Network Working Group Thomas D. Nadeau, Ed.
Internet Draft Monique Morrow, Ed.
Proposed Status: Standards Track Cisco Systems, Inc.
Expiration Date: September 2007
Peter Busschbach, Ed.
Mustapha Aissaoui, Ed.
Alcatel-Lucent
Dave Allan, Ed.
Nortel Networks
March 2007
Pseudo Wire (PW) OAM Message Mapping
draft-ietf-pwe3-oam-msg-map-05.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies the mapping of defect states between a
Pseudo Wire and the Attachment Circuits (AC) of the end-to-end
emulated service. This document covers the case whereby the ACs
and the PWs are of the same type in accordance to the PWE3
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architecture [RFC3985] such that a homogenous PW service can be
constructed.
Table of Contents
1 Introduction .................................................2
2 Terminology...................................................5
3 Reference Model and Defect Locations..........................6
4 Abstract Defect States........................................7
5 PW Status and Defects.........................................8
6 PW Defect State Entry/Exit...................................16
7 AC Defect States.............................................17
8 PW Forward Defect Entry/Exit procedures......................19
9 AC Defect Entry/Exit Procedures..............................22
10 SONET Encapsulation (CEP)...................................24
11 TDM Encapsulation...........................................24
12 Appendix A: Native Service Management.......................26
13 Security Considerations.....................................27
14 Acknowledgments.............................................28
15 IANA Considerations ........................................28
16 References..................................................28
17 Authors' Addresses..........................................30
1. Introduction
This document specifies the mapping of defect states between a
Pseudo Wire and the Attachment Circuits (AC) of the end-to-end
emulated service. This document covers the case whereby the ACs
and the PWs are of the same type in accordance to the PWE3
architecture [RFC3985] such that a homogenous PW service can be
constructed. This document is motivated by the requirements put
forth in [RFC4377] and [RFC3916].
Ideally only PW and AC defects need be propagated into the Native
Service (NS), and NS OAM mechanisms are transported transparently
over the PW. Some homogenous scenarios use PW specific OAM
mechanisms to synchronize defect state between PEs due to
discontinuities in native service OAM between the AC and the PW
(e.g. FR LMI).
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.
This document covers PWE over MPLS PSN, PWE over IP PSN and PWE
over L2TP PSN.
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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. Terminology
AIS Alarm Indication Signal
AC Attachment circuit
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
NS Native Service
OAM Operations and Maintenance
PE Provider Edge
PW Pseudowire
PSN Packet Switched Network
RDI Remote Defect Indication
SDU Service Data Unit
VCC Virtual Channel Connection
VPC Virtual Path Connection
The rest of this document will follow the following conventions:
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 [RFC4023], with an MPLS
shim header used as PW demultiplexer, will be referred to as an
MPLS-IP PSN. A PSN, which makes use of L2TPv3 [RFC3931] as the
tunneling technology, will be referred to as L2TP-IP PSN.
If LSP-Ping [RFC4379] 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.
In the context of this document a PE forwards packets between an
AC and a PW. The other PE that terminates the PW is the peer PE
and the attachment circuit associated with the far end PW
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termination is the remote AC.
Defects are discussed in the context of defect states, and the
criteria to enter and exit the defect state.
The direction of defects is discussed from the perspective of the
observing PE and what the PE may explicitly know about information
transfer capabilities of the PW service.
A forward defect is one that impacts information transfer to the
observing PE. It impacts the observing PEs ability to receive
information. A forward defect MAY also imply impact on information
sent or relayed by the observer (and as it cannot receive is
therefore unknowable) and so the forward defect state is
considered to be a superset of the two defect states.
A reverse defect is one that uniquely impacts information sent or
relayed by observer.
3. 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.............|----------| |
+----+ | |==================| | +----+
^ +----+ +----+ ^
| Provider Edge 1 Provider Edge 2 |
| |
|<-------------- Emulated Service ---------------->|
Customer Customer
Edge 1 Edge 2
Figure 1: PWE3 Network Defect Locations
In all interworking scenarios described in this document, it is
assumed that at PE1 the AC and the PW are of the same type. The
procedures described in this document exclusively apply to PE1.
PE2 for a homogenous service implements the identical
functionality (although it is not required to as long as the
notifications across the PWs are consistent).
