Network Working Group M. Chadalapaka
Request for Comments: 3783 R. Elliott
Category: Informational Hewlett-Packard Co.
May 2004
Small Computer Systems Interface (SCSI)
Command Ordering Considerations with iSCSI
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
Internet Small Computer Systems Interface (iSCSI) is a Small Computer
Systems Interface (SCSI) transport protocol designed to run on top of
TCP. The iSCSI session abstraction is equivalent to the classic SCSI
"I_T nexus", which represents the logical relationship between an
Initiator and a Target (I and T) required in order to communicate via
the SCSI family of protocols. The iSCSI session provides an ordered
command delivery from the SCSI initiator to the SCSI target. This
document goes into the design considerations that led to the iSCSI
session model as it is defined today, relates the SCSI command
ordering features defined in T10 specifications to the iSCSI
concepts, and finally provides guidance to system designers on how
true command ordering solutions can be built based on iSCSI.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions and Acronyms . . . . . . . . . . . . . . . . . . . 3
2.1. Definitions. . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of the iSCSI Protocol . . . . . . . . . . . . . . . . 4
3.1. Protocol Mapping Description . . . . . . . . . . . . . . 4
3.2. The I_T Nexus Model. . . . . . . . . . . . . . . . . . . 5
3.3. Ordered Command Delivery . . . . . . . . . . . . . . . . 6
3.3.1. Questions. . . . . . . . . . . . . . . . . . . . 6
3.3.2. The Session Guarantee. . . . . . . . . . . . . . 6
3.3.3. Ordering Onus. . . . . . . . . . . . . . . . . . 7
3.3.4. Design Intent. . . . . . . . . . . . . . . . . . 7
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4. The Command Ordering Scenario. . . . . . . . . . . . . . . . . 8
4.1. SCSI Layer . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. Command Reference Number (CRN) . . . . . . . . . 8
4.1.2. Task Attributes. . . . . . . . . . . . . . . . . 8
4.1.3. Auto Contingent Allegiance (ACA) . . . . . . . . 8
4.1.4. UA Interlock . . . . . . . . . . . . . . . . . . 9
4.2. iSCSI Layer. . . . . . . . . . . . . . . . . . . . . . . 9
5. Connection Failure Considerations. . . . . . . . . . . . . . . 9
6. Command Ordering System Considerations . . . . . . . . . . . . 10
7. Reservation Considerations . . . . . . . . . . . . . . . . . . 11
8. Security Considerations. . . . . . . . . . . . . . . . . . . . 12
9. References and Bibliography. . . . . . . . . . . . . . . . . . 12
9.1. Normative References.. . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14
1. Introduction
iSCSI is a SCSI transport protocol ([iSCSI]) designed to enable
running SCSI application protocols on TCP/IP networks, including
potentially the Internet. Given the size and scope of the Internet,
iSCSI thus enables some exciting new SCSI applications. Potential
new application areas for exploiting iSCSI's value include the
following:
a) Larger (diameter) Storage Area Networks (SANs) than had been
possible until now
b) Asynchronous remote mirroring
c) Remote tape vaulting
Each of these applications takes advantage of the practically
unlimited geographical distance that iSCSI enables between a SCSI
initiator and a SCSI target. In each of these cases, because of the
long delays involved, there is a very high incentive for the
initiator to stream SCSI commands back-to-back without waiting for
the SCSI status of previous commands. Command streaming may be
employed primarily by two classes of applications - while one class
may not particularly care about ordered command execution, the other
class does rely on ordered command execution (i.e. there is an
application-level dependency on the ordering among SCSI commands).
As an example, cases b) and c) listed earlier clearly require ordered
command execution. A mirroring application does not want the writes
to be committed out of order on the remote SCSI target, so as to
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preserve the transactional integrity of the data on that target. To
summarize, SCSI command streaming, when coupled with the guarantee of
ordered command execution on the SCSI target, is extremely valuable
for a critical class of applications in long-latency networks.
This document reviews the various protocol considerations in
designing storage solutions that employ SCSI command ordering. This
document also analyzes and explains the design intent of [iSCSI] with
respect to command ordering.
2. Definitions and Acronyms
2.1. Definitions
- I_T nexus: [SAM2] defines the I_T nexus as a relationship between
a SCSI initiator port and a SCSI target port. [iSCSI] defines an
iSCSI session as the iSCSI representation of an I_T nexus. In the
iSCSI context, the I_T nexus (i.e. the iSCSI session) is a
relationship between an iSCSI initiator's end of the session (SCSI
Initiator Port) and the iSCSI target's Portal Group (SCSI Target
Port).
