Pseudo-Wire Edge-to-Edge(PWE3) Thomas D. Nadeau
Internet Draft Monique Morrow
Expiration Date: January 2005 Cisco Systems
Peter Busschbach
Lucent Technologies
Mustapha Aissaoui
Alcatel
Editors
July 2004
Pseudo Wire (PW) OAM Message Mapping
draft-ietf-pwe3-oam-msg-map-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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Drafts.
Internet-Drafts are draft documents valid for a maximum of six
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documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
Abstract
This document enumerates the OAM defect state mapping from pseudo
wire emulated edge-to-edge services over MPLS and IP transport
networks to their native attached services.
Table of Contents
Status of this Memo.............................................1
Abstract........................................................1
Table of Contents...............................................1
1 Conventions used in this document.............................2
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2 Contributors..................................................3
3 Scope.........................................................3
4 Terminology...................................................3
5 Introduction..................................................4
6 Reference Model and Defect Locations..........................4
7 PW Status and Defects.........................................5
7.1 PW Defects.................................................5
7.1.1 Packet Loss...........................................6
7.2 Defect Detection...........................................6
7.2.1 Defect Detection Tools................................6
7.2.2 Defect Detection Mechanism Applicability..............7
7.3 PW Defect Entry and Exit Procedures........................7
7.3.1 PW Down...............................................8
7.3.2 PW Up.................................................8
7.4 Alarm Messages and Consequent Actions......................9
7.5 The Use of PW Status.......................................9
7.6 The Use of L2TP SCCN and CDN..............................10
7.7 The Use of BFD Diagnostic Codes...........................10
8 Frame Relay Encapsulation....................................11
8.1 Frame Relay Management....................................11
8.2 Mapping of Defect States from a PW to a Frame Relay AC....12
8.2.1 Procedures in FR Port Mode...........................13
8.3 Frame Relay Network and Attachment Circuit Defects........13
9 ATM Encapsulation............................................13
9.1 ATM Management............................................13
9.2 Mapping ATM and PW Defect States..........................14
9.3 Mapping of Defect States from a PW to a ATM AC............15
9.3.1 Inband ATM OAM over PW...............................15
9.3.2 Out-of-Band ATM OAM over PW..........................15
9.3.3 Procedures in ATM Port Mode..........................16
9.4 ATM Network and Attachment Circuit Defects................17
9.4.1 Inband ATM OAM over PW...............................17
9.4.2 Out-of-Band ATM OAM over PW..........................17
9.4.3 Procedures in ATM Port Mode..........................18
10 SONET Encapsulation (CEP)...................................18
11 TDM Encapsulation...........................................18
12 Ethernet Encapsulation......................................19
13 Security Considerations.....................................20
14 Acknowledgments.............................................20
15 References..................................................20
16 Intellectual Property Disclaimer............................21
17 Full Copyright Statement....................................21
18 Authors' Addresses..........................................22
1 Conventions used in this document
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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 RFC 2119.
2 Contributors
Thomas D. Nadeau, tnadeau@cisco.com
Monique Morrow, mmorrow@cisco.com
Peter B. Busschbach, busschbach@lucent.com
Mustapha Aissaoui, mustapha.aissaoui@alcatel.com
Matthew Bocci, matthew.bocci@alcatel.co.uk
David Watkinson, david.watkinson@alcatel.com
Yuichi Ikejiri, y.ikejiri@ntt.com
Kenji Kumaki, kekumaki@kddi.com
Satoru Matsushima, satoru@ft.solteria.net
3 Scope
This document specifies the mapping of defect states between a
Pseudo Wire and Attachment Circuits (AC) of the end-to-end
emulated service. This document covers the case of PW and ACs of
the same type in accordance to the PWE3 architecture [PWEARCH].
This document covers both PWE over MPLS PSN and PWE over IP PSN.
4 Terminology
AIS Alarm Indication Signal
AOM Administration, Operation and Maintenance
BDI Backward Defect Indication
CC Continuity Check
CE Customer Edge
CPCS Common Part Convergence Sublayer
DLC Data Link Connection
FDI Forward Defect Indication
FRBS Frame Relay Bearer Service
IWF Interworking Function
LB Loopback
NE Network Element
OAM Operations and Maintenance
PE Provider Edge
PW Pseudowire
PSN Packet Switched Network
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RDI Remote Defect Indicator
SDU Service Data Unit
VCC Virtual Channel Connection
VPC Virtual Path Connection
The rest of this document will follow the following convention:
If LSP-Ping is run over a PW as described in [VCCV] it will be
referred to as VCCV-Ping.
If BFD is run over a PW as described in [VCCV] it will be referred
to as VCCV-BFD.
5 Introduction
This document describes how PW defects can be detected; how alarm
information is exchanged between PEs; and how defects detected in
pseudo-wires are mapped to OAM messages native to the emulated
services and vice versa.
The objective of this document is to standardize the behavior of
PEs with respects to failures on PWs and ACs, so that there is no
ambiguity about the alarms generated and consequent actions
undertaken by PEs in response to specific failure conditions.
6 Reference Model and Defect Locations
Error! Reference source not found.Figure 1 illustrates the PWE3
network reference model with an indication of the possible defect
locations. This model will be referenced in the remainder of this
document for describing the OAM procedures.
