Internet Engineering Task Force (IETF) I. Busi, Ed.
Request for Comments: 6371 Alcatel-Lucent
Category: Informational D. Allan, Ed.
ISSN: 2070-1721 Ericsson
September 2011
Operations, Administration, and Maintenance Framework for
MPLS-Based Transport Networks
Abstract
The Transport Profile of Multiprotocol Label Switching (MPLS-TP) is a
packet-based transport technology based on the MPLS Traffic
Engineering (MPLS-TE) and pseudowire (PW) data-plane architectures.
This document describes a framework to support a comprehensive set of
Operations, Administration, and Maintenance (OAM) procedures that
fulfill the MPLS-TP OAM requirements for fault, performance, and
protection-switching management and that do not rely on the presence
of a control plane.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunications Union Telecommunication
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and Pseudowire Emulation Edge-to-Edge
(PWE3) architectures to support the capabilities and functionalities
of a packet transport network as defined by the ITU-T.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are 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/rfc6371.
Busi & Allan Informational [Page 1]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Copyright Notice
Copyright (c) 2011 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. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Conventions Used in This Document ...............................5
2.1. Terminology ................................................5
2.2. Definitions ................................................7
3. Functional Components ..........................................10
3.1. Maintenance Entity and Maintenance Entity Group ...........10
3.2. MEG Nesting: SPMEs and Tandem Connection Monitoring .......13
3.3. MEG End Points (MEPs) .....................................14
3.4. MEG Intermediate Points (MIPs) ............................18
3.5. Server MEPs ...............................................20
3.6. Configuration Considerations ..............................21
3.7. P2MP Considerations .......................................21
3.8. Further Considerations of Enhanced Segment Monitoring .....22
4. Reference Model ................................................23
4.1. MPLS-TP Section Monitoring (SMEG) .........................26
4.2. MPLS-TP LSP End-to-End Monitoring Group (LMEG) ............27
4.3. MPLS-TP PW Monitoring (PMEG) ..............................27
4.4. MPLS-TP LSP SPME Monitoring (LSMEG) .......................28
4.5. MPLS-TP MS-PW SPME Monitoring (PSMEG) .....................30
4.6. Fate-Sharing Considerations for Multilink .................31
5. OAM Functions for Proactive Monitoring .........................32
5.1. Continuity Check and Connectivity Verification ............33
5.1.1. Defects Identified by CC-V .........................35
5.1.2. Consequent Action ..................................37
5.1.3. Configuration Considerations .......................38
5.2. Remote Defect Indication ..................................40
5.2.1. Configuration Considerations .......................40
5.3. Alarm Reporting ...........................................41
5.4. Lock Reporting ............................................42
5.5. Packet Loss Measurement ...................................44
5.5.1. Configuration Considerations .......................45
Busi & Allan Informational [Page 2]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
5.5.2. Sampling Skew ......................................45
5.5.3. Multilink Issues ...................................45
5.6. Packet Delay Measurement ..................................46
5.6.1. Configuration Considerations .......................46
5.7. Client Failure Indication .................................47
5.7.1. Configuration Considerations .......................47
6. OAM Functions for On-Demand Monitoring .........................48
6.1. Connectivity Verification .................................48
6.1.1. Configuration Considerations .......................49
6.2. Packet Loss Measurement ...................................50
6.2.1. Configuration Considerations .......................50
6.2.2. Sampling Skew ......................................50
6.2.3. Multilink Issues ...................................50
6.3. Diagnostic Tests ..........................................50
6.3.1. Throughput Estimation ..............................51
6.3.2. Data-Plane Loopback ................................52
6.4. Route Tracing .............................................54
6.4.1. Configuration Considerations .......................54
6.5. Packet Delay Measurement ..................................54
6.5.1. Configuration Considerations .......................55
7. OAM Functions for Administration Control .......................55
7.1. Lock Instruct .............................................55
7.1.1. Locking a Transport Path ...........................56
7.1.2. Unlocking a Transport Path .........................56
8. Security Considerations ........................................57
9. Acknowledgments ................................................58
10. References ....................................................58
10.1. Normative References .....................................58
10.2. Informative References ...................................59
11. Contributing Authors ..........................................60
1. Introduction
As noted in the MPLS Transport Profile (MPLS-TP) framework RFCs (RFC
5921 [8] and RFC 6215 [9]), MPLS-TP is a packet-based transport
technology based on the MPLS Traffic Engineering (MPLS-TE) and
pseudowire (PW) data-plane architectures defined in RFC 3031 [1], RFC
3985 [2], and RFC 5659 [4].
MPLS-TP utilizes a comprehensive set of Operations, Administration,
and Maintenance (OAM) procedures for fault, performance, and
protection-switching management that do not rely on the presence of a
control plane.
In line with [15], existing MPLS OAM mechanisms will be used wherever
possible, and extensions or new OAM mechanisms will be defined only
where existing mechanisms are not sufficient to meet the
requirements. Some extensions discussed in this framework may end up
Busi & Allan Informational [Page 3]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
as aspirational capabilities and may be determined to be not
tractably realizable in some implementations. Extensions do not
deprecate support for existing MPLS OAM capabilities.
The MPLS-TP OAM framework defined in this document provides a
protocol-neutral description of the required OAM functions and of the
data-plane OAM architecture to support a comprehensive set of OAM
procedures that satisfy the MPLS-TP OAM requirements of RFC 5860
[11]. In this regard, it defines similar OAM functionality as for
existing Synchronous Optical Network / Synchronous Digital Hierarchy
(SONET/SDH) and Optical Transport Network (OTN) OAM mechanisms (e.g.,
[19]).
The MPLS-TP OAM framework is applicable to Sections, Label Switched
Paths (LSPs), Multi-Segment Pseudowires (MS-PWs), and Sub-Path
Maintenance Elements (SPMEs). It supports co-routed and associated
bidirectional P2P transport paths as well as unidirectional P2P and
P2MP transport paths.
OAM packets that instrument a particular direction of a transport
path are subject to the same forwarding treatment (i.e., fate-share)
as the user data packets and in some cases, where Explicitly TC-
encoded-PSC LSPs (E-LSPs) are employed, may be required to have
common per-hop behavior (PHB) Scheduling Class (PSC) End-to-End (E2E)
with the class of traffic monitored. In case of Label-Only-Inferred-
PSC LSP (L-LSP), only one class of traffic needs to be monitored, and
therefore the OAM packets have common PSC with the monitored traffic
class.
OAM packets can be distinguished from the used data packets using the
Generic Associated Channel Label (GAL) and Associated Channel Header
(ACH) constructs of RFC 5586 [7] for LSP, SPME, and Section, or the
ACH construct of RFC 5085 [3] and RFC 5586 [7] for (MS-)PW. OAM
packets are never fragmented and are not combined with user data in
the same packet payload.
This framework makes certain assumptions as to the utility and
frequency of different classes of measurement that naturally suggest
different functions are implemented as distinct OAM flows or packets.
This is dictated by the combination of the class of problem being
detected and the need for timeliness of network response to the
problem. For example, fault detection is expected to operate on an
entirely different time base than performance monitoring, which is
also expected to operate on an entirely different time base than in-
band management transactions.
Busi & Allan Informational [Page 4]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
The remainder of this memo is structured as follows:
Section 2 covers the definitions and terminology used in this memo.
Section 3 describes the functional component that generates and
processes OAM packets.
Section 4 describes the reference models for applying OAM functions
to Sections, LSP, MS-PW, and their SPMEs.
Sections 5, 6, and 7 provide a protocol-neutral description of the
OAM functions, defined in RFC 5860 [11], aimed at clarifying how the
OAM protocol solutions will behave to achieve their functional
objectives.
Section 8 discusses the security implications of OAM protocol design
in the MPLS-TP context.
The OAM protocol solutions designed as a consequence of this document
are expected to comply with the functional behavior described in
Sections 5, 6, and 7. Alternative solutions to required functional
behaviors may also be defined.
OAM specifications following this OAM framework may be provided in
different documents to cover distinct OAM functions.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunication Union Telecommunication
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and PWE3 architectures to support the
capabilities and functionalities of a packet transport network as
defined by the ITU-T.
2. Conventions Used in This Document
2.1. Terminology
AC Attachment Circuit
AIS Alarm Indication Signal
CC Continuity Check
CC-V Continuity Check and Connectivity Verification
CV Connectivity Verification
DBN Domain Border Node
Busi & Allan Informational [Page 5]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
E-LSP Explicitly TC-encoded-PSC LSP
ICC ITU Carrier Code
LER Label Edge Router
LKR Lock Report
L-LSP Label-Only-Inferred-PSC LSP
LM Loss Measurement
LME LSP Maintenance Entity
LMEG LSP ME Group
LSP Label Switched Path
LSR Label Switching Router
LSME LSP SPME ME
LSMEG LSP SPME ME Group
ME Maintenance Entity
MEG Maintenance Entity Group
MEP Maintenance Entity Group End Point
MIP Maintenance Entity Group Intermediate Point
NMS Network Management System
PE Provider Edge
PHB Per-Hop Behavior
PM Performance Monitoring
PME PW Maintenance Entity
PMEG PW ME Group
PSC PHB Scheduling Class
PSME PW SPME ME
Busi & Allan Informational [Page 6]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
PSMEG PW SPME ME Group
PW Pseudowire
SLA Service Level Agreement
SME Section Maintenance Entity
SMEG Section ME Group
SPME Sub-Path Maintenance Element
S-PE Switching Provider Edge
TC Traffic Class
T-PE Terminating Provider Edge
2.2. Definitions
This document uses the terms defined in RFC 5654 [5].
This document uses the term 'per-hop behavior' as defined in RFC 2474
[16].
This document uses the term 'LSP' to indicate either a service LSP or
a transport LSP (as defined in RFC 5921 [8]).
This document uses the term 'Section' exclusively to refer to the n=0
case of the term 'Section' defined in RFC 5960 [10].
This document uses the term 'Sub-Path Maintenance Element (SPME)' as
defined in RFC 5921 [8].
This document uses the term 'traffic profile' as defined in RFC 2475
[13].
Where appropriate, the following definitions are aligned with ITU-T
recommendation Y.1731 [21] in order to have a common, unambiguous
terminology. They do not however intend to imply a certain
implementation but rather serve as a framework to describe the
necessary OAM functions for MPLS-TP.
Adaptation function: The adaptation function is the interface between
the client (sub-)layer and the server (sub-)layer.
Branch Node: A node along a point-to-multipoint transport path that
is connected to more than one downstream node.
Busi & Allan Informational [Page 7]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Bud Node: A node along a point-to-multipoint transport path that is
at the same time a branch node and a leaf node for this transport
path.
Data-plane loopback: An out-of-service test where a transport path at
either an intermediate or terminating node is placed into a data-
plane loopback state, such that all traffic (including both payload
and OAM) received on the looped back interface is sent on the reverse
direction of the transport path.
Note: The only way to send an OAM packet to a node that has been
put into data-plane loopback mode is via Time to Live (TTL)
expiry, irrespective of whether the node is hosting MIPs or MEPs.
Domain Border Node (DBN): An intermediate node in an MPLS-TP LSP that
is at the boundary between two MPLS-TP OAM domains. Such a node may
be present on the edge of two domains or may be connected by a link
to the DBN at the edge of another OAM domain.
Down MEP: A MEP that receives OAM packets from, and transmits them
towards, the direction of a server layer.
Forwarding Engine: An abstract functional component, residing in an
LSR, that forwards the packets from an ingress interface toward the
egress interface(s).
In-Service: The administrative status of a transport path when it is
unlocked.
Interface: An interface is the attachment point to a server
(sub-)layer, e.g., a MPLS-TP Section or MPLS-TP tunnel.
Intermediate Node: An intermediate node transits traffic for an LSP
or a PW. An intermediate node may originate OAM flows directed to
downstream intermediate nodes or MEPs.
Loopback: See data-plane loopback and OAM loopback definitions.
Maintenance Entity (ME): Some portion of a transport path that
requires management bounded by two points (called MEPs), and the
relationship between those points to which maintenance and monitoring
operations apply (details in Section 3.1).
Maintenance Entity Group (MEG): The set of one or more maintenance
entities that maintain and monitor a section or a transport path in
an OAM domain.
Busi & Allan Informational [Page 8]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
MEP: A MEG End Point (MEP) is capable of initiating (source MEP) and
terminating (sink MEP) OAM packets for fault management and
performance monitoring. MEPs define the boundaries of an ME (details
in Section 3.3).
MIP: A MEG intermediate point (MIP) terminates and processes OAM
packets that are sent to this particular MIP and may generate OAM
packets in reaction to received OAM packets. It never generates
unsolicited OAM packets itself. A MIP resides within a MEG between
MEPs (details in Section 3.3).
OAM domain: A domain, as defined in [5], whose entities are grouped
for the purpose of keeping the OAM confined within that domain. An
OAM domain contains zero or more MEGs.
Note: Within the rest of this document, the term "domain" is used
to indicate an "OAM domain".
OAM flow: The set of all OAM packets originating with a specific
source MEP that instrument one direction of a MEG (or possibly both
in the special case of data-plane loopback).
OAM loopback: The capability of a node to be directed by a received
OAM packet to generate a reply back to the sender. OAM loopback can
work in-service and can support different OAM functions (e.g.,
bidirectional on-demand connectivity verification).
OAM Packet: A packet that carries OAM information between MEPs and/or
MIPs in a MEG to perform some OAM functionality (e.g., connectivity
verification).
Originating MEP: A MEP that originates an OAM transaction packet
(toward a target MIP/MEP) and expects a reply, either in-band or out-
of-band, from that target MIP/MEP. The originating MEP always
generates the OAM request packets in-band and expects and processes
only OAM reply packets returned by the target MIP/MEP.
Out-of-Service: The administrative status of a transport path when it
is locked. When a path is in a locked condition, it is blocked from
carrying client traffic.
