason
In this case, the server will allocate the new RMB and then perform
the flows described in Section 3.5.5.2.1 ("Adding a New RMB to an
SMC-R Link Group"). Once that processing is complete, the server
then provides the full RoCE information, including the new RKey, for
this connection in an SMC Confirm CLC message to the client.
3.5.2.2. SMC-R Acceptance
Upon receiving the SMC Accept CLC message from the server, the client
examines the RoCE information provided by the server to determine
whether this is a first contact for a new SMC-R link group or a
subsequent contact for an existing SMC-R link group. It is a
subsequent contact if the server-side peer ID, GID, MAC, and QP
number provided in the packet match a known SMC-R link, and the first
contact flag is not set. If this is not the case -- for example, the
GID and MAC match but the QP is new -- then the server is creating a
new, parallel SMC-R link group, and this is treated as a first
contact.
A different RMB RToken does not indicate a first contact, as the
server may have allocated a new RMB or may be using several RMBs for
this SMC-R link. The client needs the server's RMB information only
for its RDMA writes to the server, and since there is no requirement
for symmetric RMBs, this information is simply control information
for the RDMA writes on this SMC-R link.
The client must validate that the RMB element being provided by the
server is not in use by another TCP connection on this SMC-R link
group. This validation must validate the new <rtoken, index> across
Fox, et al. Informational [Page 40]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
all known <rtoken, index> on this link group. See Section 4.4.2
("RMB Element Reuse and Conflict Resolution") for the case in which
the server tries to use an RMB element that is already in use on this
link group.
Once the client has determined that this TCP connection is a
subsequent contact over an existing SMC-R link, it performs an RMB
allocation process similar to what the server did: it either
(1) allocates an element from an RMB already associated with this
SMC-R link or (2) allocates a new RMB, associates it with this SMC-R
link, and then chooses an element out of it.
If the client allocates a new RMB for this TCP connection, it
performs the processing described in Section 3.5.5.2.1 ("Adding a New
RMB to an SMC-R Link Group"). Once that processing is complete, the
client provides its full RoCE information for this TCP connection in
an SMC Confirm CLC message.
Because an SMC-R link with a verified connected QP already exists and
is being reused, there is no need for verification or alternate QP
selection flows or timers.
3.5.2.3. SMC-R Confirmation
When the server receives the client's SMC Confirm CLC message on a
subsequent contact, it verifies the following:
o The RMB element provided by the client is not already in use by
another TCP connection on this SMC-R link group (see Section 4.4.2
("RMB Element Reuse and Conflict Resolution") for the case in
which it is).
o The MAC/GID/QP information provided by the client matches an
active link within the link group. The client is free to select
any valid/active link. The client is not required to select the
same link as the server.
If this validation passes, the server stores the client's RMB
information for this connection, and the RoCE setup of the TCP
connection is complete.
3.5.2.4. TCP Data Flow Race with SMC Confirm CLC Message
On a subsequent contact TCP/IP connection, a peer may send data as
soon as it has received the peer RMB information for the connection.
There are no additional RoCE confirmation flows, since the QPs on the
SMC-R link are already reliably connected and verified.
Fox, et al. Informational [Page 41]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
In the majority of cases, the first data will flow from the client to
the server. The client must send the SMC Confirm CLC message before
sending any connection data over the chosen SMC-R link; however, the
client need not wait for confirmation of this message, and in fact
there will be no such confirmation. Since the server is required to
have the RMB fully set up and ready to receive data from the client
before sending an SMC Accept CLC message, the client can begin
sending data over the SMC-R link immediately upon completing the send
of the SMC Confirm CLC message.
It is possible that data from the client will arrive at the
server-side RMB before the SMC Confirm CLC message from the client
has been processed. In this case, the server must handle this race
condition and not provide the arrived TCP data to the socket
application until the SMC Confirm CLC message has been received and
fully processed, opening the socket.
If the server has initial data to send to the client that is not a
response to the client (this case should be rare), it can send the
data immediately upon receiving and processing the SMC Confirm CLC
message from the client. The client must have opened the TCP socket
to the client application upon sending the SMC Confirm CLC message so
the client will be ready to process data from the server.
3.5.3. First Contact Variation: Creating a Parallel Link Group
Recall that parallel SMC-R links within an SMC-R link group are not
supported. These are multiple SMC-R links within a link group that
use the same network path. However, multiple SMC-R link groups
between the same peers are supported. This means that if multiple
SMC-R links over the same RoCE path are desired, it is necessary to
use multiple SMC-R link groups. While not a recommended practice,
this could be done for platform-specific reasons, like QP separation
of different workloads. Only the server can drive the creation of
multiple SMC-R link groups between peers.
At a high level, when the server decides to create an additional
SMC-R link group with a client with which it already has an SMC-R
link group, the flows are basically the same as the normal
"first contact" case described above. The following text provides
more detail and clarification of processing in this case.
When the server receives the SMC Proposal CLC message from the client
and, using the MAC/GID information, determines that it already has an
SMC-R link group with this client, the server can either reuse the
existing SMC-R link group (detailed in Section 3.5.2 ("Subsequent
Contact") above) or create a new SMC-R link group in addition to the
existing one.
Fox, et al. Informational [Page 42]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
If the server decides to create a new SMC-R link group, it does the
same processing it would have done for first contact: allocate QP and
RMB resources as well as alternate QP resources, and communicate the
QP and RMB information to the client in the SMC Accept CLC message
with the first contact flag set.
When the client receives the server's SMC Accept CLC message with the
new QP information and the first contact flag set, it knows that the
server is creating a new SMC-R link group even though it already has
an SMC-R link group with the server. In this case, the client will
also allocate a new QP for this new SMC-R link, allocate an RMB for
it, and generate an RKey for it.
Note that multiple SMC-R link groups between the same peers must
access different RMB resources, so new RMBs will be required. Using
the same RMBs that are in use in another SMC-R link group is not
permitted.
The client then associates its new QP with the server's new QP and
sends its SMC Confirm CLC message back to the server providing the
new QP/RMB information, and then sets its confirmation timer for the
new SMC-R link.
When the server receives the client's SMC Confirm CLC message, it
associates its QP with the client's QP as learned from the SMC
Confirm CLC message and sends a confirmation LLC message. The rest
of the flow, with the confirmation QP and setup of additional SMC-R
links, unfolds just like the "first contact" case.
3.5.4. Normal SMC-R Link Termination
The normal socket API trigger points are used by the SMC-R layer to
initiate SMC-R connection termination flows. The main design point
for SMC-R normal connection flows is to use the SMC-R protocol to
first shut down the SMC-R connection and free up any SMC-R RDMA
resources, and then allow the normal TCP connection termination
protocol (i.e., FIN processing) to drive cleanup of the TCP
connection that exists on the IP fabric. This design point is very
important in ensuring that RDMA resources such as the RMBEs are only
freed and reused when both SMC-R endpoints are completely done with
their RDMA write operations to the partner's RMBE.
When the last TCP connection over an SMC-R link group terminates, the
link group can be terminated. Similar to creation of SMC-R links and
link groups, the primary responsibility for determining that normal
termination is needed and initiating it lies with the server.
Fox, et al. Informational [Page 43]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Implementations may opt to set timers to keep SMC-R link groups up
for a specified time after the last TCP connection ends, to avoid
churn in cases where TCP connections come and go regularly.
The link or link group may also be terminated as a result of a
command initiated by the operator. This command can be entered at
either the client or the server. If entered at the client, the
client requests that the server perform link or link group
termination, and the responsibility for doing so ultimately lies with
the server.
When the server determines that the SMC-R link group is to be
terminated, it sends a DELETE LINK LLC message to the client, with a
flag set indicating that all links in the link group are to be
terminated. After receiving confirmation from the adapter that the
DELETE LINK LLC message has been sent, the server can clean up its
end of the link group (QPs, RMBs, etc.). Upon receipt of the DELETE
LINK message from the server, the client must immediately comply and
clean up its end of the link group. Any TCP connections that the
client believes to be active on the link group must be immediately
terminated.
The client can request that the server delete the link group as well.
The client does this by sending a DELETE LINK message to the server,
indicating that cleanup of all links is requested. The server must
comply by sending a DELETE LINK to the client and processing as
described in the previous paragraph. If there are TCP connections
active on the link group when the server receives this request, they
are immediately terminated by sending a RST flow over the IP fabric.
3.5.5. Link Group Management Flows
3.5.5.1. Adding and Deleting Links in an SMC-R Link Group
The server has the lead role in managing the composition of the link
group. Links are added to the link group by the server. The client
may notify the server of new conditions that may result in the server
adding a new link, but the server is ultimately responsible. In
general, links are deleted from the link group by the server;
however, in certain error cases the client may inform the server that
a link must be deleted and treat it as deleted without waiting for
action from the server. These flows are detailed in the sections
that follow.
Fox, et al. Informational [Page 44]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.5.1.1. Server-Initiated ADD LINK Processing
As described in previous sections, the server initiates an ADD LINK
exchange to create redundancy in a newly created link group. Once a
link group is established, the server may also initiate ADD LINK for
other reasons, including:
o Availability of additional resources on the server host to support
an additional SMC-R link. This may include the provisioning of an
additional RNIC, more storage becoming available to support
additional QP resources, operator command, or any other
implementation-dependent reason. Note that in order to be
available for an existing link group a new RNIC must be attached
to the same RoCE LAN that the link group is using.
o Receipt of notification from the client that additional resources
on the client are available to support an additional SMC-R link.
See Section 3.5.5.1.2 ("Client-Initiated ADD LINK Processing").
Server-initiated ADD LINK processing in an established SMC-R link
group is the same as the ADD LINK processing described in
Section 3.5.1.6 ("Second SMC-R Link Setup"), with the following
changes:
o If an asymmetric SMC-R link already exists in the link group, a
second asymmetric link will not be created. Only one asymmetric
link is permitted in a link group.
o TCP data flow on already-existing link(s) in the link group is not
halted or otherwise affected during the process of setting up the
additional link.
The server will not initiate ADD LINK processing if the link group
already has the maximum number of links negotiated by the partners.
3.5.5.1.2. Client-Initiated ADD LINK Processing
If an additional RNIC becomes available for an existing SMC-R link
group on the client's side, the client notifies the server by sending
an ADD LINK request LLC message to the server. Unlike an ADD LINK
request sent by the server to the client, this ADD LINK request
merely informs the server that the client has a new RNIC. If the
link group lacks redundancy or has redundancy only on an asymmetric
link with a single RNIC on the client side, the server must initiate
an ADD LINK exchange in response to this message, to create or
improve the link group's redundancy.
Fox, et al. Informational [Page 45]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
If the link group already has symmetric-link redundancy but has fewer
than the negotiated maximum number of links, the server may respond
by initiating an ADD LINK exchange to create a new link using the
client's new resource but is not required to do so.
If the link group already has the negotiated maximum number of links,
the server must ignore the client's ADD LINK request LLC message.
Because the server is not required to respond to the client's
ADD LINK LLC message in all cases, the client must not wait for a
response or throw an error if one does not come.
3.5.5.1.3. Server-Initiated DELETE LINK Processing
Reasons that a server may delete a link include the following:
o The link has not been used for TCP connections for an
implementation-defined time interval, and deleting the link will
not cause the link group to lack redundancy.
o Errors in resources supporting the link occur. These errors may
include, but are not limited to, RNIC errors, QP errors, and
software errors.
o The RNIC supporting this SMC-R link is being taken down, either
because of an error case or because of an operator or software
command.
If a link being deleted is supporting TCP connections and there are
one or more surviving links in the link group, the TCP connections
are moved to the surviving links. For more information on this
processing, see Section 2.3 ("SMC-R Resilience and Load Balancing").
The server deletes a link from the link group by sending a
DELETE LINK request LLC message to the client over any of the usable
links in the link group. Because the DELETE LINK LLC message
specifies which link is to be deleted, it may flow over any link in
the link group. The server must not clean up its RoCE resources for
the link until the client responds.
The client responds to the server's DELETE LINK request LLC message
by sending the server a DELETE LINK response LLC message. The client
must respond positively; it cannot decline to delete the link. Once
the server has received the client's DELETE LINK response, both sides
may clean up their resources for the link.
Fox, et al. Informational [Page 46]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Either a positive write completion or some other indication from the
RNIC on the client's side is sufficient to indicate to the client
that the server has received the DELETE LINK response.
Host X Host Y
+-------------------+ +-------------------+
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 9 |
|RToken X| |Failed|<--X----X----X----X-->| | |
| | | | | | |
| \/ +------+ +------+ |
|+--------+ | | |
|| Deleted| | | |
|| RMB | | | |
|| | | | |
|+--------+ | | |
| /\ +------+ +------+ |
|RToken Z| | | SMC-R Link 2 | | |
| | |RNIC 3|<-------------------->|RNIC 4| |
| QP 64| | | | QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
DELETE LINK(request, link number = 1,
................................................>
reason code = RNIC failure)
DELETE LINK(response, link number = 1)
<................................................
(Note: Architecturally, this exchange can flow over either
SMC-R link but most likely flows over Link 2, since
the RNIC for Link 1 has failed.)
Figure 10: Server-Initiated DELETE LINK Flow
Fox, et al. Informational [Page 47]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.5.1.4. Client-Initiated DELETE LINK Request
The client may request that the server delete a link for the same
reasons that the server may delete a link, except for inactivity
timeout.
Because the client depends on the server to delete links, there are
two types of delete requests from client to server:
o Orderly: The client is requesting that the server delete the link
when able. This would result from an operator command to bring
down the RNIC or some other nonfatal reason. In this case, the
server is required to delete the link but may not do it right
away.
o Disorderly: The server must delete the link right away, because
the client has experienced a fatal error with the link.
In either case, the server responds by initiating a DELETE LINK
exchange with the client, as described in the previous section. The
difference between the two is whether the server must do so
immediately or can delay for an opportunity to gracefully delete the
link.
Fox, et al. Informational [Page 48]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Host X Host Y
+-------------------+ +-------------------+
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 9 |
|RToken X| | |<---X--X--X--X--X--X->|Failed| |
| | | | | | |
| \/ +------+ +------+ |
|+--------+ | | |
|| Deleted| | | |
|| RMB | | | |
|| | | | |
|+--------+ | | |
| /\ +------+ +------+ |
|RToken Z| | | SMC-R Link 2 | | |
| | |RNIC 3|<-------------------->|RNIC 4| |
| QP 64| | | | QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
DELETE LINK(request, link number = 1, disorderly,
<...............................................
reason code = RNIC failure)
DELETE LINK(request, link number = 1,
................................................>
reason code = RNIC failure)
DELETE LINK(response, link number = 1)
<................................................
(Note: Architecturally, this exchange can flow over either
SMC-R link but most likely flows over Link 2, since
the RNIC for Link 1 has failed.)
Figure 11: Client-Initiated DELETE LINK Flow
3.5.5.2. Managing Multiple RKeys over Multiple SMC-R Links in a
Link Group
After the initial contact sequence completes and the number of TCP
connections increases, it is possible that the SMC peers could add
more RMBs to the link group. Recall that each peer independently
manages its RMBs. Also recall that an RMB's RToken is specific to a
QP, which means that when there are multiple SMC-R links in a link
group, each RMB accessed with the link group requires a separate
RToken for each SMC-R link in the group.
Fox, et al. Informational [Page 49]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Each RMB that is added to a link must be added to all links within
the link group. The set of RMBs created for the link is called the
"RToken set". The RTokens must be exchanged with the peer. As RMBs
are added and deleted, the RToken set must remain in sync.
3.5.5.2.1. Adding a New RMB to an SMC-R Link Group
A new RMB can be added to an SMC-R link group on either the client
side or the server side. When an additional RMB is added to an
existing SMC-R link group, that RMB must be associated with the QPs
for each link in the link group. Therefore, when an RMB is added to
an SMC-R link group, its RMB RToken for each SMC-R link's QP must be
communicated to the peer.
The tokens for a new RMB added to an existing SMC-R link group are
communicated using CONFIRM RKEY LLC messages, as shown in Figure 12.
The RToken set is specified as pairs: an SMC-R link number, paired
with the new RMB's RToken over that SMC-R link. To preserve failover
capability, any TCP connection that uses a newly added RMB cannot go
active until all RTokens for the RMB have been communicated for all
of the links in the link group.
Fox, et al. Informational [Page 50]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Host X Host Y
+-------------------+ +-------------------+
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 9 |
|RToken X| | |<-------------------->| | |
| | | | | | |
| \/ +------+ +------+ |
|+--------+ | | |
|| New | | | |
|| RMB | | | |
|| | | | |
|+--------+ | | |
| /\ +------+ +------+ |
|RToken Z| | | SMC-R Link 2 | | |
| | |RNIC 3|<-------------------->|RNIC 4| |
| QP 64| | | | QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
CONFIRM RKEY(request, Add,
................................................>
RToken set((Link 1,RToken X),(Link 2,RToken Z)))
CONFIRM RKEY(response, Add,
<................................................
RToken set((Link 1,RToken X),(Link 2,RToken Z)))
(Note: This exchange can flow over either SMC-R link.)
Figure 12: Add RMB to Existing Link Group
Implementations may choose to proactively add RMBs to link groups in
anticipation of need. For example, an implementation may add a new
RMB when a certain usage threshold (e.g., percentage used) for all of
its existing RMBs has been exceeded.
A new RMB may also be added to an existing link group on an as-needed
basis -- for example, when a new TCP connection is added to the link
group but there are no available RMB elements. In this case, the CLC
exchange is paused while the peer that requires the new RMB adds it.
An example of this is illustrated in Figure 13.