ACs PSN tunnel ACs
+----+ +----+
+----+ | PE1|==================| PE2| +----+
| |---(a)---(b)..(c)......PW1..(d)..(c)..(f)---(e)---| |
| CE1| (N1) | | | | (N2) |CE2 |
| |----------|............PW2.............|----------| |
+----+ | |==================| | +----+
^ +----+ +----+ ^
| Provider Edge 1 Provider Edge 2 |
| |
|<-------------- Emulated Service ---------------->|
Customer Customer
Edge 1 Edge 2
Figure 1: PWE3 Network Defect Locations
The following is a brief description of the defect locations:
(a) Defect in the first L2 network (N1). This covers any defect
in the N1 which impacts all or a subset of ACs terminating in
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PE1. The defect is conveyed to PE1 and to the remote L2
network (N2) using a L2 specific OAM defect indication.
(b) Defect on a PE1 AC interface.
(c) Defect on a PE PSN interface.
(d) Defect in the PSN network. This covers any defect in the PSN
which impacts all or a subset of the PSN tunnels and PWs
terminating in a PE. The defect is conveyed to the PE using a
PSN and/or a PW specific OAM defect indication. Note that
control plane, i.e., signaling and routing, messages do not
necessarily follow the path of the user plane messages.
Defect in the control plane are detected and conveyed
separately through control plane mechanisms. However, in some
cases, they have an impact on the status of the PW as
explained in the next section.
(e) Defect in the second L2 network (N2). This covers any defect
in N2 which impacts all or a subset of ACs terminating in
PE2. The defect is conveyed to PE2 and to the remote L2
network (N1) using a L2 specific OAM defect indication.
(f) Defect on a PE2 AC interface.
7 PW Status and Defects
This section describes possible PW defects, ways to detect them
and consequent actions.
7.1 PW Defects
Possible defects that impact PWs are the following.
. Physical layer defect in the PSN interface
. PSN tunnel failure which results in a loss of connectivity
between ingress and egress PE
. Control session failures between ingress and egress PE
In case of a MPLS PSN there are additional defects:
. PW labeling error, which is due to a defect in the ingress PE,or
to an over-writing of the PW label value somewhere along the LSP
path.
. LSP tunnel Label swapping errors or LSP tunnel label merging
errors in the MPLS network. This could result in the termination
of a PW at the wrong egress PE.
. Unintended self-replication; e.g., due to loops or denial-of-
service attacks.
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7.1.1 Packet Loss
Persistent congestion in the PSN or in a PE could impact the
proper operation of the emulated service.
A PE can detect packet loss resulting from congestion through
several methods. If a PE uses the sequence number field in the
PWE3 Control Word for a specific Pseudo Wire [PWEARCH], it has the
ability to detect packet loss. [CONGESTION] discusses other
possible mechanisms to detect congestion between PWs.
Generally, there are congestion alarms which are raised in the
node and to the management system when congestion occurs. The
decision to declare the PW Down and to re-signal it through
another path is usually at the discretion of the network operator.
7.2 Defect Detection
7.2.1 Defect Detection Tools
To detect the defects listed in 7.1, Service Providers have a
variety of options available:
Physical Layer defect detection mechanisms such as SONET/SDH LOS,
LOF,and AIS/FERF.
PSN Defect Detection Mechanisms:
For PWE3 over an IP PSN, with L2TP as encapsulation protocol, the
defect detection mechanisms described in [L2TPv3] apply.
Furthermore, the tools Ping and Traceroute, based on ICMP Echo
Messages apply [ICMP].
For PWE3 over an MPLS PSN, several tools can be used.
. LSP-Ping and LSP-Traceroute( [LSPPING]) for LSP tunnel
connectivity verification.
. LSP-Ping with Bi-directional Forwarding Detection ([BFD]) for
LSP tunnel continuity checking.
.Furthermore, if RSVP-TE is used to setup the PSN Tunnels between
ingress and egress PE, the hello protocol can be used to detect
loss of connectivity (see [RSVP-TE]), but only at the control
plane.
PW specific defect detection mechanisms:
[VCCV] describes how LSP-Ping and BFD can be used over individual
PWs for connectivity verification and continuity checking
respectively. When used as such, we will refer to them as VCCV-
Ping and VCCV-BFD respectively.
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7.2.2 Defect Detection Mechanism Applicability
The discussion below is intended to give some perspective how
tools mentioned in the previous section can be used to detect
failures.
Observations:
. Tools like LSP-Ping and BFD can be run periodically or on
demand. If used for defect detection, as opposed to diagnostic
usage, they must be run periodically.
. Control protocol failure indications, e.g. detected through L2TP
Keep-alive messages or the RSVP-TE Hello messages, can be used to
detect many network failures. However, control protocol failures
do not necessarily coincide with data plane failures. Therefore, a
defect detection mechanism in the data plane is required to
protect against all potential data plane failures. Furthermore,
fault diagnosis mechanisms for data plane failures are required to
further analyze detected failures.
. For PWE3 over an MPLS PSN, it is effective to run a defect
detection mechanism over a PSN Tunnel frequently and run one over
every individual PW within that PSN Tunnel less frequently.
However in case the PSN traffic is distributed over Equal Cost
Multi Paths (ECMP), it may be difficult to guarantee that PSN OAM
messages follow the same path as a specific PW. A Service Provider
might therefore decide to focus on defect detection over PWs.
