Operations and Management Area Working Group T. Mizrahi
Internet Draft Marvell
Intended status: Informational July 12, 2010
Expires: January 2011
An Overview of
Operations, Administration, and Maintenance (OAM) Mechanisms
draft-ietf-opsawg-oam-overview-01.txt
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Abstract
Operations, Administration, and Maintenance (OAM) is a general term
that refers to detecting and reporting link failures. OAM mechanisms
have been defined for various layers in the protocol stack, and are
used with a variety of protocols.
This document presents an overview of the OAM mechanisms that have
been defined and are currently being defined by the IETF, as well as
a comparison to other OAM mechanisms that have been defined by the
IEEE and ITU-T.
Table of Contents
1. Introduction................................................4
2. Conventions used in this document............................8
3. Basic Terminology...........................................8
3.1. Abbreviations..........................................8
3.2. Terminology used in OAM Standards.......................9
3.2.1. General Terms......................................9
3.2.2. OAM Maintenance Entities...........................9
3.2.3. OAM Maintenance Points............................10
3.2.4. OAM Link Failures.................................10
3.2.5. Summary of OAM Terms used in the Standards.........10
4. OAM Functions..............................................12
4.1. ICMP Ping.............................................12
4.2. Bidirectional Forwarding Detection (BFD)...............12
4.2.1. Overview.........................................12
4.2.2. BFD Control.......................................12
4.2.3. BFD Echo.........................................13
4.3. LSP Ping..............................................13
4.4. PWE3 Virtual Circuit Connectivity Verification (VCCV)...13
4.5. IP Performance Metrics (IPPM)..........................14
4.5.1. Overview.........................................14
4.5.2. OWAMP/TWAMP Control...............................14
4.5.3. OWAMP/TWAMP Test..................................14
4.6. ITU-T Y.1711..........................................14
4.6.1. Overview.........................................14
4.6.2. Connectivity Verification (CV)....................15
4.6.3. Fast Failure Detection (FFD)......................15
4.6.4. Forward Defect Indication (FDI)...................15
4.6.5. Backward Defect Indication (BDI)..................15
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4.7. ITU-T Y.1731..........................................16
4.7.1. Overview.........................................16
4.7.2. ETH-CC...........................................16
4.7.3. ETH-LB...........................................17
4.7.4. ETH-TST..........................................17
4.7.5. ETH-LT...........................................17
4.7.6. ETH-AIS..........................................17
4.7.7. ETH-LCK..........................................17
4.7.8. ETH-RDI..........................................18
4.7.9. ETH-APS..........................................18
4.7.10. ETH-LM..........................................18
4.7.11. ETH-DM..........................................18
4.8. IEEE 802.1ag..........................................19
4.8.1. Overview.........................................19
4.8.2. Continuity Check..................................19
4.8.3. Loopback.........................................19
4.8.4. Linktrace........................................20
4.9. IEEE 802.3ah..........................................20
4.9.1. Overview.........................................20
4.9.2. Remote Failure Indication.........................20
4.9.3. Remote Loopback...................................20
4.9.4. Link Monitoring...................................20
4.10. MPLS-TP OAM..........................................20
4.10.1. Overview........................................20
4.10.2. Continuity Checks................................21
4.10.3. Connectivity Verification........................21
4.10.4. Diagnostic Tests.................................21
4.10.5. Route Tracing....................................22
4.10.6. Lock Instruct....................................22
4.10.7. Lock Reporting...................................22
4.10.8. Alarm Reporting..................................22
4.10.9. Remote Defect Indication.........................22
4.10.10. Client Failure Indication.......................22
4.10.11. Packet Loss Measurement.........................22
4.10.12. Packet Delay Measurement........................22
4.11. Summary of OAM Functions..............................22
4.12. Summary of Unidirectional Connectivity Check Mechanisms24
5. Security Considerations.....................................25
6. IANA Considerations........................................25
7. Acknowledgments............................................25
8. References.................................................25
8.1. Normative References...................................25
8.2. Informative References.................................28
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1. Introduction
OAM is a general term that refers to detecting and reporting link
failures and defects. The term OAM has been used over the years in
several different contexts, as discussed in [OAM Soup]. In the
context of this document OAM refers to Operations, Administration,
and Maintenance, i.e., this document refers to OAM in the context of
monitoring communication links. Other aspects associated with the OAM
acronym, such as management, are not described in this document.
OAM was originally used in the world of telephony, and has been
adopted in packet based networks. OAM mechanisms are used in various
layers in the protocol stack, and are applied to a variety of
different protocols.
The IETF has defined OAM for several protocols, and is currently
working on defining several new OAM protocols. A summary of these
protocols, old and new, is listed below:
o MPLS LSP Ping, as defined in [LSP Ping] is an OAM mechanism for
point to point MPLS LSPs. The IETF is currently working on an
extension to the LSP Ping for point to multipoint MPLS - [P2MP
Ping].
o Virtual Circuit Connectivity Check (VCCV) for Pseudowires, as
defined in [VCCV].
o ICMP Echo request, also known as Ping, as defined in [ICMPv4], and
[ICMPv6]. ICMP Ping is a very simple and basic mechanism in
failure diagnosis, and is not traditionally associated with OAM,
but it is presented in this document for the sake of completeness,
since both LSP Ping and VCCV are to some extent based on ICMP
Ping.
o Bidirectional Forwarding Detection (BFD) is a family of standards
that are currently being defined by the IETF. BFD is intended to
be a generic OAM mechanism that can be used with various
encapsulation types, and in various medium types.
o OAM for MPLS-TP is currently being defined in the MPLS working
group.
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o IP Performance Metrics (IPPM) is a working group in the IETF that
defined common metrics for performance measurement, as well as a
protocol for measuring delay and packet loss in IP networks.