Fox, et al. Informational [Page 51]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Host X -- Server Host Y -- Client
+-------------------+ +--------------------+
| Peer ID = PS1 | | Peer ID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 64 |
|RToken X| |MAC MA|<-------------------->|MAC MB| | |
| | |GID GA| |GID GB| |RToken Y2|
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| | | Subnet S1 | | New | |
|| RMB | | | | RMB | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
| | |RNIC 3| SMC-R Link 2 |RNIC 4| |RToken W2|
| | |MAC MC|<-------------------->|MAC MD| | |
| QP 9 |GID GC| |GID GD| QP 65 |
| +------+ +------+ |
+-------------------+ +--------------------+
SYN / SYN-ACK / ACK TCP three-way handshake with TCP option
<--------------------------------------------------------->
SMC Proposal(PC1,MB,GB,S1)
<--------------------------------------------------------
SMC Accept(PS1,not 1st contact,MA,GA,QP8,RToken=X,RMB elem index)
--------------------------------------------------------->
CONFIRM RKEY(request, Add,
<........................................................
RToken set((Link 1,RToken Y2),(Link 2,RToken W2)))
CONFIRM RKEY(response, Add,
........................................................>
RToken set((Link 1,RToken Y2),(Link 2,RToken W2)))
SMC Confirm(PC1,MB,GB,QP64,RToken=Y2, RMB element index)
<--------------------------------------------------------
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Figure 13: Client Adds RMB during TCP Connection Setup
Fox, et al. Informational [Page 52]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.5.2.2. Deleting an RMB from an SMC-R Link Group
Either peer can delete one or more of its RMBs as long as it is not
being used for any TCP connections. Ideally, an SMC-R peer would use
a timer to avoid freeing an RMB immediately after the last TCP
connection stops using it, to keep the RMB available for later TCP
connections and avoid thrashing with addition and deletion of RMBs.
Once an SMC-R peer decides to delete an RMB, it sends a DELETE RKEY
LLC message to its peer. It can then free the RMB once it receives
a response from the peer. Multiple RMBs can be deleted in a
DELETE RKEY exchange.
Note that in a DELETE RKEY message, it is not necessary to specify
the full RToken for a deleted RMB. The RMB's RKey over one link in
the link group is sufficient to specify which RMB is being deleted.
Host X Host Y
+-------------------+ +-------------------+
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 9 |
|RToken X| | |<-------------------->| | |
| | | | | | |
| \/ +------+ +------+ |
|+--------+ | | |
|| Deleted| | | |
|| RMB | | | |
|| | | | |
|+--------+ | | |
| /\ +------+ +------+ |
|RToken Z| | | SMC-R Link 2 | | |
| | |RNIC 3|<-------------------->|RNIC 4| |
| QP 9 | | | | |
| +------+ +------+ |
+-------------------+ +-------------------+
DELETE RKEY(request, RKey list(RKey X))
................................................>
DELETE RKEY(response, RKey list(RKey X))
<................................................
(Note: This exchange can flow over either SMC-R link.)
Figure 14: Delete RMB from SMC-R Link Group
Fox, et al. Informational [Page 53]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.5.2.3. Adding a New SMC-R Link to a Link Group with Multiple RMBs
When a new SMC-R link is added to an existing link group, there could
be multiple RMBs on each side already associated with the link group.
There could also be a different number of RMBs on one side than on
the other, because each peer manages its RMBs independently. Each of
these RMBs will require a new RToken to be used on the new SMC-R
link, and those new RTokens must then be communicated to the peer.
This requires two-way communication, as the server will have to
communicate its RTokens to the client and vice versa.
RTokens are communicated between peers in pairs. Each RToken pair
consists of:
o The RToken for the RMB, as is already known on an existing SMC-R
link in the link group.
o The RToken for the same RMB, to be used on the new SMC-R link.
These pairs are required to ensure that each peer knows which RTokens
across QPs are equivalent.
The ADD LINK request and response LLC messages do not have enough
space to contain any RToken pairs. ADD LINK CONTINUATION LLC
messages are used to communicate these pairs, as shown in Figure 15.
The ADD LINK CONTINUATION LLC messages are sent on the same SMC-R
link that the ADD LINK LLC messages were sent over, and in both the
ADD LINK and ADD LINK CONTINUATION LLC messages the first RToken in
each RToken pair will be the RToken for the RMB as known on the SMC-R
link over which the LLC message is being sent.
Fox, et al. Informational [Page 54]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Host X -- Server Host Y -- Client
+-------------------+ +-------------------+
| Peer ID = PS1 | | Peer ID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 64 |
|RKey set| |MAC MA|<-------------------->|MAC MB| |RKey set|
|X,Y,Z | |GID GA| |GID GB| |Q,R,S,T |
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| 3 RMBs | | | | 4 RMBs | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
|RKey set| |RNIC 3| SMC-R Link 2 |RNIC 4| | RKey set|
|U,V,W | |MAC MC|<-------------------->|MAC MD| | L,M,N,P |
| QP 9 |GID GC| (being added) |GID GD| QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
ADD LINK request (QP9,MC,GC, link number = 2)
............................................>
ADD LINK response (QP65,MD,GD, link number = 2)
<............................................
ADD LINK CONTINUATION req(RToken pairs=((X,U),(Y,V),(Z,W)))
............................................>
ADD LINK CONTINUATION rsp(RToken pairs=((Q,L),(R,M),(S,N),(T,P)))
<.............................................
CONFIRM LINK req/rsp exchange on Link 2
<.............................................>
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Figure 15: Exchanging RKeys when a New Link Is Added to a Link Group
Fox, et al. Informational [Page 55]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.5.3. Serialization of LLC Exchanges, and Collisions
LLC flows can be divided into two main groups for serialization
considerations.
The first group is LLC messages that are independent and can flow at
any time. These are one-time, unsolicited messages that either do
not have a required response or have a simple response that does not
interfere with the operations of another group of messages. These
messages are as follows:
o TEST LINK from either the client or the server: This message
requires a TEST LINK response to be returned but does not affect
the configuration of the link group or the RKeys.
o ADD LINK from the client to the server: This message is provided
as an "FYI" to the server to let it know that the client has an
additional RNIC available. The server is not required to act upon
or respond to this message.
o DELETE LINK from the client to the server: This message informs
the server that either (1) the client has experienced an error or
problem that requires a link or link group to be terminated or
(2) an operator has commanded that a link or link group be
terminated. The server does not respond directly to the message;
rather, it initiates a DELETE LINK exchange as a result of
receiving it.
o DELETE LINK from the server to the client, with the "delete entire
link group" flag set: This message informs the client that the
entire link group is being deleted.
The second group is LLC messages that are part of an exchange of LLC
messages that affects link group configuration; this exchange must
complete before another exchange of LLC messages that affects link
group configuration can be processed. When a peer knows that one of
these exchanges is in progress, it must not start another exchange.
These exchanges are as follows:
o ADD LINK / ADD LINK response / ADD LINK CONTINUATION / ADD LINK
CONTINUATION response / CONFIRM LINK / CONFIRM LINK response: This
exchange, by adding a new link, changes the configuration of the
link group.
o DELETE LINK / DELETE LINK response initiated by the server,
without the "delete entire link group" flag set: This exchange, by
deleting a link, changes the configuration of the link group.
Fox, et al. Informational [Page 56]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
o CONFIRM RKEY / CONFIRM RKEY response or DELETE RKEY / DELETE RKEY
response: This exchange changes the RMB configuration of the link
group. RKeys cannot change while links are being added or deleted
(while an ADD LINK or DELETE LINK is in progress). However,
CONFIRM RKEY and DELETE RKEY are unique in that both the client
and server can independently manage (add or remove) their own
RMBs. This allows each peer to concurrently change their RKeys
and therefore concurrently send CONFIRM RKEY or DELETE RKEY
requests. The concurrent CONFIRM RKEY or DELETE RKEY requests can
be independently processed and do not represent a collision.
Because the server is in control of the configuration of the link
group, many timing windows and collisions are avoided, but there are
still some that must be handled.
3.5.5.3.1. Collisions with ADD LINK / CONFIRM LINK Exchange
Colliding LLC message: TEST LINK
Action to resolve: Send immediate TEST LINK reply.
Colliding LLC message: ADD LINK from client to server
Action to resolve: Server ignores the ADD LINK message. When
client receives server's ADD LINK, client will consider that
message to be in response to its ADD LINK message and the flow
works. Since both client and server know not to start this
exchange if an ADD LINK operation is already underway, this can
only occur if the client sends this message before receiving the
server's ADD LINK and this message crosses with the server's ADD
LINK message; therefore, the server's ADD LINK arrives at the
client immediately after the client sent this message.
Colliding LLC message: DELETE LINK from client to server, specific
link specified
Action to resolve: Server queues the DELETE LINK message and
processes it after the ADD LINK exchange completes. If it is an
orderly link termination, it can wait until after this exchange
continues. If it is disorderly and the link affected is the one
that the current exchange is using, the server will discover the
outage when a message in this exchange fails.
Colliding LLC message: DELETE LINK from client to server, entire link
group to be deleted
Action to resolve: Immediately clean up the link group.
Fox, et al. Informational [Page 57]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Colliding LLC message: CONFIRM RKEY from client
Action to resolve: Send a negative CONFIRM RKEY response to the
client. Once the current exchange finishes, client will have to
recompute its RKey set to include the new link and then start a
new CONFIRM RKEY exchange.
3.5.5.3.2. Collisions during DELETE LINK Exchange
Colliding LLC message: TEST LINK from either peer
Action to resolve: Send immediate TEST LINK response.
Colliding LLC message: ADD LINK from client to server
Action to resolve: Server queues the ADD LINK and processes it
after the current exchange completes.
Colliding LLC message: DELETE LINK from client to server (specific
link)
Action to resolve: Server queues the DELETE LINK message and
processes it after the current exchange completes. If it is an
orderly link termination, it can wait until after this exchange
continues. If it is disorderly and the link affected is the one
that the current exchange is using, the server will discover the
outage when a message in this exchange fails.
Colliding LLC message: DELETE LINK from either client or server,
deleting the entire link group
Action to resolve: Immediately clean up the link group.
Colliding LLC message: CONFIRM RKEY from client to server
Action to resolve: Send a negative CONFIRM RKEY response to the
client. Once the current exchange finishes, client will have to
recompute its RKey set to include the new link and then start a
new CONFIRM RKEY exchange.
Fox, et al. Informational [Page 58]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3.5.5.3.3. Collisions during CONFIRM RKEY Exchange
Colliding LLC message: TEST LINK
Action to resolve: Send immediate TEST LINK reply.
Colliding LLC message: ADD LINK from client to server
Action to resolve: Queue the ADD LINK, and process it after the
current exchange completes.
Colliding LLC message: ADD LINK from server to client (CONFIRM RKEY
exchange was initiated by the client, and it crossed with the server
initiating an ADD LINK exchange)
Action to resolve: Process the ADD LINK. Client will receive a
negative CONFIRM RKEY from the server and will have to redo this
CONFIRM RKEY exchange after the ADD LINK exchange completes.
Colliding LLC message: DELETE LINK from client to server, specific
link to be deleted (CONFIRM RKEY exchange was initiated by the
server, and it crossed with the client's DELETE LINK request)
Action to resolve: Server queues the DELETE LINK message and
processes it after the CONFIRM RKEY exchange completes. If it is
an orderly link termination, it can wait until after this exchange
continues. If it is disorderly and the link affected is the one
that the current exchange is using, the server will discover the
outage when a message in this exchange fails.
Colliding LLC message: DELETE LINK from server to client, specific
link deleted (CONFIRM RKEY exchange was initiated by the client, and
it crossed with the server's DELETE LINK)
Action to resolve: Process the DELETE LINK. Client will receive a
negative CONFIRM RKEY from the server and will have to redo this
CONFIRM RKEY exchange after the ADD LINK exchange completes.
Colliding LLC message: DELETE LINK from either client or server,
entire link group deleted
Action to resolve: Immediately clean up the link group.
Colliding LLC message: CONFIRM LINK from the peer that did not start
the current CONFIRM LINK exchange
Action to resolve: Queue the request, and process it after the
current exchange completes.
Fox, et al. Informational [Page 59]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
4. SMC-R Memory-Sharing Architecture
4.1. RMB Element Allocation Considerations
Each TCP connection using SMC-R must be allocated an RMBE by each
SMC-R peer. This allocation is performed by each endpoint
independently to allow each endpoint to select an RMBE that best
matches the characteristics on its TCP socket endpoint. The RMBE
associated with a TCP socket endpoint must have a receive buffer that
is at least as large as the TCP receive buffer size in effect for
that connection. The receive buffer size can be determined by what
is specified explicitly by the application using setsockopt() or
implicitly via the system-configured default value. This will allow
sufficient data to be RDMA-written by the SMC-R peer to fill an
entire receive buffer size's worth of data on a given data flow.
Given that each RMB must have fixed-length RMBEs, this implies that
an SMC-R endpoint may need to maintain multiple RMBs of various sizes
for SMC-R connections on a given SMC-R link and can then select an
RMBE that most closely fits a connection.
4.2. RMB and RMBE Format
An RMB is a virtual memory buffer whose backing real memory is
pinned. The RMB is subdivided into a whole number of equal-sized RMB
Elements (RMBEs). Each RMBE begins with a 4-byte eye catcher for
diagnostic and service purposes, followed by the receive data buffer.
The contents of this diagnostic eye catcher are implementation
dependent and should be used by the local SMC-R peer to check for
overlay errors by verifying an intact eye catcher with every RMBE
access.
The RMBE is a wrapping receive buffer for receiving RDMA writes from
the peer. Cursors, as described below, are exchanged between peers
to manage and track RDMA writes and local data reads from the RMBE
for a TCP connection.
4.3. RMBE Control Information
RMBE control information consists of consumer cursors, producer
cursors, wrap counts, CDC message sequence numbers, control flags
such as urgent data and "writer blocked" indicators, and TCP
connection information such as termination flags. This information
is exchanged between SMC-R peers using CDC messages, which are passed
using RoCE SendMsg. A TCP/IP stack implementing SMC-R must receive
and store this information in its internal data structures, as it is
used to manage the RMBE and its data buffer.
Fox, et al. Informational [Page 60]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The format and contents of the CDC message are described in detail in
Appendix A.4 ("Connection Data Control (CDC) Message Format"). The
following is a high-level description of what this control
information contains.
o Connection state flags such as sending done, connection closed,
failover data validation, and abnormal close.
o A sequence number that is managed by the sender. This sequence
number starts at 1, is increased each send, and wraps to 0. This
sequence number tracks the CDC message sent and is not related to
the number of bytes sent. It is used for failover data
validation.
o Producer cursor: a wrapping offset into the receiver's RMBE data
area. Set by the peer that is writing into the RMBE, it points to
where the writing peer will write the next byte of data into an
RMBE. This cursor is accompanied by a wrap sequence number to
help the RMBE owner (the receiver) identify full window size
wrapping writes. Note that this cursor must account for (i.e.,
skip over) the RMBE eye catcher that is in the beginning of the
data area.
o Consumer cursor: a wrapping offset into the receiver's RMBE data
area. Set by the owner of the RMBE (the peer that is reading from
it), this cursor points to the offset of the next byte of data to
be consumed by the peer in its own RMBE. The sender cannot write
beyond this cursor into the receiver's RMBE without causing data
loss. Like the producer cursor, this is accompanied by a wrap
count to help the writer identify full window size wrapping reads.
Note that this cursor must account for (i.e., skip over) the RMBE
eye catcher that is in the beginning of the data area.
o Data flags such as urgent data, writer blocked indicator, and
cursor update requests.
4.4. Use of RMBEs
4.4.1. Initializing and Accessing RMBEs
The RMBE eye catcher is initialized by the RMB owner prior to
assigning it to a specific TCP connection and communicating its RMB
index to the SMC-R partner. After an RMBE index is communicated to
the SMC-R partner, the RMBE can only be referenced in "read-only
mode" by the owner, and all updates to it are performed by the remote
SMC-R partner via RDMA write operations.
Fox, et al. Informational [Page 61]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Initialization of an RMBE must include the following:
o Zeroing out the entire RMBE receive buffer, which helps minimize
data integrity issues (e.g., data from a previous connection
somehow being presented to the current connection).
o Setting the beginning RMBE eye catcher. This eye catcher plays an
important role in helping detect accidental overlays of the RMBE.
The RMB owner should always validate these eye catchers before
each new reference to the RMBE. If the eye catchers are found to
be corrupted, the local host must reset the TCP connection
associated with this RMBE and log the appropriate diagnostic
information.
4.4.2. RMB Element Reuse and Conflict Resolution
RMB elements can be reused once their associated TCP and SMC-R
connections are terminated. Under normal and abnormal SMC-R
connection termination processing, both SMC-R peers must explicitly
acknowledge that they are done using an RMBE before that element can
be freed and reassigned to another SMC-R connection instance. For
more details on SMC-R connection termination, refer to Section 4.8.
However, there are some error scenarios where this two-way explicit
acknowledgment may not be completed. In these scenarios, an RMBE
owner may choose to reassign this RMBE to a new SMC-R connection
instance on this SMC-R link group. When this occurs, the partner
SMC-R peer must detect this condition during SMC-R Rendezvous
processing when presented with an RMBE that it believes is already in
use for a different SMC-R connection. In this case, the SMC-R peer
must abort the existing SMC-R connection associated with this RMBE.
The abort processing resets the TCP connection (if it is still
active), but it must not attempt to perform any RDMA writes to this
RMBE and must also ignore any data sitting in the local RMBE
associated with the existing connection. It then proceeds to free up
the local RMBE and notify the local application that the connection
is being abnormally reset.
The remote SMC-R peer then proceeds to normal processing for this new
SMC-R connection.
Fox, et al. Informational [Page 62]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
4.5. SMC-R Protocol Considerations
The following sections describe considerations for the SMC-R protocol
as compared to TCP.
4.5.1. SMC-R Protocol Optimized Window Size Updates
An SMC-R receiver host sends its consumer cursor information to the
sender to convey the progress that the receiving application has made
in consuming the sent data. The difference between the writer's
producer cursor and the associated receiver's consumer cursor
indicates the window size available for the sender to write into.