. In MPLS networks, execution of LSP Ping would detect MPLS label
errors, since it requests the receiving node to match the label
with the original FEC that was used in the LSP set up. BFD can
also be used since it relies on discriminators. A label error
would result in a mismatch between the expected discriminator and
the actual discriminator in the BFD control messages.
. For PWE3 over an MPLS PSN, PEs could detect PSN label errors
through the execution of LSP-Ping. However, use of VCCV is
preferred as it is a more accurate detection tool for pseudowires.
Furthermore, it can be run using a BFD mode which allows it to be
used as a light-weight detection mechanism for PWs. If, due to a
label error in the PSN, a PW would be terminated on the wrong
egress PE, PEs would detect this through the execution of VCCV.
LSP ping and/or LSP trace could then be used to diagnose the
detected failure.
Based on these observations, it is clear that a service provider
has the disposal of a variety of tools. There are many factors
that influence which combination of tools best meets its needs.
7.3 PW Defect Entry and Exit Procedures
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PWs can fail in a single direction or in both directions. PEs
SHOULD keep track of the status of each individual direction. In
other words, a PE SHOULD be able to distinguish between the
following states: "PW UP", "PW Transmit Direction Down", "PW
Receive Direction Down", "PW Receive and Transmit Down".
The next two sections discuss under which conditions a PE enters
and exits these states. To avoid an unnecessarily complicated
description, only the states "PW UP" and "PW DOWN" are discussed
without further analysis whether it applies to one or two
directions of the PW.
7.3.1 PW Down
A PE will consider a PW down if one of the following occurs
. It detects a physical layer alarm on the PSN interface over
which the PW is riding and cannot re-establish the PW over another
PSN interface.
. It detects loss of connectivity on the PSN tunnel over which the
PW is riding and cannot re-establish the PW over another PSN
tunnel. This includes label swapping errors and label merging
errors.
. It receives a message from its peer indicating a PW defect,
which could be one of the following:
o PW Status indicating "PW Receive Fault"; "PW Transmit
Fault"; or "PW not forwarding"
o An L2TP SCCN or CDN message
o It detects a loss of PW connectivity, including label
errors, through VCCV.
Note that if the control session between the PEs fails, the PW is
torn down and needs to be re-established.
7.3.2 PW Up
When a PE determines that all previously existing failures have
disappeared, it SHOULD send a message to its peer to indicate
this. E.g. if the original failure was conveyed through a PW
Status message, the PE should send a PW Status message indicating
"PW Forwarding (clear all failures)"
When a PE receives a PW Status message indicating "PW Forwarding",
while it still considers a PW down, and if all previously existing
failures, if any, have disappeared, it SHOULD respond with a PW
Status message indicating "PW Forwarding".
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For PWE3 over a MPLS PSN, a PE will exit the PW down state when
the following conditions are true:
. All defects it had previously detected, as described in Section
7.3.1, have disappeared, and
. It has received a PW Status message from its peer indicating "PW
Forwarding"
For a PWE3 over a IP PSN, a PE will exit the PW down state when
the following conditions are true:
. All defects it had previously detected, as described in Section
7.3.1, have disappeared, and
. A L2TPv3 session is successfully established to carry the PW
packets.
[BFD] and [L2TPv3] define the procedures to exit the PW Down state
if the original failure notification was done through BFD or L2TP
messages, respectively.
7.4 Alarm Messages and Consequent Actions
When a PE changes the status of a PW to DOWN, it SHOULD inform its
peer, by using:
. For PWE3 on MPLS PSN, PW Status messages as defined in
[CONTROL].
. For PWE3 on IP PSN, L2TPv3 messages Stop Control-Connection
Notification (SCCN) and Call Disconnect Notify (CDN) as defined in
[L2TPv3]
Furthermore, in either case, if VCCV-BFD is used, the diagnostic
code in the VCCV-BFD Control message can be used to exchange alarm
information.
In general, PW Status messages or L2TP SCCN and CDN should be used
to communicate failures. VCCV-BFD alarm indications should only be
used in specific cases, as explained in 4.6.
Both PEs will translate the PW alarms to the appropriate failure
indications on the affected ACs. The exact procedures depend on
the emulated protocols and will be discussed in the next sections.
7.5 The Use of PW Status
This document specifies the use of PW status signaling for the
purpose of conveying the status of a PW and attached ACs between
PEs.
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At the PW setup, a PE will enter in a negotiation with its remote
peer of the use of the PW status by inserting the PW Status TLV in
the label mapping message. If the negotiation process results in
the usage of the PW status TLV, then the actual PW status is
determined by the PW status TLV that was sent within the initial
PW label mapping. Subsequent updates of PW status are conveyed
through the notification message [CONTROL].
PW Status messages are used to report the following defects:
. Defects detected through defect detection mechanisms in the MPLS
PSN
. Loss of connectivity detected through VCCV-Ping
. Defects within the PE that result in an inability to forward
traffic between ACs and PW
If the PW defect is related to one forwarding direction only, the
PE shall either use "PW Receive Fault" or "PW Transmit Fault". In
all other cases it shall use "PW Not Forwarding".
Besides reporting PW defects, PW status is used to propagate AC
defects. When and how to use those messages is dependent on the
emulated protocol and will be explained in the subsequent
paragraphs (5.3 and 6.3).