While performance measurement is not directly related to link
failures, it is often associated with OAM. Alternative protocols
for performance measurement are defined, for example, in MPLS-TP
OAM [MPLS-TP OAM], and in Ethernet OAM [ITU-T Y.1731].
In addition to the OAM mechanisms defined by the IETF, the IEEE and
ITU-T have also defined various OAM mechanisms. These various
mechanisms defined by the three standard organizations are often
tightly coupled, and have had a mutual effect on each other. For
example, the emerging MPLS-TP OAM is in many ways based on [ITU-T
Y.1731]. The ITU-T and IETF have both defined OAM mechanisms for MPLS
LSPs, [ITU-T Y.1711] and [LSP Ping]. The following OAM standards by
the IEEE and ITU-T are to some extent linked to IETF OAM mechanisms
listed above, and are also discussed in this document:
o OAM mechanisms for Ethernet based networks have been defined by
both the ITU-T in [ITU-T Y.1731], and by the IEEE in [IEEE
802.1ag]. The IEEE 802.3 standard defines OAM for one-hop Ethernet
links [IEEE 802.3ah].
o The ITU-T has defined OAM for MPLS LSPs in [ITU-T Y.1711].
This document summarizes the OAM mechanisms defined in the standards
above. The focus is on OAM mechanisms defined by the IETF, compared
with the relevant OAM mechanisms defined by the ITU-T and IEEE. We
first present a comparison of the terminology used in various OAM
standards, and then summarize the OAM functions that each OAM
standard provides.
Table 1 summarizes the OAM standards discussed in this document.
+-----------+--------------------------------------+---------------+
| | Title |Standard |
+-----------+--------------------------------------+---------------+
|ICMPv4 Ping| Internet Control Message Protocol | RFC 792 |
| | | |
+-----------+--------------------------------------+---------------+
|ICMPv6 Ping| Internet Control Message Protocol | RFC 4443 |
| | (ICMPv6) for the Internet Protocol | |
| | Version 6 (IPv6) Specification | |
+-----------+--------------------------------------+---------------+
|BFD | Bidirectional Forwarding Detection | RFC 5880 |
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| +--------------------------------------+---------------+
| | Bidirectional Forwarding Detection | RFC 5881 |
| | (BFD) for IPv4 and IPv6 (Single Hop) | |
| +--------------------------------------+---------------+
| | Generic Application of Bidirectional | RFC 5882 |
| | Forwarding Detection | |
| +--------------------------------------+---------------+
| | Bidirectional Forwarding Detection | RFC 5883 |
| | (BFD) for Multihop Paths | |
| +--------------------------------------+---------------+
| | Bidirectional Forwarding Detection | RFC 5884 |
| | for MPLS Label Switched Paths (LSPs) | |
| +--------------------------------------+---------------+
| | Bidirectional Forwarding Detection | RFC 5885 |
| | for the Pseudowire Virtual Circuit | |
| | Connectivity Verification (VCCV) | |
+-----------+--------------------------------------+---------------+
|IETF MPLS | Operations and Management (OAM) | RFC 4377 |
|OAM | Requirements for Multi-Protocol Label| |
|(LSP Ping) | Switched (MPLS) Networks | |
| +--------------------------------------+---------------+
| | A Framework for Multi-Protocol | RFC 4378 |
| | Label Switching (MPLS) Operations | |
| | and Management (OAM) | |
| +--------------------------------------+---------------+
| | Detecting Multi-Protocol Label | RFC 4379 |
| | Switched (MPLS) Data Plane Failures | |
| +--------------------------------------+---------------+
| | Operations and Management (OAM) | RFC 4687 |
| | Requirements for Point-to-Multipoint | |
| | MPLS Networks | |
+-----------+--------------------------------------+---------------+
|PW VCCV | Pseudowire Virtual Circuit | RFC 5085 |
| | Connectivity Verification (VCCV): | |
| | A Control Channel for Pseudowires | |
+-----------+--------------------------------------+---------------+
|IPPM | Framework for IP Performance Metrics | RFC 2330 |
| +--------------------------------------+---------------+
| | IPPM Metrics for Measuring | RFC 2678 |
| | Connectivity | |
| +--------------------------------------+---------------+
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| | A One-way Delay Metric for IPPM | RFC 2679 |
| +--------------------------------------+---------------+
| | A One-way Packet Loss Metric for IPPM| RFC 2680 |
| +--------------------------------------+---------------+
| | A Round-trip Delay Metric for IPPM | RFC 2681 |
| +--------------------------------------+---------------+
| | A One-way Active Measurement Protocol| RFC 4656 |
| | (OWAMP) | |
| +--------------------------------------+---------------+
| | A Two-Way Active Measurement Protocol| RFC 5357 |
| | (TWAMP) | |
+-----------+--------------------------------------+---------------+
|ITU-T | Operation & Maintenance mechanism |[ITU-T Y.1711] |
|MPLS OAM | for MPLS networks | |
| +--------------------------------------+---------------+
| | Assignment of the 'OAM Alert Label' | RFC 3429 |
| | for Multiprotocol Label Switching | |
| | Architecture (MPLS) Operation and | |
| | Maintenance (OAM) Functions | |
+-----------+--------------------------------------+---------------+
|ITU-T | OAM Functions and Mechanisms for |[ITU-T Y.1731] |
|Ethernet | Ethernet-based Networks | |
|OAM | | |
+-----------+--------------------------------------+---------------+
|MPLS-TP | Requirements for OAM in MPLS | RFC 5860 |
|OAM +--------------------------------------+---------------+
| | MPLS Generic Associated Channel | RFC 5586 |
+-----------+--------------------------------------+---------------+
|IEEE | Connectivity Fault Management |[IEEE 802.1ag] |
|CFM | | |
+-----------+--------------------------------------+---------------+
|IEEE | Media Access Control Parameters, |[IEEE 802.3ah] |
|802.3 | Physical Layers, and Management | |
|link level | Parameters for Subscriber Access | |
|OAM | Networks | |
+-----------+--------------------------------------+---------------+
Table 1 Summary of OAM Standards
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2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
3. Basic Terminology