This is somewhat similar to TCP window update processing and
therefore has some similar considerations, such as silly window
syndrome avoidance, whereby TCP has an optimization that minimizes
the overhead of very small, unproductive window size updates
associated with suboptimal socket applications consuming very small
amounts of data on every receive() invocation. For SMC-R, the
receiver only updates its consumer cursor via a unique CDC message
under the following conditions:
o The current window size (from a sender's perspective) is less than
half of the receive buffer space, and the consumer cursor update
will result in a minimum increase in the window size of 10% of the
receive buffer space. Some examples:
a. Receive buffer size: 64K, current window size (from a sender's
perspective): 50K. No need to update the consumer cursor.
Plenty of space is available for the sender.
b. Receive buffer size: 64K, current window size (from a sender's
perspective): 30K, current window size from a receiver's
perspective: 31K. No need to update the consumer cursor; even
though the sender's window size is < 1/2 of the 64K, the window
update would only increase that by 1K, which is < 1/10th of the
64K buffer size.
c. Receive buffer size: 64K, current window size (from a sender's
perspective): 30K, current window size from a receiver's
perspective: 64K. The receiver updates the consumer cursor
(sender's window size is < 1/2 of the 64K; the window update
would increase that by > 6.4K).
Fox, et al. Informational [Page 63]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
o The receiver must always include a consumer cursor update whenever
it sends a CDC message to the partner for another flow (i.e., send
flow in the opposite direction). This allows the window size
update to be delivered with no additional overhead. This is
somewhat similar to TCP DelayAck processing and quite effective
for request/response data patterns.
o If a peer has set the B-bit in a CDC message, then any consumption
of data by the receiver causes a CDC message to be sent, updating
the consumer cursor until a CDC message with that bit cleared is
received from the peer.
o The optimized window size updates are overridden when the sender
sets the Consumer Cursor Update Requested flag in a CDC message to
the receiver. When this indicator is on, the consumer must send a
consumer cursor update immediately when data is consumed by the
local application or if the cursor has not been updated for a
while (i.e., local copy of the consumer cursor does not match the
last consumer cursor value sent to the partner). This allows the
sender to perform optional diagnostics for detecting a stalled
receiver application (data has been sent but not consumed). It is
recommended that the Consumer Cursor Update Requested flag only be
sent for diagnostic procedures, as it may result in non-optimal
data path performance.
4.5.2. Small Data Sends
The SMC-R protocol makes no special provisions for handling small
data segments sent across a stream socket. Data is always sent if
sufficient window space is available. In contrast to the TCP Nagle
algorithm, there are no special provisions in SMC-R for coalescing
small data segments.
An implementation of SMC-R can be configured to optimize its sending
processing by coalescing outbound data for a given SMC-R connection
so that it can reduce the number of RDMA write operations it
performs, in a fashion similar to Nagle's algorithm. However, any
such coalescing would require a timer on the sending host that would
ensure that data was eventually sent. Also, the sending host would
have to opt out of this processing if Nagle's algorithm had been
disabled (programmatically or via system configuration).
Fox, et al. Informational [Page 64]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
4.5.3. TCP Keepalive Processing
TCP keepalive processing allows applications to direct the local
TCP/IP host to periodically "test" the viability of an idle TCP
connection. Since SMC-R connections have a TCP representation along
with an SMC-R representation, there are unique keepalive processing
considerations:
o SMC-R-layer keepalive processing: If keepalive is enabled for an
SMC-R connection, the local host maintains a keepalive timer that
reflects how long an SMC-R connection has been idle. The local
host also maintains a timestamp of last activity for each SMC-R
link (for any SMC-R connection on that link). When it is
determined that an SMC-R connection has been idle longer than the
keepalive interval, the host checks to see whether or not the
SMC-R link has been idle for a duration longer than the keepalive
timeout. If both conditions are met, the local host then performs
a TEST LINK LLC command to test the viability of the SMC-R link
over the RoCE fabric (RC-QPs). If a TEST LINK LLC command
response is received within a reasonable amount of time, then the
link is considered viable, and all connections using this link are
considered viable as well. If, however, a response is not
received in a reasonable amount of time or there's a failure in
sending the TEST LINK LLC command, then this is considered a
failure in the SMC-R link, and failover processing to an alternate
SMC-R link must be triggered. If no alternate SMC-R link exists
in the SMC-R link group, then all of the SMC-R connections on this
link are abnormally terminated by resetting the TCP connections
represented by these SMC-R connections. Given that multiple SMC-R
connections can share the same SMC-R link, implementing an SMC-R
link-level probe using the TEST LINK LLC command will help reduce
the amount of unproductive keepalive traffic for SMC-R
connections; as long as some SMC-R connections on a given SMC-R
link are active (i.e., have had I/O activity within the keepalive
interval), then there is no need to perform additional link
viability testing.
Fox, et al. Informational [Page 65]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
o TCP-layer keepalive processing: Traditional TCP "keepalive"
packets are not as relevant for SMC-R connections, given that the
TCP path is not used for these connections once the SMC-R
Rendezvous processing is completed. All SMC-R connections by
default have associated TCP connections that are idle. Are TCP
keepalive probes still needed for these connections? There are
two main scenarios to consider:
1. TCP keepalives that are used to determine whether or not the
peer TCP endpoint is still active. This is not needed for
SMC-R connections, as the SMC-R-level keepalives mentioned
above will determine whether or not the remote endpoint
connections are still active.
2. TCP keepalives that are used to ensure that TCP connections
traversing an intermediate proxy maintain an active state. For
example, stateful firewalls typically maintain state
representing every valid TCP connection that traverses the
firewall. These types of firewalls are known to expire idle
connections by removing their state in the firewall to conserve
memory. TCP keepalives are often used in this scenario to
prevent firewalls from timing out otherwise idle connections.
When using SMC-R, both endpoints must reside in the same
Layer 2 network (i.e., the same subnet). As a result,
firewalls cannot be injected in the path between two SMC-R
endpoints. However, other intermediate proxies, such as
TCP/IP-layer load balancers, may be injected in the path of two
SMC-R endpoints. These types of load balancers also maintain
connection state so that they can forward TCP connection
traffic to the appropriate cluster endpoint. When using SMC-R,
these TCP connections will appear to be completely idle, making
them susceptible to potential timeouts at the load-balancing
proxy. As a result, for this scenario, TCP keepalives may
still be relevant.
The following are the TCP-level keepalive processing requirements for
SMC-R-enabled hosts:
o SMC-R peers should allow TCP keepalives to flow on the TCP path of
SMC-R connections based on existing TCP keepalive configuration
and programming options. However, it is strongly recommended that
platforms provide the ability to specify very granular keepalive
timers (for example, single-digit-second timers) and should
consider providing a configuration option that limits the minimum
keepalive timer that will be used for TCP-layer keepalives on
SMC-R connections. This is important to minimize the amount of
TCP keepalive packets transmitted in the network for SMC-R
connections.
Fox, et al. Informational [Page 66]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
o SMC-R peers must always respond to inbound TCP-layer keepalives
(by sending ACKs for these packets) even if the connection is
using SMC-R. Typically, once a TCP connection has completed the
SMC-R Rendezvous processing and is using SMC-R for data flows, no
new inbound TCP segments are expected on that TCP connection,
other than TCP termination segments (FIN, RST, etc.). TCP
keepalives are the one exception that must be supported. Also,
since TCP keepalive probes do not carry any application-layer
data, this has no adverse impact on the application's inbound data
stream.
4.6. TCP Connection Failover between SMC-R Links
A peer may change which SMC-R link within a link group it sends its
writes over in the event of a link failure. Since each peer
independently chooses which link to send writes over for a specific
TCP connection, this process is done independently by each peer.
4.6.1. Validating Data Integrity
Even though RoCE is a reliable transport, there is a small subset of
failure modes that could cause unrecoverable loss of data. When an
RNIC acknowledges receipt of an RDMA write to its peer, that creates
a write completion event to the sending peer, which allows the sender
to release any buffers it is holding for that write. In normal
operation and in most failures, this operation is reliable.
However, there are failure modes possible in which a receiving RNIC
has acknowledged an RDMA write but then was not able to place the
received data into its host memory -- for example, a sudden,
disorderly failure of the interface between the RNIC and the host.
While rare, these types of events must be guarded against to ensure
data integrity. The process for switching SMC-R links during
failover, as described in this section, guards against this
possibility and is mandatory.
Each peer must track the current state of the CDC sequence numbers
for a TCP connection. The sender must keep track of the sequence
number of the CDC message that described the last write acknowledged
by the peer RNIC, or Sequence Sent (SS). In other words, SS
describes the last write that the sender believes its peer has
successfully received. The receiver must keep track of the sequence
number of the CDC message that described the last write that it has
successfully received (i.e., the data has been successfully placed
into an RMBE), or Sequence Received (SR).
Fox, et al. Informational [Page 67]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
When an RNIC fails and the sender changes SMC-R links, the sender
must first send a CDC message with the F-bit (failover validation
indicator; see Appendix A.4) set over the new SMC-R link. This is
the failover data validation message. The sequence number in this
CDC message is equal to SS. The CDC message key, the length, and the
SMC-R alert token are the only other fields in this CDC message that
are significant. No reply is expected from this validation message,
and once the sender has sent it, the sender may resume sending on the
new SMC-R link as described in Section 4.6.2.
Upon receipt of the failover validation message, the receiver must
verify that its SR value for the TCP connection is equal to or
greater than the sequence number in the failover validation message.
If so, no further action is required, and the TCP connection resumes
on the new SMC-R link. If SR is less than the sequence number value
in the validation message, data has been lost, and the receiver must
immediately reset the TCP connection.
4.6.2. Resuming the TCP Connection on a New SMC-R Link
When a connection is moved to a new SMC-R link and the failover
validation message has been sent, the sender can immediately resume
normal transmission. In order to preserve the application message
stream, the sender must replay any RDMA writes (and their associated
CDC messages) that were in progress or failed when the previous SMC-R
link failed, before sending new data on the new SMC-R link. The
sender has two options for accomplishing this:
o Preserve the sequence numbers "as is": Retry all failed and
pending operations as they were originally done, including
reposting all associated RDMA write operations and their
associated CDC messages without making any changes. Then resume
sending new data using new sequence numbers.
o Combine pending messages and possibly add new data: Combine failed
and pending messages into a single new write with a new sequence
number. This allows the sender to combine pending messages into
fewer operations. As a further optimization, this write can also
include new data, as long as all failed and pending data are also
included. If this approach is taken, the sequence number must be
increased beyond the last failed or pending sequence number.
Fox, et al. Informational [Page 68]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
4.7. RMB Data Flows
The following sections describe the RDMA wire flows for the SMC-R
protocol after a TCP connection has switched into SMC-R mode (i.e.,
SMC-R Rendezvous processing is complete and a pair of RMB elements
has been assigned and communicated by the SMC-R peers). The ladder
diagrams below include the following:
o RMBE control information kept by each peer. Only a subset of the
information is depicted, specifically only the fields that reflect
the stream of data written by Host A and read by Host B.
o Time line 0-x, which shows the wire flows in a time-relative
fashion.
o Note that RMBE control information is only shown in a time
interval if its value changed (otherwise, assume that the value is
unchanged from the previously depicted value).
o The local copy of the producer cursors and consumer cursors that
is maintained by each host is not depicted in these figures. Note
that the cursor values in the diagram reflect the necessity of
skipping over the eye catcher in the RMBE data area. They start
and wrap at 4, not 0.
4.7.1. Scenario 1: Send Flow, Window Size Unconstrained
SMC Host A SMC Host B
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flags
4 0 0 0 4 0 0
0 0 1 ---------------> 1 0 0 0
RDMA-WR Data
(4:1003)
4 0 2 ...............> 2 1004 0 0
CDC Message
Figure 16: Scenario 1: Send Flow, Window Size Unconstrained
Scenario assumptions:
o Kernel implementation.
o New SMC-R connection; no data has been sent on the connection.
Fox, et al. Informational [Page 69]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
o Host A: Application issues send for 1000 bytes to Host B.
o Host B: RMBE receive buffer size is 10,000; application has issued
a recv for 10,000 bytes.
Flow description:
1. The application issues a send() for 1000 bytes; the SMC-R layer
copies data into a kernel send buffer. It then schedules an RDMA
write operation to move the data into the peer's RMBE receive
buffer, at relative position 4-1003 (to skip the 4-byte
eye catcher in the RMBE data area). Note that no immediate data
or alert (i.e., interrupt) is provided to Host B for this RDMA
operation.
2. Host A sends a CDC message to update the producer cursor to
byte 1004. This CDC message will deliver an interrupt to Host B.
At this point, the SMC-R layer can return control back to the
application. Host B, once notified of the completion of the
previous RDMA operation, locates the RMBE associated with the RMBE
alert token that was included in the message and proceeds to
perform normal receive-side processing, waking up the suspended
application read thread, copying the data into the application's
receive buffer, etc. It will use the producer cursor as an
indicator of how much data is available to be delivered to the
local application. After this processing is complete, the SMC-R
layer will also update its local consumer cursor to match the
producer cursor (i.e., indicating that all data has been
consumed). Note that a message to the peer updating the consumer
cursor is not needed at this time, as the window size is
unconstrained (> 1/2 of the receive buffer size). The window size
is calculated by taking the difference between the producer cursor
and the consumer cursor in the RMBEs (10,000 - 1004 = 8996).
Fox, et al. Informational [Page 70]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
4.7.2. Scenario 2: Send/Receive Flow, Window Size Unconstrained
SMC Host A SMC Host B
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flags
4 0 0 0 4 0 0
0 0 1 ---------------> 1 0 0 0
RDMA-WR Data
(4:1003)
4 0 2 ...............> 2 1004 0 0
CDC Message
0 0 3 <-------------- 3 1004 0 0
RDMA-WR Data
(4:503)
1004 0 4 <.............. 4 1004 0 0
CDC Message
Figure 17: Scenario 2: Send/Receive Flow, Window Size Unconstrained
Scenario assumptions:
o New SMC-R connection; no data has been sent on the connection.
o Host A: Application issues send for 1000 bytes to Host B.
o Host B: RMBE receive buffer size is 10,000; application has
already issued a recv for 10,000 bytes. Once the receive is
completed, the application sends a 500-byte response to Host A.
Flow description:
1. The application issues a send() for 1000 bytes; the SMC-R layer
copies data into a kernel send buffer. It then schedules an RDMA
write operation to move the data into the peer's RMBE receive
buffer, at relative position 4-1003. Note that no immediate data
or alert (i.e., interrupt) is provided to Host B for this RDMA
operation.
2. Host A sends a CDC message to update the producer cursor to
byte 1004. This CDC message will deliver an interrupt to Host B.
At this point, the SMC-R layer can return control back to the
application.
Fox, et al. Informational [Page 71]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
3. Host B, once notified of the receipt of the previous CDC message,
locates the RMBE associated with the RMBE alert token and proceeds
to perform normal receive-side processing, waking up the suspended
application read thread, copying the data into the application's
receive buffer, etc. After this processing is complete, the SMC-R
layer will also update its local consumer cursor to match the
producer cursor (i.e., indicating that all data has been
consumed). Note that an update of the consumer cursor to the peer
is not needed at this time, as the window size is unconstrained
(> 1/2 of the receive buffer size). The application then performs
a send() for 500 bytes to Host A. The SMC-R layer will copy the
data into a kernel buffer and then schedule an RDMA write into the
partner's RMBE receive buffer. Note that this RDMA write
operation includes no immediate data or notification to Host A.
4. Host B sends a CDC message to update the partner's RMBE control
information with the latest producer cursor (set to 503 and not
shown in the diagram above) and to also inform the peer that the
consumer cursor value is now 1004. It also updates the local
current consumer cursor and the last sent consumer cursor to 1004.
This CDC message includes notification, since we are updating our
producer cursor; this requires attention by the peer host.
4.7.3. Scenario 3: Send Flow, Window Size Constrained
SMC Host A SMC Host B
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flags
4 0 0 0 4 0 0
4 0 1 ---------------> 1 4 0 0
RDMA-WR Data
(4:3003)
4 0 2 ...............> 2 3004 0 0
CDC Message
4 0 3 3 3004 0 0
4 0 4 ---------------> 4 3004 0 0
RDMA-WR Data
(3004:7003)
4 0 5 ................> 5 7004 0 0
CDC Message
7004 0 6 <................ 6 7004 0 0
CDC Message
Figure 18: Scenario 3: Send Flow, Window Size Constrained
Fox, et al. Informational [Page 72]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Scenario assumptions:
o New SMC-R connection; no data has been sent on this connection.
o Host A: Application issues send for 3000 bytes to Host B and then
another send for 4000 bytes.
o Host B: RMBE receive buffer size is 10,000. Application has
already issued a recv for 10,000 bytes.
Flow description:
1. The application issues a send() for 3000 bytes; the SMC-R layer
copies data into a kernel send buffer. It then schedules an RDMA
write operation to move the data into the peer's RMBE receive
buffer, at relative position 4-3003. Note that no immediate data
or alert (i.e., interrupt) is provided to Host B for this RDMA
operation.
2. Host A sends a CDC message to update its producer cursor to
byte 3003. This CDC message will deliver an interrupt to Host B.
At this point, the SMC-R layer can return control back to the
application.
3. Host B, once notified of the receipt of the previous CDC message,
locates the RMBE associated with the RMBE alert token and proceeds
to perform normal receive-side processing, waking up the suspended
application read thread, copying the data into the application's
receive buffer, etc. After this processing is complete, the SMC-R
layer will also update its local consumer cursor to match the
producer cursor (i.e., indicating that all data has been
consumed). It will not, however, update the partner with this
information, as the window size is not constrained
(10,000 - 3000 = 7000 bytes of available space). The application
on Host B also issues a new recv() for 10,000 bytes.
