Pseudo-Wire Edge-to-Edge(PWE3) Thomas D. Nadeau
Internet Draft Monique Morrow
Expiration Date: April 2005 Cisco Systems
Peter Busschbach
Lucent Technologies
Mustapha Aissaoui
Alcatel
Editors
October 2004
Pseudo Wire (PW) OAM Message Mapping
draft-ietf-pwe3-oam-msg-map-01.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
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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.............................3
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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........................8
7.3.1 PW Down...............................................8
7.3.2 PW Up.................................................9
7.4 Alarm Messages and Consequent Actions.....................10
7.5 The Use of PW Status......................................10
7.6 The Use of L2TP STOPCCN and CDN...........................11
7.7 The Use of BFD Diagnostic Codes...........................11
8 Frame Relay Encapsulation....................................12
8.1 Frame Relay Management....................................12
8.2 FR AC State...............................................13
8.3 Mapping of Defect States from a PW to a Frame Relay AC....13
8.3.1 Procedures in FR Port Mode...........................14
8.4 Frame Relay Network and Attachment Circuit Defects........14
9 ATM Encapsulation............................................15
9.1 ATM Management............................................15
9.2 ATM AC State..............................................16
9.3 Mapping ATM and PW Defect States..........................16
9.4 Mapping of Defect States from a PW to a ATM AC............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..........................19
9.5 ATM Network and Attachment Circuit Defects................19
9.5.1 Inband ATM OAM over PW...............................19
9.5.2 Out-of-Band ATM OAM over PW..........................19
9.5.3 Procedures in ATM Port Mode..........................20
10 SONET Encapsulation (CEP)...................................20
11 TDM Encapsulation...........................................20
12 Ethernet Encapsulation......................................21
12.1 Ethernet AC State........................................22
12.2 Mapping of Defect States from a PW to a Ethernet AC......22
12.3 Frame Relay Network and Attachment Circuit Defects.......22
13 Security Considerations.....................................22
14 Acknowledgments.............................................22
15 References..................................................22
16 Intellectual Property Disclaimer............................24
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17 Full Copyright Statement....................................24
18 Authors' Addresses..........................................25
1 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 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
David Allan, dallan@nortelnetworks.com
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
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IWF Interworking Function
LB Loopback
NE Network Element
OAM Operations and Maintenance
PE Provider Edge
PW Pseudowire
PSN Packet Switched Network
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:
The PW can ride over three types of Packet Switched Network (PSN).
A PSN which makes use of LSPs as the tunneling technology to
forward the PW packets will be referred to as an MPLS PSN. A PSN
which makes use of MPLS-in-IP tunneling [MPLS-in-IP], with a MPLS
shim header used as PW demultiplexer, will be referred to as an
MPLS-IP PSN. A PSN which makes use of L2TPv3 [L2TPv3] as the
tunneling technology will be referred to as L2TP-IP PSN.
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
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.............|----------| |
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+----+ | |==================| | +----+
^ +----+ +----+ ^
| 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
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
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In case of an MPLS PSN and an MPLS-IP 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.
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 L2TP-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 and an MPLS-IP PSN, several tools can be
used.
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. 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.
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 and an MPLS-IP 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
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would result in a mismatch between the expected discriminator and
the actual discriminator in the BFD control messages.
. For PWE3 over an MPLS PSN and an MPLS-IP 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, i.e., VCCV-BFD, 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
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. 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 "Local PSN-facing PW (ingress)
Receive Receive Fault"; "Local PSN-facing PW (egress) Transmit
Fault"; or "PW not forwarding"
o In the case of an L2TP-IP, this is a L2TP StopCCN or CDN
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message. A StopCCN message indicates that the control connection
has been shut down by the remote PE [L2TPv3]. This is typically
used for defects in the PSN which impact both the control
connection and the individual data plane sessions. On reception of
this message, a PE closes the control connection and will clear
all the sessions managed by this control connection. Since each
session carries a single PW, the state of the corresponding PWs is
changed to DOWN. A CDN message indicates that the remote peer
requests the disconnection of a specific session [L2TPv3]. In this
case only the state of the corresponding PW is changed to DOWN.
This is typically used for local defects in a PE which impact only
a specific session and the corresponding PW.
o It detects a loss of PW connectivity, including label
errors, through VCCV.
Note that if the PW control session between the PEs fails, the PW
is torn down and needs to be re-established. However, the
consequent actions towards the ACs are the same as if the PW state
were DOWN.
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".