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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 the native service 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
(which is considered a remote AC defect in the context of
procedures outlined in this draft). The defect is conveyed to
PE2 and to the remote L2 network (N1) using the native
service OAM defect indication.
(f) Defect on a PE2 AC interface (which is also considered a
remote AC defect in the context of this draft).
4. Abstract Defect States
PE1 is obliged to track four abstract defect states that reflect
the observed state of both directions of the PW service on both
the AC and the PW sides. Faults may impact only one or both
directions of the PW.
The observed state is a combination of faults directly detected by
PE1, or faults it has been made aware of via notifications.
+-----+
----AC forward---->| |-----PW reverse---->
CE1 | PE1 | PE2/CE2
<---AC reverse-----| |<----PW forward-----
+-----+
(arrows indicate direction of user traffic impacted by a defect)
Figure 2: Forward and Reverse Defect States and Notifications
PE1 will directly detect or be notified of AC forward and PW
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forward defects as they occur upstream of PE1 and impact traffic
being sent to PE1.
In Figure 2, PE1 may be notified of a forward defect in the AC by
receiving a Forward Defect indication, e.g., ATM AIS, from CE1.
This defect impacts the ability of PE1 to receive user traffic
from CE1 on the AC. PE1 can also directly detect this defect if it
resulted from a failure of the receive side in the local port or
link over which the AC is configured.
Similarly, PE1 may detect or be notified of a forward defect in
the PW by receiving a Forward Defect indication from PE2. This
notification can either be a Local PSN-facing PW (egress)
Transmit Fault or a Local Attachment Circuit (ingress) Receive
Fault. This defect impacts the ability of PE1 to receive user
traffic from CE2.
Note that the AC or PW Forward Defect notification is sent in the
same direction as the user traffic impacted by the defect.
PE1 will only be notified of AC reverse and PW reverse defects as
they universally will be detected by other devices and only impact
traffic that has already been relayed by PE1. In Figure 2, PE1 may
be notified of a reverse defect in the AC by receiving a Reverse
Defect indication, e.g., ATM RDI, from CE1. This defect impacts
the ability of PE1 to send user traffic to CE1 on the AC.
Similarly, PE1 may be notified of a reverse defect in the PW by
receiving a Reverse Defect indication from PE2. This notification
can either be a Local PSN-facing PW (ingress) Receive Fault or a
Local Attachment Circuit (egress) Transmit Fault. This defect
impacts the ability of PE1 to send user traffic to CE2.
Note that the AC or PW Reverse Defect notification is sent in the
reverse direction to the user traffic impacted by the defect.
The procedures outlined in this document define the entry and exit
criteria for each of the four states with respect to the set of
potential ACs and PWs within the document scope and the consequent
actions that PE1 must perform to properly interwork those
notifications. The abstract defect states used by PE1 are common
to all potential interworking combinations of PWs and ACs.
When a PE has multiple sources of notifications from a peer (e.g.
PSN control plane, LDP control plane, BFD), it is obliged to track
all sources, but with respect to consequent actions the forward
state ALWAYS has precedence over the reverse state.
5. PW Status and Defects
This section describes possible PW defects, ways to detect them
and consequent actions.
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5.1 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 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.
5.1.1 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 [RFC3985], it has the
ability to detect packet loss. Translation of congestion detection
ato PW defect states is outside the scope of this specification.
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 select another path is
usually at the discretion of the network operator.
5.2 Defect Detection and Notification
5.2.1 Defect Detection Tools
To detect the defects listed in 7.1, Service Providers have a
variety of options available:
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Physical Layer defect detection and notification 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 [RFC3931] apply.
Furthermore, the tools Ping and Traceroute, based on ICMP Echo
Messages apply [RFC792].
For PWE3 over an MPLS PSN and an MPLS-IP PSN, several tools can be
used.
- LSP-Ping and LSP-Traceroute( [RFC4379]) 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 [RFC3209], 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.
Furthermore, the detection of a fault could occur at different
points in the network and there are several ways the observing PE
determines a fault exists:
a. egress PE detection of failure (e.g. BFD)
b. ingress PE detection of failure (e.g. LSP-PING)
c. ingress PE notification of failure (e.g. RSVP Path-err)
5.2.2 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
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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
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.