4. On Host A, the application issues a send() for 4000 bytes. The
SMC-R layer copies the data into a kernel buffer and schedules an
async RDMA write into the peer's RMBE receive buffer at relative
position 3003-7004. Note that no alert is provided to Host B for
this flow.
5. Host A sends a CDC message to update the producer cursor to
byte 7004. This CDC message will deliver an interrupt to Host B.
At this point, the SMC-R layer can return control back to the
application.
Fox, et al. Informational [Page 73]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
6. Host B, once notified of the receipt of the previous CDC message,
locates the RMBE associated with the RMBE alert token and proceeds
to perform normal receive-side processing, waking up the suspended
application read thread, copying the data into the application's
receive buffer, etc. After this processing is complete, the SMC-R
layer will also update its local consumer cursor to match the
producer cursor (i.e., indicating that all data has been
consumed). It will then determine whether or not it needs to
update the consumer cursor to the peer. The available window size
is now 3000 (10,000 - (producer cursor - last sent consumer
cursor)), which is < 1/2 of the receive buffer size
(10,000/2 = 5000), and the advance of the window size is > 10% of
the window size (1000). Therefore, a CDC message is issued to
update the consumer cursor to Peer A.
4.7.4. Scenario 4: Large Send, Flow Control, Full Window Size Writes
SMC Host A SMC Host B
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flags
1004 1 0 0 1004 1 0
1004 1 1 ---------------> 1 1004 1 0
RDMA-WR Data
(1004:9999)
1004 1 2 ---------------> 2 1004 1 0
RDMA-WR Data
(4:1003)
1004 1 3 ...............> 3 1004 2 Wrt
CDC Message Blk
1004 2 4 <............... 4 1004 2 Wrt
CDC Message Blk
1004 2 5 ---------------> 5 1004 2 Wrt
RDMA-WR Data Blk
(1004:9999)
1004 2 6 ---------------> 6 1004 2 Wrt
RDMA-WR Data Blk
(4:1003)
1004 2 7 ...............> 7 1004 3 Wrt
CDC Message Blk
1004 3 8 <............... 8 1004 3 Wrt
CDC Message Blk
Figure 19: Scenario 4: Large Send, Flow Control,
Full Window Size Writes
Fox, et al. Informational [Page 74]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Scenario assumptions:
o Kernel implementation.
o Existing SMC-R connection, Host B's receive window size is fully
open (peer consumer cursor = peer producer cursor).
o Host A: Application issues send for 20,000 bytes to Host B.
o Host B: RMBE receive buffer size is 10,000; application has issued
a recv for 10,000 bytes.
Flow description:
1. The application issues a send() for 20,000 bytes; the SMC-R layer
copies data into a kernel send buffer (assumes that send buffer
space of 20,000 is available for this connection). It then
schedules an RDMA write operation to move the data into the peer's
RMBE receive buffer, at relative position 1004-9999. Note that no
immediate data or alert (i.e., interrupt) is provided to Host B
for this RDMA operation.
2. Host A then schedules an RDMA write operation to fill the
remaining 1000 bytes of available space in the peer's RMBE receive
buffer, at relative position 4-1003. Note that no immediate data
or alert (i.e., interrupt) is provided to Host B for this RDMA
operation. Also note that an implementation of SMC-R may optimize
this processing by combining steps 1 and 2 into a single
RDMA write operation (with two different data sources).
3. Host A sends a CDC message to update the producer cursor to
byte 1004. Since the entire receive buffer space is filled, the
producer writer blocked flag (the "Wrt Blk" indicator (flag) in
Figure 19) is set and the producer cursor wrap sequence number
(the producer "Wrap Seq#" in Figure 19) is incremented. This CDC
message will deliver an interrupt to Host B. At this point, the
SMC-R layer can return control back to the application.
4. Host B, once notified of the receipt of the previous CDC message,
locates the RMBE associated with the RMBE alert token and proceeds
to perform normal receive-side processing, waking up the suspended
application read thread, copying the data into the application's
receive buffer, etc. In this scenario, Host B notices that the
producer cursor has not been advanced (same value as the consumer
cursor); however, it notices that the producer cursor wrap
sequence number is different from its local value (1), indicating
that a full window of new data is available. All of the data in
the receive buffer can be processed, with the first segment
Fox, et al. Informational [Page 75]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
(1004-9999) followed by the second segment (4-1003). Because the
producer writer blocked indicator was set, Host B schedules a CDC
message to update its latest information to the peer: consumer
cursor (1004), consumer cursor wrap sequence number (the current
value of 2 is used).
5. Host A, upon receipt of the CDC message, locates the TCP
connection associated with the alert token and, upon examining the
control information provided, notices that Host B has consumed all
of the data (based on the consumer cursor and the consumer cursor
wrap sequence number) and initiates the next RDMA write to fill
the receive buffer at offset 1003-9999.
6. Host A then moves the next 1000 bytes into the beginning of the
receive buffer (4-1003) by scheduling an RDMA write operation.
Note that at this point there are still 8 bytes remaining to be
written.
7. Host A then sends a CDC message to set the producer writer blocked
indicator and to increment the producer cursor wrap sequence
number (3).
8. Host B, upon notification, completes the same processing as step 4
above, including sending a CDC message to update the peer to
indicate that all data has been consumed. At this point, Host A
can write the final 8 bytes to Host B's RMBE into
positions 1004-1011 (not shown).
Fox, et al. Informational [Page 76]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
4.7.5. Scenario 5: Send Flow, Urgent Data, Window Size Unconstrained
SMC Host A SMC Host B
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flag
1000 1 0 0 1000 1 0
1000 1 1 ---------------> 1 1000 1 0
RDMA-WR Data
(1000:1499)
1000 1 2 ...............> 2 1500 1 UrgP
CDC Message UrgA
1500 1 3 <............... 3 1500 1 UrgP
CDC Message UrgA
1500 1 4 ---------------> 4 1500 1 UrgP
RDMA-WR Data UrgA
(1500:2499)
1500 1 5 ...............> 5 2500 1 0
CDC Message
Figure 20: Scenario 5: Send Flow, Urgent Data, Window Size Open
Scenario assumptions:
o Kernel implementation.
o Existing SMC-R connection; window size open (unconstrained); all
data has been consumed by receiver.
o Host A: Application issues send for 500 bytes with urgent data
indicator (out of band) to Host B, then sends 1000 bytes of
normal data.
o Host B: RMBE receive buffer size is 10,000; application has issued
a recv for 10,000 bytes and is also monitoring the socket for
urgent data.
Flow description:
1. The application issues a send() for 500 bytes of urgent data; the
SMC-R layer copies data into a kernel send buffer. It then
schedules an RDMA write operation to move the data into the peer's
RMBE receive buffer, at relative position 1000-1499. Note that no
immediate data or alert (i.e., interrupt) is provided to Host B
for this RDMA operation.
Fox, et al. Informational [Page 77]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
2. Host A sends a CDC message to update its producer cursor to
byte 1500 and to turn on the producer Urgent Data Pending (UrgP)
and Urgent Data Present (UrgA) flags. This CDC message will
deliver an interrupt to Host B. At this point, the SMC-R layer
can return control back to the application.
3. Host B, once notified of the receipt of the previous CDC message,
locates the RMBE associated with the RMBE alert token, notices
that the Urgent Data Pending flag is on, and proceeds with out-of-
band socket API notification -- for example, satisfying any
outstanding select() or poll() requests on the socket by
indicating that urgent data is pending (i.e., by setting the
exception bit on). The urgent data present indicator allows
Host B to also determine the position of the urgent data (the
producer cursor points 1 byte beyond the last byte of urgent
data). Host B can then perform normal receive-side processing
(including specific urgent data processing), copying the data into
the application's receive buffer, etc. Host B then sends a CDC
message to update the partner's RMBE control area with its latest
consumer cursor (1500). Note that this CDC message must occur,
regardless of the current local window size that is available.
The partner host (Host A) cannot initiate any additional RDMA
writes until it receives acknowledgment that the urgent data has
been processed (or at least processed/remembered at the SMC-R
layer).
4. Upon receipt of the message, Host A wakes up, sees that the peer
consumed all data up to and including the last byte of urgent
data, and now resumes sending any pending data. In this case, the
application had previously issued a send for 1000 bytes of normal
data, which would have been copied in the send buffer, and control
would have been returned to the application. Host A now initiates
an RDMA write to move that data to the peer's receive buffer at
position 1500-2499.
5. Host A then sends a CDC message to update its producer cursor
value (2500) and to turn off the Urgent Data Pending and Urgent
Data Present flags. Host B wakes up, processes the new data
(resumes application, copies data into the application receive
buffer), and then proceeds to update the local current consumer
cursor (2500). Given that the window size is unconstrained, there
is no need for a consumer cursor update in the peer's RMBE.
Fox, et al. Informational [Page 78]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
4.7.6. Scenario 6: Send Flow, Urgent Data, Window Size Closed
SMC Host A SMC Host B
RMBE A Info RMBE B Info
(Consumer Cursors) (Producer Cursors)
Cursor Wrap Seq# Time Time Cursor Wrap Seq# Flag
1000 1 0 0 1000 2 Wrt
Blk
1000 1 1 ...............> 1 1000 2 Wrt
CDC Message Blk
UrgP
1000 2 2 <............... 2 1000 2 Wrt
CDC Message Blk
UrgP
1000 2 3 ---------------> 3 1000 2 Wrt
RDMA-WR Data Blk
(1000:1499) UrgP
1000 2 4 ...............> 4 1500 2 UrgP
CDC Message UrgA
1500 2 5 <............... 5 1500 2 UrgP
CDC Message UrgA
1500 2 6 ---------------> 6 1500 2 UrgP
RDMA-WR Data UrgA
(1500:2499)
1000 2 7 ...............> 7 2500 2 0
CDC Message
Figure 21: Scenario 6: Send Flow, Urgent Data, Window Size Closed
Scenario assumptions:
o Kernel implementation.
o Existing SMC-R connection; window size closed; writer is blocked.
o Host A: Application issues send for 500 bytes with urgent data
indicator (out of band) to Host B, then sends 1000 bytes of
normal data.
o Host B: RMBE receive buffer size is 10,000; application has no
outstanding recv() (for normal data) and is monitoring the socket
for urgent data.
Fox, et al. Informational [Page 79]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Flow description:
1. The application issues a send() for 500 bytes of urgent data; the
SMC-R layer copies data into a kernel send buffer (if available).
Since the writer is blocked (window size closed), it cannot send
the data immediately. It then sends a CDC message to notify the
peer of the Urgent Data Pending (UrgP) indicator (the writer
blocked indicator remains on as well). This serves as a signal to
Host B that urgent data is pending in the stream. Control is also
returned to the application at this point.
2. Host B, once notified of the receipt of the previous CDC message,
locates the RMBE associated with the RMBE alert token, notices
that the Urgent Data Pending flag is on, and proceeds with out-of-
band socket API notification -- for example, satisfying any
outstanding select() or poll() requests on the socket by
indicating that urgent data is pending (i.e., by setting the
exception bit on). At this point, it is expected that the
application will enter urgent data mode processing, expeditiously
processing all normal data (by issuing recv API calls) so that it
can get to the urgent data byte. Whether the application has this
urgent mode processing or not, at some point, the application will
consume some or all of the pending data in the receive buffer.
When this occurs, Host B will also send a CDC message to update
its consumer cursor and consumer cursor wrap sequence number to
the peer. In the example above, a full window's worth of data was
consumed.
3. Host A, once awakened by the message, will notice that the window
size is now open on this connection (based on the consumer cursor
and the consumer cursor wrap sequence number, which now matches
the producer cursor wrap sequence number) and resume sending of
the urgent data segment by scheduling an RDMA write into relative
position 1000-1499.
4. Host A then sends a CDC message to advance its producer cursor
(1500) and to also notify Host B of the Urgent Data Present (UrgA)
indicator (and turn off the writer blocked indicator). This
signals to Host B that the urgent data is now in the local receive
buffer and that the producer cursor points to the last byte of
urgent data.
5. Host B wakes up, processes the urgent data, and, once the urgent
data is consumed, sends a CDC message to update its consumer
cursor (1500).
Fox, et al. Informational [Page 80]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
6. Host A wakes up, sees that Host B has consumed the sequence number
associated with the urgent data, and then initiates the next RDMA
write operation to move the 1000 bytes associated with the next
send() of normal data into the peer's receive buffer at
position 1500-2499. Note that the send API would have likely
completed earlier in the process by copying the 1000 bytes into a
send buffer and returning back to the application, even though we
could not send any new data until the urgent data was processed
and acknowledged by Host B.
7. Host A sends a CDC message to advance its producer cursor to 2500
and to reset the Urgent Data Pending and Urgent Data Present
flags. Host B wakes up and processes the inbound data.
4.8. Connection Termination
Just as SMC-R connections are established using a combination of TCP
connection establishment flows and SMC-R protocol flows, the
termination of SMC-R connections also uses a similar combination of
SMC-R protocol termination flows and normal TCP connection
termination flows. The following sections describe the SMC-R
protocol normal and abnormal connection termination flows.
4.8.1. Normal SMC-R Connection Termination Flows
Normal SMC-R connection flows are triggered via the normal stream
socket API semantics, namely by the application issuing a close() or
shutdown() API. Most applications, after consuming all incoming data
and after sending any outbound data, will then issue a close() API to
indicate that they are done both sending and receiving data. Some
applications, typically a small percentage, make use of the
shutdown() API that allows them to indicate that the application is
done sending data, receiving data, or both sending and receiving
data. The main use of this API is scenarios where a TCP application
wants to alert its partner endpoint that it is done sending data but
is still receiving data on its socket (shutdown for write). Issuing
shutdown() for both sending and receiving data is really no different
than issuing a close() and can therefore be treated in a similar
fashion. Shutdown for read is typically not a very useful operation
and in normal circumstances does not trigger any network flows to
notify the partner TCP endpoint of this operation.
These same trigger points will be used by the SMC-R layer to initiate
SMC-R connection termination flows. The main design point for SMC-R
normal connection flows is to use the SMC-R protocol to first shut
down the SMC-R connection and free up any SMC-R RDMA resources, and
then allow the normal TCP connection termination protocol (i.e., FIN
processing) to drive cleanup of the TCP connection. This design
Fox, et al. Informational [Page 81]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
point is very important in ensuring that RDMA resources such as
the RMBEs are only freed and reused when both SMC-R endpoints
are completely done with their RDMA write operations to the
partner's RMBE.
1
+-----------------+
|-------------->| CLOSED |<-------------|
3D | | | | 4D
| +-----------------+ |
| | |
| 2 | |
| V |
+----------------+ +-----------------+ +----------------+
|AppFinCloseWait | | ACTIVE | |PeerFinCloseWait|
| | | | | |
+----------------+ +-----------------+ +----------------+
| | | |
| Active Close | 3A | 4A | Passive Close |
| V | V |
| +--------------+ | +-------------+ |
|--<----|PeerCloseWait1| | |AppCloseWait1|--->----|
3C | | | | | | | 4C
| +--------------+ | +-------------+ |
| | | | |
| | 3B | 4B | |
| V | V |
| +--------------+ | +-------------+ |
|--<----|PeerCloseWait2| | |AppCloseWait2|--->----|
| | | | |
+--------------+ | +-------------+
|
|
Figure 22: SMC-R Connection States
Figure 22 describes the states that an SMC-R connection typically
goes through. Note that there are variations to these states that
can occur when an SMC-R connection is abnormally terminated, similar
in a way to when a TCP connection is reset. The following are the
high-level state transitions for an SMC-R connection:
1. An SMC-R connection begins in the Closed state. This state is
meant to reflect an RMBE that is not currently in use (was
previously in use but no longer is, or was never allocated).
Fox, et al. Informational [Page 82]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
2. An SMC-R connection progresses to the Active state once the SMC-R
Rendezvous processing has successfully completed, RMB element
indices have been exchanged, and SMC-R links have been activated.
In this state, the TCP connection is fully established, rendezvous
processing has been completed, and SMC-R peers can begin the
exchange of data via RDMA.
3. Active close processing (on the SMC-R peer that is initiating the
connection termination).
A. When an application on one of the SMC-R connection peers issues
a close(), a shutdown() for write, or a shutdown() for both
read and write, the SMC-R layer on that host will initiate
SMC-R connection termination processing. First, if a close()
or shutdown(both) is issued, it will check to see that there's
no data in the local RMB element that has not been read by the
application. If unread data is detected, the SMC-R connection
must be abnormally reset; for more details on this, refer to
Section 4.8.2 ("Abnormal SMC-R Connection Termination Flows").
If no unread data is pending, it then checks to see whether or
not any outstanding data is waiting to be written to the peer,
or if any outstanding RDMA writes for this SMC-R connection
have not yet completed. If either of these two scenarios is
true, an indicator that this connection is in a pending close
state is saved in internal data structures representing this
SMC-R connection, and control is returned to the application.
If all data to be written to the partner has completed, this
peer will send a CDC message to notify the peer of either the
PeerConnectionClosed indicator (close or shutdown for both was
issued) or the PeerDoneWriting indicator. This will provide an
interrupt to inform that partner SMC-R peer that the connection
is terminating. At this point, the local side of the SMC-R
connection transitions in the PeerCloseWait1 state, and control
can be returned to the application. If this process could not
be completed synchronously (the pending close condition
mentioned above), it is completed when all RDMA writes for data
and control cursors have been completed.
B. At some point, the SMC-R peer application (passive close) will
consume all incoming data, realize that that partner is done
sending data on this connection, and proceed to initiate its
own close of the connection once it has completed sending all
data from its end. The partner application can initiate this
connection termination processing via close() or shutdown()
APIs. If the application does so by issuing a shutdown() for
write, then the partner SMC-R layer will send a CDC message to
notify the peer (the active close side) of the PeerDoneWriting
indicator. When the "active close" SMC-R peer wakes up as a
Fox, et al. Informational [Page 83]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
result of the previous CDC message, it will notice that the
PeerDoneWriting indicator is now on and transition to the
PeerCloseWait2 state. This state indicates that the peer is
done sending data and may still be reading data. At this
point, the "active close" peer will also need to ensure that
any outstanding recv() calls for this socket are woken up and
remember that no more data is forthcoming on this connection
(in case the local connection was shutdown() for write only).