- PDU (Protocol Data Unit): An iSCSI initiator and iSCSI target
communicate using iSCSI protocol messages. These messages are
called "iSCSI protocol data units" (iSCSI PDUs).
- SCSI device: A SCSI device is an entity that contains one or more
SCSI ports that are connected to a service delivery subsystem and
supports SCSI application protocols. In the iSCSI context, the
SCSI Device is the component within an iSCSI Node that provides
the SCSI functionality. The SCSI Device Name is defined to be the
iSCSI Name of the node.
- Session: A group of logically related iSCSI connections that link
an initiator with a target form a session (equivalent to a SCSI
I-T nexus). The number of participating iSCSI connections within
an iSCSI session may vary over time. The multiplicity of
connections at the iSCSI level is completely hidden for the SCSI
layer - each SCSI port in an I_T nexus sees only one peer SCSI
port across all the connections of a session.
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2.2. Acronyms
Acronym Definition
--------------------------------------------------------------
ACA Auto Contingent Allegiance
ASC Additional Sense Code
ASCQ Additional Sense Code Qualifier
CRN Command Reference Number
IETF Internet Engineering Task Force
ISID Initiator Session Identifier
ITT Initiator Task Tag
LU Logical Unit
LUN Logical Unit Number
NIC Network Interface Card
PDU Protocol Data Unit
TMF Task Management Function
TSIH Target Session Identifying Handle
SAN Storage Area Network
SCSI Small Computer Systems Interface
TCP Transmission Control Protocol
UA Unit Attention
WG Working Group
3. Overview of the iSCSI Protocol
3.1. Protocol Mapping Description
The iSCSI protocol is a mapping of the SCSI remote procedure
invocation model (see [SAM2]) over the TCP protocol.
SCSI's notion of a task maps to an iSCSI task. Each iSCSI task is
uniquely identified within that I_T nexus by a 32-bit unique
identifier called Initiator Task Tag (ITT). The ITT is both an iSCSI
identifier of the task and a classic SCSI task tag.
SCSI commands from the initiator to the target are carried in iSCSI
requests called SCSI Command PDUs. SCSI status back to the initiator
is carried in iSCSI responses called SCSI Response PDUs. SCSI Data-
out from the initiator to the target is carried in SCSI Data-Out
PDUs, and the SCSI Data-in back to the initiator is carried in SCSI
Data-in PDUs.
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3.2. The I_T Nexus Model
In the iSCSI model, the SCSI I_T nexus maps directly to the iSCSI
session, which is an iSCSI protocol abstraction spanning one or more
TCP connections. The iSCSI protocol defines the semantics in order
to realize one logical flow of bidirectional communication on the I_T
nexus, potentially spanning multiple TCP connections (as many as
2^16). The multiplicity of iSCSI connections is thus completely
contained at the iSCSI layer, while the SCSI layer is presented with
a single I_T nexus, even in a multi-connection session. A session
between a pair of given iSCSI nodes is identified by the session
identifier (SSID) and each connection within a given session is
uniquely identified by a connection identifier (CID) in iSCSI. The
SSID itself has two components - Initiator Session Identifier (ISID)
and a Target Session Identifying Handler (TSIH) - each identifying
one end of the same session.
There are four crucial functional facets of iSCSI that together
present this single logical flow abstraction to the SCSI layer, even
with an iSCSI session spanning across multiple iSCSI connections.
a) Ordered command delivery: A sequence of SCSI commands that is
striped across all the connections in the session is
"reordered" by the target iSCSI layer into an identical
sequence based on a Command Sequence Number (CmdSN) that is
unique across the session. The goal is to achieve bandwidth
aggregation from multiple TCP connections, but to still make it
appear to the target SCSI layer as if all the commands had
travelled in one flow.
b) Connection allegiance: All the PDU exchanges for a SCSI
Command, up to and including the SCSI Response PDU for the
Command, are required to flow on the same iSCSI connection at
any given time. This again is intended to hide the multi-
connection nature of a session because the SCSI layer on either
side will never see the PDU contents out of order (e.g., status
cannot bypass read data for an initiator).