Path Segment: It is either a segment or a concatenated segment, as
defined in RFC 5654 [5].
Signal Degrade: A condition declared by a MEP when the data
forwarding capability associated with a transport path has
deteriorated, as determined by performance monitoring (PM). See also
ITU-T recommendation G.806 [14].
Busi & Allan Informational [Page 9]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Signal Fail: A condition declared by a MEP when the data forwarding
capability associated with a transport path has failed, e.g., loss of
continuity. See also ITU-T recommendation G.806 [14].
Sink MEP: A MEP acts as a sink MEP for an OAM packet when it
terminates and processes the packets received from its associated
MEG.
Source MEP: A MEP acts as source MEP for an OAM packet when it
originates and inserts the packet into the transport path for its
associated MEG.
Tandem Connection: A tandem connection is an arbitrary part of a
transport path that can be monitored (via OAM) independent of the
end-to-end monitoring (OAM). The tandem connection may also include
the forwarding engine(s) of the node(s) at the boundaries of the
tandem connection. Tandem connections may be nested but cannot
overlap. See also ITU-T recommendation G.805 [20].
Target MEP/MIP: A MEP or a MIP that is targeted by OAM transaction
packets and that replies to the originating MEP that initiated the
OAM transactions. The target MEP or MIP can reply either in-band or
out-of-band. The target sink MEP function always receives the OAM
request packets in-band, while the target source MEP function only
generates the OAM reply packets that are sent in-band.
Up MEP: A MEP that transmits OAM packets towards, and receives them
from, the direction of the forwarding engine.
3. Functional Components
MPLS-TP is a packet-based transport technology based on the MPLS and
PW data plane architectures ([1], [2], and [4]) and is capable of
transporting service traffic where the characteristics of information
transfer between the transport path end points can be demonstrated to
comply with certain performance and quality guarantees.
In order to describe the required OAM functionality, this document
introduces a set of functional components.
3.1. Maintenance Entity and Maintenance Entity Group
MPLS-TP OAM operates in the context of Maintenance Entities (MEs)
that define a relationship between two points of a transport path to
which maintenance and monitoring operations apply. The two points
that define a maintenance entity are called Maintenance Entity Group
End Points (MEPs). The collection of one or more MEs that belongs to
the same transport path and that are maintained and monitored as a
Busi & Allan Informational [Page 10]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
group are known as a Maintenance Entity Group (MEG). In between
MEPs, there are zero or more intermediate points, called Maintenance
Entity Group Intermediate Points (MIPs). MEPs and MIPs are
associated with the MEG and can be shared by more than one ME in a
MEG.
An abstract reference model for an ME is illustrated in Figure 1
below.
+-+ +-+ +-+ +-+
|A|----|B|----|C|----|D|
+-+ +-+ +-+ +-+
Figure 1: ME Abstract Reference Model
The instantiation of this abstract model to different MPLS-TP
entities is described in Section 4. In Figure 1, nodes A and D can
be Label Edge Routers (LERs) for an LSP or the Terminating Provider
Edges (T-PEs) for an MS-PW, nodes B and C are LSRs for an LSP or
Switching PEs (S-PEs) for an MS-PW. MEPs reside in nodes A and D,
while MIPs reside in nodes B and C and may reside in A and D. The
links connecting adjacent nodes can be physical links, (sub-)layer
LSPs/SPMEs, or server-layer paths.
This functional model defines the relationships between all OAM
entities from a maintenance perspective and it allows each
Maintenance Entity to provide monitoring and management for the
(sub-)layer network under its responsibility and efficient
localization of problems.
An MPLS-TP Maintenance Entity Group may be defined to monitor the
transport path for fault and/or performance management.
The MEPs that form a MEG bound the scope of an OAM flow to the MEG
(i.e., within the domain of the transport path that is being
monitored and managed). There are two exceptions to this:
1) A misbranching fault may cause OAM packets to be delivered to a
MEP that is not in the MEG of origin.
2) An out-of-band return path may be used between a MIP or a MEP and
the originating MEP.
In case of a unidirectional point-to-point transport path, a single
unidirectional Maintenance Entity is defined to monitor it.
Busi & Allan Informational [Page 11]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
In case of associated bidirectional point-to-point transport paths,
two independent unidirectional Maintenance Entities are defined to
independently monitor each direction. This has implications for
transactions that terminate at or query a MIP, as a return path from
MIP to the originating MEP does not necessarily exist in the MEG.
In case of co-routed bidirectional point-to-point transport paths, a
single bidirectional Maintenance Entity is defined to monitor both
directions congruently.
In case of unidirectional point-to-multipoint transport paths, a
single unidirectional Maintenance Entity for each leaf is defined to
monitor the transport path from the root to that leaf.
In all cases, portions of the transport path may be monitored by the
instantiation of SPMEs (see Section 3.2).
The reference model for the P2MP MEG is represented in Figure 2.
+-+
/--|D|
/ +-+
+-+
/--|C|
+-+ +-+/ +-+\ +-+
|A|----|B| \--|E|
+-+ +-+\ +-+ +-+
\--|F|
+-+
Figure 2: Reference Model for P2MP MEG
In the case of P2MP transport paths, the OAM measurements are
independent for each ME (A-D, A-E, and A-F):
o Fault conditions - some faults may impact more than one ME
depending on where the failure is located;
o Packet loss - packet dropping may impact more than one ME
depending from where the packets are lost;
o Packet delay - will be unique per ME.
Each leaf (i.e., D, E, and F) terminates OAM flows to monitor the ME
between itself and the root while the root (i.e., A) generates OAM
packets common to all the MEs of the P2MP MEG. All nodes may
implement a MIP in the corresponding MEG.
Busi & Allan Informational [Page 12]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
3.2. MEG Nesting: SPMEs and Tandem Connection Monitoring
In order to verify and maintain performance and quality guarantees,
there is a need to apply OAM functionality not only on a transport
path granularity (e.g., LSP or MS-PW), but also on arbitrary parts of
transport paths, defined as tandem connections, between any two
arbitrary points along a transport path.
Sub-Path Maintenance Elements (SPMEs), as defined in [8], are
hierarchical LSPs instantiated to provide monitoring of a portion of
a set of transport paths (LSPs or MS-PWs) that follow the same path
between the ingress and the egress of the SPME. The operational
aspects of instantiating SPMEs are out of scope of this memo.
SPMEs can also be employed to meet the requirement to provide tandem
connection monitoring (TCM), as defined by ITU-T Recommendation G.805
[20].
TCM for a given path segment of a transport path is implemented by
creating an SPME that has a 1:1 association with the path segment of
the transport path that is to be monitored.
In the TCM case, this means that the SPME used to provide TCM can
carry one and only one transport path, thus allowing direct
correlation between all fault management and performance monitoring
information gathered for the SPME and the monitored path segment of
the end-to-end transport path.
There are a number of implications to this approach:
1) The SPME would use the uniform model [23] of Traffic Class (TC)
code point copying between sub-layers for Diffserv such that the
E2E markings and PHB treatment for the transport path were
preserved by the SPMEs.
2) The SPME normally would use the short-pipe model for TTL handling
[6] (no TTL copying between sub-layers) such that the TTL distance
to the MIPs for the E2E entity would not be impacted by the
presence of the SPME, but it should be possible for an operator to
specify use of the uniform model.
Note that points 1 and 2 above assume that the TTL copying mode and
TC copying modes are independently configurable for an LSP.
The TTL distance to the MIPs plays a critical role for delivering
packets to these MIPs as described in Section 3.4.
Busi & Allan Informational [Page 13]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
There are specific issues with the use of the uniform model of TTL
copying for an SPME:
1. A MIP in the SPME sub-layer is not part of the transport-path MEG;
hence, only an out-of-band return path for OAM originating in the
transport-path MEG that addressed an SPME MIP might be available.
2. The instantiation of a lower-level MEG or protection-switching
actions within a lower-level MEG may change the TTL distances to
MIPs in the higher-level MEGs.
The end points of the SPME are MEPs and limit the scope of an OAM
flow within the MEG that the MEPs belong to (i.e., within the domain
of the SPME that is being monitored and managed).
When considering SPMEs, it is important to consider that the
following properties apply to all MPLS-TP MEGs (regardless of whether
they instrument LSPs, SPMEs, or MS-PWs):
o They can be nested but not overlapped, e.g., a MEG may cover a
path segment of another MEG and may also include the forwarding
engine(s) of the node(s) at the edge(s) of the path segment.
However, when MEGs are nested, the MEPs and MIPs in the SPME are
no longer part of the encompassing MEG.
o It is possible that MEPs of MEGs that are nested reside on a
single node but again are implemented in such a way that they do
not overlap.
o Each OAM flow is associated with a single MEG.
o When an SPME is instantiated after the transport path has been
instantiated, the TTL distance to the MIPs may change for the
short-pipe model of TTL copying, and may change for the uniform
model if the SPME is not co-routed with the original path.
3.3. MEG End Points (MEPs)
MEG End Points (MEPs) are the source and sink points of a MEG. In
the context of an MPLS-TP LSP, only LERs can implement MEPs, while in
the context of an SPME, any LSR of the MPLS-TP LSP can be an LER of
SPMEs that contributes to the overall monitoring infrastructure of
the transport path. Regarding PWs, only T-PEs can implement MEPs;
while for SPMEs supporting one or more PWs, both T-PEs and S-PEs can
implement SPME MEPs. Any MPLS-TP LSR can implement a MEP for an
MPLS-TP Section.
Busi & Allan Informational [Page 14]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
MEPs are responsible for originating almost all of the proactive and
on-demand monitoring OAM functionality for the MEG. There is a
separate class of notifications (such as Lock Report (LKR) and Alarm
Indication Signal (AIS)) that are originated by intermediate nodes
and triggered by server-layer events. A MEP is capable of
originating and terminating OAM packets for fault management and
performance monitoring. These OAM packets are carried within the
Generic Associated Channel (G-ACh) with the proper encapsulation and
an appropriate channel type as defined in RFC 5586 [7]. A MEP
terminates all the OAM packets it receives from the MEG it belongs to
and silently discards those that do not. (Note that in the
particular case of Connectivity Verification (CV) processing, a CV
packet from an incorrect MEG will result in a mis-connectivity defect
and there are further actions taken.) The MEG the OAM packet belongs
to is associated with the MPLS or PW label, whether the label is used
to infer the MEG or the content of the OAM packet is an
implementation choice. In the case of an MPLS-TP Section, the MEG is
inferred from the port on which an OAM packet was received with the
GAL at the top of the label stack.
OAM packets may require the use of an available "out-of-band" return
path (as defined in [8]). In such cases, sufficient information is
required in the originating transaction such that the OAM reply
packet can be constructed and properly forwarded to the originating
MEP (e.g., IP address).
Each OAM solution document will further detail the applicability of
the tools it defines as a proactive or on-demand mechanism as well as
its usage when:
o The "in-band" return path exists and it is used.
o An "out-of-band" return path exists and it is used.
o Any return path does not exist or is not used.
Once a MEG is configured, the operator can configure which proactive
OAM functions to use on the MEG, but the MEPs are always enabled.
MEPs terminate all OAM packets received from the associated MEG. As
the MEP corresponds to the termination of the forwarding path for a
MEG at the given (sub-)layer, OAM packets never leak outside of a MEG
in a properly configured fault-free implementation.
Busi & Allan Informational [Page 15]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
A MEP of an MPLS-TP transport path coincides with transport path
termination and monitors it for failures or performance degradation
(e.g., based on packet counts) in an end-to-end scope. Note that
both the source MEP and sink MEP coincide with transport paths'
source and sink terminations.
The MEPs of an SPME are not necessarily coincident with the
termination of the MPLS-TP transport path. They are used to monitor
a path segment of the transport path for failures or performance
degradation (e.g., based on packet counts) only within the boundary
of the MEG for the SPME.
An MPLS-TP sink MEP passes a fault indication to its client
(sub-)layer network as a consequent action of fault detection. When
the client layer is not MPLS-TP, the consequent actions in the client
layer (e.g., ignore or generate client-layer-specific OAM
notifications) are outside the scope of this document.
A node hosting a MEP can either support per-node MEP or per-interface
MEP(s). A per-node MEP resides in an unspecified location within the
node, while a per-interface MEP resides on a specific side of the
forwarding engine. In particular, a per-interface MEP is called an
"Up MEP" or a "Down MEP" depending on its location relative to the
forwarding engine. An "Up MEP" transmits OAM packets towards, and
receives them from, the direction of the forwarding engine, while a
"Down MEP" receives OAM packets from, and transmits them towards, the
direction of a server layer.
Busi & Allan Informational [Page 16]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Source node Up MEP Destination node Up MEP
------------------------ ------------------------
| | | |
|----- -----| |----- -----|
| MEP | | | | | | MEP |
| | ---- | | | | ---- | |
| In |->-| FW |->-| Out |->- ->-| In |->-| FW |->-| Out |
| i/f | ---- | i/f | | i/f | ---- | i/f |
|----- -----| |----- -----|
| | | |
------------------------ ------------------------
(1) (2)
Source node Down MEP Destination node Down MEP
------------------------ ------------------------
| | | |
|----- -----| |----- -----|
| | | MEP | | MEP | | |
| | ---- | | | | ---- | |
| In |->-| FW |->-| Out |->- ->-| In |->-| FW |->-| Out |
| i/f | ---- | i/f | | i/f | ---- | i/f |
|----- -----| |----- -----|
| | | |
------------------------ ------------------------
(3) (4)
Figure 3: Examples of Per-Interface MEPs
Figure 3 describes four examples of per-interface Up MEPs: an Up
Source MEP in a source node (case 1), an Up Sink MEP in a destination
node (case 2), a Down Source MEP in a source node (case 3), and a
Down Sink MEP in a destination node (case 4).