C. This flow is a common transition from 3A or 3B above. When the
SMC-R peer (passive close) consumes all data and updates all
necessary cursors to the peer, and the application closes its
socket (close or shutdown for both), it will send a CDC message
to the peer (the active close side) with the
PeerConnectionClosed indicator set. At this point, the
connection can transition back to the Closed state if the local
application has already closed (or issued shutdown for both)
the socket. Once in the Closed state, the RMBE can now be
safely reused for a new SMC-R connection. When the
PeerConnectionClosed indicator is turned on, the SMC-R peer is
indicating that it is done updating the partner's RMBE.
D. Conditional state: If the local application has not yet issued
a close() or shutdown(both), we need to wait until the
application does so. Once it does, the local host will send a
CDC message to notify the peer of the PeerConnectionClosed
indicator and then transition to the Closed state.
4. Passive close processing (on the SMC-R peer that receives an
indication that the partner is closing the connection).
A. Upon receipt of a CDC message, the SMC-R layer will detect that
the PeerConnectionClosed indicator or PeerDoneWriting indicator
is on. If any outstanding recv() calls are pending, they are
completed with an indicator that the partner has closed the
connection (zero-length data presented to the application). If
there is any pending data to be written and
PeerConnectionClosed is on, then an SMC-R connection reset must
be performed. The connection then enters the AppCloseWait1
state on the passive close side waiting for the local
application to initiate its own close processing.
B. If the local application issues a shutdown() for writing, then
the SMC-R layer will send a CDC message to notify the partner
of the PeerDoneWriting indicator and then transition the local
side of the SMC-R connection to the AppCloseWait2 state.
Fox, et al. Informational [Page 84]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
C. When the application issues a close() or shutdown() for both,
the local SMC-R peer will send a message informing the peer of
the PeerConnectionClosed indicator and transition to the Closed
state if the remote peer has also sent the local peer the
PeerConnectionClosed indicator. If the peer has not sent the
PeerConnectionClosed indicator, we transition into the
PeerFinCloseWait state.
D. The local SMC-R connection stays in this state until the peer
sends the PeerConnectionClosed indicator in a CDC message.
When the indicator is sent, we transition to the Closed state
and are then free to reuse this RMBE.
Note that each SMC-R peer needs to provide some logic that will
prevent being stranded in a termination state indefinitely. For
example, if an Active Close SMC-R peer is in a PeerCloseWait (1 or 2)
state waiting for the remote SMC-R peer to update its connection
termination status, it needs to provide a timer that will prevent it
from waiting in that state indefinitely should the remote SMC-R peer
not respond to this termination request. This could occur in error
scenarios -- for example, if the remote SMC-R peer suffered a failure
prior to being able to respond to the termination request or the
remote application is not responding to this connection termination
request by closing its own socket. This latter scenario is similar
to the TCP FINWAIT2 state, which has been known to sometimes cause
issues when remote TCP/IP hosts lose track of established connections
and neglect to close them. Even though the TCP standards do not
mandate a timeout from the TCP FINWAIT2 state, most TCP/IP
implementations assign a timeout for this state. A similar timeout
will be required for SMC-R connections. When this timeout occurs,
the local SMC-R peer performs TCP reset processing for this
connection. However, no additional RDMA writes to the partner RMBE
can occur at this point (we have already indicated that we are done
updating the peer's RMBE). After the TCP connection is reset, the
RMBE can be returned to the free pool for reallocation. See
Section 4.4.2 for more details.
Also note that it is possible to have two SMC-R endpoints initiate an
Active close concurrently. In that scenario, the flows above still
apply; however, both endpoints follow the active close path (path 3).
Fox, et al. Informational [Page 85]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
4.8.2. Abnormal SMC-R Connection Termination Flows
Abnormal SMC-R connection termination can occur for a variety of
reasons, including the following:
o The TCP connection associated with an SMC-R connection is reset.
In TCP, either endpoint can send a RST segment to abort an
existing TCP connection when error conditions are detected for the
connection or the application overtly requests that the connection
be reset.
o Normal SMC-R connection termination processing has unexpectedly
stalled for a given connection. When the stall is detected
(connection termination timeout condition), an abnormal SMC-R
connection termination flow is initiated.
In these scenarios, it is very important that resources associated
with the affected SMC-R connections are properly cleaned up to ensure
that there are no orphaned resources and that resources can reliably
be reused for new SMC-R connections. Given that SMC-R relies heavily
on the RDMA write processing, special care needs to be taken to
ensure that an RMBE is no longer being used by an SMC-R peer before
logically reassigning that RMBE to a new SMC-R connection.
When an SMC-R peer initiates a TCP connection reset, it also
initiates an SMC-R abnormal connection flow at the same time. The
SMC-R peers explicitly signal their intent to abnormally terminate an
SMC-R connection and await explicit acknowledgment that the peer has
received this notification and has also completed abnormal connection
termination on its end. Note that TCP connection reset processing
can occur in parallel to these flows.
Fox, et al. Informational [Page 86]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
+-----------------+
|-------------->| CLOSED |<-------------|
| | | |
| +-----------------+ |
| |
| |
| |
| +-----------------------+ |
| | Any state | |
|1B | (before setting | 2B|
| | PeerConnectionClosed | |
| | indicator in | |
| | peer's RMBE) | |
| +-----------------------+ |
| 1A | | 2A |
| Active Abort | | Passive Abort |
| V V |
| +--------------+ +--------------+ |
|-------|PeerAbortWait | | Process Abort|------|
| | | |
+--------------+ +--------------+
Figure 23: SMC-R Abnormal Connection Termination State Diagram
Figure 23 above shows the SMC-R abnormal connection termination state
diagram:
1. Active abort designates the SMC-R peer that is initiating the TCP
RST processing. At the time that the TCP RST is sent, the active
abort side must also do the following:
A. Send the PeerConnAbort indicator to the partner in a CDC
message, and then transition to the PeerAbortWait state.
During this state, it will monitor this SMC-R connection
waiting for the peer to send its corresponding PeerConnAbort
indicator but will ignore any other activity in this connection
(i.e., new incoming data). It will also generate an
appropriate error to any socket API calls issued against this
socket (e.g., ECONNABORTED, ECONNRESET).
B. Once the peer sends the PeerConnAbort indicator to the local
host, the local host can transition this SMC-R connection to
the Closed state and reuse this RMBE. Note that the SMC-R peer
that goes into the active abort state must provide some
protection against staying in that state indefinitely should
the remote SMC-R peer not respond by sending its own
PeerConnAbort indicator to the local host. While this should
be a rare scenario, it could occur if the remote SMC-R peer
Fox, et al. Informational [Page 87]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
(passive abort) suffered a failure right after the local SMC-R
peer (active abort) sent the PeerConnAbort indicator. To
protect against these types of failures, a timer can be set
after entering the PeerAbortWait state, and if that timer pops
before the peer has sent its local PeerConnAbort indicator (to
the active abort side), this RMBE can be returned to the free
pool for possible reallocation. See Section 4.4.2 for more
details.
2. Passive abort designates the SMC-R peer that is the recipient of
an SMC-R abort from the peer designated by the PeerConnAbort
indicator being sent by the peer in a CDC message. Upon receiving
this request, the local peer must do the following:
A. Using the appropriate error codes, indicate to the socket
application that this connection has been aborted, and then
purge all in-flight data for this connection that is waiting to
be read or waiting to be sent.
B. Send a CDC message to notify the peer of the PeerConnAbort
indicator and, once that is completed, transition this RMBE to
the Closed state.
If an SMC-R peer receives a TCP RST for a given SMC-R connection, it
also initiates SMC-R abnormal connection termination processing if it
has not already been notified (via the PeerConnAbort indicator) that
the partner is severing the connection. It is possible to have two
SMC-R endpoints concurrently be in an active abort role for a given
connection. In that scenario, the flows above still apply but both
endpoints take the active abort path (path 1).
4.8.3. Other SMC-R Connection Termination Conditions
The following are additional conditions that have implications for
SMC-R connection termination:
o An SMC-R peer being gracefully shut down. If an SMC-R peer
supports a graceful shutdown operation, it should attempt to
terminate all SMC-R connections as part of shutdown processing.
This could be accomplished via LLC DELETE LINK requests on all
active SMC-R links.
o Abnormal termination of an SMC-R peer. In this example, there may
be no opportunity for the host to perform any SMC-R cleanup
processing. In this scenario, it is up to the remote peer to
detect a RoCE communications failure with the failing host. This
Fox, et al. Informational [Page 88]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
could trigger SMC-R link switchover, but that would also generate
RoCE errors, causing the remote host to eventually terminate all
existing SMC-R connections to this peer.
o Loss of RoCE connectivity between two SMC-R peers. If two peers
are no longer reachable across any links in their SMC-R link
group, then both peers perform a TCP reset for the connections,
generate an error to the local applications, and free up all QP
resources associated with the link group.
5. Security Considerations
5.1. VLAN Considerations
The concepts and access control of virtual LANs (VLANs) must be
extended to also cover the RoCE network traffic flowing across the
Ethernet.
The RoCE VLAN configuration and access permissions must mirror the IP
VLAN configuration and access permissions over the Converged Enhanced
Ethernet fabric. This means that hosts, routers, and switches that
have access to specific VLANs on the IP fabric must also have the
same VLAN access across the RoCE fabric. In other words, the SMC-R
connectivity will follow the same virtual network access permissions
as normal TCP/IP traffic.
5.2. Firewall Considerations
As mentioned above, the RoCE fabric inherits the same VLAN
topology/access as the IP fabric. RoCE is a Layer 2 protocol that
requires both endpoints to reside in the same Layer 2 network (i.e.,
VLAN). RoCE traffic cannot traverse multiple VLANs, as there is no
support for routing RoCE traffic beyond a single VLAN. As a result,
SMC-R communications will also be confined to peers that are members
of the same VLAN. IP-based firewalls are typically inserted between
VLANs (or physical LANs) and rely on normal IP routing to insert
themselves in the data path. Since RoCE (and by extension SMC-R) is
not routable beyond the local VLAN, there is no ability to insert a
firewall in the network path of two SMC-R peers.
5.3. Host-Based IP Filters
Because SMC-R maintains the TCP three-way handshake for connection
setup before switching to RoCE out of band, existing IP filters that
control connection setup flows remain effective in an SMC-R
environment. IP filters that operate on traffic flowing in an active
TCP connection are not supported, because the connection data does
not flow over IP.
Fox, et al. Informational [Page 89]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
5.4. Intrusion Detection Services
Similar to IP filters, intrusion detection services that operate on
TCP connection setups are compatible with SMC-R with no changes
required. However, once the TCP connection has switched to RoCE out
of band, packets are not available for examination.
5.5. IP Security (IPsec)
IP security is not compatible with SMC-R, because there are no IP
packets on which to operate. TCP connections that require IP
security must opt out of SMC-R.
5.6. TLS/SSL
Transport Layer Security/Secure Socket Layer (TLS/SSL) is preserved
in an SMC-R environment. The TLS/SSL layer resides above the SMC-R
layer, and outgoing connection data is encrypted before being passed
down to the SMC-R layer for RDMA write. Similarly, incoming
connection data goes through the SMC-R layer encrypted and is
decrypted by the TLS/SSL layer as it is today.
The TLS/SSL handshake messages flow over the TCP connection after the
connection has switched to SMC-R, and so they are exchanged using
RDMA writes by the SMC-R layer, transparently to the TLS/SSL layer.
6. IANA Considerations
The scarcity of TCP option codes available for assignment is
understood, and this architecture uses experimental TCP options
following the conventions of [RFC6994] ("Shared Use of Experimental
TCP Options").
TCP ExID 0xE2D4C3D9 has been registered with IANA as a TCP Experiment
Identifier. See Section 3.1.
If this protocol achieves wide acceptance, a discrete option code may
be requested by subsequent versions of this protocol.
Fox, et al. Informational [Page 90]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
7. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013,
<http://www.rfc-editor.org/info/rfc6994>.
[RoCE] InfiniBand, "RDMA over Converged Ethernet specification",
<https://cw.infinibandta.org/wg/Members/documentRevision/
download/7149>.
Fox, et al. Informational [Page 91]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Appendix A. Formats
A.1. TCP Option
The SMC-R TCP option is formatted in accordance with [RFC6994]
("Shared Use of Experimental TCP Options"). The ExID value is
IBM-1047 (EBCDIC) encoding for "SMCR".
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind = 254 | Length = 6 | x'E2' | x'D4' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24: SMC-R TCP Option Format
A.2. CLC Messages
The following rules apply to all CLC messages:
General rules on formats:
o Reserved fields must be set to zero and not validated.
o Each message has an eye catcher at the start and another
eye catcher at the end. These must both be validated by the
receiver.
o SMC version indicator: The only SMC-R version defined in this
architecture is version 1. In the future, if peers have a
mismatch of versions, the lowest common version number is used.
Fox, et al. Informational [Page 92]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.2.1. Peer ID Format
All CLC messages contain a peer ID that uniquely identifies an
instance of a TCP/IP stack. This peer ID is required to be
universally unique across TCP/IP stacks and instances (including
restarts) of TCP/IP stacks.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Instance ID | RoCE MAC (first 2 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RoCE MAC (last 4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: Peer ID Format
Instance ID
A 2-byte instance count that ensures that if the same RNIC MAC is
later used in the peer ID for a different TCP/IP stack -- for
example, if an RNIC is redeployed to another stack -- the values
are unique. It also ensures that if a TCP/IP stack is restarted,
the instance ID changes. The value is implementation defined,
with one suggestion being 2 bytes of the system clock.
RoCE MAC
The RoCE MAC address for one of the peer's RNICs. Note that in a
virtualized environment this will be the virtual MAC of one of the
peer's RNICs.
Fox, et al. Informational [Page 93]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.2.2. SMC Proposal CLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length |Version| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Client's Peer ID -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Client's preferred GID -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Client's preferred RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |Offset to mask/prefix area (0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Area for future growth .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Subnet Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Mask Lgth| Reserved |Num IPv6 prfx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Array of IPv6 prefixes (variable length) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26: SMC Proposal CLC Message Format
Fox, et al. Informational [Page 94]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The fields present in the SMC Proposal CLC message are:
Eye catchers
Like all CLC messages, the SMC Proposal has beginning and ending
eye catchers to aid with verification and parsing. The hex digits
spell "SMCR" in IBM-1047 (EBCDIC).
Type
CLC message Type 1 indicates SMC Proposal.
Length
The length of this CLC message. If this is an IPv4 flow, this
value is 52. Otherwise, it is variable, depending upon how many
prefixes are listed.
Version
Version of the SMC-R protocol. Version 1 is the only currently
defined value.
Client's Peer ID
As described in Appendix A.2.1 above.
Client's preferred RoCE GID
The IPv6 address of the client's preferred RNIC on the RoCE
fabric.
Client's preferred RoCE MAC address
The MAC address of the client's preferred RNIC on the RoCE fabric.
It is required, as some operating systems do not have neighbor
discovery or ARP support for RoCE RNICs.
Offset to mask/prefix area
Provides the number of bytes that must be skipped after this
field, to access the IPv4 Subnet Mask field and the fields that
follow it. Allows for future growth of this signal. In this
version of the architecture, this value is always zero.
Fox, et al. Informational [Page 95]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Area for future growth
In this version of the architecture, this field does not exist.
This indicates where additional information may be inserted into
the signal in the future. The "Offset to mask/prefix area" field
must be used to skip over this area.
IPv4 Subnet Mask
If this message is flowing over an IPv4 TCP connection, the value
of the subnet mask associated with the interface over which the
client sent this message. If this is an IPv6 flow, this field is
all zeros.
This field, along with all fields that follow it in this signal,
must be accessed by skipping the number of bytes listed in the
"Offset to mask/prefix area" field after the end of that field.
IPv4 Mask Lgth
If this message is flowing over an IPv4 TCP connection, the number
of significant bits in the IPv4 Subnet Mask field. If this is an
IPv6 flow, this field is zero.
Num IPv6 prfx
If this message is flowing over an IPv6 TCP connection, the number
of IPv6 prefixes that follow, with a maximum value of 8. If this
is an IPv4 flow, this field is zero and is immediately followed by
the ending eye catcher.
Fox, et al. Informational [Page 96]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Array of IPv6 prefixes
For IPv6 TCP connections, a list of the IPv6 prefixes associated
with the network over which the client sent this message, up to a
maximum of eight prefixes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 prefix value +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length |
+-+-+-+-+-+-+-+-+
Figure 27: Format for IPv6 Prefix Array Element
Fox, et al. Informational [Page 97]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.2.3. SMC Accept CLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length = 68 |Version|F|Rsrvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Server's Peer ID -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Server's RoCE GID -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Server's RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Server QP (bytes 1-2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---+
|Srvr QP byte 3 | Server RMB RKey (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Srvr RMB byte 4|Server RMB indx| Srvr RMB alert tkn (bytes 1-2)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Srvr RMB alert tkn (bytes 3-4)|Bsize | MTU | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Server's RMB virtual address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Server's initial packet sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28: SMC Accept CLC Message Format
Fox, et al. Informational [Page 98]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The fields present in the SMC Accept CLC message are:
Eye catchers
Like all CLC messages, the SMC Accept has beginning and ending
eye catchers to aid with verification and parsing. The hex digits
spell "SMCR" in IBM-1047 (EBCDIC).
Type
CLC message Type 2 indicates SMC Accept.
Length
The SMC Accept CLC message is 68 bytes long.
Version
Version of the SMC-R protocol. Version 1 is the only currently
defined value.