c) Task set management function handling: [iSCSI] specifies an
ordered sequence of steps for the iSCSI layer on the SCSI
target in handling the two SCSI task management functions
(TMFs) that manage SCSI task sets. The two TMFs are ABORT TASK
SET that aborts all active tasks in a session, and CLEAR TASK
SET that clears the tasks in the task set. The goal of the
sequence of steps is to guarantee that the initiator receives
the SCSI Response PDUs of all unaffected tasks before the TMF
Response itself arrives, regardless of the number of
connections in the iSCSI session. This operational model is
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again intended to preserve the single flow abstraction to the
SCSI layer.
d) Immediate task management function handling: Even when a TMF
request is marked as "immediate" (i.e. only has a position in
the command stream, but does not consume a CmdSN), [iSCSI]
defines semantics that require the target iSCSI layer to ensure
that the TMF request is executed as if the commands and the TMF
request were all flowing on a single logical channel. This
ensures that the TMF request will act on tasks that it was
meant to manage.
The following sections will analyze the "Ordered command delivery"
aspect in more detail, since command ordering is the focus of this
document.
3.3. Ordered Command Delivery
3.3.1. Questions
A couple of important questions related to iSCSI command ordering
were considered early on in the design of the iSCSI protocol. The
questions were:
a) What should be the command ordering behavior required of iSCSI
implementations in the presence of transport errors, such as
errors that corrupt the data in a fashion that is not detected
by the TCP checksum (e.g., two offsetting bit flips in the same
bit position), but is detected by the iSCSI CRC digest?
b) Should [iSCSI] require both initiators and targets to use
ordered command delivery?
Since the answers to these questions are critical to the
understanding of the ordering behavior required by the iSCSI
protocol, the following sub-sections consider them in more detail.
3.3.2. The Session Guarantee
The final disposition of question a) in section 3.3.1 was reflected
in [RFC3347], "iSCSI MUST specify strictly ordered delivery of SCSI
commands over an iSCSI session between an initiator/target pair, even
in the presence of transport errors." Stated differently, an iSCSI
digest failure, or an iSCSI connection termination, must not cause
the iSCSI layer on a target to allow executing the commands in an
order different from that intended (as indicated by the CmdSN order)
by the initiator. This design choice is enormously helpful in
building storage systems and solutions that can now always assume
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command ordering to be a service characteristic of an iSCSI
substrate.
Note that by taking the position that an iSCSI session always
guarantees command ordering, [iSCSI] was indirectly implying that the
principal reason for the multi-connection iSCSI session abstraction
was to allow ordered bandwidth aggregation for an I_T nexus. In
deployment models where this cross-connection ordering mandated by
[iSCSI] is deemed expensive, a serious consideration should be given
to deploying multiple single-connection sessions instead.
3.3.3. Ordering Onus
The final resolution of b) in section 3.3.1 by the iSCSI protocol
designers was in favor of not always requiring the initiators to use
command ordering. This resolution is reflected in dropping the
mandatory ACA usage requirement on the initiators, and allowing an
ABORT TASK TMF to plug a command hole etc., since these are conscious
choices an initiator makes in favor of not using ordered command
delivery. The net result can be discerned by a careful reader of
[iSCSI] - the onus of ensuring ordered command delivery is always on
the iSCSI targets, while the initiators may or may not utilize
command ordering. iSCSI targets, being the servers in the client-
server model, do not really attempt to establish whether or not a
client (initiator) intends to take advantage of command ordering
service, but instead simply always provide the guaranteed delivery
service. The rationale here is that there are inherent SCSI and
application-level dependencies, as we shall see in building a command
ordered solution, that are beyond the scope of [iSCSI], to mandate or
even discern the intent with respect to the usage of command
ordering.
3.3.4. Design Intent
To summarize the design intent of [iSCSI]:
The service delivery subsystem (see [SAM2]) abstraction provided by
an iSCSI session is guaranteed to have the intrinsic property of
ordered delivery of commands to the target SCSI layer under all
conditions. Consequently, the guarantee of the ordered command
delivery is across the entire I_T nexus spanning all the LUs that the
nexus is authorized to access. It is the initiator's discretion as
to whether or not this property will be used.
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4. The Command Ordering Scenario
A storage systems designer working with SCSI and iSCSI has to
consider the following protocol features in SCSI and iSCSI layers,
each of which has a role to play in realizing the command ordering
goal.
4.1. SCSI Layer
The SCSI application layer has several tools to enforce ordering.