The usage of per-interface Up MEPs extends the coverage of the ME for
both fault and performance monitoring closer to the edge of the
domain and determines that the location of a failure or performance
degradation is within a node or on a link between two adjacent nodes.
Each OAM solution document will further detail the implications of
the tools it defines when used with per-interface or per-node MEPs,
if necessary.
It may occur that multiple MEPs for the same MEG are on the same
node, and are all Up MEPs, each on one side of the forwarding engine,
such that the MEG is entirely internal to the node.
Busi & Allan Informational [Page 17]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
It should be noted that an ME may span nodes that implement per-node
MEPs and per-interface MEPs. This guarantees backward compatibility
with most of the existing LSRs that can implement only a per-node
MEP. In fact, in many current implementations, label operations are
largely performed on the ingress interface; hence, the exposure of
the GAL as top label will occur at the ingress interface.
Note that a MEP can only exist at the beginning and end of a
(sub-)layer in MPLS-TP. If there is a need to monitor some portion
of that LSP or PW, a new sub-layer (in the form of an SPME) must be
created that permits MEPs and associated MEGs to be created.
In the case where an intermediate node sends an OAM packet to a MEP,
it uses the top label of the stack at that point.
3.4. MEG Intermediate Points (MIPs)
A MEG Intermediate Point (MIP) is a function located at a point
between the MEPs of a MEG for a PW, LSP, or SPME.
A MIP is capable of reacting to some OAM packets and forwarding all
the other OAM packets while ensuring fate-sharing with user data
packets. However, a MIP does not initiate unsolicited OAM packets,
but may be addressed by OAM packets initiated by one of the MEPs of
the MEG. A MIP can generate OAM packets only in response to OAM
packets that it receives from the MEG it belongs to. The OAM packets
generated by the MIP are sent to the originating MEP.
An intermediate node within a MEG can either:
o support per-node MIPs (i.e., a single MIP per node in an
unspecified location within the node); or
o support per-interface MIPs (i.e., two or more MIPs per node on
both sides of the forwarding engine).
Support of per-interface or per-node MIPs is an implementation
choice. It is also possible that a node could support per-interface
MIPs on some MEGs and per-node MIPs on other MEGs for which it is a
transit node.
Busi & Allan Informational [Page 18]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Intermediate node
------------------------
| |
|----- -----|
| MIP | | MIP |
| | ---- | |
->-| In |->-| FW |->-| Out |->-
| i/f | ---- | i/f |
|----- -----|
| |
------------------------
Figure 4: Example of Per-Interface MIPs
Figure 4 describes an example of two per-interface MIPs at an
intermediate node of a point-to-point MEG.
Using per-interface MIPs allows the network operator to determine
that the location of a failure or performance degradation is within a
node or on a link between two adjacent nodes.
When sending an OAM packet to a MIP, the source MEP should set the
TTL field to indicate the number of hops necessary to reach the node
where the MIP resides.
The source MEP should also include target MIP information in the OAM
packets sent to a MIP to allow proper identification of the MIP
within the node. The MEG the OAM packet belongs to is associated
with the MPLS label, whether the label is used to infer the MEG or
the content of the OAM packet is an implementation choice. In the
latter case, the MPLS label is checked to be the expected one.
The use of TTL expiry to deliver OAM packets to a specific MIP is not
a fully reliable delivery mechanism because the TTL distance of a MIP
from a MEP can change. Any MPLS-TP node silently discards any OAM
packet that is received with an expired TTL and that is not addressed
to any of its MIPs or MEPs. An MPLS-TP node that does not support
OAM is also expected to silently discard any received OAM packet.
Packets directed to a MIP may not necessarily carry specific MIP
identification information beyond that of TTL distance. In this
case, a MIP would promiscuously respond to all MEP queries on its
MEG. This capability could be used for discovery functions (e.g.,
route tracing as defined in Section 6.4) or when it is desirable to
leave to the originating MEP the job of correlating TTL and MIP
identifiers and noting changes or irregularities (via comparison with
information previously extracted from the network).
Busi & Allan Informational [Page 19]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
MIPs are associated to the MEG they belong to, and their identity is
unique within the MEG. However, their identity is not necessarily
unique to the MEG, e.g., all nodal MIPs in a node can have a common
identity.
A node hosting a MEP can also support per-interface Up MEPs and per-
interface MIPs on either side of the forwarding engine.
Once a MEG is configured, the operator can enable/disable the MIPs on
the nodes within the MEG. All the intermediate nodes and possibly
the end nodes host MIP(s). Local policy allows them to be enabled
per function and per MEG. The local policy is controlled by the
management system, which may delegate it to the control plane. A
disabled MIP silently discards any received OAM packets.
3.5. Server MEPs
A server MEP is a MEP of a MEG that is either:
o defined in a layer network that is "below", which is to say
encapsulates and transports the MPLS-TP layer network being
referenced; or
o defined in a sub-layer of the MPLS-TP layer network that is
"below", which is to say encapsulates and transports the sub-layer
being referenced.
A server MEP can coincide with a MIP or a MEP in the client (MPLS-TP)
(sub-)layer network.
A server MEP also provides server-layer OAM indications to the
client/server adaptation function between the client (MPLS-TP)
(sub-)layer network and the server (sub-)layer network. The
adaptation function maintains state on the mapping of MPLS-TP
transport paths that are set up over that server (sub-)layer's
transport path.
For example, a server MEP can be:
o a non-MPLS MEP at a termination point of a physical link (e.g.,
802.3, an SDH Virtual Circuit, or OTN Optical Data Unit (ODU)),
for the MPLS-TP Section layer network, defined in Section 4.1;
o an MPLS-TP Section MEP for MPLS-TP LSPs, defined in Section 4.2;
o an MPLS-TP LSP MEP for MPLS-TP PWs, defined in Section 4.3;
Busi & Allan Informational [Page 20]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
o an MPLS-TP SPME MEP used for LSP path segment monitoring, as
defined in Section 4.4, for MPLS-TP LSPs or higher-level SPMEs
providing LSP path segment monitoring; or
o an MPLS-TP SPME MEP used for PW path segment monitoring, as
defined in Section 4.5, for MPLS-TP PWs or higher-level SPMEs
providing PW path segment monitoring.
The server MEP can run appropriate OAM functions for fault detection
within the server (sub-)layer network and provides a fault indication
to its client MPLS-TP layer network via the client/server adaptation
function. When the server layer is not MPLS-TP, server MEP OAM
functions are simply assumed to exist but are outside the scope of
this document.
3.6. Configuration Considerations
When a control plane is not present, the management plane configures
these functional components. Otherwise, they can be configured by
either the management plane or the control plane.
Local policy allows disabling the usage of any available "out-of-
band" return path, as defined in [8], irrespective of what is
requested by the node originating the OAM packet.
SPMEs are usually instantiated when the transport path is created by
either the management plane or the control plane (if present).
Sometimes an SPME can be instantiated after the transport path is
initially created.
3.7. P2MP Considerations
All the traffic sent over a P2MP transport path, including OAM
packets generated by a MEP, is sent (multicast) from the root to all
the leaves. As a consequence:
o To send an OAM packet to all leaves, the source MEP can send a
single OAM packet that will be delivered by the forwarding plane
to all the leaves and processed by all the leaves. Hence, a
single OAM packet can simultaneously instrument all the MEs in a
P2MP MEG.
o To send an OAM packet to a single leaf, the source MEP sends a
single OAM packet that will be delivered by the forwarding plane
to all the leaves but contains sufficient information to identify
a target leaf, and therefore is processed only by the target leaf
and can be silently discarded by the other leaves.
Busi & Allan Informational [Page 21]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
o To send an OAM packet to a single MIP, the source MEP sends a
single OAM packet with the TTL field indicating the number of hops
necessary to reach the node where the MIP resides. This packet
will be delivered by the forwarding plane to all intermediate
nodes at the same TTL distance of the target MIP and to any leaf
that is located at a shorter distance. The OAM packet must
contain sufficient information to identify the target MIP and
therefore is processed only by the target MIP and can be silently
discarded by the others.
o In order to send an OAM packet to M leaves (i.e., a subset of all
the leaves), the source MEP sends M different OAM packets targeted
to each individual leaf in the group of M leaves. Aggregating or
subsetting mechanisms are outside the scope of this document.
A bud node with a Down MEP or a per-node MEP will both terminate and
relay OAM packets. Similar to how fault coverage is maximized by the
explicit utilization of Up MEPs, the same is true for MEPs on a bud
node.
P2MP paths are unidirectional; therefore, any return path to an
originating MEP for on-demand transactions will be out-of-band. A
mechanism to target "on-demand" transactions to a single MEP or MIP
is required as it relieves the originating MEP of an arbitrarily
large processing load and of the requirement to filter and discard
undesired responses. This is because normally TTL exhaustion will
address all MIPs at a given distance from the source, and failure to
exhaust TTL will address all MEPs.
3.8. Further Considerations of Enhanced Segment Monitoring
Segment monitoring, like any in-service monitoring, in a transport
network should meet the following network objectives:
1. The monitoring and maintenance of existing transport paths has to
be conducted in service without traffic disruption.
2. Segment monitoring must not modify the forwarding of the segment
portion of the transport path.
SPMEs defined in Section 3.2 meet the above two objectives, when they
are pre-configured or pre-instantiated as exemplified in Section 3.6.
However, sometimes pre-design and pre-configuration of all the
considered patterns of SPME are not preferable in real operation due
to the burden of design works, a number of header consumptions,
bandwidth consumption, and so on.
Busi & Allan Informational [Page 22]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
When SPMEs are configured or instantiated after the transport path
has been created, network objective (1) can be met: application and
removal of SPME to a faultless monitored transport entity can be
performed in such a way as not to introduce any loss of traffic,
e.g., by using a non-disruptive "make before break" technique.
However, network objective (2) cannot be met due to new assignment of
MPLS labels. As a consequence, generally speaking, the results of
SPME monitoring are not necessarily correlated with the behavior of
traffic in the monitored entity when it does not use SPME. For
example, application of SPME to a problematic/faulty monitoring
entity might "fix" the problem encountered by the latter -- for as
long as SPME is applied. And vice versa, application of SPME to a
faultless monitored entity may result in making it faulty -- again,
as long as SPME is applied.
Support for a more sophisticated segment-monitoring mechanism
(temporal and hitless segment monitoring) to efficiently meet the two
network objectives may be necessary.
One possible option to instantiate non-intrusive segment monitoring
without the use of SPMEs would require the MIPs selected as
monitoring end points to implement enhanced functionality and state
for the monitored transport path.
For example, the MIPs need to be configured with the TTL distance to
the peer or with the address of the peer, when out-of-band return
paths are used.
A further issue that would need to be considered is events that
result in changing the TTL distance to the peer monitoring entity,
such as protection events that may temporarily invalidate OAM
information gleaned from the use of this technique.
Further considerations on this technique are outside the scope of
this document.
4. Reference Model
The reference model for the MPLS-TP OAM framework builds upon the
concept of a MEG, and its associated MEPs and MIPs, to support the
functional requirements specified in RFC 5860 [11].
The following MPLS-TP MEGs are specified in this document:
o A Section Maintenance Entity Group (SMEG), allowing monitoring and
management of MPLS-TP Sections (between MPLS LSRs).
Busi & Allan Informational [Page 23]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
o An LSP Maintenance Entity Group (LMEG), allowing monitoring and
management of an end-to-end LSP (between LERs).
o A PW Maintenance Entity Group (PMEG), allowing monitoring and
management of an end-to-end Single-Segment Pseudowire (SS-PW) or
MS-PW (between T-PEs).
o An LSP SPME ME Group (LSMEG), allowing monitoring and management
of an SPME (between a given pair of LERs and/or LSRs along an
LSP).
o A PW SPME ME Group (PSMEG), allowing monitoring and management of
an SPME (between a given pair of T-PEs and/or S-PEs along an
(MS-)PW).
The MEGs specified in this MPLS-TP OAM framework are compliant with
the architecture framework for MPLS-TP [8] that includes both MS-PWs
[4] and LSPs [1].
Hierarchical LSPs are also supported in the form of SPMEs. In this
case, each LSP in the hierarchy is a different sub-layer network that
can be monitored, independently from higher- and lower-level LSPs in
the hierarchy, on an end-to-end basis (from LER to LER) by an SPME.
It is possible to monitor a portion of a hierarchical LSP by
instantiating a hierarchical SPME between any LERs/LSRs along the
hierarchical LSP.
Busi & Allan Informational [Page 24]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Native |<------------------ MS-PW1Z ---------------->| Native
Layer | | Layer
Service | |<LSP13>| |<-LSP3X->| |<LSPXZ>| | Service
(AC1) V V V V V V V V (AC2)
+----+ +---+ +----+ +----+ +---+ +----+
+----+ |T-PE| |LSR| |S-PE| |S-PE| |LSR| |T-PE| +----+
| | | 1 | | 2 | | 3 | | X | | Y | | Z | | |
| | | |=======| |=========| |=======| | | |
| CE1|--|.......PW13......|...PW3X..|......PWXZ.......|---|CE2 |
| | | |=======| |=========| |=======| | | |
| | | | | | | | | | | | | | | |
+----+ | | | | | | | | | | | | +----+
+----+ +---+ +----+ +----+ +---+ +----+
. . . .
| | | |
|<--- Domain 1 -->| |<--- Domain Z -->|
^----------------- PW1Z PMEG ----------------^
^--- PW13 PSMEG --^ ^--- PWXZ PSMEG --^
^-------^ ^-------^
LSP13 LMEG LSPXZ LMEG
^--^ ^--^ ^---------^ ^--^ ^--^
Sec12 Sec23 Sec3X SecXY SecYZ
SMEG SMEG SMEG SMEG SMEG
^---^ ME
^ MEP
==== LSP
.... PW
T-PE 1: Terminating Provider Edge 1
LSR 2: Label Switching Router 2
S-PE 3: Switching Provider Edge 3
S-PE X: Switching Provider Edge X
LSR Y: Label Switching Router Y
T-PE Z: Terminating Provider Edge Z
Figure 5: Reference Model for the MPLS-TP OAM Framework
Figure 5 depicts a high-level reference model for the MPLS-TP OAM
framework. The figure depicts portions of two MPLS-TP-enabled
network domains, Domain 1 and Domain Z. In Domain 1, T-PE 1 is
adjacent to LSR 2 via the MPLS-TP Section Sec12, and LSR 2 is
adjacent to S-PE 3 via the MPLS-TP Section Sec23. Similarly, in
Domain Z, S-PE X is adjacent to LSR Y via the MPLS-TP Section SecXY,
and LSR Y is adjacent to T-PE Z via the MPLS-TP Section SecYZ. In
addition, S-PE 3 is adjacent to S-PE X via the MPLS-TP Section Sec3X.