5.3 Overview of fault notifications
For a MPLS PSN and a IP PSN using MPLS-in-IP [RFC4023], a
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PW that are established and maintained using LDP SHOULD use
LDP status signaling messages as the default mechanism
for AC, PW status and defect notification [RFC4447]. If
a combination of VCCV+BFD [VCCV] status and LDP status are
used, LDP status MUST take precidence over VCCV-BFD status
as as outlined below in Section 5.3.4. For PWs established
using other means such as static configuration, inband signaling
using VCCV-BFD [VCCV] SHOULD be used to convey AC and PW
status.
For a IP PSN using L2TPv3, i.e., a L2TP-IP PSN, StopCCN and CDN
messages are used for conveying defects in the PSN and PW
respectively, while the Set-Link-Info (SLI) messages are used to
convey status and defects in the AC and local L2 network.
5.3.1 Use of Native Service notifications
In the context of this document, ATM and unstructured SONET/TDM
PWs are the only examples of a PW that has native service
notification capability. Frame relay does have the FR OAM
specification [FRF.19], but this is not commonly deployed. All
other PWs use PW specific notification mechanisms.
ATM PWs may optionally also use PW specific notification
mechanisms.
In normal, i.e., defect-free, operation, all the types of ATM OAM
cells described in Section 12.2 are either terminated at the PE,
for OAM segments terminating in the AC endpoint, or transparently
carried over the PSN tunnel [RFC4714]. This is referred to as
inband ATM OAM over PW 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 PEs which cannot generate and/or transmit ATM OAM cells over
the ATM PW. This is referred to as Out-of-band ATM OAM over PW.
Ethernet OAm is not covered in this specification.
5.3.2 The Use of PW Status for MPLS and MPLS-IP PSNs
For a MPLS PSN and a IP PSN using MPLS-in-IP [RFC4023], a
PW that are established and maintained using LDP SHOULD use
LDP status signaling messages as the default mechanism
for AC, PW status and defect notification [RFC4447]. If
a combination of VCCV+BFD [VCCV] status and LDP status are
used, LDP status MUST take precidence over VCCV-BFD status
as as outlined below in Section 5.3.4. For PWs established
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using other means such as static configuration, inband signaling
using VCCV-BFD [VCCV] SHOULD be used to convey AC and PW
status.
[RFC4446] defines the following valid PW status codepoints.
[RFC4447] specifies that Pseudo Wire forwarding is used to
clear all faults and that Pseudo Wire Not Forwarding is used to
convey any other defects that cannot be represented by the other
codepoints. The remaining codepoints map to the forward defect
and reverse defect defined in this document as follows:
Forward defect - corresponds to the logical OR of
Local Attachment Circuit (ingress)
Receive Fault and Local PSN-facing
PW (egress) Transmit Fault
Reverse defect - corresponds to the logical OR of
Local Attachment Circuit (egress)
Transmit Fault and Local PSN-facing
PW (ingress) Receive Fault
PW status is used to convey the defect view of the PW local to the
originating PE. This is the local PW state. This state is conveyed
in the form of a forward defect or a reverse defect.
Thus PW status (when available) shall be used to report the
following failures:
- Failures detected through defect detection mechanisms in the
MPLS and MPLS-IP PSN
- Failures detected through VCCV-Ping
- Failures within the PE that result in an inability to forward
traffic between ACs and PW
- State of the AC when the PE does not have native service OAM
capability or emulation of native service OAM capability is
prohibitive. This state is conveyed in the form of a forward
defect or a reverse defect.
Note that there are a couple of situations which require PW label
withdrawal as opposed to a PW status notification by the PE. The
first one is when the PW is taken administratively down in
accordance to [RFC4447]. The second one is when the Target
LDP session established between the two PEs is lost. In the
latter case, the PW labels will need to be re-signaled when the
Targeted LDP session is re-established.
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5.3.3 The Use of L2TP STOPCCN and CDN
[RFC3931] describes the use of STOPCCN and CDN messages to exchange
alarm information between PEs. A StopCCN message indicates that
the control connection has been shut down by the remote PE
[RFC3931]. 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 [RFC3931]. 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.
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-Ping
- 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.
5.3.4 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. This section describes the
behavior of the PE nodes with respect to using one or both methods
for detecting and propagating defect state.
For a MPLS-PSN, the PEs negotiate the use of the VCCV
capabilities when the label mapping messages are exchanged to
establish the two directions of the PW. An OAM capability TLV
is signaled as part of the PW FEC interface parameters TLV.