7.6 The Use of L2TP SCCN and CDN
[L2TPv3] describes the use of SCCN and CDN messages to exchange
alarm information between PEs. Like PW Status, SCCN and CDN
messages shall be used to report the following failures:
. Failures detected through defect detection mechanisms in the IP
PSN
. Failures detected through VCCV (except for VCCV-BFD)
. Failures within the PE that result in an inability to forward
traffic between ACs and PW
In L2TP, the Set-Link-Info (SLI) message is used to convey
failures on the ACs.
7.7 The Use of BFD Diagnostic Codes
[BFD] defines a set of diagnostic codes that partially overlap
with failures that can be communicated through PW Status messages
or L2TP SCCN and CDN. To avoid ambiguous situations, these
messages SHOULD be used for all failures that are detected through
means other than BFD.
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For VCCV-BFD, therefore, only the following diagnostic codes
apply:
Code Message
---- ------------------------------
0 No Diagnostic
1 Control Detection Time Expired
3 Neighbor Signaled Session Down
7 Administratively Down
[VCCV] states that, when used over PWs, the asynchronous mode of
BFD should be used. Diagnostic code 2 (Echo Function Failed) does
not apply to the asynchronous mode, but to the Demand Mode.
All other BFD diagnostic codes refer to failures that can be
communicated through PW Status or L2TP SCCN and CDN.
The VCCV-BFD procedures are as follows:
When the downstream PE (PE1) does not receive control messages
from the upstream PE (PE2) during a certain number of transmission
intervals (a number provisioned by the operator), it declares that
the PW in its receive direction is down. PE1 sends a message to
PE2 with H=0 (i.e. "I do not hear you") and with diagnostic code
1. In turn, PE2 declares the PW is down in its transmit direction
and it uses diagnostic code 3 in its control messages to PE2.
When a PW is taken administratively down, the PEs will exchange PW
Status messages with code "Pseudo Wire Not Forwarding" or L2TP CDN
messages with code "Session disconnected for administrative
reasons". In addition, exchange of BFD control messages MUST be
suspended. To that end, the PEs MUST send control messages with
H=0 and diagnostic code 7.
Note, according to [BFD], control messages with an incorrect
discriminator field must be discarded. However, since such an
occurrence might be caused by swapped or mismerged LSPs, it would
be better if a new diagnostic code were introduced to discriminate
between missing control packets and packets with an incorrect
discriminator. In other words, in most cases one would communicate
PW failures through PW Status messages except for a well-defined
set of exceptions where BFD is used. How PW defects that can be
detected through the use of BFD or through other means, are mapped
to defect indications on the ACs is described in sections 8 and
subsequent sections.
8 Frame Relay Encapsulation
8.1 Frame Relay Management
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The management of Frame Relay Bearer Service (FRBS) connections
can be accomplished through two distinct methodologies:
1. Based on ITU-T Q.933 Annex A, Link Integrity Verification
procedure, where STATUS and STATUS ENQUIRY signaling messages are
sent using DLCI=0 over a given UNI and NNI physical link. [ITU-T
Q.933]
2. Based on FRBS LMI, and similar to ATM ILMI where LMI is common
in private Frame Relay networks.
In addition, ITU-T I.620 addresses Frame Relay loopback, but the
deployment of this standard is relatively limited. [ITU-T I.620]
It is possible to use either, or both, of the above options to
manage Frame Relay interfaces. This document will refer
exclusively to Q.933 messages.
The status of any provisioned Frame Relay PVC may be updated
through:
. STATUS messages in response to STATUS ENQUIRY messages, these
are mandatory.
. Optional unsolicited STATUS updates independent of STATUS
ENQUIRY (typically under the control of management system, these
updates can be sent periodically (continuous monitoring) or only
upon detection of specific defects based on configuration.
In Frame Relay, a DLC is either up or down. There is no
distinction between different directions.
8.2 Mapping of Defect States from a PW to a Frame Relay AC
The following are the OAM procedures for defects in locations (c)
and (d) in Figure 1.
In case a PE keeps track of the status of individual Frame Relay
PVCs (which often, but not necessarily, coincides with the usage
of 1-1 encapsulation mode), the following procedures apply:
When PE1 determines that one or both directions of a PW is down it
will indicate this on the corresponding FR ACs by sending Active
bit = 0 in the full status report (and optionally in the
asynchronous status message), as per Q.933 annex A. In addition,
PE1 should send a PW status signaling message, ôPW Not Forwardingö
to the remote PE to convey the status of the affected PWs. PE2
should in turn generate a full status report with the Active bit =
0 (and optionally in the asynchronous status message), as per
Q.933 annex A, into N2 for the corresponding FR ACs.
When the PE determines that the PW is up, it will indicate this on
the AC through STATUS messages that indicate that the FR PVCs
linked to that PW are active.
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8.2.1 Procedures in FR Port Mode
In case of pure port mode, STATUS ENQUIRY and STATUS messages are
transported transparently over the PW. A PW Failure will therefore
result in timeouts at the Frame Relay devices at one or both sites
of the emulated interface.
8.3 Frame Relay Network and Attachment Circuit Defects
The following are the OAM procedures for defects in locations (a)
and (b) in Figure 1. The handling of a defect in locations (e) and
(f) is similar to that of locations (a) and (b) respectively.