4.1.1. Command Reference Number (CRN)
CRN is an ordered sequence number which, when enabled for a device
server, increments by one for each I_T_L nexus (see [SAM2]). The one
notable drawback with CRN is that there is no SCSI-generic way (such
as through mode pages) to enable or disable the CRN feature. [SAM2]
also leaves the usage semantics of CRN for the SCSI transport
protocol, such as iSCSI, to specify. [iSCSI] chose not to support
the CRN feature for various reasons.
4.1.2. Task Attributes
[SAM2] defines the following four task attributes - SIMPLE, ORDERED,
HEAD OF QUEUE, and ACA. Each task to an LU may be assigned an
attribute. [SAM2] defines the ordering constraints that each of
these attributes conveys to the device server that is servicing the
task. In particular, judicious use of ORDERED and SIMPLE attributes
applied to a stream of pipelined commands could convey the precise
execution schema for the commands that the initiator issues, provided
the commands are received in the same order on the target.
4.1.3. Auto Contingent Allegiance (ACA)
ACA is an LU-level condition that is triggered when a command (with
the NACA bit set to 1) completes with CHECK CONDITION. When ACA is
triggered, it prevents all commands other than those with the ACA
attribute from executing until the CLEAR ACA task management function
is executed, while blocking all the other tasks already in the task
set. See [SAM2] for the detailed semantics of ACA. Since ACA is
closely tied to the notion of a task set, one would ideally have to
select the scope of the task set (by setting the TST bit to 1 in the
control mode page of the LU) to be per-initiator in order to prevent
command failures in one I_T_L nexus from impacting other I_T_L
nexuses through ACA.
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4.1.4. UA Interlock
When UA interlock is enabled, the logical unit does not clear any
standard Unit Attention condition reported with autosense, and in
addition, establishes a Unit Attention condition when a task is
terminated with one of BUSY, TASK SET FULL, or RESERVATION CONFLICT
statuses. This so-called "interlocked UA" is cleared only when the
device server executes an explicit REQUEST SENSE ([SPC3]) command
from the same initiator. From a functionality perspective, the scope
of UA interlock today is slightly different from ACA's because it
enforces ordering behavior for completion statuses other than CHECK
CONDITION, but otherwise conceptually has the same design intent as
ACA. On the other hand, ACA is somewhat more sophisticated because
it allows special "cleanup" tasks (ones with ACA attribute) to
execute when ACA is active. One of the principal reasons UA
interlock came into being was that SCSI designers wanted a command
ordering feature without the side effects of using the aforementioned
TST bit in the control mode page.
4.2. iSCSI Layer
As noted in section 3.2 and section 3.3, the iSCSI protocol enforces
and guarantees ordered command delivery per iSCSI session using the
CmdSN, and this is an attribute of the SCSI transport layer. Note
further that any command ordering solution that seeks to realize
ordering from the initiator SCSI layer to the target SCSI layer would
be of practical value only when the command ordering is guaranteed by
the SCSI transport layer. In other words, the related SCSI
application layer protocol features such as ACA etc. are based on the
premise of an ordered SCSI transport. Thus, iSCSI's command ordering
is the last piece in completing the puzzle of building solutions that
rely on ordered command execution, by providing the crucial guarantee
that all the commands handed to the initiator iSCSI layer will be
transported and handed to the target SCSI layer in the same order.
5. Connection Failure Considerations
[iSCSI] mandates that when an iSCSI connection fails, the active
tasks on that connection must be terminated if not recovered within a
certain negotiated time limit. When an iSCSI target does terminate
some subset of tasks due to iSCSI connection dynamics, there is a
danger that the SCSI layer would simply move on to the next tasks
waiting to be processed and execute them out-of-order unbeknownst to
the initiator SCSI layer. To preclude this danger, [iSCSI] further
mandates the following:
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a) The tasks terminated due to the connection failure must be
internally terminated by the iSCSI target "as if" due to a
CHECK CONDITION. While this particular completion status is
never communicated back to the initiator, the "as if" is still
meaningful and required because if the initiator were using ACA
as the command ordering mechanism of choice, a SCSI-level ACA
will be triggered due to this mandatory CHECK CONDITION. This
addresses the aforementioned danger.
b) After the tasks are terminated due to the connection failure,
the iSCSI target must report a Unit Attention condition on the
next command processed on any connection for each affected
I_T_L nexus of that session. This is required because if the
initiator were using UA interlock as the command ordering
mechanism of choice, a SCSI-level UA will trigger a UA-
interlock. This again addresses the aforementioned danger.
iSCSI targets must report this UA with the status of CHECK
CONDITION, and the ASC/ASCQ value of 47h/7Fh ("SOME COMMANDS
CLEARED BY ISCSI PROTOCOL EVENT").