For PWE3 over a MPLS PSN and a MPLS-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
. It has received a PW Status message from its peer indicating "PW
Forwarding"
For a PWE3 over a L2TP-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.
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[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 an MPLS PSN or on an MPLS-IP PSN, PW Status messages
as defined in [CONTROL].
. For PWE3 on L2TP-IP PSN, L2TPv3 messages Stop Control-Connection
Notification (STOPCCN) 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 STOPCCN 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.
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
or MPLS-IP PSN
. Loss of connectivity detected through VCCV-Ping
. Defects within the PE that result in an inability to forward
traffic between ACs and PW
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If the PW defect is related to one forwarding direction only, the
PE shall either use "Local PSN-facing PW (ingress) Receive Fault"
or "Local PSN-facing PW (egress) 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 Section 8 and in
subsequent sections..
7.6 The Use of L2TP STOPCCN and CDN
[L2TPv3] describes the use of STOPCCN and CDN messages to exchange
alarm information between PEs. Like PW Status, STOPCCN and CDN
messages shall be used to report the following failures:
. Failures detected through defect detection mechanisms in the
L2TP-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 STOPCCN and CDN messages. To avoid ambiguous situations,
these messages SHOULD be used for all failures that are detected
through means other than BFD.
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 STOPCCN and CDN.
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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.
In conclusion, one would communicate PW defects through PW Status
messages, or L2TP STOPCCN and CDN messages in all cases, 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 section 8 and in subsequent sections.
8 Frame Relay Encapsulation
8.1 Frame Relay Management
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.
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. 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 FR AC State
A PE changes the state of an FR AC to DOWN if any of the following
conditions are met:
(i) A PVC is not ædeletedÆ from the Frame Relay network and
the Frame Relay network explicitly indicates in a full
status report (and optionally by the asynchronous status
message) that this Frame Relay PVC is æinactiveÆ. In this
case, this status maps across the PE to the corresponding
PW only.
(ii) The LIV indicates that the link from the PE to the Frame
Relay network is down. In this case, the link down
indication maps across the PE to all corresponding PWs.
(iii) A physical layer alarm is detected on the FR interface. In
this case, this status maps across the PE to all
corresponding PWs.
A PE exits the FR AC Down state when all defects it had previously
detected have disappeared.
8.3 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:
a. PE1 MUST change the state of the affected PWs to DOWN for
the direction of the defect.
b. PE1 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 N1 for the corresponding FR
ACs.
c. If both directions of the PW are down, PE1 MUST generate a
PW status message indicating ææPW not forwardingÆÆ. If only
the Transmit direction is down, PE1 MUST generate a PW
status message indicating ææLocal PSN-facing PW (egress)
Transmit FaultÆÆ.
d. If only the Receive direction of the PW is down, PE1 MUST
generate a PW status message indicating ææLocal PSN-facing
PW (ingress) Receive FaultÆÆ.
e. On reception of the PW status message, PE2 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.
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For PWE3 over L2TP-IP, the following operations MUST be performed
when PE1 detects a defect in locations (c) or (d):
a. PE1 MUST change the state of the affected PWs to DOWN for
the direction of the defect.
b. PE1 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 N1 for the corresponding FR
ACs.
c. PE1 MUST send an L2TPv3 CDN message or a StopCCN message.
d. On reception of the CDN or StopCCN message, PE2 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.
When the PW state changes back to UP, a PE MUST generate a full
status report (and optionally in the asynchronous status message),
indicating a ææactiveÆÆ status for the corresponding FR AC.
In addition, it MUST generate a PW Status message indicating
ææPseudo Wire forwarding (clear all failures)ÆÆ for PWE3 over a MPLS
PSN and a MPLS-IP PSN. For PWE3 or an L2TP-IP, the PW UP state is
the result of the successful re-establishment of a L2TPv3 session
to carry the PW packets.
This will result in clearing the alarm states in the remote PE, in
CE1, and in CE2
8.3.1 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 of the Q.933 link and PVC management protocol
at the Frame Relay devices at one or both sites of the emulated
interface.
8.4 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 or MPLS-IP PSN, a PE that detects or is
notified of a defect in locations (a) or (b) MUST change the local
state 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 =
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0 (and optionally in the asynchronous status message), as per
Q.933 annex A, into N2 for the corresponding FR ACs.