Busi & Allan Informational [Page 25]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Figure 5 also shows a bidirectional MS-PW (MS-PW1Z) between AC1 on
T-PE1 and AC2 on T-PE Z. The MS-PW consists of three bidirectional
PW path segments: 1) PW13 path segment between T-PE 1 and S-PE 3 via
the bidirectional LSP13 LSP, 2) PW3X path segment between S-PE 3 and
S-PE X via the bidirectional LSP3X LSP, and 3) PWXZ path segment
between S-PE X and T-PE Z via the bidirectional LSPXZ LSP.
The MPLS-TP OAM procedures that apply to a MEG are expected to
operate independently from procedures on other MEGs. Yet, this does
not preclude that multiple MEGs may be affected simultaneously by the
same network condition -- for example, a fiber cut event.
Note that there are no constraints imposed by this OAM framework on
the number or type (P2P, P2MP, LSP, or PW), of MEGs that may be
instantiated on a particular node. In particular, when looking at
Figure 5, it should be possible to configure one or more MEPs on the
same node if that node is the end point of one or more MEGs.
Figure 5 does not describe a PW3X PSMEG because typically SPMEs are
used to monitor an OAM domain (like PW13 and PWXZ PSMEGs) rather than
the segment between two OAM domains. However, the OAM framework does
not pose any constraints on the way SPMEs are instantiated as long as
they are not overlapping.
The subsections below define the MEGs specified in this MPLS-TP OAM
architecture framework document. Unless otherwise stated, all
references to domains, LSRs, MPLS-TP Sections, LSPs, pseudowires, and
MEGs in this section are made in relation to those shown in Figure 5.
4.1. MPLS-TP Section Monitoring (SMEG)
An MPLS-TP Section MEG (SMEG) is an MPLS-TP maintenance entity
intended to monitor an MPLS-TP Section. An SMEG may be configured on
any MPLS-TP section. SMEG OAM packets must fate-share with the user
data packets sent over the monitored MPLS-TP Section.
An SMEG is intended to be deployed for applications where it is
preferable to monitor the link between topologically adjacent (next
hop in this layer network) MPLS-TP LSRs rather than monitoring the
individual LSP or PW path segments traversing the MPLS-TP Section and
where the server-layer technology does not provide adequate OAM
capabilities.
Busi & Allan Informational [Page 26]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Figure 5 shows five Section MEGs configured in the network between
AC1 and AC2:
1. Sec12 MEG associated with the MPLS-TP Section between T-PE 1 and
LSR 2,
2. Sec23 MEG associated with the MPLS-TP Section between LSR 2 and
S-PE 3,
3. Sec3X MEG associated with the MPLS-TP Section between S-PE 3 and
S-PE X,
4. SecXY MEG associated with the MPLS-TP Section between S-PE X and
LSR Y, and
5. SecYZ MEG associated with the MPLS-TP Section between LSR Y and
T-PE Z
4.2. MPLS-TP LSP End-to-End Monitoring Group (LMEG)
An MPLS-TP LSP MEG (LMEG) is an MPLS-TP maintenance entity group
intended to monitor an end-to-end LSP between its LERs. An LMEG may
be configured on any MPLS LSP. LMEG OAM packets must fate-share with
user data packets sent over the monitored MPLS-TP LSP.
An LMEG is intended to be deployed in scenarios where it is desirable
to monitor an entire LSP between its LERs, rather than, say,
monitoring individual PWs.
Figure 5 depicts two LMEGs configured in the network between AC1 and
AC2: 1) the LSP13 LMEG between T-PE 1 and S-PE 3, and 2) the LSPXZ
LMEG between S-PE X and T-PE Z. Note that the presence of a LSP3X
LMEG in such a configuration is optional, and hence, not precluded by
this framework. For instance, the network operator may prefer to
monitor the MPLS-TP Section between the two LSRs rather than the
individual LSPs.
4.3. MPLS-TP PW Monitoring (PMEG)
An MPLS-TP PW MEG (PMEG) is an MPLS-TP maintenance entity intended to
monitor a SS-PW or MS-PW between its T-PEs. A PMEG can be configured
on any SS-PW or MS-PW. PMEG OAM packets must fate-share with the
user data packets sent over the monitored PW.
A PMEG is intended to be deployed in scenarios where it is desirable
to monitor an entire PW between a pair of MPLS-TP-enabled T-PEs
rather than monitoring the LSP that aggregates multiple PWs between
PEs.
Busi & Allan Informational [Page 27]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Figure 5 depicts an MS-PW (MS-PW1Z) consisting of three path segments
(PW13, PW3X, and PWXZ) and its associated end-to-end PMEG (PW1Z
PMEG).
4.4. MPLS-TP LSP SPME Monitoring (LSMEG)
An MPLS-TP LSP SPME MEG (LSMEG) is an MPLS-TP SPME with an associated
maintenance entity group intended to monitor an arbitrary part of an
LSP between the MEPs instantiated for the SPME, independent from the
end-to-end monitoring (LMEG). An LSMEG can monitor an LSP path
segment, and it may also include the forwarding engine(s) of the
node(s) at the edge(s) of the path segment.
When an SPME is established between non-adjacent LSRs, the edges of
the SPME become adjacent at the LSP sub-layer network and any LSR
that was previously in between becomes an LSR for the SPME.
Multiple hierarchical LSMEGs can be configured on any LSP. LSMEG OAM
packets must fate-share with the user data packets sent over the
monitored LSP path segment.
A LSME can be defined between the following entities:
o The LER and LSR of a given LSP.
o Any two LSRs of a given LSP.
An LSMEG is intended to be deployed in scenarios where it is
preferable to monitor the behavior of a part of an LSP or set of LSPs
rather than the entire LSP itself, for example, when there is a need
to monitor a part of an LSP that extends beyond the administrative
boundaries of an MPLS-TP-enabled administrative domain.
Busi & Allan Informational [Page 28]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
|<-------------------- PW1Z ------------------->|
| |
| |<-------------LSP1Z LSP------------->| |
| |<-LSP13->| |<LSP3X>| |<-LSPXZ->| |
V V V V V V V V
+----+ +---+ +----+ +----+ +---+ +----+
+----+ | PE | |LSR| |DBN | |DBN | |LSR| | PE | +----+
| | | 1 | | 2 | | 3 | | X | | Y | | Z | | |
| |AC1| |=====================================| |AC2| |
| CE1|---|.....................PW1Z......................|---|CE2 |
| | | |=====================================| | | |
| | | | | | | | | | | | | | | |
+----+ | | | | | | | | | | | | +----+
+----+ +---+ +----+ +----+ +---+ +----+
. . . .
| | | |
|<---- Domain 1 --->| |<---- Domain Z --->|
^---------^ ^---------^
LSP13 LSMEG LSPXZ LSMEG
^-------------------------------------^
LSP1Z LMEG
DBN: Domain Border Node
PE 1: Provider Edge 1
LSR 2: Label Switching Router 2
DBN 3: Domain Border Node 3
DBN X: Domain Border Node X
LSR Y: Label Switching Router Y
PE Z: Provider Edge Z
Figure 6: MPLS-TP LSP SPME MEG (LSMEG)
Figure 6 depicts a variation of the reference model in Figure 5 where
there is an end-to-end LSP (LSP1Z) between PE 1 and PE Z. LSP1Z
consists of, at least, three LSP Concatenated Segments: LSP13, LSP3X,
and LSPXZ. In this scenario, there are two separate LSMEGs
configured to monitor the LSP1Z: 1) a LSMEG monitoring the LSP13
Concatenated Segment on Domain 1 (LSP13 LSMEG), and 2) a LSMEG
monitoring the LSPXZ Concatenated Segment on Domain Z (LSPXZ LSMEG).
It is worth noticing that LSMEGs can coexist with the LMEG monitoring
the end-to-end LSP and that LSMEG MEPs and LMEG MEPs can be
coincident in the same node (e.g., PE 1 node supports both the LSP1Z
LMEG MEP and the LSP13 LSMEG MEP).
Busi & Allan Informational [Page 29]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
4.5. MPLS-TP MS-PW SPME Monitoring (PSMEG)
An MPLS-TP MS-PW SPME Monitoring MEG (PSMEG) is an MPLS-TP SPME with
an associated maintenance entity group intended to monitor an
arbitrary part of an MS-PW between the MEPs instantiated for the
SPME, independently of the end-to-end monitoring (PMEG). A PSMEG can
monitor a PW path segment, and it may also include the forwarding
engine(s) of the node(s) at the edge(s) of the path segment. A PSMEG
is no different than an SPME; it is simply named as such to discuss
SPMEs specifically in a PW context.
When SPME is established between non-adjacent S-PEs, the edges of the
SPME become adjacent at the MS-PW sub-layer network, and any S-PE
that was previously in between becomes an LSR for the SPME.
S-PE placement is typically dictated by considerations other than
OAM. S-PEs will frequently reside at operational boundaries such as
the transition from distributed control plane (CP) to centralized
Network Management System (NMS) control or at a routing area
boundary. As such, the architecture would appear not to have the
flexibility that arbitrary placement of SPME segments would imply.
Support for an arbitrary placement of PSMEG would require the
definition of additional PW sub-layering. Multiple hierarchical
PSMEGs can be configured on any MS-PW. PSMEG OAM packets fate-share
with the user data packets sent over the monitored PW path Segment.
A PSMEG does not add hierarchical components to the MPLS
architecture; it defines the role of existing components for the
purposes of discussing OAM functionality.
A PSME can be defined between the following entities:
o The T-PE and any S-PE of a given MS-PW.
o Any two S-PEs of a given MS-PW.
Note that, in line with the SPME description in Section 3.2, when a
PW SPME is instantiated after the MS-PW has been instantiated, the
TTL distance of the MIPs may change and MIPs in the PW SPME are no
longer part of the encompassing MEG. This means that the S-PE nodes
hosting these MIPs are no longer S-PEs but P nodes at the SPME LSP
level. The consequences are that the S-PEs hosting the PSMEG MEPs
become adjacent S-PEs. This is no different than the operation of
SPMEs in general.
A PSMEG is intended to be deployed in scenarios where it is
preferable to monitor the behavior of a part of an MS-PW rather than
the entire end-to-end PW itself, for example, when monitoring an MS-
Busi & Allan Informational [Page 30]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
PW path segment within a given network domain of an inter-domain MS-
PW.
Figure 5 depicts an MS-PW (MS-PW1Z) consisting of three path
segments: PW13, PW3X, and PWXZ with two separate PSMEGs: 1) a PSMEG
monitoring the PW13 MS-PW path segment on Domain 1 (PW13 PSMEG) and
2) a PSMEG monitoring the PWXZ MS-PW path segment on Domain Z with
(PWXZ PSMEG).
It is worth noticing that PSMEGs can coexist with the PMEG monitoring
the end-to-end MS-PW and that PSMEG MEPs and PMEG MEPs can be
coincident in the same node (e.g., T-PE 1 node supports both the PW1Z
PMEG MEP and the PW13 PSMEG MEP).
4.6. Fate-Sharing Considerations for Multilink
Multilink techniques are in use today and are expected to continue to
be used in future deployments. These techniques include Ethernet
link aggregation [22] and the use of link bundling for MPLS [18]
where the option to spread traffic over component links is supported
and enabled. While the use of link bundling can be controlled at the
MPLS-TP layer, use of link aggregation (or any server-layer-specific
multilink) is not necessarily under the control of the MPLS-TP layer.
Other techniques may emerge in the future. These techniques
frequently share the characteristic that an LSP may be spread over a
set of component links and therefore be reordered, but no flow within
the LSP is reordered (except when very infrequent and minimally
disruptive load rebalancing occurs).
The use of multilink techniques may be prohibited or permitted in any
particular deployment. If multilink techniques are used, the
deployment can be considered to be only partially MPLS-TP compliant;
however, this is unlikely to prevent their use.
The implications for OAM are that not all components of a multilink
will be exercised, independent server-layer OAM being required to
exercise the aggregated link components. This has further
implications for MIP and MEP placement, as per-interface MIPs or Down
MEPs on a multilink interface are akin to a layer violation, as they
instrument at the granularity of the server layer. The implications
for reduced OAM loss measurement functionality are documented in
Sections 5.5.3 and 6.2.3.
Busi & Allan Informational [Page 31]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
5. OAM Functions for Proactive Monitoring
In this document, proactive monitoring refers to OAM operations that
are either configured to be carried out periodically and continuously
or preconfigured to act on certain events such as alarm signals.
Proactive monitoring is usually performed "in-service". Such
transactions are universally MEP to MEP in operation, while
notifications can be node to node (e.g., some MS-PW transactions) or
node to MEPs (e.g., AIS). The control and measurement considerations
are:
1. Proactive monitoring for a MEG is typically configured at the
creation time of the transport path.
2. The operational characteristics of in-band measurement
transactions (e.g., CV, Loss Measurement (LM), etc.) are
configured at the MEPs.