3.1. Abbreviations
AIS Alarm Indication Signal
APS Automatic Protection Switching
BDI Backward Defect Indication
BFD Bidirectional Forwarding Detection
CC Continuity Check
CCM Continuity Check Message
CV Connectivity Verification
DM Delay Measurement
DTE Data Terminal Equipment
FDI Forward Defect Indication
FFD Fast Failure Detection
ICMP Internet Control Message Protocol
L2TP Layer Two Tunneling Protocol
LCCE L2TP Control Connection Endpoint
LM Loss Measurement
LSP Label Switching Path
LSR Label Switching Router
MA Maintenance Association
ME Maintenance Entity
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MEG Maintenance Entity Group
MEP Maintenance End Point
MIP Maintenance Intermediate Point
MP Maintenance Point
MPLS Multiprotocol Label Switching
MPLS-TP MPLS Transport Profile
OAM Operations, Administration, and Maintenance
PE Provider Edge
PW Pseudowire
PWE3 Pseudowire Emulation Edge-to-Edge
RDI Remote Defect Indication
TTSI Trail Termination Source Identifier
VCCV Virtual Circuit Connectivity Verification
3.2. Terminology used in OAM Standards
3.2.1. General Terms
A wide variety of terms is used in various OAM standards. Each of the
OAM standards listed in the reference section includes a section that
defines the relevant terms. A thesaurus of terminology for MPLS-TP
terms is presented in [MPLS-TP Term], and provides a good summary of
some of the OAM related terminology.
This section presents a comparison of the terms used in various OAM
standards, without fully quoting the definition of each term. For a
formal definition of each term, refer to the references at the end of
this document. The comparison focuses on three basic terms, and is
summarized in section 3 ..2.5.
3.2.2. OAM Maintenance Entities
A Maintenance Entity (ME) can be either a point-to-point or a point-
to-multipoint relationship between two or more Maintenance Points.
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The connectivity between these Maintenance Points is mangaged and
monitored by the OAM protocol.
The term Maintenance Entity (ME) is defined in ITU-T standards (e.g.
[ITU-T Y.1731]). Various terms are used to refer to an ME. For
example, in MPLS terminology, an ME is simply referred to as an LSP.
BFD does not explicitly use a term that is equivalent to ME, but
rather uses the term "session", referring to the relationship between
two nodes using a BFD protocol.
3.2.3. OAM Maintenance Points
A Maintenance Point (MP) is a node that uses an OAM protocol. A
Maintenance End Point (MEP) is one of the end points of an ME. A
Maintenance Intermediate Point (MIP) is a point between two MEPs,
that is able to respond to OAM frames, but does not initiate them.
The terms MEP and MIP are defined in ITU-T standards (e.g. [ITU-T
Y.1731]). The term Maintenance Point is a general term for MEPs and
MIPs, and is used in [IEEE 802.1ag].
3.2.4. OAM Link Failures
The terms Failure, Fault, and Defect are intermittently used in the
standards. In some standards, such as [IEEE 802.1ag], there is no
distinction between these terms, while in other standards each of
these terms refers to a different type of malfunction.
The ITU-T distinguishes between these terms in [ITU-T G.806]. The
term Fault refers to an inability to perform a required action, e.g.,
an unsuccessful attempt to deliver a packet. The term Defect refers
to an interruption in the normal operation, such as a consecutive
period of time where no packets are delivered successfully. The term
Failure refers to the termination of the required function. While a
Defect typically refers to a limited period of time, a failure refers
to a long period of time.
3.2.5. Summary of OAM Terms used in the Standards
Table 2 provides a comparison of the terminology used in different
OAM standards.
+-----------+-------------+-----------+----------------------------+
| |Maintenance |Maintenance|Link Failure Terminology |
| |Point |Entity | |
| |Terminology |Terminology| |
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+-----------+-------------+-----------+----------------------------+
|ICMPv4 Ping|-Host | | |
| |-Gateway | | |
+ --------- + ----------- + --------- + -------------------------- +
|ICMPv6 Ping| Node | | |
+ --------- + ----------- + --------- + -------------------------- +
|BFD | System | Session |-Failure |
| | | |-Session is declared down |
+ --------- + ----------- + --------- + -------------------------- +
|LSP Ping | LSR | LSP |-Failure |
| | | |-Fault = typically a local |
| | | | isolated failure |
+ --------- + ----------- + --------- + -------------------------- +
|PW VCCV |-PE | PW |-Failure |
| |-LCCE | |-Fault |
+ --------- + ----------- + --------- + -------------------------- +
|IPPM |-Host |-Path | Connectivity is indicated |
| |-End system |-Measuremen| by a Boolean value. Thus, |
| | | t session | a failure is referred to as|
| | | | a path with a measurement |
| | | | value "false". |
+ --------- + ----------- + --------- + -------------------------- +
|ITU-T | LSR | LSP |-Fault, Defect, Failure: as |
|Y.1711 | | | defined in [ITU-T G.806] |
+ --------- + ----------- + --------- + -------------------------- +
|ITU-T |-MEP | ME |-Fault, Defect, Failure: as |
|Y.1731 |-MIP | | defined in [ITU-T G.806] |
| | | | |
+ --------- + ----------- + --------- + -------------------------- +
|MPLS-TP |-End Point |-LSP |-Fault, Defect, Failure: as |
|OAM |-Intermediate|-PW | defined in [ITU-T G.806] |
| |Point |-Section | |
+ --------- + ----------- + --------- + -------------------------- +
|IEEE |-MEP | ME |-Failure |
|802.1ag |-MIP | |-Fault |
| |-MP | |-Defect |
+ --------- + ----------- + --------- + -------------------------- +
|IEEE | DTE | Link |-Failure |
|802.3ah | | |-Fault |
+-----------+-------------+-----------+----------------------------+
Table 2 Summary of OAM Terms
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4. OAM Functions
4.1. ICMP Ping
ICMP provides a bidirectional connectivity check for the Internet
Protocol. The originator transmits an echo request packet, and the
receiver replies with an echo reply. ICMP ping is defined in two
variants, [ICMPv4] is used for IPv4, and [ICMPv6] is used for IPv6.