As explained in [CONTROL], if a PE detects that a Frame Relay PVC
is "inactive", as defined in [ITU-T Q933] Annex A.5, it will
convey this information to its peer using a PW status message. The
remote PE SHOULD generate the corresponding errors and alarms on
the egress Frame Relay PVC
For PWE3 over MPLS PSN, a PE that detects or is notified of a
defect in locations (a) or (b) MUST change the local status of the
corresponding FR ACs to DOWN in PE1 and MUST send a PW Status
message indicating both "AC Receive Fault" and "AC Transmit
Fault". On reception of this PW status message, the egress PE MUST
generate a full status report with the Active bit = 0 (and
optionally in the asynchronous status message), as per Q.933 annex
A, into N2 for the corresponding FR ACs.
For PWE3 over IP PSN, a PE that detects or is notified of a defect
in locations (a) or (b) MUST change the local status of the
corresponding FR ACs to DOWN in PE1 and MUST send an L2TP Set-Link
Info (LSI) message with a Circuit Status Attribute Value Pair
(AVP) indicating "inactive". On reception of this LSI message, the
egress PE MUST generate a full status report with the Active bit =
0 (and optionally in the asynchronous status message), as per
Q.933 annex A, into N2 for the corresponding FR ACs.
9 ATM Encapsulation
9.1 ATM Management
ATM management and OAM mechanisms are much more evolved than those
of Frame Relay. There are five broad management-related
categories, including fault management (FT), Performance
management (PM), configuration management (CM), Accounting
management (AC), and Security management (SM). ITU-T
Recommendation I.610 describes the functions for the operation and
maintenance of the physical layer and the ATM layer, that is,
management at the bit and cell levels ([ITU-T I.610]). Because of
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its scope, this document will concentrate on ATM fault management
functions. Fault management functions include the following:
1) Alarm indication signal (AIS)
2) Remote Defect indication (RDI).
3) Continuity Check (CC).
4) Loopback (LB)
Some of the basic ATM fault management functions are described as
follows: Alarm indication signal (AIS) sends a message in the same
direction as that of the signal, to the effect that an error has
been detected.
Remote defect indication (RDI) sends a message to the transmitting
terminal that an error has been detected. RDI is also referred to
as the far-end reporting failure. Alarms related to the physical
layer are indicated using path AIS/RDI. Virtual path AIS/RDI and
virtual channel AIS/RDI are also generated for the ATM layer.
OAM cells (F4 and F5 cells) are used for the control of virtual
paths and virtual channels with regard to their performance and
availability. F4 cells are used to monitor a VPC, F5 cells for a
VCC. OAM cells in the F4 and F5 flows are used for monitoring a
segment of the network and end-to-end monitoring. OAM cells in F4
flows have the same VPI as that of the connection being monitored.
OAM cells in F5 flows have the same VPI and VCI as that of the
connection being monitored. The AIS and RDI messages of the F4
and F5 flows are sent to the other network nodes via the VPC or
the VCC to which the message refers. The type of error and its
location can be indicated in the OAM cells. Continuity check is
another fault management function. To check whether a VCC that has
been idle for a period of time is still functioning, the network
elements can send continuity-check cells along that VCC.
9.2 Mapping ATM and PW Defect States
In normal, i.e., defect-free, operation, all the types of ATM OAM
cells described in Section 9.1 are either terminated at the PE,
for OAM segments terminating in the AC endpoint, or transparently
carried over the PSN tunnel [PWE3-ATM]. This is referred to as
ôinband ATM OAM over PWö and is the default method.
An optional out-of band method based on relaying the ATM defect
state over a PW specific defect indication mechanism is provided
for PEÆs which cannot generate and/or transmit ATM OAM cells over
the ATM PW. This is referred to as ôOut-of-band ATM OAM over PWö.
Note that the out-of-band method assumes that the end-to-end
circuit consists of three independent segments, <VCC1, ATM PW,
VCC2>, with defect states relayed across the boundary of these
segments. An important consequence of this is that when a PE is
notified of a defect in the remote ATM network, in the remote AC,
or in the PW, it will always generate a F4/F5 AIS message towards
the local ATM network and local CE regardless of the stated
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direction of the defect. At the same time, the PE should not relay
over the PW the defect state of a received F4/F5 RDI from the
local CE if it is sourcing a F4/F5 AIS on the same AC towards that
CE. These conditions maintain the independence of the three defect
loops while relaying the defect states end-to-end. The procedures
in sections 9.3.2 and 9.4.2 satisfy these two conditions.
9.3 Mapping of Defect States from a PW to a ATM AC
The following are the OAM procedures for defects in locations (c)
and (d) in Figure 1.
9.3.1 Inband ATM OAM over PW
When PE1 detects a defect in locations (c) or (d) it MUST change
the status of the affected PWs to DOWN for the direction of the
defect. If both directions of the PW are down or if only the
Receive direction of the PW is down, PE1 MUST generate AIS on the
affected ACs to convey this status to the ATM network (N1) and CE1
[PWE3-ATM]. CE1 will reply with a F4/F5 RDI which gets forwarded
by PE1 over the PW. PE2 will receive the RDI message only if the
forwards direction of the PW, i.e., PE1-to-PE2, is not affected by
the defect. In this case, PE2 MUST forward the RDI message to CE2
through the ATM network (N2).
If only the PW Transmit direction is DOWN at PE1, this is
generally detected by PE2 through a PSN or a PW continuity
checking or connectivity verification mechanism as explained in
Section 7.3.1. PE1 is notified through the return path of that
specific mechanism. In this case, PE2 will follow the same
procedures described above for a defect in the PW Receive
direction.