3. Server-layer events are reported by OAM packets originating at
intermediate nodes.
4. The measurements resulting from proactive monitoring are typically
reported outside of the MEG (e.g., to a management system) as
notification events such as faults or indications of performance
degradations (such as signal degrade conditions).
5. The measurements resulting from proactive monitoring may be
periodically harvested by an NMS.
Proactive fault reporting is assumed to be subject to unreliable
delivery and soft-state, and it needs to operate in cases where a
return path is not available or faulty. Therefore, periodic
repetition is assumed to be used for reliability, instead of
handshaking.
Delay measurement also requires periodic repetition to allow
estimation of the packet delay variation for the MEG.
For statically provisioned transport paths, the above information is
statically configured; for dynamically established transport paths,
the configuration information is signaled via the control plane or
configured via the management plane.
The operator may enable/disable some of the consequent actions
defined in Section 5.1.2.
Busi & Allan Informational [Page 32]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
5.1. Continuity Check and Connectivity Verification
Proactive Continuity Check functions, as required in Section 2.2.2 of
RFC 5860 [11], are used to detect a loss of continuity (LOC) defect
between two MEPs in a MEG.
Proactive Connectivity Verification functions, as required in Section
2.2.3 of RFC 5860 [11], are used to detect an unexpected connectivity
defect between two MEGs (e.g., mismerging or misconnection), as well
as unexpected connectivity within the MEG with an unexpected MEP.
Both functions are based on the (proactive) generation, at the same
rate, of OAM packets by the source MEP that are processed by the peer
sink MEP(s). As a consequence, in order to save OAM bandwidth
consumption, CV, when used, is linked with CC into Continuity Check
and Connectivity Verification (CC-V) OAM packets.
In order to perform proactive Connectivity Verification, each CC-V
OAM packet also includes a globally unique Source MEP identifier,
whose value needs to be configured on the source MEP and on the peer
sink MEP(s). In some cases, to avoid the need to configure the
globally unique Source MEP identifier, it is preferable to perform
only proactive Continuity Check. In this case, the CC-V OAM packet
does not need to include any globally unique Source MEP identifier.
Therefore, a MEG can be monitored only for CC or for both CC and CV.
CC-V OAM packets used for CC-only monitoring are called CC OAM
packets, while CC-V OAM packets used for both CC and CV are called CV
OAM packets.
As a consequence, it is not possible to detect misconnections between
two MEGs monitored only for continuity as neither the OAM packet type
nor the OAM packet content provides sufficient information to
disambiguate an invalid source. To expand:
o For a CC OAM packet leaking into a CC monitored MEG -
undetectable.
o For a CV OAM packet leaking into a CC monitored MEG - reception of
CV OAM packets instead of a CC OAM packets (e.g., with the
additional Source MEP identifier) allows detecting the fault.
o For a CC OAM packet leaking into a CV monitored MEG - reception of
CC OAM packets instead of CV OAM packets (e.g., lack of additional
Source MEP identifier) allows detecting the fault.
o For a CV OAM packet leaking into a CV monitored MEG - reception of
CV OAM packets with different Source MEP identifier permits fault
to be identified.
Busi & Allan Informational [Page 33]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Having a common packet format for CC-V OAM packets would simplify
parsing in a sink MEP to properly detect all the misconfiguration
cases described above.
MPLS-TP OAM supports different formats of MEP identifiers to address
different environments. When an alternative to IP addressing is
desired (e.g., MPLS-TP is deployed in transport network environments
where consistent operations with other transport technologies defined
by the ITU-T are required), the ITU Carrier Code (ICC)-based format
for MEP identification is used: this format is under definition in
[25]. When MPLS-TP is deployed in an environment where IP
capabilities are available and desired for OAM, the IP-based MEP
identification is used: this format is described in [24].
CC-V OAM packets are transmitted at a regular, operator-configurable
rate. The default CC-V transmission periods are application
dependent (see Section 5.1.3).
Proactive CC-V OAM packets are transmitted with the "minimum loss
probability PHB" within the transport path (LSP, PW) they are
monitoring. For E-LSPs, this PHB is configurable on the network
operator's basis, while for L-LSPs this is determined as per RFC 3270
[23]. PHBs can be translated at the network borders by the same
function that translates them for user data traffic. The implication
is that CC-V fate-shares with much of the forwarding implementation,
but not all aspects of PHB processing are exercised. Either on-
demand tools are used for finer-grained fault finding or an
implementation may utilize a CC-V flow per PHB to ensure a CC-V flow
fate-shares with each individual PHB.
In a co-routed or associated, bidirectional point-to-point transport
path, when a MEP is enabled to generate proactive CC-V OAM packets
with a configured transmission rate, it also expects to receive
proactive CC-V OAM packets from its peer MEP at the same transmission
rate. This is because a common SLA applies to all components of the
transport path. In a unidirectional transport path (either point-to-
point or point-to-multipoint), the source MEP is enabled only to
generate CC-V OAM packets, while each sink MEP is configured to
expect these packets at the configured rate.
MIPs, as well as intermediate nodes not supporting MPLS-TP OAM, are
transparent to the proactive CC-V information and forward these
proactive CC-V OAM packets as regular data packets.
During path setup and tear down, situations arise where CC-V checks
would give rise to alarms, as the path is not fully instantiated. In
order to avoid these spurious alarms, the following procedures are
recommended. At initialization, the source MEP function (generating
Busi & Allan Informational [Page 34]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
proactive CC-V packets) should be enabled prior to the corresponding
sink MEP function (detecting continuity and connectivity defects).
When disabling the CC-V proactive functionality, the sink MEP
function should be disabled prior to the corresponding source MEP
function.
It should be noted that different encapsulations are possible for
CC-V packets, and therefore it is possible that in case of
misconfigurations or mis-connectivity, CC-V packets are received with
an unexpected encapsulation.
There are practical limitations to detecting unexpected
encapsulation. It is possible that there are misconfiguration or
mis-connectivity scenarios where OAM packets can alias as payload,
e.g., when a transport path can carry an arbitrary payload without a
pseudowire.
When CC-V packets are received with an unexpected encapsulation that
can be parsed by a sink MEP, the CC-V packet is processed as if it
were received with the correct encapsulation. If it is not a
manifestation of a mis-connectivity defect, a warning is raised (see
Section 5.1.1.4). Otherwise, the CC-V packet may be silently
discarded as unrecognized and a LOC defect may be detected (see
Section 5.1.1.1).
The defect conditions are described in no specific order.
5.1.1. Defects Identified by CC-V
Proactive CC-V functions allow a sink MEP to detect the defect
conditions described in the following subsections. For all of the
described defect cases, a sink MEP should notify the equipment fault
management process of the detected defect.
Sequential consecutive loss of CC-V packets is considered indicative
of an actual break and not of congestive loss or physical-layer
degradation. The loss of 3 packets in a row (implying a detection
interval that is 3.5 times the insertion time) is interpreted as a
true break and a condition that will not clear by itself.
A CC-V OAM packet is considered to carry an unexpected globally
unique Source MEP identifier if it is a CC OAM packet received by a
sink MEP monitoring the MEG for CV; it is a CV OAM packet received by
a sink MEP monitoring the MEG for CC, or it is a CV OAM packet
received by a sink MEP monitoring the MEG for CV but carrying a
unique Source MEP identifier that is different that the expected one.
Conversely, the CC-V packet is considered to have an expected
globally unique Source MEP identifier; it is a CC OAM packet received
Busi & Allan Informational [Page 35]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
by a sink MEP monitoring the MEG for CC, or it is a CV OAM packet
received by a sink MEP monitoring the MEG for CV and carrying a
unique Source MEP identifier that is equal to the expected one.
5.1.1.1. Loss of Continuity Defect
When proactive CC-V is enabled, a sink MEP detects a loss of
continuity (LOC) defect when it fails to receive proactive CC-V OAM
packets from the source MEP.
o Entry criteria: If no proactive CC-V OAM packets from the source
MEP (and in the case of CV, this includes the requirement to have
the expected globally unique Source MEP identifier) are received
within the interval equal to 3.5 times the receiving MEP's
configured CC-V reception period.
o Exit criteria: A proactive CC-V OAM packet from the source MEP
(and again in the case of CV, with the expected globally unique
Source MEP identifier) is received.
5.1.1.2. Mis-Connectivity Defect
When a proactive CC-V OAM packet is received, a sink MEP identifies a
mis-connectivity defect (e.g., mismerge, misconnection, or unintended
looping) when the received packet carries an unexpected globally
unique Source MEP identifier.
o Entry criteria: The sink MEP receives a proactive CC-V OAM packet
with an unexpected globally unique Source MEP identifier or with
an unexpected encapsulation.
o Exit criteria: The sink MEP does not receive any proactive CC-V
OAM packet with an unexpected globally unique Source MEP
identifier for an interval equal at least to 3.5 times the longest
transmission period of the proactive CC-V OAM packets received
with an unexpected globally unique Source MEP identifier since
this defect has been raised. This requires the OAM packet to
self-identify the CC-V periodicity, as not all MEPs can be
expected to have knowledge of all MEGs.
5.1.1.3. Period Misconfiguration Defect
If proactive CC-V OAM packets are received with the expected globally
unique Source MEP identifier but with a transmission period different
than the locally configured reception period, then a CC-V period
misconfiguration defect is detected.
Busi & Allan Informational [Page 36]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
o Entry criteria: A MEP receives a CC-V proactive packet with the
expected globally unique Source MEP identifier but with a
transmission period different than its own CC-V-configured
transmission period.
o Exit criteria: The sink MEP does not receive any proactive CC-V
OAM packet with the expected globally unique Source MEP identifier
and an incorrect transmission period for an interval equal at
least to 3.5 times the longest transmission period of the
proactive CC-V OAM packets received with the expected globally
unique Source MEP identifier and an incorrect transmission period
since this defect has been raised.
5.1.1.4. Unexpected Encapsulation Defect
If proactive CC-V OAM packets are received with the expected globally
unique Source MEP identifier but with an unexpected encapsulation,
then a CC-V unexpected encapsulation defect is detected.
It should be noted that there are practical limitations to detecting
unexpected encapsulation (see Section 5.1.1).
o Entry criteria: A MEP receives a CC-V proactive packet with the
expected globally unique Source MEP identifier but with an
unexpected encapsulation.
o Exit criteria: The sink MEP does not receive any proactive CC-V
OAM packet with the expected globally unique Source MEP identifier
and an unexpected encapsulation for an interval equal at least to
3.5 times the longest transmission period of the proactive CC-V
OAM packets received with the expected globally unique Source MEP
identifier and an unexpected encapsulation since this defect has
been raised.
5.1.2. Consequent Action
A sink MEP that detects any of the defect conditions defined in
Section 5.1.1 declares a defect condition and performs the following
consequent actions.
If a MEP detects a mis-connectivity defect, it blocks all the traffic
(including also the user data packets) that it receives from the
misconnected transport path.
If a MEP detects a LOC defect that is not caused by a period
misconfiguration, it should block all the traffic (including also the
user data packets) that it receives from the transport path, if this
consequent action has been enabled by the operator.
Busi & Allan Informational [Page 37]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
It is worth noticing that the OAM requirements document [11]
recommends that CC-V proactive monitoring be enabled on every MEG in
order to reliably detect connectivity defects. However, CC-V
proactive monitoring can be disabled by an operator for a MEG. In
the event of a misconnection between a transport path that is
proactively monitored for CC-V and a transport path that is not, the
MEP of the former transport path will detect a LOC defect
representing a connectivity problem (e.g., a misconnection with a
transport path where CC-V proactive monitoring is not enabled)
instead of a continuity problem, with a consequence of delivery of
traffic to an incorrect destination. For these reasons, the traffic
block consequent action is applied even when a LOC condition occurs.
This block consequent action can be disabled through configuration.
This deactivation of the block action may be used for activating or
deactivating the monitoring when it is not possible to synchronize
the function activation of the two peer MEPs.
If a MEP detects a LOC defect (Section 5.1.1.1) or a mis-connectivity
defect (Section 5.1.1.2), it declares a signal fail condition of the
ME.
It is a matter of local policy whether or not a MEP that detects a
period misconfiguration defect (Section 5.1.1.3) declares a signal
fail condition of the ME.
The detection of an unexpected encapsulation defect does not have any
consequent action: it is just a warning for the network operator. An
implementation able to detect an unexpected encapsulation but not
able to verify the source MEP ID may choose to declare a mis-
connectivity defect.
5.1.3. Configuration Considerations
At all MEPs inside a MEG, the following configuration information
needs to be configured when a proactive CC-V function is enabled:
o MEG-ID: the MEG identifier to which the MEP belongs.
o MEP-ID: the MEP's own identity inside the MEG.
o list of the other MEPs in the MEG. For a point-to-point MEG, the
list would consist of the single MEP ID from which the OAM packets
are expected. In case of the root MEP of a P2MP MEG, the list is
composed of all the leaf MEP IDs inside the MEG. In case of the
leaf MEP of a P2MP MEG, the list is composed of the root MEP ID
(i.e., each leaf needs to know the root MEP ID from which it
expects to receive the CC-V OAM packets).
Busi & Allan Informational [Page 38]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
o PHB for E-LSPs. It identifies the per-hop behavior of a CC-V
packet. Proactive CC-V packets are transmitted with the "minimum
loss probability PHB" previously configured within a single
network operator. This PHB is configurable on network operator's
basis. PHBs can be translated at the network borders.
o transmission rate. The default CC-V transmission periods are
application dependent (depending on whether they are used to
support fault management, performance monitoring, or protection-
switching applications):
* Fault Management: default transmission period is 1 s (i.e.,
transmission rate of 1 packet/second).