F-bit
First contact flag: A 1-bit flag that indicates that the server
believes this TCP connection is the first SMC-R contact for this
link group.
Server's Peer ID
As described in Appendix A.2.1 above.
Server's RoCE GID
The IPv6 address of the RNIC that the server chose for this SMC-R
link.
Server's RoCE MAC address
The MAC address of the server's RNIC for the SMC-R link. It is
required, as some operating systems do not have neighbor discovery
or ARP support for RoCE RNICs.
Server's QP number
The number for the reliably connected queue pair that the server
created for this SMC-R link.
Fox, et al. Informational [Page 99]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Server's RMB RKey
The RDMA RKey for the RMB that the server created or chose for
this TCP connection.
Server's RMB element index
Indexes which element within the server's RMB will represent this
TCP connection.
Server's RMB element alert token
A platform-defined, architecturally opaque token that identifies
this TCP connection. Added by the client as immediate data on
RDMA writes from the client to the server to inform the server
that there is data for this connection to retrieve from the
RMB element.
Bsize:
Server's RMB element buffer size in 4-bit compressed notation:
x = 4 bits. Actual buffer size value is (2^(x + 4)) * 1K.
Smallest possible value is 16K. Largest size supported by this
architecture is 512K.
MTU
An enumerated value indicating this peer's QP MTU size. The two
peers exchange their MTU values, and whichever value is smaller
will be used for the QP. This field should only be validated in
the first contact exchange.
The enumerated MTU values are:
0: reserved
1: 256
2: 512
3: 1024
4: 2048
5: 4096
6-15: reserved
Fox, et al. Informational [Page 100]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Server's RMB virtual address
The virtual address of the server's RMB as assigned by the
server's RNIC.
Server's initial packet sequence number
The starting packet sequence number that this peer will use when
sending to the other peer, so that the other peer can prepare its
QP for the sequence number to expect.
Fox, et al. Informational [Page 101]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.2.4. SMC Confirm CLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length = 68 |Version| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Client's Peer ID -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- Client's RoCE GID -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Client's RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Client QP (bytes 1-2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---+
|Clnt QP byte 3 | Client RMB RKey (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Clnt RMB byte 4|Client RMB indx| Clnt RMB alert tkn (bytes 1-2)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Clnt RMB alert tkn (bytes 3-4)|Bsize | MTU | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Client's RMB Virtual Address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Client's initial packet sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29: SMC Confirm CLC Message Format
The SMC Confirm CLC message is nearly identical to the SMC Accept,
except that it contains client information and lacks a first contact
flag.
Fox, et al. Informational [Page 102]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The fields present in the SMC Confirm CLC message are:
Eye catchers
Like all CLC messages, the SMC Confirm has beginning and ending
eye catchers to aid with verification and parsing. The hex digits
spell "SMCR" in IBM-1047 (EBCDIC).
Type
CLC message Type 3 indicates SMC Confirm.
Length
The SMC Confirm CLC message is 68 bytes long.
Version
Version of the SMC-R protocol. Version 1 is the only currently
defined value.
Client's Peer ID
As described in Appendix A.2.1 above.
Client's RoCE GID
The IPv6 address of the RNIC that the client chose for this SMC-R
link.
Client's RoCE MAC address
The MAC address of the client's RNIC for the SMC-R link. It is
required, as some operating systems do not have neighbor discovery
or ARP support for RoCE RNICs.
Client's QP number
The number for the reliably connected queue pair that the client
created for this SMC-R link.
Client's RMB RKey
The RDMA RKey for the RMB that the client created or chose for
this TCP connection.
Fox, et al. Informational [Page 103]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Client's RMB element index
Indexes which element within the client's RMB will represent this
TCP connection.
Client's RMB element alert token
A platform-defined, architecturally opaque token that identifies
this TCP connection. Added by the server as immediate data on
RDMA writes from the server to the client to inform the client
that there is data for this connection to retrieve from the
RMB element.
Bsize:
Client's RMB element buffer size in 4-bit compressed notation:
x = 4 bits. Actual buffer size value is (2^(x + 4)) * 1K.
Smallest possible value is 16K. Largest size supported by this
architecture is 512K.
MTU
An enumerated value indicating this peer's QP MTU size. The two
peers exchange their MTU values, and whichever value is smaller
will be used for the QP. The values are enumerated in
Appendix A.2.3. This value should only be validated in the first
contact exchange.
Client's RMB Virtual Address
The virtual address of the client's RMB as assigned by the
server's RNIC.
Client's initial packet sequence number
The starting packet sequence number that this peer will use when
sending to the other peer, so that the other peer can prepare its
QP for the sequence number to expect.
Fox, et al. Informational [Page 104]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.2.5. SMC Decline CLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Length = 28 |Version|S|Rsrvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Sender's Peer ID -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer Diagnosis Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| x'E2' | x'D4' | x'C3' | x'D9' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 30: SMC Decline CLC Message Format
The fields present in the SMC Decline CLC message are:
Eye catchers
Like all CLC messages, the SMC Decline has beginning and ending
eye catchers to aid with verification and parsing. The hex digits
spell "SMCR" in IBM-1047 (EBCDIC).
Type
CLC message Type 4 indicates SMC Decline.
Length
The SMC Decline CLC message is 28 bytes long.
Version
Version of the SMC-R protocol. Version 1 is the only currently
defined value.
S-bit
Sync Bit. Indicates that the link group is out of sync and the
receiving peer must clean up its representation of the link group.
Fox, et al. Informational [Page 105]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Sender's Peer ID
As described in Appendix A.2.1 above.
Peer Diagnosis Information
4 bytes of diagnosis information provided by the peer. These
values are defined by the individual peers, and it is necessary to
consult the peer's system documentation to interpret the results.
A.3. LLC Messages
LLC messages are sent over an existing SMC-R link using RoCE SendMsg
and are always 44 bytes long so that they fit into the space
available in a single WQE without requiring the receiver to post
receive buffers. If all 44 bytes are not needed, they are padded out
with zeros. LLC messages are in a request/response format. The
message type is the same for request and response, and a flag
indicates whether a message is flowing as a request or a response.
The two high-order bits of an LLC message opcode indicate how it is
to be handled by a peer that does not support the opcode.
If the high-order bits of the opcode are b'00', then the peer must
support the LLC message and indicate a protocol error if it does not.
If the high-order bits of the opcode are b'10', then the peer must
silently discard the LLC message if it does not support the opcode.
This requirement is included to allow for toleration of advanced, but
optional, functionality.
High-order bits of b'11' indicate a Connection Data Control (CDC)
message as described in Appendix A.4.
Fox, et al. Informational [Page 106]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.3.1. CONFIRM LINK LLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length = 44 | Reserved |R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+- -+
| Sender's RoCE GID |
+- -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |Sender's QP number, bytes 1-2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sender QP byte3| Link number |Sender's link userID, bytes 1-2|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sender's link userID, bytes 3-4| Max links | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Reserved -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 31: CONFIRM LINK LLC Message Format
The CONFIRM LINK LLC message is required to be exchanged between the
server and client over a newly created SMC-R link to complete the
setup of an SMC-R link. Its purpose is to confirm that the RoCE path
is actually usable.
On first contact, this message flows after the server receives the
SMC Confirm CLC message from the client over the IP connection. For
additional links added to an SMC-R link group, it flows after the
ADD LINK and ADD LINK CONTINUATION exchange. This flow provides
confirmation that the queue pair is in fact usable. Each peer echoes
its RoCE information back to the other.
Fox, et al. Informational [Page 107]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The contents of the CONFIRM LINK LLC message are:
Type
Type 1 indicates CONFIRM LINK.
Length
The CONFIRM LINK LLC message is 44 bytes long.
R
Reply flag. When set, indicates that this is a CONFIRM LINK
reply.
Sender's RoCE MAC address
The MAC address of the sender's RNIC for the SMC-R link. It is
required, as some operating systems do not have neighbor discovery
or ARP support for RoCE RNICs.
Sender's RoCE GID
The IPv6 address of the RNIC that the sender is using for this
SMC-R link.
Sender's QP number
The number for the reliably connected queue pair that the sender
created for this SMC-R link.
Link number
An identifier assigned by the server that uniquely identifies the
link within the link group. This identifier is ONLY unique within
a link group. Provided by the server and echoed back by the
client.
Link user ID
An opaque, implementation-defined identifier assigned by the
sender and provided to the receiver solely for purposes of
display, diagnosis, network management, etc. The link user ID
should be unique across the sender's entire software space,
including all other link groups.
Fox, et al. Informational [Page 108]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Max links
The maximum number of links the sender can support in a link
group. The maximum for this link group is the smaller of the
values provided by the two peers.
A.3.2. ADD LINK LLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length = 44 | Rsrvd |RsnCode|R|Z| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's RoCE |
+- MAC address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+- -+
| Sender's RoCE GID |
+- -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |Sender's QP number, bytes 1-2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sender QP byte3| Link number |Rsrvd | MTU |Initial PSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial PSN (continued) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+
| Reserved |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 32: ADD LINK LLC Message Format
The ADD LINK LLC message is sent over an existing link in the link
group when a peer wishes to add an SMC-R link to an existing SMC-R
link group. It is sent by the server to add a new SMC-R link to the
group, or by the client to request that the server add a new link --
for example, when a new RNIC becomes active. When sent from the
client to the server, it represents a request that the server
initiate an ADD LINK exchange.
Fox, et al. Informational [Page 109]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
This message is sent immediately after the initial SMC-R link in the
group completes, as described in Section 3.5.1 ("First Contact"). It
can also be sent over an existing SMC-R link group at any time as new
RNICs are added and become available. Therefore, there can be as few
as one new RMB RToken to be communicated, or several. RTokens will
be communicated using ADD LINK CONTINUATION messages.
The contents of the ADD LINK LLC message are:
Type
Type 2 indicates ADD LINK.
Length
The ADD LINK LLC message is 44 bytes long.
RsnCode
If the Z (rejection) flag is set, this field provides the reason
code. Values can be:
X'1' - no alternate path available: set when the server
provides the same MAC/GID as an existing SMC-R link in
the group, and the client does not have any additional
RNICs available (i.e., the server is attempting to set
up an asymmetric link but none is available).
X'2' - Invalid MTU value specified.
R
Reply flag. When set, indicates that this is an ADD LINK reply.
Z
Rejection flag. When set on reply, indicates that the server's
ADD LINK was rejected by the client. When this flag is set, the
reason code will also be set.
Sender's RoCE MAC address
The MAC address of the sender's RNIC for the new SMC-R link. It
is required, as some operating systems do not have neighbor
discovery or ARP support for RoCE RNICs.
Fox, et al. Informational [Page 110]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Sender's RoCE GID
The IPv6 address of the RNIC that the sender is using for the new
SMC-R link.
Sender's QP number
The number for the reliably connected queue pair that the sender
created for the new SMC-R link.
Link number
An identifier for the new SMC-R link. This is assigned by the
server and uniquely identifies the link within the link group.
This identifier is ONLY unique within a link group. Provided by
the server and echoed back by the client.
MTU
An enumerated value indicating this peer's QP MTU size. The two
peers exchange their MTU values, and whichever value is smaller
will be used for the QP. The values are enumerated in
Appendix A.2.3.
Initial PSN
The starting packet sequence number (PSN) that this peer will use
when sending to the other peer, so that the other peer can prepare
its QP for the sequence number to expect.
Fox, et al. Informational [Page 111]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.3.3. ADD LINK CONTINUATION LLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length = 44 | Reserved |R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Linknum | NumRTokens | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- RKey/RToken pair -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- RKey/RToken pair or zeros -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 33: ADD LINK CONTINUATION LLC Message Format
When a new SMC-R link is added to an SMC-R link group, it is
necessary to communicate the new link's RTokens for the RMBs that the
SMC-R link group can access. This message follows the ADD LINK and
provides the RTokens.
The server kicks off this exchange by sending the first ADD LINK
CONTINUATION LLC message, and the server controls the exchange as
described below.
o If the client and the server require the same number of ADD LINK
CONTINUATION messages to communicate their RTokens, the server
starts the exchange by sending the first ADD LINK CONTINUATION
request to the client with its (the server's) RTokens. The client
then responds with an ADD LINK CONTINUATION response with its
RTokens, and so on until the exchange is completed.
Fox, et al. Informational [Page 112]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
o If the server requires more ADD LINK CONTINUATION messages than
the client, then after the client has communicated all of its
RTokens, the server continues to send ADD LINK CONTINUATION
request messages to the client. The client continues to respond,
using empty (number of RTokens to be communicated = 0) ADD LINK
CONTINUATION response messages.
o If the client requires more ADD LINK CONTINUATION messages than
the server, then after communicating all of its RTokens, the
server will continue to send empty ADD LINK CONTINUATION messages
to the client to solicit replies with the client's RTokens, until
all have been communicated.
The contents of the ADD LINK CONTINUATION LLC message are:
Type
Type 3 indicates ADD LINK CONTINUATION.
Length
The ADD LINK CONTINUATION LLC message is 44 bytes long.
R
Reply flag. When set, indicates that this is an ADD LINK
CONTINUATION reply.
LinkNum
The link number of the new link within the SMC-R link group for
which RKeys are being communicated.
NumRTokens
Number of RTokens remaining to be communicated (including the ones
in this message). If the value is less than or equal to 2, this
is the last message. If it is greater than 2, another
continuation message will be required, and its value will be the
value in this message minus 2, and so on until all RKeys are
communicated. The maximum value for this field is 255.
Fox, et al. Informational [Page 113]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
RKey/RToken pairs (two or less)
These consist of an RKey for an RMB that is known on the SMC-R
link over which this message was sent (the reference RKey), paired
with the same RMB's RToken over the new SMC-R link. A full RToken
is not required for the reference, because it is only being used
to distinguish which RMB it applies to, not address it.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reference RKey |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New RKey |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- New Virtual Address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 34: RKey/RToken Pair Format
The contents of the RKey/RToken pair are:
Reference RKey
The RKey of the RMB as it is already known on the SMC-R link over
which this message is being sent. Required so that the peer knows
with which RMB to associate the new RToken.
New RKey
The RKey of this RMB as it is known over the new SMC-R link.
New Virtual Address
The virtual address of this RMB as it is known over the new
SMC-R link.
Fox, et al. Informational [Page 114]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.3.4. DELETE LINK LLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Length = 44 | Reserved |R|A|O| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Linknum | reason code (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RsnCode byte 4 | |
+-+-+-+-+-+-+-+-+ -+
| |
+- -+
| |
+- -+
| |
+- Reserved -+
| |
+- -+
| |
+- -+
| |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 35: DELETE LINK LLC Message Format
When the client or server detects that a QP or SMC-R link goes down
or needs to come down, it sends this message over one of the other
links in the link group.
When the DELETE LINK is sent from the client, it only serves as a
notification, and the client expects the server to respond by sending
a DELETE LINK request. To avoid races, only the server will initiate
the actual DELETE LINK request and response sequence that results
from notification from the client.
The server can also initiate the DELETE LINK without notification
from the client if it detects an error or if orderly link termination
was initiated.
The client may also request termination of the entire link group, and
the server may terminate the entire link group using this message.
Fox, et al. Informational [Page 115]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The contents of the DELETE LINK LLC message are:
Type
Type 4 indicates DELETE LINK.
Length
The DELETE LINK LLC message is 44 bytes long.
R
Reply flag. When set, indicates that this is a DELETE LINK reply.
A
"All" flag. When set, indicates that all links in the link group
are to be terminated. This terminates the link group.
O
Orderly flag. Indicates orderly termination. Orderly termination
is generally caused by an operator command rather than an error on
the link. When the client requests orderly termination, the
server may wait to complete other work before terminating.
LinkNum
The link number of the link to be terminated. If the A flag is
set, this field has no meaning and is set to 0.
RsnCode
The termination reason code. Currently defined reason codes are:
Request reason codes:
X'00010000' = Lost path
X'00020000' = Operator initiated termination
X'00030000' = Program initiated termination (link inactivity)
X'00040000' = LLC protocol violation
X'00050000' = Asymmetric link no longer needed
Fox, et al. Informational [Page 116]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Response reason code:
X'00100000' = Unknown link ID (no link)
A.3.5. CONFIRM RKEY LLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 | Length = 44 | Reserved |R|0|Z|C|Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NumTkns | New RMB RKey for this link (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|ThisLink byte 4| |
+-+-+-+-+-+-+-+-+ -+
| New RMB virtual address for this link |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ -+
| |
+- Other link RMB specification or zeros -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+
| |
+- -+
| Other link RMB specification or zeros |
+- +-+-+-+-+-+-+-+-+
| | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 36: CONFIRM RKEY LLC Message Format
The CONFIRM RKEY flow can be sent at any time from either the client
or the server, to inform the peer that an RMB has been created or
deleted. The creator of a new RMB must inform its peer of the new
RMB's RToken for all SMC-R links in the SMC-R link group.
For RMB creation, the creator sends this message over the SMC-R link
that the first TCP connection that uses the new RMB is using. This
message contains the new RMB RToken for the SMC-R link over which
the message is sent. It then lists the sender's SMC-R links in the
link group paired with the new RToken for the new RMB for that link.
This message can communicate the new RTokens for three QPs: the QP
for the link over which this message is sent, and two others. If
there are more than three links in the SMC-R link group, a
CONFIRM RKEY CONTINUATION will be required.
Fox, et al. Informational [Page 117]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The peer responds by simply echoing the message with the response
flag set. If the response is a negative response, the sender must
recalculate the RToken set and start a new CONFIRM RKEY exchange from
the beginning. The timing of this retry is controlled by the C flag,
as described below.
The contents of the CONFIRM RKEY LLC message are:
Type
Type 6 indicates CONFIRM RKEY.
Length
The CONFIRM RKEY LLC message is 44 bytes long.
R
Reply flag. When set, indicates that this is a CONFIRM RKEY
reply.
0
Reserved bit.
Z
Negative response flag.