4.2. Bidirectional Forwarding Detection (BFD)
4.2.1. Overview
While multiple OAM mechanisms have been defined for various protocols
in the protocol stack, Bidirectional Forwarding Detection [BFD],
defined by the IETF BFD working group, is a generic OAM mechanism
that can be deployed over various encapsulating protocols, and in
various medium types. The IETF has defined variants of the protocol
for IP ([BFD IP], [BFD Multi]), for MPLS LSPs [BFD LSP], and for PWE3
[BFD VCCV]. BFD for MPLS-TP is currently evolving in the MPLS working
group (e.g. [MPLS-TP Ping BFD]).
BFD includes two main OAM functions, using two types of BFD packets:
BFD Control packets, and BFD Echo packets.
4.2.2. BFD Control
BFD supports a unidirectional connectivity check, using BFD control
packets. BFD control packets are be sent in one of two modes:
o Asynchronous mode: in this mode BFD control packets are sent
periodically. When the receiver detects that no BFD control packet
have been received during a predetermined period of time, a
failure is detected.
o Demand mode: in this mode, BFD control packets are sent on-demand.
Upon need, a system initiates a series of BFD control packets to
verify the link. BFD control packets are sent independently in
each direction of the link.
The transmission interval of BFD packets that are sent periodically,
is a result of negotiation between the two systems. Each BFD Control
packet includes the desired transmission interval, and the desired
reception interval, allowing the two systems to agree on common
intervals.
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If no BFD Control packets are received during a fixed period of time
called the Detection Time, the session is declared to be down. The
detection time is a function of the negotiated transmission time, and
a parameter called Detect Mult. Detect Mult determines the number of
missing BFD Control packets that cause the session to be declared as
down. This parameter is included in the BFD Control packet.
The BFD Control packet also includes two fields that specify the
transmitting and receiving systems, called My Discriminator and Your
Discriminator, respectively.
4.2.3. BFD Echo
The echo function is a bidirectional connectivity check. A BFD echo
packet is sent to a peer system, and is looped back to the
originator. The echo function can be used proactively, or on-demand.
4.3. LSP Ping
The IETF MPLS working group has defined OAM for MPLS LSPs. The
requirements and framework of this effort was defined in [MPLS OAM
FW] and [MPLS OAM], respectively. The corresponding OAM mechanism
that was defined in this context is LSP Ping [LSP Ping]. LSP ping is
used to detect data plain failures in MPLS LSPs. The transmitting LSR
sends an echo request to a remote LSR, and in turn receives an echo
reply. LSP ping is used in one of two modes:
o "Ping" mode: In this mode LSP ping is used for end-to-end
connectivity verification between two LSRs.
o "Traceroute" mode: This mode is used for hop-by-hop fault
localization.
4.4. PWE3 Virtual Circuit Connectivity Verification (VCCV)
VCCV, as defined in [VCCV], maintains the connectivity status of a
pseudowire. VCCV is supported for both MPLS PWs and L2TPv3 PWs.
VCCV supports two possible Connectivity Verification (CV) types,
i.e., two modes of operation:
o ICMP Ping: In this mode the CV is performed using an ICMP ping
packet format, as defined in [ICMPv4] or [ICMPv6].
o LSP Ping: In this mode the LSP Ping packet format, as defined in
[LSP Ping] is used for CV.
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4.5. IP Performance Metrics (IPPM)
4.5.1. Overview
The IPPM working group [IPPM FW] in the IETF defines common criteria
and metrics for measuring performance of IP traffic. Some of the key
RFCs published by this working group have defined metrics for
measuring connectivity [rfc2678], delay [RFC2679, RFC 2681], and
packet loss [RFC2681].
The IPPM working group has defined not only metrics for performance
measurement, but also protocols that define how the measurement is
carried out. The One-way Active Measurement Protocol [OWAMP] and the
Two-Way Active Measurement Protocol [TWAMP] define a method and
protocol for measuring delay and packet loss in IP networks.
OWAMP and TWAMP use two separate protocols: a Control plane protocol,
and a Test plane protocol.
4.5.2. OWAMP/TWAMP Control
Each of these standards defines a Control protocol. This protocol is
layered over TCP, and is used to initiate measurement sessions, and
to communicate their results.
4.5.3. OWAMP/TWAMP Test
The Test protocol is layered over UDP, and is used to measure delay
and packet loss between the session endpoints. The Test session is
initiated by a Request/Response negotiation, followed by a set of
active test packets that are used for the measurement.
4.6. ITU-T Y.1711
4.6.1. Overview
As mentioned above (4.3.), the IETF defined LSP Ping as an OAM
mechanism for MPLS. The ITU-T has also defined an OAM protocol for
MPLS, defined in [ITU-T Y.1711]. The standard defines mechanisms for
connectivity verification and fast failure detection, as well as
mechanism for reporting defects that have been identified in an LSP.
MPLS OAM packets per Y.1711 are detected by a reserved MPLS label
value. The reserved value is 14, and is defined in [OAM Label] as the
'OAM Alert Label'.