* Performance Management: default transmission period is 100 ms
(i.e., transmission rate of 10 packets/second). CC-V
contributes to the accuracy of performance monitoring
statistics by permitting the defect-free periods to be properly
distinguished as described in Sections 5.5.1 and 5.6.1.
* Protection Switching: If protection switching with CC-V, defect
entry criteria of 12 ms is required (for example, in
conjunction with the requirement to support 50 ms recovery time
as indicated in RFC 5654 [5]), then an implementation should
use a default transmission period of 3.33 ms (i.e.,
transmission rate of 300 packets/second). Sometimes, the
requirement of 50 ms recovery time is associated with the
requirement for a CC-V defect entry criteria period of 35 ms;
in these cases a transmission period of 10 ms (i.e.,
transmission rate of 100 packets/second) can be used.
Furthermore, when there is no need for so small CC-V defect
entry criteria periods, a larger transmission period can be
used.
It should be possible for the operator to configure these
transmission rates for all applications, to satisfy specific network
requirements.
Note that the reception period is the same as the configured
transmission rate.
For management-provisioned transport paths, the above parameters are
statically configured; for dynamically signaled transport paths, the
configuration information is distributed via the control plane.
The operator should be able to enable/disable some of the consequent
actions. Which consequent actions can be enabled/disabled is
described in Section 5.1.2.
Busi & Allan Informational [Page 39]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
5.2. Remote Defect Indication
The Remote Defect Indication (RDI) function, as required in Section
2.2.9 of RFC 5860 [11], is an indicator that is transmitted by a sink
MEP to communicate to its source MEP that a signal fail condition
exists. In case of co-routed and associated bidirectional transport
paths, RDI is associated with proactive CC-V, and the RDI indicator
can be piggy-backed onto the CC-V packet. In case of unidirectional
transport paths, the RDI indicator can be sent only using an out-of-
band return path if it exists and its usage is enabled by policy
actions.
When a MEP detects a signal fail condition (e.g., in case of a
continuity or connectivity defect), it should begin transmitting an
RDI indicator to its peer MEP. When incorporated into CC-V, the RDI
information will be included in all proactive CC-V packets that it
generates for the duration of the signal fail condition's existence.
A MEP that receives packets from a peer MEP with the RDI information
should determine that its peer MEP has encountered a defect condition
associated with a signal fail condition.
MIPs as well as intermediate nodes not supporting MPLS-TP OAM are
transparent to the RDI indicator and forward OAM packets that include
the RDI indicator as regular data packets, i.e., the MIP should not
perform any actions nor examine the indicator.
When the signal fail condition clears, the MEP should stop
transmitting the RDI indicator to its peer MEP. When incorporated
into CC-V, the RDI indicator will not be set for subsequent
transmission of proactive CC-V packets. A MEP should clear the RDI
defect upon reception of an RDI indicator cleared.
5.2.1. Configuration Considerations
In order to support RDI, the indication may be carried in a unique
OAM packet or may be embedded in a CC-V packet. The in-band RDI
transmission rate and PHB of the OAM packets carrying RDIs should be
the same as that configured for CC-V to allow both far-end and near-
end defect conditions being resolved in a timeframe that has the same
order of magnitude. This timeframe is application specific as
described in Section 5.1.3. Methods of the out-of-band return paths
will dictate how out-of-band RDIs are transmitted.
Busi & Allan Informational [Page 40]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
5.3. Alarm Reporting
The Alarm Reporting function, as required in Section 2.2.8 of RFC
5860 [11], relies upon an Alarm Indication Signal (AIS) packet to
suppress alarms following detection of defect conditions at the
server (sub-)layer.
When a server MEP asserts a signal fail condition, it notifies that
to the co-located MPLS-TP client/server adaptation function that then
generates OAM packets with AIS information in the downstream
direction to allow the suppression of secondary alarms at the MPLS-TP
MEP in the client (sub-)layer.
The generation of packets with AIS information starts immediately
when the server MEP asserts a signal fail condition. These periodic
OAM packets, with AIS information, continue to be transmitted until
the signal fail condition is cleared.
It is assumed that to avoid spurious alarm generation a MEP detecting
a loss of continuity defect (see Section 5.1.1.1) will wait for a
hold-off interval prior to asserting an alarm to the management
system. Therefore, upon receiving an OAM packet with AIS
information, an MPLS-TP MEP enters an AIS defect condition and
suppresses reporting of alarms to the NMS on the loss of continuity
with its peer MEP, but it does not block traffic received from the
transport path. A MEP resumes loss of continuity alarm generation
upon detecting loss of continuity defect conditions in the absence of
AIS condition.
MIPs, as well as intermediate nodes, do not process AIS information
and forward these AIS OAM packets as regular data packets.
For example, let's consider a fiber cut between T-PE 1 and LSR 2 in
the reference network of Figure 5. Assuming that all of the MEGs
described in Figure 5 have proactive CC-V enabled, a LOC defect is
detected by the MEPs of Sec12 SMEG, LSP13 LMEG, PW1 PSMEG, and PW1Z
PMEG; however, in a transport network, only the alarm associated to
the fiber cut needs to be reported to an NMS, while all secondary
alarms should be suppressed (i.e., not reported to the NMS or
reported as secondary alarms).
If the fiber cut is detected by the MEP in the physical layer (in LSR
2), LSR 2 can generate the proper alarm in the physical layer and
suppress the secondary alarm associated with the LOC defect detected
on Sec12 SMEG. As both MEPs reside within the same node, this
process does not involve any external protocol exchange. Otherwise,
Busi & Allan Informational [Page 41]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
if the physical layer does not have enough OAM capabilities to detect
the fiber cut, the MEP of Sec12 SMEG in LSR 2 will report a LOC
alarm.
In both cases, the MEP of Sec12 SMEG in LSR 2 notifies the adaptation
function for LSP13 LMEG that then generates AIS packets on the LSP13
LMEG in order to allow its MEP in S-PE 3 to suppress the LOC alarm.
S-PE 3 can also suppress the secondary alarm on PW13 PSMEG because
the MEP of PW13 PSMEG resides within the same node as the MEP of
LSP13 LMEG. The MEP of PW13 PSMEG in S-PE 3 also notifies the
adaptation function for PW1Z PMEG that then generates AIS packets on
PW1Z PMEG in order to allow its MEP in T-PE Z to suppress the LOC
alarm.
The generation of AIS packets for each MEG in the MPLS-TP client
(sub-)layer is configurable (i.e., the operator can enable/disable
the AIS generation).
The AIS condition is cleared if no AIS packet has been received in
3.5 times the AIS transmission period.
The AIS transmission period is traditionally one per second, but an
option to configure longer periods would be also desirable. As a
consequence, OAM packets need to self-identify the transmission
period such that proper exit criteria can be established.
AIS packets are transmitted with the "minimum loss probability PHB"
within a single network operator. For E-LSPs, this PHB is
configurable on network operator's basis, while for L-LSPs, this is
determined as per RFC 3270 [23].
5.4. Lock Reporting
The Lock Reporting function, as required in Section 2.2.7 of RFC 5860
[11], relies upon a Lock Report (LKR) packet used to suppress alarms
following administrative locking action in the server (sub-)layer.
When a server MEP is locked, the MPLS-TP client (sub-)layer
adaptation function generates packets with LKR information to allow
the suppression of secondary alarms at the MEPs in the client
(sub-)layer. Again, it is assumed that there is a hold-off for any
loss of continuity alarms in the client-layer MEPs downstream of the
node originating the Lock Report. In case of client (sub-)layer co-
routed bidirectional transport paths, the LKR information is sent on
both directions. In case of client (sub-)layer unidirectional
transport paths, the LKR information is sent only in the downstream
direction. As a consequence, in case of client (sub-)layer point-to-
multipoint transport paths, the LKR information is sent only to the
Busi & Allan Informational [Page 42]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
MEPs that are downstream from the server (sub-)layer that has been
administratively locked. Client (sub-)layer associated bidirectional
transport paths behave like co-routed bidirectional transport paths
if the server (sub-)layer that has been administratively locked is
used by both directions; otherwise, they behave like unidirectional
transport paths.
The generation of packets with LKR information starts immediately
when the server MEP is locked. These periodic packets, with LKR
information, continue to be transmitted until the locked condition is
cleared.
Upon receiving a packet with LKR information, an MPLS-TP MEP enters
an LKR defect condition and suppresses the loss of continuity alarm
associated with its peer MEP but does not block traffic received from
the transport path. A MEP resumes loss of continuity alarm
generation upon detecting loss of continuity defect conditions in the
absence of the LKR condition.
MIPs, as well as intermediate nodes, do not process the LKR
information; they forward these LKR OAM packets as regular data
packets.
For example, let's consider the case where the MPLS-TP Section
between T-PE 1 and LSR 2 in the reference network of Figure 5 is
administratively locked at LSR 2 (in both directions).
Assuming that all the MEGs described in Figure 5 have proactive CC-V
enabled, a LOC defect is detected by the MEPs of LSP13 LMEG, PW1
PSMEG, and PW1Z PMEG; however, in a transport network all these
secondary alarms should be suppressed (i.e., not reported to the NMS
or reported as secondary alarms).
The MEP of Sec12 SMEG in LSR 2 notifies the adaptation function for
LSP13 LMEG that then generates LKR packets on the LSP13 LMEG in order
to allow its MEPs in T-PE 1 and S-PE 3 to suppress the LOC alarm.
S-PE 3 can also suppress the secondary alarm on PW13 PSMEG because
the MEP of PW13 PSMEG resides within the same node as the MEP of
LSP13 LMEG. The MEP of PW13 PSMEG in S-PE 3 also notifies the
adaptation function for PW1Z PMEG that then generates AIS packets on
PW1Z PMEG in order to allow its MEP in T-PE Z to suppress the LOC
alarm.
The generation of LKR packets for each MEG in the MPLS-TP client
(sub-)layer is configurable (i.e., the operator can enable/disable
the LKR generation).
Busi & Allan Informational [Page 43]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
The locked condition is cleared if no LKR packet has been received
for 3.5 times the transmission period.
The LKR transmission period is traditionally one per second, but an
option to configure longer periods would be also desirable. As a
consequence, OAM packets need to self-identify the transmission
period such that proper exit criteria can be established.
LKR packets are transmitted with the "minimum loss probability PHB"
within a single network operator. For E-LSPs, this PHB is
configurable on network operator's basis, while for L-LSPs, this is
determined as per RFC 3270 [23].
5.5. Packet Loss Measurement
Packet Loss Measurement (LM) is one of the capabilities supported by
the MPLS-TP Performance Monitoring (PM) function in order to
facilitate reporting of Quality of Service (QoS) information for a
transport path as required in Section 2.2.11 of RFC 5860 [11]. LM is
used to exchange counter values for the number of ingress and egress
packets transmitted and received by the transport path monitored by a
pair of MEPs.
Proactive LM is performed by periodically sending LM OAM packets from
a MEP to a peer MEP and by receiving LM OAM packets from the peer MEP
(if a co-routed or associated bidirectional transport path) during
the lifetime of the transport path. Each MEP performs measurements
of its transmitted and received user data packets. These
measurements are then correlated in real time with the peer MEP in
the ME to derive the impact of packet loss on a number of performance
metrics for the ME in the MEG. The LM transactions are issued such
that the OAM packets will experience the same PHB scheduling class as
the measured traffic while transiting between the MEPs in the ME.
For a MEP, near-end packet loss refers to packet loss associated with
incoming data packets (from the far-end MEP), while far-end packet
loss refers to packet loss associated with egress data packets
(towards the far-end MEP).
Proactive LM can be operated in two ways:
o One-way: a MEP sends an LM OAM packet to its peer MEP containing
all the required information to facilitate near-end packet loss
measurements at the peer MEP.
o Two-way: a MEP sends an LM OAM packet with an LM request to its
peer MEP, which replies with an LM OAM packet as an LM response.
The request/response LM OAM packets contain all the required
Busi & Allan Informational [Page 44]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
information to facilitate both near-end and far-end packet loss
measurements from the viewpoint of the originating MEP.
One-way LM is applicable to both unidirectional and bidirectional
(co-routed or associated) transport paths, while two-way LM is
applicable only to bidirectional (co-routed or associated) transport
paths.
MIPs, as well as intermediate nodes, do not process the LM
information; they forward these proactive LM OAM packets as regular
data packets.
5.5.1. Configuration Considerations
In order to support proactive LM, the transmission rate and, for
E-LSPs, the PHB class (associated with the LM OAM packets originating
from a MEP) need to be configured as part of the LM provisioning. LM
OAM packets should be transmitted with the PHB that yields the lowest
drop precedence within the measured PHB Scheduling Class (see RFC
3260 [17]), in order to maximize reliability of measurement within
the traffic class.
If that PHB class is not an ordered aggregate where the ordering
constraint is all packets with the PHB class being delivered in
order, LM can produce inconsistent results.
Performance monitoring (e.g., LM) is only relevant when the transport
path is defect free. CC-V contributes to the accuracy of PM
statistics by permitting the defect-free periods to be properly
distinguished. Therefore, support of proactive LM has implications
on the CC-V transmission period (see Section 5.1.3).
5.5.2. Sampling Skew
If an implementation makes use of a hardware forwarding path that
operates in parallel with an OAM processing path, whether hardware or
software based, the packet and byte counts may be skewed if one or
more packets can be processed before the OAM processing samples
counters. If OAM is implemented in software, this error can be quite
large.
5.5.3. Multilink Issues
If multilink is used at the ingress or egress of a transport path,
there may not be a single packet-processing engine where an LM packet
can be injected or extracted as an atomic operation while having
accurate packet and byte counts associated with the packet.
Busi & Allan Informational [Page 45]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
In the case where multilink is encountered along the route of the
transport path, the reordering of packets within the transport path
can cause inaccurate LM results.