When the PW status changes back to UP, a PE MUST cease the
generation of the F4/F5 messages on the AC towards the CE. This
will result in clearing the AIS or RDI states in the remote PE, in
CE1, and in CE2.
9.3.2 Out-of-Band ATM OAM over PW
For PWE3 over MPLS PSN, the following operations MUST be performed
when PE1 detects a defect in locations (c) or (d):
a. PE1 MUST change the status of the affected PWs to DOWN for
the direction of the defect.
b. If both directions of the PW are down, PE1 MUST generate a
PW status message indicating ôPW not forwardingö.
c. If only the Receive direction of the PW is down, PE1 MUST
generate a PW status message indicating ôLocal PSN-facing
PW (ingress) Receive Faultö.
d. PE1 MUST generate a F4/F5 AIS on the affected ACs to
convey this status to the ATM network (N1) and CE1 [PWE3-
ATM].
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e. CE1 replies with a F4/F5 RDI. PE1 MUST terminate the F4/F5
RDI since it is sourcing a AIS over the same AC towards
CE1.
a. On reception of the PW status message, PE2 MUST generate a
F4/F5 AIS on the related ATM ACs towards CE2.
f. The termination point of the ATM VCC or VPC in the far-end
CE, i.e., CE2, generates a F4/F5 RDI in response to the
received F4/F5 AIS. PE2 MUST terminate the F4/F5 RDI since
it is sourcing a AIS over the same AC towards CE2.
For PWE3 over IP PSN, the following operations MUST be performed
when PE1 detects a defect in locations (c) or (d):
a. PE1 MUST change the status of the affected PWs to DOWN for
the direction of the defect.
b. If both directions of the PW are down, or if only the
Receive direction of the PW is down, PE1 MUST send an
L2TPv3 SCCN or CDN message.
c. PE1 MUST generate a F4/F5 AIS on the affected ACs to
convey this status to the ATM network (N1) and CE1 [PWE3-
ATM].
d. CE1 replies with a F4/F5 RDI. PE1 MUST terminate the F4/F5
RDI since it is sourcing a AIS over the same AC towards
CE1.
e. On reception of the SSCN or CDN message, PE2 MUST generate
a F4/F5 AIS on the related ATM ACs towards CE2.
f. The termination point of the ATM VCC or VPC in the far-end
CE, i.e., CE2, generates a F4/F5 RDI in response to the
received F4/F5 AIS. PE2 MUST terminate the F4/F5 RDI since
it is sourcing a AIS over the same AC towards CE2.
If only the PW Transmit direction is DOWN at PE1, this is
generally detected by PE2 through a PSN or a PW continuity
checking or connectivity verification mechanism as explained in
Section 7.3.1. PE1 is notified through the return path of that
specific mechanism. In this case, PE2 will follow the same
procedures described above for a defect in the PW Receive
direction.
When the PW status changes back to UP, a PE MUST cease the
generation of the F4/F5 messages on the AC towards the CE. In
addition, it MUST generate a PW Status message indicating ôPseudo
Wire forwarding (clear all failures)ö for PWE3 over a MPLS PSN.
For PWE3 or an IP PSN, the actions are TBD.
This will result in clearing the AIS or RDI states in the remote
PE, in CE1, and in CE2.
9.3.3 Procedures in ATM Port Mode
In case of transparent mapping,(i.e.: "port mode"),
where the PE does not keep track of the status of individual ATM
VPCs or VCCs, a PE does not know which VPCs and/or VCCs are
active. In such a case there is a need for another defect
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indication mechanism on the AC. This is beyond the scope of this
document.
9.4 ATM Network and Attachment Circuit Defects
The following are the OAM procedures for defects in locations (a)
and (b) in Figure 1. The handling of a defect in locations (e) and
(f) is similar to that of locations (a) and (b) respectively.
9.4.1 Inband ATM OAM over PW
PE1 MUST transparently carry the F4/F5 AIS or RDI cells received
on the corresponding ATM AC (defect a) or the F4/F5 AIS generated
locally (defect b) over the corresponding ATM PW. The termination
point of the ATM VCC or VPC in the far-end CE, i.e., CE2,
generates a F4/F5 RDI in response to a F4/F5 AIS. PE2 MUST forward
the RDI over the PW and PE1 MUST forward it over the corresponding
AC. CE1 does not reply to a received F4/F5 RDI message.
9.4.2 Out-of-Band ATM OAM over PW
If PE1 cannot generate and/or transmit ATM OAM cells over the ATM
PW, it may use the following procedure.
For PWE3 over MPLS PSN, the following operations MUST be performed
when PE1 receives a F4/F5 AIS or RDI from the ATM network (defect
a) or when it detects a defect in the Receive or Transmit
direction of the ATM AC (defect b):
a. PE1 MUST send a PW Status message indicating "AC Receive
Fault" for a received F4/F5 AIS.
b. PE1 MUST send a PW status message indicating "AC Transmit
Fault" for a received F4/F5 RDI only if it is not sourcing
a F4/F5 AIS over the same AC towards CE1.
c. PE1 MUST generate a F4/F5 RDI on the related ACs towards
CE1 in response to a received F4/F5 AIS only.
d. On reception of the PW status message, PE2 MUST generate
a F4/F5 AIS on the related ATM ACs towards CE2.
e. The termination point of the ATM VCC or VPC in the far-
end, i.e., CE2, generates a F4/F5 RDI in response to the
received F4/F5 AIS. PE2 MUST terminate the F4/F5 RDI since
it is sourcing a AIS over the same AC towards CE2.