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4.6.2. Connectivity Verification (CV)
The CV function is used to detect connectivity defects in an LSP. CV
frames are sent proactively at a rate of 1 per second. Each frame
contains the Trail-Termination Source Identifier (TTSI), indicating
the identity of the transmitting LSR.
The CV function can detect any of the following defect conditions.
o Loss of Connectivity Verification (LOCV): A loss of connectivity
is detected when no CV OAM packets are received in a period of 3
consecutive transmission periods.
o TTSI Mismatch: A TTSI mismatch is detected when a CV frame with an
unexpected TTSI is received.
o TTSI Mismerge: A TTSI mismerge is detected when the CV frames
received in a given LSP contain some frame with an expected TTSI,
and some frames with an unexpected TTSI.
o Excess: An excess is detected when at least 5 CV frames are
received during a period of 3 consecutive transmission periods.
4.6.3. Fast Failure Detection (FFD)
The FFD function is a proactive function, used for fast detection of
connectivity defects. While CV is typically sufficient for path
failure detection and reporting, protection switching mechanisms
typically require faster detection. FFD is very similar to CV in
terms of the packet format, and the possible defect conditions, but
FFD allows a configurable transmission frequency. The default
transmission rate of FFD frames is 20 per second, i.e., every 50 ms,
allowing fast detection for protection switching applications.
4.6.4. Forward Defect Indication (FDI)
The FDI function is used by an LSR to report a defect to affected
client layers, allowing them to suppress alarms about this defect. An
FDI packets are sent at a rate of 1 per second.
4.6.5. Backward Defect Indication (BDI)
The BDI function is used to inform the LSR at an LSP trail
termination source point about a defect condition in the forward
direction of an LSP. The LSR at the LSP trail termination sink point
transmits the BDI to the upstream LSR through the return path. BDI
packets are sent at the same transmission rate as FDI.
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4.7. ITU-T Y.1731
4.7.1. Overview
The [ITU-T Y.1731] is a protocol for Ethernet OAM. It is presented in
this document as a reference point, since the OAM mechanisms that are
currently being defined by the IETF for MPLS-TP are in many ways
based on this standard. The standard defines various OAM functions,
including unidirectional and bidirectional continuity check, and
functions for performance monitoring.
4.7.2. ETH-CC
The Ethernet Continuity Check function is a proactive function that
allows a MEP to detect loss of continuity with any of the other MEPs
in the MEG. This function also allows detection of other defect
conditions, such as unintended connectivity between two MEGs. The
ETH-CC function is used for one of three possible applications: fault
management, performance monitoring (see 4.6.10.), and protection
switching.
Continuity Check Messages (CCM) are transmitted periodically at a
constant rate. There are 7 possible transmission periods, from 3.33
ms to 10 min. When the ETH-CC function detects a defect, it reports
one of the following defect conditions:
o Loss of continuity (LOC): Occurs when at least when no CCM
messages have been received from a peer MEP during a period of 3.5
times the configured transmission period.
o Unexpected MEG level: The MEG level is a 3-bit number that defines
the level of hierarchy of the MEG. This defect condition occurs
when a CCM is received from a peer MEP with a MEG level that is
lower than the expected MEG level.
o Mismerge: Occurs when a CCM is received from a peer MEP with an
unexpected MEG ID.
o Unexpected MEP: Occurs when a CCM is received from a peer MEP with
an unexpected transmitting MEP ID.
o Unexpected period: Occurs when the transmission period field in
the CCM does not match the expected transmission period value.
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4.7.3. ETH-LB
The Ethernet loopback function verifies connectivity with a peer MEP
or MIP. The loopback function is performed on-demand, by sending a
loopback message (LBM) to the peer MEP or MIP. The peer node then
responds with a loopback reply (LBR).
More precisely, it is used for one of two purposes:
o Bidirectional connectivity test.
o Bidirectional in-service / out-of-service test. The in-service
mode refers to a test that is run under traffic, while the out-of-
service test requires other traffic to be halted.
4.7.4. ETH-TST
The test function is very similar to the loopback function, but is
unidirectional, i.e., the ETH-TST PDUs are terminated by the receiver
rather than being looped back to the sender.
4.7.5. ETH-LT
The Ethernet linktrace is an on-demand function that is used for path
discovery to a given target, or for locating a failure in a broken
path.
4.7.6. ETH-AIS
The Alarm Indication Signal indicates that a MEG should suppress
alarms about a defect condition at a lower MEG level, i.e., since a
defect has occurred in a lower hierarchy in the network, it should
not be reported by the current node.
A MEP that detects a failure periodically sends AIS messages to
higher hierarchies. AIS messages are sent periodically at a
recommended rate of 1 packet per second, until the defect condition
is resolved.
4.7.7. ETH-LCK
The lock function is used for administrative locking. A MEP can
initiate administrative locking, resulting in interruption of data,
e.g., for out-of-service ETH-LB or ETH-TST.
A MEP that initiates an administrative locking notifies its peer MEPs
to halt all relevant traffic until administrative/diagnostic
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condition is removed. ETH-LCK frames are used to report to higher MEG
levels about the lock. The LCK frame, much like an AIS frame,
indicates to the receiving MEP that it should suppress alarms about
the locked link.
4.7.8. ETH-RDI
The Remote Defect Indication allows the sender to indicate that it
encountered a defect conditions. The receiving MEPs are then aware
that there is a defect condition in the MEG.
4.7.9. ETH-APS
The Y.1731 standard defines the frame format for Automatic Protection
Switching frames. The protection switching operations are defined in
other ITU-T standards.
4.7.10. ETH-LM
The loss measurement function allows a MEP to measure the packet loss
rate from/to a given MEP in the MEG. Each MEP maintains counters of
transmitted and received in-profile packets to/from each of its peer
MEPs. These counters are incorporated in the ETH-LM frames, allowing
the MEPs to compute the packet loss rate.