5.6. Packet Delay Measurement
Packet Delay Measurement (DM) is one of the capabilities supported by
the MPLS-TP PM function in order to facilitate reporting of QoS
information for a transport path as required in Section 2.2.12 of RFC
5860 [11]. Specifically, proactive DM is used to measure the long-
term packet delay and packet delay variation in the transport path
monitored by a pair of MEPs.
Proactive DM is performed by sending periodic DM OAM packets from a
MEP to a peer MEP and by receiving DM OAM packets from the peer MEP
(if a co-routed or associated bidirectional transport path) during a
configurable time interval.
Proactive DM can be operated in two ways:
o One-way: a MEP sends a DM OAM packet to its peer MEP containing
all the required information to facilitate one-way packet delay
and/or one-way packet delay variation measurements at the peer
MEP. Note that this requires precise time synchronization at
either MEP by means outside the scope of this framework.
o Two-way: a MEP sends a DM OAM packet with a DM request to its peer
MEP, which replies with a DM OAM packet as a DM response. The
request/response DM OAM packets contain all the required
information to facilitate two-way packet delay and/or two-way
packet delay variation measurements from the viewpoint of the
originating MEP.
One-way DM is applicable to both unidirectional and bidirectional
(co-routed or associated) transport paths, while two-way DM is
applicable only to bidirectional (co-routed or associated) transport
paths.
MIPs, as well as intermediate nodes, do not process the DM
information; they forward these proactive DM OAM packets as regular
data packets.
5.6.1. Configuration Considerations
In order to support proactive DM, the transmission rate and, for
E-LSPs, the PHB (associated with the DM OAM packets originating from
a MEP) need to be configured as part of the DM provisioning. DM OAM
packets should be transmitted with the PHB that yields the lowest
Busi & Allan Informational [Page 46]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
drop precedence within the measured PHB Scheduling Class (see RFC
3260 [17]).
Performance monitoring (e.g., DM) is only relevant when the transport
path is defect free. CC-V contributes to the accuracy of PM
statistics by permitting the defect-free periods to be properly
distinguished. Therefore, support of proactive DM has implications
on the CC-V transmission period (see Section 5.1.3).
5.7. Client Failure Indication
The Client Failure Indication (CFI) function, as required in Section
2.2.10 of RFC 5860 [11], is used to help process client defects and
propagate a client signal defect condition from the process
associated with the local attachment circuit where the defect was
detected (typically the source adaptation function for the local
client interface). It is propagated to the process associated with
the far-end attachment circuit (typically the source adaptation
function for the far-end client interface) for the same transmission
path, in case the client of the transport path does not support a
native defect/alarm indication mechanism, e.g., AIS.
A source MEP starts transmitting a CFI to its peer MEP when it
receives a local client signal defect notification via its local
client signal fail indication. Mechanisms to detect local client
signal fail defects are technology specific. Similarly, mechanisms
to determine when to cease originating client signal fail indication
are also technology specific.
A sink MEP that has received a CFI reports this condition to its
associated client process via its local CFI function. Consequent
actions toward the client attachment circuit are technology specific.
There needs to be a 1:1 correspondence between the client and the
MEG; otherwise, when multiple clients are multiplexed over a
transport path, the CFI packet requires additional information to
permit the client instance to be identified.
MIPs, as well as intermediate nodes, do not process the CFI
information; they forward these proactive CFI OAM packets as regular
data packets.
5.7.1. Configuration Considerations
In order to support CFI indication, the CFI transmission rate and,
for E-LSPs, the PHB of the CFI OAM packets should be configured as
part of the CFI configuration.
Busi & Allan Informational [Page 47]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
6. OAM Functions for On-Demand Monitoring
In contrast to proactive monitoring, on-demand monitoring is
initiated manually and for a limited amount of time, usually for
operations such as diagnostics to investigate a defect condition.
On-demand monitoring covers a combination of "in-service" and "out-
of-service" monitoring functions. The control and measurement
implications are:
1. A MEG can be directed to perform an "on-demand" functions at
arbitrary times in the lifetime of a transport path.
2. "Out-of-service" monitoring functions may require a priori
configuration of both MEPs and intermediate nodes in the MEG
(e.g., data-plane loopback) and the issuance of notifications into
client layers of the transport path being removed from service
(e.g., lock reporting)
3. The measurements resulting from "on-demand" monitoring are
typically harvested in real time, as they are frequently initiated
manually. These do not necessarily require different harvesting
mechanisms than for harvesting proactive monitoring telemetry.
The functions that are exclusively out-of-service are those described
in Section 6.3. The remainder are applicable to both in-service and
out-of-service transport paths.
6.1. Connectivity Verification
The on-demand connectivity verification function, as required in
Section 2.2.3 of RFC 5860 [11], is a transaction that flows from the
originating MEP to a target MIP or MEP to verify the connectivity
between these points.
Use of on-demand CV is dependent on the existence of a bidirectional
ME or an associated return ME, or the availability of an out-of-band
return path, because it requires the ability for target MIPs and MEPs
to direct responses to the originating MEPs.
One possible use of on-demand CV would be to perform fault management
without using proactive CC-V, in order to preserve network resources,
e.g., bandwidth, processing time at switches. In this case, network
management periodically invokes on-demand CV.
Busi & Allan Informational [Page 48]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
An additional use of on-demand CV would be to detect and locate a
problem of connectivity when a problem is suspected or known to be
based on other tools. In this case, the functionality will be
triggered by the network management in response to a status signal or
alarm indication.
On-demand CV is based upon generation of on-demand CV packets that
should uniquely identify the MEG that is being checked. The on-
demand functionality may be used to check either an entire MEG (end-
to-end) or between the originating MEP and a specific MIP. This
functionality may not be available for associated bidirectional
transport paths or unidirectional paths, as the MIP may not have a
return path to the originating MEP for the on-demand CV transaction.
When on-demand CV is invoked, the originating MEP issues a sequence
of on-demand CV packets that uniquely identifies the MEG being
verified. The number of packets and their transmission rate should
be pre-configured at the originating MEP to take into account normal
packet-loss conditions. The source MEP should use the mechanisms
defined in Sections 3.3 and 3.4 when sending an on-demand CV packet
to a target MEP or target MIP, respectively. The target MEP/MIP
shall return a reply on-demand CV packet for each packet received.
If the expected number of on-demand CV reply packets is not received
at the originating MEP, this is an indication that a connectivity
problem may exist.
On-demand CV should have the ability to carry padding such that a
variety of MTU sizes can be originated to verify the MTU transport
capability of the transport path.
MIPs that are not targeted by on-demand CV packets, as well as
intermediate nodes, do not process the CV information; they forward
these on-demand CV OAM packets as regular data packets.
6.1.1. Configuration Considerations
For on-demand CV, the originating MEP should support the
configuration of the number of packets to be transmitted/received in
each sequence of transmissions and their packet size.
In addition, when the CV packet is used to check connectivity toward
a target MIP, the number of hops to reach the target MIP should be
configured.
For E-LSPs, the PHB of the on-demand CV packets should be configured
as well. This permits the verification of correct operation of QoS
queuing as well as connectivity.
Busi & Allan Informational [Page 49]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
6.2. Packet Loss Measurement
On-demand Packet Loss Measurement (LM) is one of the capabilities
supported by the MPLS-TP Performance Monitoring function in order to
facilitate the diagnosis of QoS performance for a transport path, as
required in Section 2.2.11 of RFC 5860 [11].
On-demand LM is very similar to proactive LM described in Section
5.5. This section focuses on the differences between on-demand and
proactive LM.
On-demand LM is performed by periodically sending LM OAM packets from
a MEP to a peer MEP and by receiving LM OAM packets from the peer MEP
(if a co-routed or associated bidirectional transport path) during a
pre-defined monitoring period. Each MEP performs measurements of its
transmitted and received user data packets. These measurements are
then correlated to evaluate the packet-loss performance metrics of
the transport path.
Use of packet loss measurement in an out-of-service transport path
requires a traffic source such as a test device that can inject
synthetic traffic.
6.2.1. Configuration Considerations
In order to support on-demand LM, the beginning and duration of the
LM procedures, the transmission rate, and, for E-LSPs, the PHB class
(associated with the LM OAM packets originating from a MEP) must be
configured as part of the on-demand LM provisioning. LM OAM packets
should be transmitted with the PHB that yields the lowest drop
precedence as described in Section 5.5.1.
6.2.2. Sampling Skew
The same considerations described in Section 5.5.2 for the proactive
LM are also applicable to on-demand LM implementations.
6.2.3. Multilink Issues
Multilink issues are as described in Section 5.5.3.
6.3. Diagnostic Tests
Diagnostic tests are tests performed on a MEG that has been taken out
of service.
Busi & Allan Informational [Page 50]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
6.3.1. Throughput Estimation
Throughput estimation is an on-demand out-of-service function, as
required in Section 2.2.5 of RFC 5860 [11], that allows verifying the
bandwidth/throughput of an MPLS-TP transport path (LSP or PW) before
it is put in service.
Throughput estimation is performed between MEPs and between a MEP and
a MIP. It can be performed in one-way or two-way modes.
According to RFC 2544 [12], this test is performed by sending OAM
test packets at increasing rates (up to the theoretical maximum),
computing the percentage of OAM test packets received, and reporting
the rate at which OAM test packets begin to drop. In general, this
rate is dependent on the OAM test packet size.
When configured to perform such tests, a source MEP inserts OAM test
packets with a specified packet size and transmission pattern at a
rate to exercise the throughput.
The throughput test can create congestion within the network, thus
impacting other transport paths. However, the test traffic should
comply with the traffic profile of the transport path under test, so
the impact of the test will not be worse than the impact caused by
the customers, whose traffic would be sent over that transport path,
sending the traffic at the maximum rate allowed by their traffic
profiles. Therefore, throughput tests are not applicable to
transport paths that do not have a defined traffic profile, such as
LSPs in a context where statistical multiplexing is leveraged for
network capacity dimensioning.
For a one-way test, the remote sink MEP receives the OAM test packets
and calculates the packet loss. For a two-way test, the remote MEP
loops the OAM test packets back to the original MEP, and the local
sink MEP calculates the packet loss.
It is worth noting that two-way throughput estimation is only
applicable to bidirectional (co-routed or associated) transport paths
and can only evaluate the minimum of available throughput of the two
directions. In order to estimate the throughput of each direction
uniquely, two one-way throughput estimation sessions have to be set
up. One-way throughput estimation requires coordination between the
transmitting and receiving test devices as described in Section 6 of
RFC 2544 [12].
It is also worth noting that if throughput estimation is performed on
transport paths that transit oversubscribed links, the test may not
produce comprehensive results if viewed in isolation because the
Busi & Allan Informational [Page 51]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
impact of the test on the surrounding traffic needs to also be
considered. Moreover, the estimation will only reflect the bandwidth
available at the moment when the measure is made.
MIPs that are not targeted by on-demand test OAM packets, as well as
intermediate nodes, do not process the throughput test information;
they forward these on-demand test OAM packets as regular data
packets.
6.3.1.1. Configuration Considerations
Throughput estimation is an out-of-service tool. The diagnosed MEG
should be put into a locked state before the diagnostic test is
started.
A MEG can be put into a locked state either via an NMS action or
using the Lock Instruct OAM tool as defined in Section 7.
At the transmitting MEP, provisioning is required for a test signal
generator that is associated with the MEP. At a receiving MEP,
provisioning is required for a test signal detector that is
associated with the MEP.
In order to ensure accurate measurement, care needs to be taken to
enable throughput estimation only if all the MEPs within the MEG can
process OAM test packets at the same rate as the payload data rates
(see Section 6.3.1.2).
6.3.1.2. Limited OAM Processing Rate
If an implementation is able to process payload at much higher data
rates than OAM test packets, then accurate measurement of throughput
using OAM test packets is not achievable. Whether OAM packets can be
processed at the same rate as payload is implementation dependent.
6.3.1.3. Multilink Considerations
If multilink is used, then it may not be possible to perform
throughput measurement, as the throughput test may not have a
mechanism for utilizing more than one component link of the
aggregated link.
6.3.2. Data-Plane Loopback
Data-plane loopback is an out-of-service function, as required in
Section 2.2.5 of RFC 5860 [11]. This function consists in placing a
transport path, at either an intermediate or terminating node, into a
data-plane loopback state, such that all traffic (including both
Busi & Allan Informational [Page 52]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
payload and OAM) received on the looped back interface is sent on the
reverse direction of the transport path. The traffic is looped back
unmodified except for normal per-hop processing such as TTL
decrement.
The data-plane loopback function requires that the MEG is locked such
that user data traffic is prevented from entering/exiting that MEG.
Instead, test traffic is inserted at the ingress of the MEG. This
test traffic can be generated from an internal process residing
within the ingress node or injected by external test equipment
connected to the ingress node.
It is also normal to disable proactive monitoring of the path as the
MEP located upstream with respect to the node set in the data-plane
loopback mode will see all the OAM packets originated by itself, and
this may interfere with other measurements.
The only way to send an OAM packet (e.g., to remove the data-plane
loopback state) to the MIPs or MEPs hosted by a node set in the data-
plane loopback mode is via TTL expiry. It should also be noted that
MIPs can be addressed with more than one TTL value on a co-routed
bidirectional path set into data-plane loopback.
If the loopback function is to be performed at an intermediate node,
it is only applicable to co-routed bidirectional paths. If the
loopback is to be performed end to end, it is applicable to both co-
routed bidirectional and associated bidirectional paths.
It should be noted that data-plane loopback function itself is
applied to data-plane loopback points that can reside on different
interfaces from MIPs/MEPs. Where a node implements data-plane
loopback capability and whether it implements it in more than one
point is implementation dependent.
6.3.2.1. Configuration Considerations
Data-plane loopback is an out-of-service tool. The MEG that defines
a diagnosed transport path should be put into a locked state before
the diagnostic test is started. However, a means is required to
permit the originated test traffic to be inserted at the ingress MEP
when data-plane loopback is performed.