C
Configuration Retry bit. If this is a negative response and this
flag is set, the originator should recalculate the RKey set and
retry this exchange as soon as the current configuration change is
completed. If this flag is not set on a negative response, the
originator must wait for the next natural stimulus (for example, a
new TCP connection started that requires a new RMB) before
retrying.
NumTkns
The number of other link/RToken pairs, including those provided in
this message, to be communicated. Note that this value does not
include the RToken for the link on which this message was sent
(i.e., the maximum value is 2). If this value is 3 or less, this
is the only message in the exchange. If this value is greater
than 3, a CONFIRM RKEY CONTINUATION message will be required.
Fox, et al. Informational [Page 118]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Note: In this version of the architecture, eight is the maximum
number of links supported in a link group.
New RMB RKey for this link
The new RMB's RKey as assigned on the link over which this message
is being sent.
New RMB virtual address for this link
The new RMB's virtual address as assigned on the link over which
this message is being sent.
Other link RMB specification
The new RMB's specification on the other links in the link group,
as shown in Figure 37.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link number | RMB's RKey for the specified link (bytes 1-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|New RKey byte 4| |
+-+-+-+-+-+-+-+-+ -+
| RMB's virtual address for the specified link |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
Figure 37: Format of Link Number/RKey Pairs
Link number
The link number for a link in the link group.
RMB's RKey for the specified link
The RKey used to reach the RMB over the link whose number was
specified in the Link number field.
RMB's virtual address for the specified link
The virtual address used to reach the RMB over the link whose
number was specified in the Link number field.
Fox, et al. Informational [Page 119]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.3.6. CONFIRM RKEY CONTINUATION LLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 8 | Length = 44 | Reserved |R|0|Z| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NumTknsLeft | |
+-+-+-+-+-+-+-+-+ -+
| |
+- Other link RMB specification -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+ -+
| |
+- Other link RMB specification or zeros -+
| |
+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+
| |
+- -+
| Other link RMB specification or zeros |
+- +-+-+-+-+-+-+-+-+
| | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 38: CONFIRM RKEY CONTINUATION LLC Message Format
The CONFIRM RKEY CONTINUATION LLC message is used to communicate any
additional RMB RTokens that did not fit into the CONFIRM RKEY
message. Each of these messages can hold up to three RMB RTokens.
The NumTknsLeft field indicates how many RMB RTokens are to be
communicated, including the ones in this message. If the value is 3
or less, this is the last message of the group. If the value is 4 or
higher, additional CONFIRM RKEY CONTINUATION messages will follow,
and the NumTknsLeft value will be a countdown until all are
communicated.
Like the CONFIRM RKEY message, the peer responds by echoing the
message back with the reply flag set.
Fox, et al. Informational [Page 120]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The contents of the CONFIRM RKEY CONTINUATION LLC message are:
Type
Type 8 indicates CONFIRM RKEY CONTINUATION.
Length
The CONFIRM RKEY CONTINUATION LLC message is 44 bytes long.
R
Reply flag. When set, indicates that this is a CONFIRM RKEY
CONTINUATION reply.
0
Reserved bit.
Z
Negative response flag.
NumTknsLeft
The number of link/RToken pairs, including those provided in this
message, that are remaining to be communicated. If this value is
3 or less, this is the last message in the exchange. If this
value is greater than 3, another CONFIRM RKEY CONTINUATION message
will be required. Note that in this version of the architecture,
eight is the maximum number of links supported in a link group.
Other link RMB specification
The new RMB's specification on other links in the link group, as
shown in Figure 37.
Fox, et al. Informational [Page 121]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.3.7. DELETE RKEY LLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Length = 44 | Reserved |R|0|Z| Rsrvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Count | Error Mask | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First deleted RKey |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Second deleted RKey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Third deleted RKey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fourth deleted RKey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fifth deleted RKey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sixth deleted RKey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seventh deleted RKey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Eighth deleted RKey or zeros |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 39: DELETE RKEY LLC Message Format
The DELETE RKEY flow can be sent at any time from either the client
or the server, to inform the peer that one or more RMBs have been
deleted. Because the peer already knows every RMB's RKey on each
link in the link group, this message only specifies one RKey for each
RMB being deleted. The RKey provided for each deleted RMB will be
its RKey as known on the SMC-R link over which this message is sent.
It is not necessary to provide the entire RToken. The RKey alone is
sufficient for identifying an existing RMB.
The peer responds by simply echoing the message with the response
flag set. If the peer did not recognize an RKey, a negative response
flag will be set; however, no aggressive recovery action beyond
logging the error will be taken.
Fox, et al. Informational [Page 122]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The contents of the DELETE RKEY LLC message are:
Type
Type 9 indicates DELETE RKEY.
Length
The DELETE RKEY LLC message is 44 bytes long.
R
Reply flag. When set, indicates that this is a DELETE RKEY reply.
0
Reserved bit.
Z
Negative response flag.
Count
Number of RMBs being deleted by this message. Maximum value is 8.
Error Mask
If this is a negative response, indicates which RMBs were not
successfully deleted. Each bit corresponds to a listed RMB; for
example, b'01010000' indicates that the second and fourth RKeys
weren't successfully deleted.
Deleted RKeys
A list of Count RKeys. Provided on the request flow and echoed
back on the response flow. Each RKey is valid on the link over
which this message is sent and represents a deleted RMB. Up to
eight RMBs can be deleted in this message.
Fox, et al. Informational [Page 123]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
A.3.8. TEST LINK LLC Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 7 | Length = 44 | Reserved |R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- User Data -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- -+
| Reserved |
+- -+
| |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 40: TEST LINK LLC Message Format
The TEST LINK request can be sent from either peer to the other on an
existing SMC-R link at any time to test that the SMC-R link is active
and healthy at the software level. A peer that receives a TEST LINK
LLC message immediately sends back a TEST LINK reply, echoing back
the user data. Refer also to Section 4.5.3 ("TCP Keepalive
Processing").
Fox, et al. Informational [Page 124]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The contents of the TEST LINK LLC message are:
Type
Type 7 indicates TEST LINK.
Length
The TEST LINK LLC message is 44 bytes long.
R
Reply flag. When set, indicates that this is a TEST LINK reply.
User Data
The receiver of this message echoes the sender's data back in a
TEST LINK response LLC message.
A.4. Connection Data Control (CDC) Message Format
The RMBE control data is communicated using Connection Data Control
(CDC) messages, which use RoCE SendMsg, similar to LLC messages.
Also, as with LLC messages, CDC messages are 44 bytes long to ensure
that they can fit into private data areas of receive WQEs without
requiring the receiver to post receive buffers.
Unlike LLC messages, this data is integral to the data path, so its
processing must be prioritized and optimized similarly to other data
path processing. While LLC messages may be processed on a slower
path than data, these messages cannot be.
Fox, et al. Informational [Page 125]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
0 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = x'FE' | Length = 44 | Sequence number |
4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SMC-R alert token |
8 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Producer cursor wrap seqno |
12 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Producer Cursor |
16 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Consumer cursor wrap seqno |
20 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Consumer Cursor |
24 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B|P|U|R|F|Rsrvd|D|C|A| Reserved |
28 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
32 +- -+
| |
36 +- Reserved -+
| |
40 +- -+
| |
44 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 41: Connection Data Control (CDC) Message Format
Type = x'FE'
This type number has the two high-order bits turned on to enable
processing to quickly distinguish it from an LLC message.
Length = 44
The length of inline data that does not require the posting of a
receive buffer.
Sequence number
A 2-byte unsigned integer that represents a wrapping sequence
number. The initial value is 1, and this value can wrap to 0.
Incremented with every control message sent, except for the
failover data validation message, and used to guard against
processing an old control message out of sequence. Also used in
failover data validation. In normal usage, if this number is less
Fox, et al. Informational [Page 126]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
than the last received value, discard this message. If greater,
process this message. Old control messages can be lost with no
ill effect but cannot be processed after newer ones.
If this is a failover validation CDC message (F flag set), then
the receiver must verify that it has received and fully processed
the RDMA write that was described by the CDC message with the
sequence number in this message. If not, the TCP connection must
be reset to guard against data loss. Details of this processing
are provided in Section 4.6.1.
SMC-R alert token
The endpoint-assigned alert token that identifies to which TCP
connection on the link group this control message refers.
Producer cursor wrap seqno
A 2-byte unsigned integer that represents a wrapping counter
incremented by the producer whenever the data written into this
RMBE receive buffer causes a wrap (i.e., the producer cursor
wraps). This is used by the receiver to determine when new data
is available even though the cursors appear unchanged, such as
when a full window size write is completed (producer cursor of
this RMBE sent by peer = local consumer cursor) or in scenarios
where the producer cursor sent for this RMBE < local consumer
cursor.
Producer Cursor
A 4-byte unsigned integer that is a wrapping offset into the RMBE
data area. Points to the next byte of data to be written by the
sender. Can advance up to the receiver's consumer cursor as known
by the sender. When the urgent data present indicator is on,
points 1 byte beyond the last byte of urgent data. When computing
this cursor, the presence of the eye catcher in the RMBE data area
must be accounted for. The first writable data location in the
RMBE is at offset 4, so this cursor begins at 4 and wraps to 4.
Consumer cursor wrap seqno
A 2-byte unsigned integer that mirrors the value of the producer
cursor wrap sequence number when the last read from this RMBE
occurred. Used as an indicator of how far along the consumer is
in reading data (i.e., processed last wrap point or not). The
producer side can use this indicator to detect whether or not more
data can be written to the partner in full window write scenarios
(where the producer cursor = consumer cursor as known on the
Fox, et al. Informational [Page 127]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
remote RMBE). In this scenario, if the consumer sequence number
equals the local producer sequence number, the producer knows that
more data can be written.
Consumer Cursor
A 4-byte unsigned integer that is a wrapping offset into the
sender's RMBE data area. Points to the offset of the next byte of
data to be consumed by the peer in its own RMBE. When computing
this cursor, the presence of the eye catcher in the RMBE data area
must be accounted for. The first writable data location in the
RMBE is at offset 4, so this cursor begins at 4 and wraps to 4.
The sender cannot write beyond this cursor into the peer's RMBE
without causing data loss.
B-bit
Writer blocked indicator: Sender is blocked for writing. If this
bit is set, sender will require explicit notification when receive
buffer space is available.
P-bit
Urgent data pending: Sender has urgent data pending for this
connection.
U-bit
Urgent data present: Indicates that urgent data is present in the
RMBE data area, and the producer cursor points to 1 byte beyond
the last byte of urgent data.
R-bit
Request for consumer cursor update: Indicates that an immediate
consumer cursor update is requested, regardless of whether or not
one is warranted according to the window size optimization
algorithm described in Section 4.5.1.
F-bit
Failover validation indicator: Sent by a peer to guard against
data loss during failover when the TCP connection is being moved
to another SMC-R link in the link group. When this bit is set,
the only other fields in the CDC message that are significant are
the Type, Length, SMC-R alert token, and Sequence number fields.
The receiver must validate that it has fully processed the RDMA
write described by the previous CDC message bearing the same
Fox, et al. Informational [Page 128]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
sequence number as this validation message. If it has, no further
action is required. If it has not, the TCP connection must be
reset. This processing is described in detail in Section 4.6.1.
D-bit
Sending done indicator: Sent by a peer when it is done writing new
data into the receiver's RMBE data area.
C-bit
PeerConnectionClosed indicator: Sent by a peer when it is
completely done with this connection and will no longer be making
any updates to the receiver's RMBE or sending any more control
messages.
A-bit
Abnormal close indicator: Sent by a peer when the connection is
abnormally terminated (for example, the TCP connection was reset).
When sent, it indicates that the peer is completely done with this
connection and will no longer be making any updates to this RMBE
or sending any more control messages. It also indicates that the
RMBE owner must flush any remaining data on this connection and
generate an error return code to any outstanding socket APIs on
this connection (same processing as receiving a RST segment on a
TCP connection).
Appendix B. Socket API Considerations
A key design goal for SMC-R is to require no application changes for
exploitation. It is confined to socket applications using stream
(i.e., TCP) sockets over IPv4 or IPv6. By virtue of the fact that
the switch to the SMC-R protocol occurs after a TCP connection is
established, no changes are required in a socket address family or in
the IP addresses and ports that the socket applications are using.
Existing socket APIs that allow applications to retrieve local and
remote socket address structures for an established TCP connection
(for example, getsockname() and getpeername()) will continue to
function as they have before. Existing DNS setup and APIs for
resolving hostnames to IP addresses and vice versa also continue to
function without any changes. In general, all of the usual socket
APIs that are used for TCP communications (send APIs, recv APIs,
etc.) will continue to function as they do today, even if SMC-R is
used as the underlying protocol.
Fox, et al. Informational [Page 129]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Each SMC-R-enabled implementation does, however, need to pay special
attention to any socket APIs that have a reliance on the underlying
TCP and IP protocols and also ensure that their behavior in an SMC-R
environment is reasonable and minimizes impact on the application.
While the basic socket API set is fairly similar across different
operating systems, there is more variability when it comes to
advanced socket API options. Each implementation needs to perform a
detailed analysis of its API options, any possible impact that SMC-R
may have, and any resultant implications. As part of that step, a
discussion or review with other implementations supporting SMC-R
would be useful to ensure consistent implementation.
B.1. setsockopt() / getsockopt() Considerations
These APIs allow socket applications to manipulate socket, transport
(TCP/UDP), and IP-level options associated with a given socket.
Typically, a platform restricts the number of IP options available to
stream (TCP) socket applications, given their connection-oriented
nature. The general guideline here is to continue processing these
APIs in a manner that allows for application compatibility. Some
options will be relevant to the SMC-R protocol and will require
special processing "under the covers". For example, the ability to
manipulate TCP send and receive buffer sizes is still valid for
SMC-R. However, other options may have no meaning for SMC-R. For
example, if an application enabled the TCP_NODELAY socket option to
disable Nagle's algorithm, it should have no real effect on SMC-R
communications, as there is no notion of Nagle's algorithm with this
new protocol. But the implementation must accept the TCP_NODELAY
option as it does today and save it so that it can be later extracted
via getsockopt() processing. Note that any TCP or IP-level options
will still have an effect on any TCP/IP packets flowing for an SMC-R
connection (i.e., as part of TCP/IP connection establishment and
TCP/IP connection termination packet flows).
Under the covers, manipulation of the TCP options will also include
the SMC-layer setting, as well as reading the SMC-R experimental
option before and after completion of the three-way TCP handshake.
Fox, et al. Informational [Page 130]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Appendix C. Rendezvous Error Scenarios
This section discusses error scenarios for setting up and managing
SMC-R links.
C.1. SMC Decline during CLC Negotiation
A peer to the SMC-R CLC negotiation can send an SMC Decline in lieu
of any expected CLC message to decline SMC and force the TCP
connection back to the IP fabric. There can be several reasons for
an SMC Decline during the CLC negotiation, including the following:
o RNIC went down
o SMC-R forbidden by local policy
o subnet (IPv4) or prefix (IPv6) doesn't match
o lack of resources to perform SMC-R
In all cases, when an SMC Decline is sent in lieu of an expected CLC
message, no confirmation is required, and the TCP connection
immediately falls back to using the IP fabric.
To prevent ambiguity between CLC messages and application data, an
SMC Decline cannot "chase" another CLC message. An SMC Decline can
only be sent in lieu of an expected CLC message. For example, if the
client sends an SMC Proposal and then its RNIC goes down, it must
wait for the SMC Accept from the server and then reply to the
SMC Accept with an SMC Decline.
This "no chase" rule means that if this TCP connection is not a first
contact between RoCE peers, a server cannot send an SMC Decline after
sending an SMC Accept -- it can only either break the TCP connection
or fail over if a problem arises in the RoCE fabric after it has sent
the SMC Accept. Similarly, once the client sends an SMC Confirm on a
TCP connection that isn't a first contact, it is committed to SMC-R
for this TCP connection and cannot fall back to IP.
C.2. SMC Decline during LLC Negotiation
For a TCP connection that represents a first contact between RoCE
pairs, it is possible for SMC to fall back to IP during the LLC
negotiation. This is possible until the first contact SMC-R link is
confirmed. For example, see Figure 42. After a first contact SMC-R
link is confirmed, fallback to IP is no longer possible. This
translates to the following rule: a first contact peer can send an
Fox, et al. Informational [Page 131]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
SMC Decline at any time during LLC negotiation until it has
successfully sent its CONFIRM LINK (request or response) flow. After
that point, it cannot fall back to IP.
Host X -- Server Host Y -- Client
+-------------------+ +-------------------+
| Peer ID = PS1 | | Peer ID = PC1 |
| +------+ +------+ |
| QP 8 |RNIC 1| SMC-R Link 1 |RNIC 2| QP 64 |
| RKey X | |MAC MA|<-------------------->|MAC MB| | |
| | |GID GA| attempted setup |GID GB| | RKey Y2|
| \/ +------+ +------+ \/ |
|+--------+ | | +--------+ |
|| RMB | | | | RMB | |
|+--------+ | | +--------+ |
| /\ +------+ +------+ /\ |
| | |RNIC 3| |RNIC 4| | RKey W2|
| | |MAC MC| |MAC MD| | |
| QP 9 |GID GC| |GID GD| QP 65 |
| +------+ +------+ |
+-------------------+ +-------------------+
SYN / SYN-ACK / ACK TCP three-way handshake with TCP option
<--------------------------------------------------------->
SMC Proposal / SMC Accept / SMC Confirm exchange
<-------------------------------------------------------->
CONFIRM LINK(request, Link 1)
.........................................................>
CONFIRM LINK(response, Link 1)
X...................................