The ETH-LM function measures the far-end loss, referring to traffic
FROM the MEP to its peer, as well as the near-end loss, referring to
traffic from the peer MEP TO the local MEP.
ETH-LM is performed in one of two possible modes:
o Single-ended LM: in this mode loss measurement is performed on-
demand. The initiator sends an LM message (LMM) to its peer MEP,
and the peer responds with an LM reply (LMR).
o Dual-ended LM: in this mode loss measurement is performed
proactively. The continuity check message (CCM) is used for
proactive LM. The LM counters are piggy-backed into the CCM, and
allow proactive loss measurement.
4.7.11. ETH-DM
The delay measurement function is an on-demand function that allows a
MEP to measure the frame delay and frame delay variation to a peer
MEP.
ETH-DM can be performed in one of two modes of operation:
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o One-way DM: in this mode, a MEP transmits a 1DM frame containing
the time of its transmission, TxTimeStampf. The receiving MEP
receives the 1DM frame and records the time of reception, RxTimef.
The receiving MEP can then compute the one-way delay by: RxTimef -
TxTimeStampf.
o Two-way DM: in this mode, a MEP transmits a delay measurement
message (DMM) containing its transmission time, TxTimeStampf. The
peer MEP receives the DMM and responds with a delay measurement
reply (DMR). Upon receiving the DMR, the initiating MEP records
the time of its reception, RxTimef, and computes the round trip
delay by: RxTimef - TxTimeStampf.
Each MEP maintains a time-of-day clock that is used for timestamping
delay measurement frames. It should be noted that in one-way DM it is
implicitly assumed that the clocks of the two peer MEPs are
synchronized by a time synchronization protocol.
4.8. IEEE 802.1ag
4.8.1. Overview
While the [ITU-T Y.1731] was defined in the ITU-T, the IEEE defined
the [IEEE 802.1ag] as a standard for connectivity fault management in
Ethernet based networks. While the two standards are to some extent
overlapping, they can also be viewed as two complementary parts of a
single Ethernet OAM picture. The two standards use a common packet
format. There are a few differences between the two standards in
terms of terminology: the term MEG level, used in Y.1731, as referred
to as Maintenance Domain level in 802.1ag; the Y.1731 standard uses
the term MEG, while the 802.1ag equivalent is Maintenance Association
(MA).
While Y.1731 defines multiple OAM functions (see section 4.6), the
802.1ag standard focuses on three main OAM functions: continuity
check, loopback, and linktrace, and defines them with great detail.
4.8.2. Continuity Check
See 4.6.2.
4.8.3. Loopback
See 4.6.3.
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4.8.4. Linktrace
See 4.6.5.
4.9. IEEE 802.3ah
4.9.1. Overview
The [IEEE 802.3ah] defines an Ethernet link-layer OAM, for single-hop
Ethernet links. The OAM functions in this standard are described
below.
4.9.2. Remote Failure Indication
This function allows a node to notify a peer about a defect in the
receive path. Some physical interfaces allow unidirectional traffic,
where even if one direction of the link fails, the reverse direction
can still be used to convey the remote failure indication.
4.9.3. Remote Loopback
The remote loopback function provides a diagnostic mode that is used
to verify the link connectivity, and to measure the packet loss rate.
When a bridge interface is configured to loopback mode, all incoming
traffic through the interface is looped and sent back to the
originator.
4.9.4. Link Monitoring
Link monitoring provides an event notification function, allowing
peer devices to communicate defect conditions and diagnostic
information.
4.10. MPLS-TP OAM
4.10.1. Overview
The MPLS working group is currently working on defining the OAM
requirements and mechanisms for MPLS-TP. The requirements of MPLS-TP
OAM are defined in [MPLS-TP OAM], and are described below.
MPLS-TP OAM traffic uses a Generic Associated Channel (G-ACh),
defined in [G-ACh]. This standard defines that MPLS-TP OAM traffic
uses:
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o An Associated Channel Header (ACH), also known as a Control Word
in the PWE3 terminology, is a 4-byte header that is added to OAM
packets.
o A Generic Associated Label (GAL). The GAL is a reserved MPLS label
value. The reserved value is 13, and identifies the packet as an
MPLS-TP OAM packet. A GAL indicates the existence of the ACH
immediately after it.
The analysis in [OAM Analysis] discusses various OAM mechanism that
were considered in order to satisfy the requirements in [MPLS-TP
OAM]. The MPLS working group currently plans to use a mixture of OAM
mechanisms that are based on various existing standards, and adapt
them to the requirements of [MPLS-TP OAM]. Some of the main building
blocks of this solution are based on:
o Bidirectional Forwarding Detection ([BFD], [BFD LSP]) for
proactive connectivity verification.
o LSP Ping as defined in [LSP Ping] for on-demand connectivity
verification.
o Y.1731 per the [ITU-T Y.1731], mainly for performance measurement.
The requirements of MPLS-TP OAM are summarized below.
4.10.2. Continuity Checks
The continuity check is a proactive function that allows an End Point
to determine whether or not it receives traffic from its peer End
Points.
4.10.3. Connectivity Verification
The connectivity verification is a function that allows an End Point
to verify its connectivity to a peer node. The connectivity check is
performed by sending a connectivity verification PDU to the peer
node, and receiving a reply within an expected time frame. This
function can be performed proactively or on-demand.
4.10.4. Diagnostic Tests
This function allows an End Point to perform an on-demand test, e.g.,
for bandwidth measurement.
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4.10.5. Route Tracing
This on-demand function is used for path discovery and for locating
link failures.
4.10.6. Lock Instruct
The lock instruct function allows an End Point to instruct its peers
to enter an administrative status where all traffic is halted except
the test traffic and OAM PDUs.