A transport path, at either an intermediate or terminating node, can
be put into data-plane loopback state via an NMS action or using an
OAM tool for data-plane loopback configuration.
Busi & Allan Informational [Page 53]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
If the data-plane loopback point is set somewhere at an intermediate
point of a co-routed bidirectional transport path, the side of the
loopback function (east/west side or both sides) needs to be
configured.
6.4. Route Tracing
It is often necessary to trace a route covered by a MEG from an
originating MEP to the peer MEP(s) including all the MIPs in between.
This may be conducted after provisioning an MPLS-TP transport path
for, e.g., troubleshooting purposes such as fault localization.
The route tracing function, as required in Section 2.2.4 of RFC 5860
[11], is providing this functionality. Based on the fate-sharing
requirement of OAM flows, i.e., OAM packets receive the same
forwarding treatment as data packets, route tracing is a basic means
to perform connectivity verification and, to a much lesser degree,
continuity check. For this function to work properly, a return path
must be present.
Route tracing might be implemented in different ways, and this
document does not preclude any of them.
Route tracing should always discover the full list of MIPs and of
peer MEPs. In case a defect exists, the route tracing function will
only be able to trace up to the defect, and it needs to be able to
return the incomplete list of OAM entities that it was able to trace
so that the fault can be localized.
6.4.1. Configuration Considerations
The configuration of the route tracing function must at least support
the setting of the number of trace attempts before it gives up.
6.5. Packet Delay Measurement
Packet Delay Measurement (DM) is one of the capabilities supported by
the MPLS-TP PM function in order to facilitate reporting of QoS
information for a transport path, as required in Section 2.2.12 of
RFC 5860 [11]. Specifically, on-demand DM is used to measure packet
delay and packet delay variation in the transport path monitored by a
pair of MEPs during a pre-defined monitoring period.
On-demand DM is performed by sending periodic DM OAM packets from a
MEP to a peer MEP and by receiving DM OAM packets from the peer MEP
(if a co-routed or associated bidirectional transport path) during a
configurable time interval.
Busi & Allan Informational [Page 54]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
On-demand DM can be operated in two modes:
o One-way: a MEP sends a DM OAM packet to its peer MEP containing
all the required information to facilitate one-way packet delay
and/or one-way packet delay variation measurements at the peer
MEP. Note that this requires precise time synchronization at
either MEP by means outside the scope of this framework.
o Two-way: a MEP sends a DM OAM packet with a DM request to its peer
MEP, which replies with a DM OAM packet as a DM response. The
request/response DM OAM packets contain all the required
information to facilitate two-way packet delay and/or two-way
packet delay variation measurements from the viewpoint of the
originating MEP.
MIPs, as well as intermediate nodes, do not process the DM
information; they forward these on-demand DM OAM packets as regular
data packets.
6.5.1. Configuration Considerations
In order to support on-demand DM, the beginning and duration of the
DM procedures, the transmission rate and, for E-LSPs, the PHB
(associated with the DM OAM packets originating from a MEP) need to
be configured as part of the DM provisioning. DM OAM packets should
be transmitted with the PHB that yields the lowest drop precedence
within the measured PHB Scheduling Class (see RFC 3260 [17]).
In order to verify different performances between long and short
packets (e.g., due to the processing time), it should be possible for
the operator to configure the packet size of the on-demand OAM DM
packet.
7. OAM Functions for Administration Control
7.1. Lock Instruct
The Lock Instruct (LKI) function, as required in Section 2.2.6 of RFC
5860 [11], is a command allowing a MEP to instruct the peer MEP(s) to
put the MPLS-TP transport path into a locked condition.
This function allows single-side provisioning for administratively
locking (and unlocking) an MPLS-TP transport path.
Note that it is also possible to administratively lock (and unlock)
an MPLS-TP transport path using two-side provisioning, where the NMS
administratively puts both MEPs into an administrative lock
condition. In this case, the LKI function is not required/used.
Busi & Allan Informational [Page 55]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
MIPs, as well as intermediate nodes, do not process the Lock Instruct
information; they forward these on-demand LKI OAM packets as regular
data packets.
7.1.1. Locking a Transport Path
A MEP, upon receiving a single-side administrative lock command from
an NMS, sends an LKI request OAM packet to its peer MEP(s). It also
puts the MPLS-TP transport path into a locked state and notifies its
client (sub-)layer adaptation function upon the locked condition.
A MEP, upon receiving an LKI request from its peer MEP, can either
accept or reject the instruction and replies to the peer MEP with an
LKI reply OAM packet indicating whether or not it has accepted the
instruction. This requires either an in-band or out-of-band return
path. The LKI reply is needed to allow the MEP to properly report to
the NMS the actual result of the single-side administrative lock
command.
If the lock instruction has been accepted, it also puts the MPLS-TP
transport path into a locked state and notifies its client
(sub-)layer adaptation function upon the locked condition.
Note that if the client (sub-)layer is also MPLS-TP, Lock Report
(LKR) generation at the client MPLS-TP (sub-)layer is started, as
described in Section 5.4.
7.1.2. Unlocking a Transport Path
A MEP, upon receiving a single-side administrative unlock command
from NMS, sends an LKI removal request OAM packet to its peer MEP(s).
The peer MEP, upon receiving an LKI removal request, can either
accept or reject the removal instruction and replies with an LK
removal reply OAM packet indicating whether or not it has accepted
the instruction. The LKI removal reply is needed to allow the MEP to
properly report to the NMS the actual result of the single-side
administrative unlock command.
If the lock removal instruction has been accepted, it also clears the
locked condition on the MPLS-TP transport path and notifies its
client (sub-)layer adaptation function of this event.
The MEP that has initiated the LKI clear procedure, upon receiving a
positive LKI removal reply, also clears the locked condition on the
MPLS-TP transport path and notifies this event to its client
(sub-)layer adaptation function.
Busi & Allan Informational [Page 56]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Note that if the client (sub-)layer is also MPLS-TP, Lock Report
(LKR) generation at the client MPLS-TP (sub-)layer is terminated, as
described in Section 5.4.
8. Security Considerations
A number of security considerations are important in the context of
OAM applications.
OAM traffic can reveal sensitive information, such as performance
data and details, about the current state of the network. Insertion
or modification of OAM transactions can mask the true operational
state of the network, and in the case of transactions for
administration control, such as lock or data-plane loopback
instructions, these can be used for explicit denial-of-service
attacks. The effect of such attacks is mitigated only by the fact
that, for in-band messaging, the managed entities whose state can be
masked is limited to those that transit the point of malicious access
to the network internals due to the fate-sharing nature of OAM
messaging. This is not true when an out-of-band return path is
employed.
The sensitivity of OAM data therefore suggests that one solution is
that some form of authentication, authorization, and encryption is in
place. This will prevent unauthorized access to vital equipment, and
it will prevent third parties from learning about sensitive
information about the transport network. However, it should be
observed that the combination of the frequency of some OAM
transactions, the need for timeliness of OAM transaction exchange,
and all permutations of unique MEP to MEP, MEP to MIP, and
intermediate-system-originated transactions mitigates against the
practical establishment and maintenance of a large number of security
associations per MEG either in advance or as required.
For this reason, it is assumed that the internal links of the network
are physically secured from malicious access such that OAM
transactions scoped to fault and performance management of individual
MEGs are not encumbered with additional security. Further, it is
assumed in multi-provider cases where OAM transactions originate
outside of an individual provider's trusted domain that filtering
mechanisms or further encapsulation will need to constrain the
potential impact of malicious transactions. Mechanisms that the
framework does not specify might be subject to additional security
considerations.
In case of misconfiguration, some nodes can receive OAM packets that
they cannot recognize. In such a case, these OAM packets should be
silently discarded in order to avoid malfunctions whose effects may
Busi & Allan Informational [Page 57]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
be similar to malicious attacks (e.g., degraded performance or even
failure). Further considerations about data-plane attacks via G-ACh
are provided in RFC 5921 [8].
9. Acknowledgments
The authors would like to thank all members of the teams (the Joint
Working Team, the MPLS Interoperability Design Team in IETF, and the
Ad Hoc Group on MPLS-TP in ITU-T) involved in the definition and
specification of the MPLS Transport Profile.
The editors gratefully acknowledge the contributions of Adrian
Farrel, Yoshinori Koike, Luca Martini, Yuji Tochio, and Manuel Paul
for the definition of per-interface MIPs and MEPs.
The editors gratefully acknowledge the contributions of Malcolm
Betts, Yoshinori Koike, Xiao Min, and Maarten Vissers for the Lock
Report and Lock Instruct descriptions.
The authors would also like to thank Alessandro D'Alessandro, Loa
Andersson, Malcolm Betts, Dave Black, Stewart Bryant, Rui Costa,
Xuehui Dai, John Drake, Adrian Farrel, Dan Frost, Xia Liang, Liu
Gouman, Peng He, Russ Housley, Feng Huang, Su Hui, Yoshionori Koike,
Thomas Morin, George Swallow, Yuji Tochio, Curtis Villamizar, Maarten
Vissers, and Xuequin Wei for their comments and enhancements to the
text.
10. References
10.1. Normative References
[1] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.
[2] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation Edge-
to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[3] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire Virtual
Circuit Connectivity Verification (VCCV): A Control Channel for
Pseudowires", RFC 5085, December 2007.
[4] Bocci, M. and S. Bryant, "An Architecture for Multi-Segment
Pseudowire Emulation Edge-to-Edge", RFC 5659, October 2009.
[5] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
Sprecher, N., and S. Ueno, "Requirements of an MPLS Transport
Profile", RFC 5654, September 2009.
Busi & Allan Informational [Page 58]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
[6] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing in
Multi-Protocol Label Switching (MPLS) Networks", RFC 3443,
January 2003.
[7] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., "MPLS
Generic Associated Channel", RFC 5586, June 2009.
[8] Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau, L., and
L. Berger, "A Framework for MPLS in Transport Networks", RFC
5921, July 2010.
[9] Bocci, M., Levrau, L., and D. Frost, "MPLS Transport Profile
User-to-Network and Network-to-Network Interfaces", RFC 6215,
April 2011.
[10] Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS
Transport Profile Data Plane Architecture", RFC 5960, August
2010.
[11] Vigoureux, M., Ed., Ward, D., Ed., and M. Betts, Ed.,
"Requirements for Operations, Administration, and Maintenance
(OAM) in MPLS Transport Networks", RFC 5860, May 2010.
[12] Bradner, S. and J. McQuaid, "Benchmarking Methodology for
Network Interconnect Devices", RFC 2544, March 1999.
[13] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W.
Weiss, "An Architecture for Differentiated Service", RFC 2475,
December 1998.
[14] ITU-T Recommendation G.806 (01/09), "Characteristics of
transport equipment - Description methodology and generic
functionality", January 2009.
10.2. Informative References
[15] Sprecher, N. and L. Fang, "An Overview of the OAM Tool Set for
MPLS based Transport Networks", Work in Progress, June 2011.
[16] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of
the Differentiated Services Field (DS Field) in the IPv4 and
IPv6 Headers", RFC 2474, December 1998.
[17] Grossman, D., "New Terminology and Clarifications for Diffserv",
RFC 3260, April 2002.
[18] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling in MPLS
Traffic Engineering (TE)", RFC 4201, October 2005.
Busi & Allan Informational [Page 59]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
[19] ITU-T Recommendation G.707/Y.1322 (01/07), "Network node
interface for the synchronous digital hierarchy (SDH)", January
2007.
[20] ITU-T Recommendation G.805 (03/00), "Generic functional
architecture of transport networks", March 2000.
[21] ITU-T Recommendation Y.1731 (02/08), "OAM functions and
mechanisms for Ethernet based networks", February 2008.
[22] IEEE Standard 802.1AX-2008, "IEEE Standard for Local and
Metropolitan Area Networks - Link Aggregation", November 2008.
[23] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P.,
Krishnan, R., Cheval, P., and J. Heinanen, "Multi-Protocol Label
Switching (MPLS) Support of Differentiated Services", RFC 3270,
May 2002.
[24] Bocci, M., Swallow, G., and E. Gray, "MPLS Transport Profile
(MPLS-TP) Identifiers", RFC 6370, September 2011.
[25] Winter, R., Ed., van Helvoort, H., and M. Betts, "MPLS-TP
Identifiers Following ITU-T Conventions", Work in Progress, July
2011.
11. Contributing Authors
Ben Niven-Jenkins
Velocix
EMail: ben@niven-jenkins.co.uk
Annamaria Fulignoli
Ericsson
EMail: annamaria.fulignoli@ericsson.com
Enrique Hernandez-Valencia
Alcatel-Lucent
EMail: Enrique.Hernandez@alcatel-lucent.com
Busi & Allan Informational [Page 60]RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Lieven Levrau
Alcatel-Lucent
EMail: Lieven.Levrau@alcatel-lucent.com
Vincenzo Sestito
Alcatel-Lucent
EMail: Vincenzo.Sestito@alcatel-lucent.com
Nurit Sprecher
Nokia Siemens Networks
EMail: nurit.sprecher@nsn.com
Huub van Helvoort
Huawei Technologies
EMail: hhelvoort@huawei.com
Martin Vigoureux
Alcatel-Lucent
EMail: Martin.Vigoureux@alcatel-lucent.com
Yaacov Weingarten
Nokia Siemens Networks
EMail: yaacov.weingarten@nsn.com
Rolf Winter
NEC
EMail: Rolf.Winter@nw.neclab.eu
Busi & Allan Informational [Page 61]
RFC 6371 OAM Framework for MPLS-Based Transport September 2011
Authors' Addresses
Dave Allan
Ericsson
EMail: david.i.allan@ericsson.com
Italo Busi
Alcatel-Lucent
EMail: Italo.Busi@alcatel-lucent.com
Busi & Allan Informational [Page 62]