6. Command Ordering System Considerations
In general, command ordering is automatically enforced if targets and
initiators comply with the iSCSI specification. However, listed
below are certain additional related implementation considerations
for the iSCSI initiators and targets to take note of.
a) Even when all iSCSI and SCSI command ordering considerations
earlier noted in this document were applied, it is beneficial
for iSCSI initiators to proactively avoid scenarios that would
otherwise lead to out-of-order command execution. This is
simply because the SCSI command ordering features such as UA
interlock are likely to be costlier in performance when they
are allowed to be triggered. [iSCSI] provides enough guidance
on how to implement this proactive detection of PDU ordering
errors.
b) The whole notion of command streaming does of course assume
that the target in question supports command queueing. An
iSCSI target desirous of supporting command ordering solutions
should ensure that the SCSI layer on the target supports
command queuing. The remote backup (tape vaulting)
applications that iSCSI enables make an especially compelling
case that tape devices should give a very serious consideration
to supporting command queuing, at least when used in
conjunction with iSCSI.
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c) An iSCSI target desirous of supporting high-performance command
ordering solutions that involve specifying a description of
execution schema should ensure that the SCSI layer on the
target in fact does support the ORDERED and SIMPLE task
attributes.
d) There is some consideration of expanding the scope of UA
interlock to encompass CHECK CONDITION status, and thus make it
the only required command ordering functionality of
implementations to build command ordering solutions. Until
this is resolved in T10, the currently defined semantics of UA
interlock and ACA warrant implementing both features by iSCSI
targets desirous of supporting command ordering solutions.
7. Reservation Considerations
[iSCSI] describes a "principle of conservative reuse" that encourages
iSCSI initiators to reuse the same ISIDs (see section 3.2) to various
SCSI target ports, in order to present the same SCSI initiator port
name to those target ports. This is in fact a very crucial
implementation consideration that must be complied with. [SPC3]
mandates the SCSI targets to associate persistent reservations and
the related registrations with the SCSI initiator port names whenever
they are required by the SCSI transport protocol. Since [iSCSI]
requires the mandatory SCSI initiator port names based on ISIDs,
iSCSI targets are required to work off the SCSI initiator port names,
and thus indirectly the ISIDs, in enforcing the persistent
reservations.
This fact has the following implications for the implementations:
a) If a persistent reservation/registration is intended to be used
across multiple SCSI ports of a SCSI device, the initiator
iSCSI implementation must use the same ISID across associated
iSCSI sessions connecting to different iSCSI target portal
groups of the SCSI device.
b) If a persistent reservation/registration is intended to be used
across the power loss of a SCSI target, the initiator iSCSI
implementation must use the same ISIDs as before in
re-establishing the associated iSCSI sessions upon subsequent
reboot in order to rely on the persist through power loss
capability.
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8. Security Considerations
For security considerations in using the iSCSI protocol, refer to the
Security Considerations section in [iSCSI]. This document does not
introduce any additional security considerations other than those
already discussed in [iSCSI].
9. References
9.1. Normative References
[iSCSI] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M. and
E. Zeidner, "Internet Small Computer Systems Inferface
(iSCSI)", RFC 3720, May 2004.
[SAM2] ANSI INCITS.366:2003 SCSI Architecture Model - 2 (SAM-2).
9.2. Informative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3347] Krueger, M. and R. Haagens, "iSCSI Requirements and Design
Considerations", RFC 3347, July 2002.
[SPC3] INCITS T10/1416-D, SCSI Primary Commands-3 (SPC-3).
10. Acknowledgments
We are grateful to the IPS working group whose work defined the iSCSI
protocol. Thanks also to David Black (EMC) who encouraged the
publication of this document. Special thanks to Randy Haagens (HP)
for his insights on the topic of command ordering. Thanks are also
due to Elizabeth Rodriguez for carefully reviewing this document.
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11. Authors' Addresses
Mallikarjun Chadalapaka
Hewlett-Packard Company
8000 Foothills Blvd.
Roseville, CA 95747-5668, USA
Phone: +1.916.785.5621
EMail: cbm@rose.hp.com
Rob Elliott
Hewlett-Packard Company
MC140801
PO Box 692000
Houston, TX 77269-2000 USA
Phone: +1.281.518.5037
EMail: elliott@hp.com
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Chadalapaka & Elliott Informational [Page 14]