4.10.7. Lock Reporting
This function allows an Intermediate Point to report to an End Point
about a lock condition.
4.10.8. Alarm Reporting
This function allows an Intermediate Point to report to an End Point
about a defect condition.
4.10.9. Remote Defect Indication
This is a proactive function that allows the sender to indicate that
it encountered a defect conditions.
4.10.10. Client Failure Indication
This function allows the MPLS-TP network to relay information about a
fault condition in a client network, allowing the failure indication
to propagate from end to end over the MPLS-TP network.
4.10.11. Packet Loss Measurement
This function measures the packet loss ratio between two peer End
Points. It can be performed proactively or on-demand.
4.10.12. Packet Delay Measurement
This function measures the frame delay between two peer End Points.
Two modes of operation are supported, one-way DM, and two-way DM.
4.11. Summary of OAM Functions
Table 3 summarizes the OAM functions that are supported in each of
the standards that were analyzed in this section.
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+-----------+-------+--------+--------+-----------+-------+--------+
| Standard |Unidire|Bidirect|Path |Defect |Perform|Other |
| |ctional|ional |Discover|Indications|ance |Function|
| |Connect|Connecti|y | |Monitor|s |
| |ivity |vity | | |ing | |
| |Check |Check | | | | |
+-----------+-------+--------+--------+-----------+-------+--------+
|ICMP Ping | | Echo | | | | |
+ --------- + ----- + ------ + ------ + --------- + ----- + ------ +
|BFD |BFD |BFD | | | | |
| |Control|Echo | | | | |
+ --------- + ----- + ------ + ------ + --------- + ----- + ------ +
|LSP Ping | |"Ping" |"Tracero| | | |
| | |mode |ute" | | | |
| | | |mode | | | |
+ --------- + ----- + ------ + ------ + --------- + ----- + ------ +
|PW VCCV | |VCCV | | | | |
+ --------- + ----- + ------ + ------ + --------- + ----- + ------ +
|IPPM | | | | |-Delay | |
| | | | | | measur| |
| | | | | | ement | |
| | | | | |-Packet| |
| | | | | | loss | |
| | | | | | measur| |
| | | | | | ement | |
+ --------- + ----- + ------ + ------ + --------- + ----- + ------ +
|ITU-T |-CV | | | | | |
|Y.1711 |-FFD | | | | | |
+ --------- + ----- + ------ + ------ + --------- + ----- + ------ +
|ITU-T |ETH-CC |ETH-LB |ETH-LT |-ETH-RDI |-ETH-LM|-ETH-LCK|
|Y.1731 | | | |-ETH-AIS |-ETH-DM|-ETH-APS|
| | | | | | |-ETH-TST|
+ --------- + ----- + ------ + ------ + --------- + ----- + ------ +
|IEEE |CC |Loopback|Linktrac| | | |
|802.1ag | | |e | | | |
+ --------- + ----- + ------ + ------ + --------- + ----- + ------ +
|IEEE | |Remote | |-Remote | | |
|802.3ah | |Loopback| | Failure | | |
| | | | | Indication| | |
| | | | |-Link | | |
| | | | | Monitoring| | |
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+ --------- + ----- + ------ + ------ + --------- + ----- + ------ +
|MPLS-TP |CC |CV |Route |-Alarm |-LM |-Diagnos|
|OAM | | |Tracing | Reporting |-DM | tic Tes|
| | | | |-Client | | s |
| | | | | Failure | |-Lock |
| | | | | Indication| | |
| | | | |-Remote | | |
| | | | | Defect | | |
| | | | | Indication| | |
+-----------+-------+--------+--------+-----------+-------+--------+
Table 3 Summary of OAM Functions
4.12. Summary of Unidirectional Connectivity Check Mechanisms
A key element in some of the OAM standards that are analyzed in this
document is the unidirectional connectivity check. It is thus
interesting to present a more detailed comparison of the connectivity
check mechanisms defined in OAM standards. Table 4 can be viewed as
an extension of Table 3, but is presented separately for convenience.
The table compares the OAM standards that support a unidirectional
connectivity check. MPLS-TP is not included in the comparison, as the
continuity check mechanism in MPLS-TP has not yet been defined.
The "Tx Interval" column in the table specifies the period between
two consequent message transmissions, while the "Source Identifier"
column specifies the name of the field in the OAM packet that is used
as the identifier of the transmitter. The "Error Codes" column
specifies the possible error codes when the unidirectional
connectivity check detects a failure.
+-----------+-------+--------+---+--------+------------------------+
| |Mechani|Tx |UC/|Source | Error |
| |sm |Interval|MC |Identifi| Codes |
| | | | |er | |
+-----------+-------+--------+---+--------+------------------------+
|BFD |BFD |Negotiat|UC |My Discr| Control Detection Time |
| |Control|ed durin| |iminator| Expired |
| | |g sessio| | | |
| | |n | | | |
+ --------- + ----- + ------ + - + ------ + ---------------------- +
|ITU-T |CV |CV: 1s |UC |TTSI |-Loss of CV (LOCV) |
|Y.1711 |FFD |FFD: par| | |-TTSI Mismatch |
| | |ameter, | | |-TTSI Mismerge |
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| | |default:| | |-Excess |
| | |50 ms | | | |
+ --------- + ----- + ------ + - + ------ + ---------------------- +
|ITU-T |CC |7 possib|UC/|MEP ID |-Loss of Continuity(LOC)|
|Y.1731 / | |le perio|MC | |-Unexpected MEG level |
|IEEE | |ds: | | |-Mismerge |
|802.1ag | |3 1/3 ms| | |-Unexpected MEP |
| | |10 ms | | |-Unexpected period |
| | |100 ms | | | |
| | |1 s | | | |
| | |10 s | | | |
| | |1 min | | | |
| | |10 min | | | |
+-----------+-------+--------+---+--------+------------------------+
Table 4 Summary of OAM Terms
5. Security Considerations
There are no security implications imposed by this document.