For PEW3 over IP PSN, the following operations MUST be performed
when PE1 receives a F4/F5 AIS or RDI from the ATM network (defect
a) or when it detects a defect in the Receive or Transmit
direction of the ATM AC (defect b):
b. PE1 MUST send an L2TP Set-Link Info (LSI) message with a
Circuit Status AVP indicating "inactive". In the case of a
received F4/F5 RDI, PE1 MUST not generate the LSI message
if it is sourcing a F4/F5 AIS over the same AC towards
CE1.
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c. PE1 MUST generate a F4/F5 RDI on the related ACs towards
CE1 in response to a received F4/F5 AIS only.
d. On reception of the L2TP LSI message, PE2 MUST generate a
F4/F5 AIS on the related ATM ACs towards CE2.
e. The termination point of the ATM VCC or VPC in the far-end
CE, i.e., CE2, generates a F4/F5 RDI in response to the
received F4/F5 AIS. PE2 MUST terminate the F4/F5 RDI since
it is sourcing a AIS over the same AC towards CE2.
9.4.3 Procedures in ATM Port Mode
In case of transparent mapping, ,(i.e.: "port mode"), where the PE
does not know which VCCs and/or VPCs are active, AIS/RDI messages
are transparently propagated to the remote ATM network without PE
intervention for defects in the ATM network (location a). For
defects in the PE ATM AC interface ,location b, the PE MUST send a
PW-STATUS message to its peer. How the peer propagates that
message on its AC is beyond the scope of this document.
10 SONET Encapsulation (CEP)
[CEP] discusses how Loss of Connectivity and other SONET/SDH
protocol failures on the PW are translated to alarms on the ACs
and vice versa. In essence, all defect management procedures are
handled entirely in the emulated protocol. There is no need for an
interaction between PW defect management and SONET layer defect
management.
11 TDM Encapsulation
From an OAM perspective, the PSN carrying a TDM PW provides the
same function as that of SONET/SDH or ATM network carrying the
same low-rate TDM stream. Hence the interworking of defect OAM is
similar.
For structure-agnostic TDM PWs, the TDM stream is to be carried
transparently across the PSN, and this requires TDM OAM
indications to be transparently transferred along with the TDM
data. For structure-aware TDM PWs the TDM structure alignment is
terminated at ingress to the PSN and regenerated at egress, and
hence OAM indications may need to be signaled by special means. In
both cases generation of the appropriate emulated OAM indication
may be required when the PSN is at fault.
Since TDM is a real-time signal, defect indications and
performance measurements may be classified into two classes,
urgent and deferrable. Urgent messages are those whose contents
may not be significantly delayed with respect to the TDM data that
they potentially impact, while deferrable messages may arrive at
the far end delayed with respect to simultaneously generated TDM
data. For example, a forward indication signifying that the TDM
data is invalid (e.g. TDM loss of signal, or MPLS loss of packets)
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is only of use when received before the TDM data is to be played
out towards the far end TDM system. It is hence classified as an
urgent message, and we can not delegate its signaling to a
separate maintenance or management flow. On the other hand, the
forward loss of multiframe synchronization, and most reverse
indications do not need to be acted upon before a particular TDM
frame is played out.
From the above discussion it is evident that the complete solution
to OAM for TDM PWs needs to have at least two, and perhaps three
components. The required functionality is transparent transfer of
native TDM OAM and urgent transfer of indications (by flags) along
with the impacted packets. Optionally there may be mapping between
TDM and PSN OAM flows.
TDM AIS generated in the TDM network due to a fault in that
network is generally carried unaltered, although the TDM
encapsulations allow for its suppression for bandwidth
conservation purposes. Similarly, when the TDM loss of signal is
detected at the PE, it will generally emulate TDM AIS.
SAToP and the two structure-aware TDM encapsulations have
converged on a common set of defect indication flags in the PW
control word. When the PE detects or is informed of lack of
validity of the TDM signal, it raises the local ("L") defect flag,
uniquely identifying the defect as originating in the TDM network.
The remote PE must ensure that TDM AIS is delivered to the remote
TDM network. When the defect lies in the MPLS network, the remote
PE fails to receive packets. The remote PE generates TDM AIS
towards its TDM network, and in addition raises the remote defect
("R") flag in its PSN-bound packets, uniquely identifying the
defect as originating in the PSN. Finally, defects in the remote
TDM network that cause RDI generation in that network, may
optionally be indicated by proper setting of the field of valid
packets in the opposite direction.
12 Ethernet Encapsulation
At this point in time, Ethernet OAM is not defined. Therefore, the
procedures for mapping PW failures to Ethernet OAM messages and
vice versa are currently rudimentary.
When an ingress PE detects that an Ethernet AC is down (because
the related ethernet physical interface is down), it SHOULD send a
PW Status message indicating both "AC Receive Fault" and "AC
Transmit Fault".
If an egress PE determines that all ACs on a specific ethernet
physical interface are affected (either because of ingress AC
failures or because of PW failures), it MAY propagate these alarms
by bringing the entire physical interface down.
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13 Security Considerations
The mapping messages described in this document do not change the
security functions inherent in the actual messages.