:
: RoCE write failure
:.................................>
SMC Decline(PC1, reason code)
<--------------------------------------------------------
Connection data flows over IP fabric
<------------------------------------------------------->
Legend:
------------ TCP/IP and CLC flows
............ RoCE (LLC) flows
Figure 42: SMC Decline during LLC Negotiation
Fox, et al. Informational [Page 132]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
C.3. The SMC Decline Window
Because SMC-R does not support fallback to IP for a TCP connection
that is already using RDMA, there are specific rules on when the
SMC Decline CLC message, which signals a fallback to IP because of an
error or problem with the RoCE fabric, can be sent during TCP
connection setup. There is a "point of no return" after which a
connection cannot fall back to IP, and RoCE errors that occur after
this point require the connection to be broken with a RST flow in the
IP fabric.
For a first contact, that point of no return is after the ADD LINK
LLC message has been successfully sent for the second SMC-R link.
Specifically, the server cannot fall back to IP after receiving
either (1) a positive write completion indication for the ADD LINK
request or (2) the ADD LINK response from the client, whichever comes
first. The client cannot fall back to IP after sending a negative
ADD LINK response, receiving a positive write complete on a positive
ADD LINK response, or receiving a CONFIRM LINK for the second SMC-R
link from the server, whichever comes first.
For a subsequent contact, that point of no return is after the last
send of the CLC negotiation completes. This, in combination with the
rule that error "chasers" are not allowed during CLC negotiation,
means that the server cannot send an SMC Decline after sending an SMC
Accept, and the client cannot send an SMC Decline after sending an
SMC Confirm.
C.4. Out-of-Sync Conditions during SMC-R Negotiation
The SMC Accept CLC message contains a first contact flag that
indicates to the client whether the server believes it is setting up
a new link group or using an existing link group. This flag is used
to detect an out-of-sync condition between the client and the server.
The scenario for such a condition is as follows: there is a single
existing SMC-R link between the peers. After the client sends the
SMC Proposal CLC message, the existing SMC-R link between the client
and the server fails. The client cannot chase the SMC Proposal CLC
message with an SMC Decline CLC message in this case, because the
client does not yet know that the server would have wanted to choose
the SMC-R link that just crashed. The QP that failed recovers before
the server returns its SMC Accept CLC message. This means that there
is a QP but no SMC-R link. Since the server had not yet learned of
the SMC-R link failure when it sent the SMC Accept CLC message, it
attempts to reuse the SMC-R link that just failed. This means that
the server would not set the first contact flag, indicating to the
client that the server thinks it is reusing an SMC-R link. However,
the client does not have an SMC-R link that matches the server's
Fox, et al. Informational [Page 133]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
specification. Because the first contact flag is off, the client
realizes it is out of sync with the server and sends an SMC Decline
to cause the connection to fall back to IP.
C.5. Timeouts during CLC Negotiation
Because the SMC-R negotiation flows as TCP data, there are built-in
timeouts and retransmits at the TCP layer for individual messages.
Implementations also must protect the overall TCP/CLC handshake with
a timer or timers to prevent connections from hanging indefinitely
due to SMC-R processing. This can be done with individual timers for
individual CLC messages or an overall timer for the entire exchange,
which may include the TCP handshake and the CLC handshake under one
timer or separate timers. This decision is implementation dependent.
If the TCP and/or CLC handshakes time out, the TCP connection must be
terminated as it would be in a legacy IP environment when connection
setup doesn't complete in a timely manner. Because the CLC flows are
TCP messages, if they cannot be sent and received in a timely
fashion, the TCP connection is not healthy and would not work if
fallback to IP were attempted.
C.6. Protocol Errors during CLC Negotiation
Protocol errors occur during CLC negotiation when a message is
received that is not expected. For example, a peer that is expecting
a CLC message but instead receives application data has experienced a
protocol error; this also indicates a likely software error, as the
two sides are out of sync. When application data is expected, this
data is not parsed to ensure that it's not a CLC message.
When a peer is expecting a CLC negotiation message, any parsing error
except a bad enumerated value in that message must be treated as
application data. The CLC negotiation messages are designed with
beginning and ending eye catchers to help verify that a CLC
negotiation message is actually the expected message. If other
parsing errors in an expected CLC message occur, such as incorrect
length fields or incorrectly formatted fields, the message must be
treated as application data.
All protocol errors, with the exception of bad enumerated values,
must result in termination of the TCP connection. No fallback to IP
is allowed in the case of a protocol error, because if the protocols
are out of sync, mismatched, or corrupted, then data and security
integrity cannot be ensured.
Fox, et al. Informational [Page 134]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The exception to this rule is enumerated values -- for example, the
QP MTU values on SMC Accept and SMC Confirm. If a reserved value is
received, the proper error response is to send an SMC Decline and
fall back to IP; this is because the use of a reserved enumerated
value indicates that the other partner likely has additional support
that the receiving partner does not have. This indicated mismatch of
SMC-R capabilities is not an integrity problem but indicates that
SMC-R cannot be used for this connection.
C.7. Timeouts during LLC Negotiation
Whenever a peer sends an LLC message to which a reply is expected, it
sets a timer after the send posts to wait for the reply. An expected
response may be a reply flavor of the LLC message (for example, a
CONFIRM LINK reply) or a new LLC message (for example, an ADD LINK
CONTINUATION expected from the server by the client if there are more
RKeys to be communicated).
On LLC flows that are part of a first contact setup of a link group,
the value of the timer is implementation dependent but should be long
enough to allow the other peer to have a write complete timeout and
2-3 retransmits of an SMC Decline on the TCP fabric. For LLC flows
that are maintaining the link group and are not part of a first
contact setup of a link group, the timers may be shorter. Upon
receipt of an expected reply, the timer is cancelled. If a timer
pops without a reply having been received, the sender must initiate a
recovery action.
During first contact processing, failure of an LLC verification timer
is a "should-not-occur" that indicates a problem with one of the
endpoints; this is because if there is a "routine" failure in the
RoCE fabric that causes an LLC verification send to fail, the sender
will get a write completion failure and will then send an SMC Decline
to the partner. The only time an LLC verification timer will expire
on a first contact is when the sender thinks the send succeeded but
it actually didn't. Because of the reliably connected nature of QP
connections on the RoCE fabric, this indicates a problem with one of
the peers, not with the RoCE fabric.
After the reliably connected queue pair for the first SMC-R link in a
link group is set up on initial contact, the client sets a timer to
wait for a RoCE verification message from the server that the QP is
actually connected and usable. If the server experiences a failure
sending its QP confirmation message, it will send an SMC Decline,
which should arrive at the client before the client's verification
timer expires. If the client's timer expires without receiving
either an SMC Decline or a RoCE message confirmation from the server,
Fox, et al. Informational [Page 135]
RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
there is a problem with either the server or the TCP fabric. In
either case, the client must break the TCP connection and clean up
the SMC-R link.
There are two scenarios in which the client's response to the QP
verification message fails to reach the server. The main difference
is whether or not the client has successfully completed the send of
the CONFIRM LINK response.
In the normal case of a problem with the RoCE path, the client will
learn of the failure by getting a write completion failure, before
the server's timer expires. In this case, the client sends an SMC
Decline CLC message to the server, and the TCP connection falls back
to IP.
If the client's send of the confirmation message receives a positive
return code but for some reason still does not reach the server, or
the client's SMC Decline CLC message fails to reach the server after
the client fails to send its RoCE confirmation message, then the
server's timer will time out and the server must break the TCP
connection by sending a RST. This is expected to be a very rare
case, because if the client cannot send its CONFIRM LINK response LLC
message, the client should get a negative return code and initiate
fallback to IP. A client receiving a positive return code on a send
that fails to reach the server should also be an extremely rare case.
C.7.1. Recovery Actions for LLC Timeouts and Failures
The following list describes recovery actions for LLC timeouts. A
write completion failure or other indication of send failure for an
LLC command is treated the same as a timeout.
LLC message: CONFIRM LINK from server (first contact, first link in
the link group)
Timer waits for: CONFIRM LINK reply from client.
Recovery action: Break the TCP connection by sending a RST, and
clean up the link. The server should have received an SMC Decline
from the client by now if the client had an LLC send failure.
LLC message: CONFIRM LINK from server (first contact, second link in
the link group)
Timer waits for: CONFIRM LINK reply from client.
Fox, et al. Informational [Page 136]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
Recovery action: The second link was not successfully set up.
Send a DELETE LINK to the client. Connection data cannot flow in
the first link in the link group, until the reply to this DELETE
LINK is received, to prevent the peers from being out of sync on
the state of the link group.
LLC message: CONFIRM LINK from server (not first contact)
Timer waits for: CONFIRM LINK reply from client.
Recovery action: Clean up the new link, and set a timer to retry.
Send a DELETE LINK to the client, in case the client has a longer
timer interval, so the client can stop waiting.
LLC message: CONFIRM LINK reply from client (first contact)
Timer waits for: ADD LINK from server.
Recovery action: Clean up the SMC-R link, and break the TCP
connection by sending a RST over the IP fabric. There is a
problem with the server. If the server had a send failure, it
should have sent an SMC Decline by now.
LLC message: ADD LINK from server (first contact)
Timer waits for: ADD LINK reply from client.
Recovery action: Break the TCP connection with a RST, and clean up
RoCE resources. The connection is past the point where the server
can fall back to IP, and if the client had a send problem it
should have sent an SMC Decline by now.
LLC message: ADD LINK from server (not first contact)
Timer waits for: ADD LINK reply from client.
Recovery action: Clean up resources (QP, RKeys, etc.) for the new
link, and treat the link over which the ADD LINK was sent as if it
had failed. If there is another link available to resend the
ADD LINK and the link group still needs another link, retry the
ADD LINK over another link in the link group.
LLC message: ADD LINK reply from client (and there are more RKeys to
be communicated)
Timer waits for: ADD LINK CONTINUATION from server.
Recovery action: Treat the same as ADD LINK timer failure.
Fox, et al. Informational [Page 137]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
LLC message: ADD LINK reply or ADD LINK CONTINUATION reply from
client (and there are no more RKeys to be communicated, for the
second link in a first contact scenario)
Timer waits for: CONFIRM LINK from the server, over the new link.
Recovery action: The setup of the new link failed. Send a
DELETE LINK to the server. Do not consider the socket opened to
the client application until receiving confirmation from the
server in the form of a DELETE LINK request for this link and
sending the reply (to prevent the partners from being out of sync
on the state of the link group).
Set a timer to send another ADD LINK to the server if there is
still an unused RNIC on the client side.
LLC message: ADD LINK reply or ADD LINK CONTINUATION reply from
client (and there are no more RKeys to be communicated)
Timer waits for: CONFIRM LINK from the server, over the new link.
Recovery action: Send a DELETE LINK to the server for the new
link, then clean up any resource allocated for the new link and
set a timer to send an ADD LINK to the server if there is still an
unused RNIC on the client side. The setup of the new link failed,
but the link over which the ADD LINK exchange occurred is
unaffected.
LLC message: ADD LINK CONTINUATION from server
Timer waits for: ADD LINK CONTINUATION reply from client.
Recovery action: Treat the same as ADD LINK timer failure.
LLC message: ADD LINK CONTINUATION reply from client (first contact,
and RMB count fields indicate that the server owes more ADD LINK
CONTINUATION messages)
Timer waits for: ADD LINK CONTINUATION from server.
Recovery action: Clean up the SMC-R link, and break the TCP
connection by sending a RST. There is a problem with the server.
If the server had a send failure, it should have sent an
SMC Decline by now.
Fox, et al. Informational [Page 138]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
LLC message: ADD LINK CONTINUATION reply from client (not first
contact, and RMB count fields indicate that the server owes more
ADD LINK CONTINUATION messages)
Timer waits for: ADD LINK CONTINUATION from server.
Recovery action: Treat as if client detected link failure on the
link that the ADD LINK exchange is using. Send a DELETE LINK to
the server over another active link if one exists; otherwise,
clean up the link group.
LLC message: DELETE LINK from client
Timer waits for: DELETE LINK request from server.
Recovery action: If the scope of the request is to delete a single
link, the surviving link over which the client sent the
DELETE LINK is no longer usable either. If this is the last link
in the link group, end TCP connections over the link group by
sending RST packets. If there are other surviving links in the
link group, resend over a surviving link. Also send a DELETE LINK
over a surviving link for the link over which the client attempted
to send the initial DELETE LINK message. If the scope of the
request is to delete the entire link group, try resending on other
links in the link group until success is achieved. If all sends
fail, tear down the link group and any TCP connections that exist
on it.
LLC message: DELETE LINK from server (scope: entire link group)
Timer waits for: Confirmation from the adapter that the message
was delivered.
Recovery action: Tear down the link group and any TCP connections
that exist on it.
LLC message: DELETE LINK from server (scope: single link)
Timer waits for: DELETE LINK reply from client.
Recovery action: The link over which the server sent the
DELETE LINK is no longer usable either. If this is the last link
in the link group, end TCP connections over the link group by
sending RST packets. If there are other surviving links in the
link group, resend over a surviving link. Also send a DELETE LINK
over a surviving link for the link over which the server attempted
to send the initial DELETE LINK message. If the scope of the
request is to delete the entire link group, try resending on other
Fox, et al. Informational [Page 139]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
links in the link group until success is achieved. If all sends
fail, tear down the link group and any TCP connections that exist
on it.
LLC message: CONFIRM RKEY from client
Timer waits for: CONFIRM RKEY reply from server.
Recovery action: Perform normal client procedures for detection of
failed link. The link over which the message was sent has failed.
LLC message: CONFIRM RKEY from server
Timer waits for: CONFIRM RKEY reply from client.
Recovery action: Perform normal server procedures for detection of
failed link. The link over which the message was sent has failed.
LLC message: TEST LINK from client
Timer waits for: TEST LINK reply from server.
Recovery action: Perform normal client procedures for detection of
failed link. The link over which the message was sent has failed.
LLC message: TEST LINK from server
Timer waits for: TEST LINK reply from client.
Recovery action: Perform normal server procedures for detection of
failed link. The link over which the message was sent has failed.
The following list describes recovery actions for invalid LLC
messages. These could be misformatted or contain out-of-sync data.
LLC message received: CONFIRM LINK from server
What it indicates: Incorrect link information.
Recovery action: Protocol error. The link must be brought down by
sending a DELETE LINK for the link over another link in the link
group if one exists. If this is a first contact, fall back to IP
by sending an SMC Decline to the server.
Fox, et al. Informational [Page 140]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
LLC message received: ADD LINK
What it indicates: Undefined enumerated MTU value.
Recovery action: Send a negative ADD LINK reply with reason
code x'2'.
LLC message received: ADD LINK reply from client
What it indicates: Client-side link information that would result
in a parallel link being set up.
Recovery action: Parallel links are not permitted. Delete the
link by sending a DELETE LINK to the client over another link in
the link group.
LLC message received: Any link group command from the server, except
DELETE LINK for the entire link group
What it indicates: Client has sent a DELETE LINK for the link on
which the message was received.
Recovery action: Ignore the LLC message. Worst case: the server
will time out. Best case: the DELETE LINK crosses with the
command from the server, and the server realizes it failed.
LLC message received: ADD LINK CONTINUATION from server or ADD LINK
CONTINUATION reply from client
What it indicates: Number of RMBs provided doesn't match count
given on initial ADD LINK or ADD LINK reply message.
Recovery action: Protocol error. Treat as if detected link
outage.
LLC message received: DELETE LINK from client
What it indicates: Link indicated doesn't exist.
Recovery action: If the link is in the process of being cleaned
up, assume timing window and ignore message. Otherwise, send a
DELETE LINK reply with reason code 1.
LLC message received: DELETE LINK from server
What it indicates: Link indicated doesn't exist.
Recovery action: Send a DELETE LINK reply with reason code 1.
Fox, et al. Informational [Page 141]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
LLC message received: CONFIRM RKEY from either client or server
What it indicates: No RKey provided for one or more of the links
in the link group.
Recovery action: Treat as if detected failure of the link(s) for
which no RKey was provided.
LLC message received: DELETE RKEY
What it indicates: Specified RKey doesn't exist.
Recovery action: Send a negative DELETE RKEY response.
LLC message received: TEST LINK reply
What it indicates: User data doesn't match what was sent in the
TEST LINK request.
Recovery action: Treat as if detected that the link has gone down.
This is a protocol error.
LLC message received: Unknown LLC type with high-order bits of opcode
equal to b'10'
What it indicates: This is an optional LLC message that the
receiver does not support.
Recovery action: Ignore (silently discard) the message.
LLC message received: Any unambiguously incorrect or out-of-sync LLC
message
What it indicates: Link is out of sync.
Recovery action: Treat as if detected that the link has gone down.
Note that an unsupported or unknown LLC opcode whose two
high-order bits are b'10' is not an error and must be silently
discarded. Any other unknown or unsupported LLC opcode is an
error.
C.8. Failure to Add Second SMC-R Link to a Link Group
When there is any failure in setting up the second SMC-R link in an
SMC-R link group, including confirmation timer expiration, the SMC-R
link group is allowed to continue without available failover.
However, this situation is extremely undesirable, and the server must
endeavor to correct it as soon as it can.
Fox, et al. Informational [Page 142]RFC 7609 IBM's Shared Memory Communications over RDMA August 2015
The server peer in the SMC-R link group must set a timer to drive it
to retry setup of a failed additional SMC-R link. The server will
immediately retry the SMC-R link setup when the first of the
following events occurs:
o The retry timer expires.
o A new RNIC becomes available to the server, on the same LAN as the
SMC-R link group.
o An ADD LINK LLC request message is received from the client; this
indicates the availability of a new RNIC on the client side.
Authors' Addresses
Mike Fox
IBM
3039 Cornwallis Rd.
Research Triangle Park, NC 27709
United States
Email: mjfox@us.ibm.com
Constantinos (Gus) Kassimis
IBM
3039 Cornwallis Rd.
Research Triangle Park, NC 27709
United States
Email: kassimis@us.ibm.com
Jerry Stevens
IBM
3039 Cornwallis Rd.
Research Triangle Park, NC 27709
United States
Email: sjerry@us.ibm.com
Fox, et al. Informational [Page 143]