6. IANA Considerations
There are no new IANA considerations implied by this document.
7. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
8. References
8.1. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LSP Ping] Kompella, K., Swallow, G., "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.
[MPLS OAM] Nadeau, T., Morrow, M., Swallow, G., Allan, D., and
Matsushima, S., "Operations and Management (OAM)
Requirements for Multi-Protocol Label Switched (MPLS)
Networks", RFC 4377, February 2006.
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[MPLS OAM FW] Allan, D., Nadeau, T., "A Framework for Multi-Protocol
Label Switching (MPLS) Operations and Management
(OAM)", RFC 4378, February 2006.
[MPLS OAM P2MP] Yasukawa, S., Farrel, A., King, D., and Nadeau, T.,
"Operations and Management (OAM) Requirements for
Point-to-Multipoint MPLS Networks", RFC 4687,
September 2006.
[OAM Label] Ohta, H., "Assignment of the 'OAM Alert Label' for
Multiprotocol Label Switching Architecture (MPLS)
Operation and Maintenance (OAM) Functions", RFC 3429,
November 2002.
[MPLS-TP OAM] Vigoureux, M., Ward, D., Betts, M., "Requirements for
OAM in MPLS Transport Networks", RFC 5860, May 2010.
[G-ACh] Bocci, M., Vigoureux, M., Bryant, S., "MPLS Generic
Associated Channel", RFC 5586, June 2009.
[VCCV] Nadeau, T., Pignataro, C., "Pseudowire Virtual Circuit
Connectivity Verification (VCCV): A Control Channel
for Pseudowires", RFC 5085, December 2007.
[ICMPv4] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[ICMPv6] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[IPPM FW] Paxson, V., Almes, G., Mahdavi, J., and Mathis, M.,
"Framework for IP Performance Metrics", RFC 2330, May
1998.
[IPPM Con] Mahdavi, J., Paxson, V., "IPPM Metrics for Measuring
Connectivity", RFC 2678, September 1999.
[IPPM 1DM] Almes, G., Kalidindi, S., Zekauskas, M., "A One-way
Delay Metric for IPPM", RFC 2679, September 1999.
[IPPM 1LM] Almes, G., Kalidindi, S., Zekauskas, M., "A One-way
Packet Loss Metric for IPPM", RFC 2680, September
1999.
[IPPM 2DM] Almes, G., Kalidindi, S., Zekauskas, M., "A Round-trip
Delay Metric for IPPM", RFC 2681, September 1999.
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[OWAMP] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and
Zekauskas, M., "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.
[TWAMP] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and
Babiarz, J., "A Two-Way Active Measurement Protocol
(TWAMP)", RFC 5357, October 2008.
[BFD] Katz, D., Ward, D., "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[BFD IP] Katz, D., Ward, D., "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
2010.
[BFD Gen] Katz, D., Ward, D., "Generic Application of
Bidirectional Forwarding Detection (BFD)", RFC 5882,
June 2010.
[BFD Multi] Katz, D., Ward, D., "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, June 2010.
[BFD LSP] Aggarwal, R., Kompella, K., Nadeau, T., and Swallow,
G., "Bidirectional Forwarding Detection (BFD) for MPLS
Label Switched Paths (LSPs)", RFC 5884, June 2010.
[BFD VCCV] Nadeau, T., Pignataro, C., "Bidirectional Forwarding
Detection (BFD) for the Pseudowire Virtual Circuit
Connectivity Verification (VCCV)", RFC 5885, June
2010.
[IEEE 802.1ag]"Connectivity Fault Management", December 2007.
[ITU-T Y.1731]"OAM Functions and Mechanisms for Ethernet-based
Networks", February 2008.
[ITU-T Y.1711]"Operation & Maintenance mechanism for MPLS networks",
February 2004.
[IEEE 802.3ah]"Media Access Control Parameters, Physical Layers, and
Management Parameters for Subscriber Access Networks",
clause 57, September 2004.
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8.2. Informative References
[P2MP Ping] Saxena, S., Farrel, A. , Yasukawa, S., "Detecting Data
Plane Failures in Point-to-Multipoint Multiprotocol
Label Switching (MPLS) - Extensions to LSP Ping",
draft-ietf-mpls-p2mp-lsp-ping, March 2010.
[OAM Soup] Andersson, L., Van Helvoort, H., Bonica, R., Romascanu,
D., Mansfield, S., "The OAM Acronym Soup", draft-ietf-
opsawg-mpls-tp-oam-def, June 2010.
[ITU-T G.806] "Characteristics of transport equipment - Description
methodology and generic functionality", January 2009.
[MPLS-TP Term]Van Helvoort, H., Andersson, L., Sprecher, N., "A
Thesaurus for the Terminology used in Multiprotocol
Label Switching Transport Profile (MPLS-TP)
drafts/RFCs and ITU-T's Transport Network
Recommendations", draft-ietf-mpls-tp-rosetta-stone,
May 2010.
[MPLS-TP Ping BFD] Bahadur, N., Aggarwal, R., Ward, D., Nadeau, T.,
Sprecher, N., Weingarten, Y., "LSP-Ping and BFD
encapsulation over ACH", draft-ietf-mpls-tp-lsp-ping-
bfd-procedures, March 2010.
[OAM Analysis] Sprecher, N., Bellagamba, E., Weingarten, Y., "MPLS-TP
OAM Analysis", draft-ietf-mpls-tp-oam-analysis, July
2010.
Authors' Addresses
Tal Mizrahi
Marvell
Email: talmi@marvell.com
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