For PWE3 over L2TP-IP PSN, a PE that detects or is notified of a
defect in locations (a) or (b) MUST change the local state 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
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
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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 ATM AC State
A PE changes the state of an ATM AC to DOWN if any of the
following conditions are met:
(i) It detects a physical layer alarm on the ATM interface or
it receives a F4 AIS/RDI for a AC inside a terminating VP.
(ii) It receives an F4/F5 AIS/RDI OAM cell indicating that the
ATM VP/VC is down in the adjacent L2 ATM network (e.g., N1
for PE1).
(iii) It detects loss of connectivity on the ATM VPC/VCC while
running ATM continuity checking (ATM CC) with the local
ATM network and CE.
A PE exits the ATM AC Down state when all defects it had
previously detected have disappeared. The exact conditions under
which a PE exits a AIS or a RDI state, or declares that
connectivity is restored via ATM CC are defined in I.610 [I.610].
9.3 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
ææinband ATM OAM over PWÆÆ 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 ææOut-of-band ATM OAM over PWÆÆ.
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
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 9.4.2 and 9.5.2 satisfy these two conditions.
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9.4 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.
9.4.1 Inband ATM OAM over PW
When PE1 detects a defect in locations (c) or (d) it MUST change
the state 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 F4/F5 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. If however, PE1 detects the defect in the transmit
direction through a time-out of a connectivity verification
mechanism such as LSP-Ping or VCCV-Ping, it MUST generate a PW
status message indicating ææLocal PSN-facing PW (egress) Transmit
FaultÆÆ and forward it to PE2. On reception of this message, 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.
9.4.2 Out-of-Band ATM OAM over PW
For PWE3 over an MPLS PSN or an MPLS-IP PSN, the following
operations MUST be performed when PE1 detects a defect in
locations (c) or (d):
a. PE1 MUST change the state 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 ææPW not forwardingÆÆ.If only
the Transmit direction is down, PE1 MUST generate a PW
status message indicating ææLocal PSN-facing PW (egress)
Transmit FaultÆÆ. In addition, PE1 MUST generate a F4/F5
RDI on the affected ACs to convey this status to the ATM
network (N1) and CE1.
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c. If only the Receive direction of the PW is down, PE1 MUST
generate a PW status message indicating ææLocal PSN-facing
PW (ingress) Receive FaultÆÆ. In addition, PE1 MUST
generate a F4/F5 AIS on the affected ACs to convey this
status to the ATM network (N1) and CE1.
d. CE1 replies with a F4/F5 RDI in response to a received
F4/F5 AIS only. PE1 MUST terminate the F4/F5 RDI since it
is sourcing a PW status message towards PE2. Note however
that the RDI defect state is treated as a separate defect
from the original PW defect state.
e. On reception of a ææPW not forwardingÆÆ or a ææLocal PSN-
facing PW (egress) Transmit FaultÆÆ status message, PE2
MUST generate a F4/F5 AIS on the related ATM ACs towards
CE2. On reception of a ææLocal PSN-facing PW (ingress)
Receive FaultÆÆ status message, PE2 MUST generate a F4/F5
RDI 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 only. PE2 MUST treat this as a separate
defect from the original PW defect and MUST generate a PW
status message indicating ææAC Transmit FaultÆÆ towards
PE1.PE1 MUST terminate the received PW status message and
does not perform any additional action since it is
sourcing a F4/F5 RDI towards CE1.
For PWE3 over L2TP-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
both directions.
b. PE1 MUST send an L2TPv3 STOPCCN 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.
d. CE1 replies with a F4/F5 RDI. PE1 MUST terminate the F4/F5
RDI since it has informed PE2 that it had disconnected the
corresponding L2TPv3 sessions.
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 has disconnected the corresponding L2TPv3 sessions.
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 ææPseudo
Wire forwarding (clear all failures)ÆÆ for PWE3 over a MPLS PSN and
a MPLS-IP PSN. For PWE3 or an L2TP-IP PSN, the PW UP state is the
result of the successful re-establishment of a L2TPv3 session to
carry the PW packets..
This will result in clearing the AIS or RDI states in the remote
PE, in CE1, and in CE2.
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9.4.3 Procedures in ATM Port Mode
In case of transparent cell transport, 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 indication
mechanism on the AC. This is beyond the scope of this document.
9.5 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.
9.5.1 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.
9.5.2 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 an MPLS PSN or an MPLS-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):
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.