14 Acknowledgments
Hari Rakotoranto, Eric Rosen, Mark Townsley, Michel Khouderchah,
Bertrand Duvivier, Vanson Lim and Chris Metz Cisco Systems
15 References
[BFD] Katz, D., Ward, D., "Bidirectional Forwarding Detection",
Internet Draft <draft-katz-ward-bfd-02.txt>, May 2004
[CEP] Malis, A., et.al., "SONET/SDH Circuit Emulation over Packet
(CEP)", Internet Draft <draft-ietf-pwe3-sonet-08.txt>, June
2004
[CONGESTION] Rosen, E., Bryant, S., Davie, B., "PWE3 Congestion
Control Framework", Internet Draft <draft-rosen-pwe3-
congestion-01.txt", March 2004
[CONTROL] Martini, L., Rosen, E., Smith, T., "Pseudowire Setup and
Maintenance using LDP", Internet Draft <draft-ietf-pwe3-
control-protocol-08.txt>, July 2004
[ICMP] Postel, J. "Internet Control Message Protocol" RFC 792
[ITU-T I.610] Recommendation I.610 "B-ISDN operation and
maintenance principles and functions", February 1999
[ITU-T I.620] Recommendation I.620 "Frame relay operation and
maintenance principles and functions", October 1996
[ITU-T Q.933] Recommendation Q.933 " ISDN Digital Subscriber
Signalling System No. 1 (DSS1) Signalling specifications
for frame mode switched and permanent virtual connection
control and status monitoring" February 2003
[L2TPv3] Lau, J., et.al. " Layer Two Tunneling Protocol (Version
3", Internet Draft <draft-ietf-l2tpext-l2tp-base-14.txt>,
June 2004
[LSPPING] Kompella, K., Pan, P., Sheth, N., Cooper, D., Swallow,
G., Wadhwa, S., Bonica, R., " Detecting MPLS Data Plane
Failures", Internet Draft < draft-ietf-mpls-lsp-ping-06.txt>,
July 2004
[OAM REQ] T. Nadeau et.al., "OAM Requirements for MPLS Networks",
Internet Draft <draft-ietf-mpls-oam-requirements-02>, June
2003
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[PWEARCH] Bryant, S., Pate, P., "PWE3 Architecture", Internet
Draft, < draft-ietf-pwe3-arch-07.txt>, March 2004
[PWEATM] Martini, L., et al., "Encapsulation Methods for Transport
of ATM Cells/Frame Over IP and MPLS Networks", Internet Draft
<draft-ietf-pwe3-atm-encap-06.txt>, July 2004
[PWREQ] Xiao, X., McPherson, D., Pate, P., "Requirements for
Pseudo Wire Emulation Edge to-Edge (PWE3)", < draft-ietf-
pwe3-requirements-08.txt>, December 2003
[RSVP-TE] Awduche, D., et.al. " RSVP-TE: Extensions to RSVP for
LSP Tunnels", RFC 3209
[VCCV] Nadeau, T., et al."Pseudo Wire Virtual Circuit Connection
Verification (VCCV)", Internet Draft <draft-ietf-pwe3-vccv-
03.txt>, October 2004.
16 Intellectual Property Disclaimer
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; neither does it represent
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Information on the IETF's procedures with respect to rights in
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or users of this specification can be obtained from the IETF
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The IETF invites any interested party to bring to its attention
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17 Full Copyright Statement
"Copyright (C) The Internet Society (2004). Except as set forth
below, authors retain all their rights.
This document and translations of it may be copied and furnished
to others, and derivative works that comment on or otherwise
explain it or assist in its implementation may be prepared,
copied, published and distributed, in whole or in part, without
restriction of any kind, provided that the above copyright notice
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and this paragraph are included on all such copies and derivative
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Internet Society or other Internet organizations, except as needed
for the purpose of developing Internet standards in which case the
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Process must be followed, or as required to translate it into
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The limited permissions granted above are perpetual and will not
be revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
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18 Authors' Addresses
Thomas D. Nadeau
Cisco Systems, Inc.
300 Beaverbrook Drive
Boxborough, MA 01824
Phone: +1-978-936-1470
Email: tnadeau@cisco.com
Monique Morrow
Cisco Systems, Inc.
Glatt-com
CH-8301 Glattzentrum
Switzerland
Email: mmorrow@cisco.com
Peter B. Busschbach
Lucent Technologies
67 Whippany Road
Whippany, NJ, 07981
Email: busschbach@lucent.com
Mustapha Aissaoui
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
Email: mustapha.aissaoui@alcatel.com
Matthew Bocci
Alcatel
Voyager Place, Shoppenhangers Rd
Maidenhead, Berks, UK SL6 2PJ
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Email: matthew.bocci@alcatel.co.uk
David Watkinson
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
Email: david.watkinson@alcatel.com
Yuichi Ikejiri
NTT Communications Corporation
1-1-6, Uchisaiwai-cho, Chiyoda-ku
Tokyo 100-8019, JAPAN
Email: y.ikejiri@ntt.com
Kenji Kumaki
KDDI Corporation
KDDI Bldg. 2-3-2
Nishishinjuku, Shinjuku-ku
Tokyo 163-8003,JAPAN
E-mail : kekumaki@kddi.com
Satoru Matsushima
Japan Telecom
JAPAN
Email: satoru@ft.solteria.net
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