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 a "AC Receive Fault" status message, PE2
MUST generate a F4/F5 AIS on the related ATM ACs towards
CE2. On reception of a ææAC Transmit FaultÆÆ status message,
PE2 MUST generate a F4/F5 RDI 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 only. PE2 MUST treat this as a separate
defect from the original remote AC defect and MUST
generate a PW status message indicating ææAC Transmit
FaultÆÆ towards PE1.PE1 MUST terminate the received PW
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status message and does not perform any additional action
since it is sourcing a F4/F5 RDI towards CE1.
For PEW3 over L2TP-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):
a. PE1 MUST send an L2TP Set-Link Info (LSI) message with a
Circuit Status AVP indicating "inactive".
b. PE1 MUST generate a F4/F5 RDI on the related ACs towards
CE1 in response to a received F4/F5 AIS only.
c. On reception of the L2TP LSI message, PE2 MUST generate a
F4/F5 AIS on the related ATM ACs towards CE2.
d. 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 treat this as a separate
defect from the original remote AC defect and MUST
generate an L2TP Set-Link Info (LSI) message with a
Circuit Status AVP indicating "inactive" towards PE1. On
its reception, PE1 MUST cease the generation of RDI and
generate a F4/F5 AIS towards CE1. CE1 will reply with a
F4/F5 RDI which if received by PE1 and is terminated since
PE1 has already sent a LSI to inform PE2 of an AC defect.
9.5.3 Procedures in ATM Port Mode
In case of transparent cell transport, 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
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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)
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
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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.
12.1 Ethernet AC State
A PE changes the state of an Ethernet AC to DOWN if any of the
following conditions are met:
(i) A physical layer alarm is detected on the Ethernet
interface.
A PE exits the Ethernet AC Down state when all defects it had
previously detected have disappeared.
12.2 Mapping of Defect States from a PW to a Ethernet AC
The procedures are the same as those described in Section 8.3 for
a FR encapsulation. The only difference is that there is no defect
notification available on the Ethernet AC. If an egress PE
determines that all ACs on a specific Ethernet physical interface
are affected, it MAY propagate these alarms by bringing the entire
physical interface down.
12.3 Frame Relay Network and Attachment Circuit Defects
The procedures are the same as those described in Section 8.4 for
a FR encapsulation. The only difference is that there is no defect
notification available on the Ethernet AC. If an egress PE
determines that all ACs on a specific Ethernet physical interface
are affected, it MAY propagate these alarms by bringing the entire
physical interface down.
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
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[CEP] Malis, A., et.al., "SONET/SDH Circuit Emulation over Packet
(CEP)", Internet Draft <draft-ietf-pwe3-sonet-09.txt>, August
2004
[CONGESTION] Rosen, E., Bryant, S., Davie, B., "PWE3 Congestion
Control Framework", Internet Draft <draft-rosen-pwe3-
congestion-02.txt", September 2004
[CONTROL] Martini, L., Rosen, E., Smith, T., "Pseudowire Setup and
Maintenance using LDP", Internet Draft <draft-ietf-pwe3-
control-protocol-11.txt>, October 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
[MPLS-in-IP] Worster. T., et al., ææEncapsulating MPLS in IP or
Generic Routing Encapsulation (GRE)ÆÆ, draft-ietf-mpls-in-ip-
or-gre-08.txt, June 2004.
[OAM REQ] T. Nadeau et.al., "OAM Requirements for MPLS Networks",
Internet Draft <draft-ietf-mpls-oam-requirements-04>,
September 2004
[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-07.txt>, Ocotber 2004
[PWREQ] Xiao, X., McPherson, D., Pate, P., "Requirements for
Pseudo Wire Emulation Edge to-Edge (PWE3)", RFC 3916,
September 2004
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[RSVP-TE] Awduche, D., et.al. " RSVP-TE: Extensions to RSVP for
LSP Tunnels", RFC 3209, December 2001
[VCCV] Nadeau, T., et al."Pseudo Wire Virtual Circuit Connection
Verification (VCCV)", Internet Draft <draft-ietf-pwe3-vccv-
03.txt>, June 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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in BCP-11. Copies of claims of rights made available for
publication and any assurances of licenses to be made available,
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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
any copyrights, patents or patent applications, or other
proprietary rights which may cover technology that may be required
to practice this standard. Please address the information to the
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17 Full Copyright Statement
"Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights."
"This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
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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
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
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Internet Draft draft-ietf-pwe3-oam-msg-map-01.txt October 2004
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
David Allan
Nortel Networks
3500 Carling Ave.,
Ottawa, Ontario, CANADA
Email: dallan@nortelnetworks.com
Nadeau, et al. Expires April 2005 [Page 26]