The CV Type Indicators field in this TLV defines a bitmask used
to indicate the specific OAM capabilities that the PE can make
use of over the PW being established. A CV type of 0x04 indicates
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that BFD is used for PW fault detection only.
In this mode, only two of the diagnostics codes specified in [BFD]
will be used: They are:
0 - No diagnostic:
1 - Control detection time expired
7 - Administratively Down
The first code indicates that the peer PE is correcty receiving
BFD control messages. The second code indicates that the peer has
stopped receiving BFD control messages. A PE shall use
"Administrative down" to bring down the BFD session when the PW is
brought down administratively. All other defects - such as AC
defects and PE internal failures that prevent it from forwarding
traffic - must be communicated through PW Status messages, in
the case of MPLS PSN or MPLS-IP PSN, or the appropriate L2TP
codes in the case of L2TP-IP PSN, as defined in 5.3.2 and 5.3.3.
A CV type of 0x08 in the OAM capabilities TLS indicates that BFD
is used for both PW fault detection and AC/PW Fault Notification.
In this case, all defects - including AC defects and PE internal
failures - are signaled through BFD.
6. PW Defect State Entry/Exit
6.1 PW Forward Defect Entry/Exit
A PE will enter the PW forward defect state if one of the
following occurs
- 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 PE2 indicating PW forward defect or
PW not forwarding, which indicates PE2 detected or was notified
of a PW fault downstream of it or that there was a remote AC
fault.
- It detects a loss of PW connectivity, including label errors,
through VCCV-BFD or VCCV-PING in no reply mode.
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
entered the forward defect state.
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PE1 will exit the forward defect state if the PW status
received from PE2 has the forward defect indication cleared,
and it has established that PW/PSN connectivity is working
in the forward direction. Note that this may result in a
transition to the PW operational or PW reverse defect states.
For a PWE3 over a L2TP-IP PSN, a PE will exit the PW forward
defect state when the following conditions are true:
- All defects it had previously detected have disappeared, and
- A L2TPv3 session is successfully established to carry the PW
packets.
6.2 PW reverse defect state entry/exit
A PE will enter the PW reverse defect state if
it receives a message from PE2 indicating PW reverse defect
which indicates PE2 detected or was notified of a PW/PSN fault
upstream of it or that there was a remote AC fault and it is not
already in the PW forward defect state.
PE1 will exit the reverse defect state if the PW status
received from PE2 contains the reverse defect indication
cleared, or it has entered the PW forward defect state.
For a PWE3 over a L2TP-IP PSN, the PW reverse defect state is not
valid and a PE can only enter the PW forward defect state.
6.2.1 PW reverse defects that require PE state synchronization
Some PW mechanisms will result in PW defects being detected by or
notified to PE1 when PE1 is upstream of the fault but the
notification did not originate with PE2. The resultant actions are
identical to that of entering the PW reverse defect state with the
addition that PE1 needs to synchronize state with PE2 and the PW
state communicated from PE1 to PE2 needs to indicate state
accordingly.
When the PSN uses RSVP-TE or proactively uses LSP-PING as a PW
fault detection mechanism, PE1 must enter to the PW reverse defect
state.
The exit criteria being when, the RSVP fault state or the LSP-PING
fault state exit criteria has been met, indicating no PW reverse
defects.
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7 AC Defect States
7.1 FR ACs
PE1 enters the AC Forward Defect state if any of the following
conditions are met:
(i) A PVC is not deleted 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 inactive. 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 AC Forward Defect state when all defects it had
previously detected have disappeared.
The AC reverse defect state is not valid for FR ACs.
7.2 ATM ACs
7.2.1 AC Forward Defect State Entry/Exit
PE1 enters the AC forward defect state if any of the following
conditions are met:
(i) It detects or is notified of a physical layer fault on the
ATM interface and/or it terminates an F4 AIS flow or has
loss of F4 CC for a VP carrying VCCs.
(ii) It terminates an F4 AIS OAM flow, in the case of a VPC,
or an F5 AIS OAM flow, in the case of a VCC, indicating
that the ATM VPC or VCC is down in the adjacent L2 ATM
network (e.g., N1 for PE1). This is applicable to the
case of the out-of-band ATM OAM over PW method only.
(iii) It detects loss of connectivity on the NS ATM VPC/VCC
while terminating ATM continuity checking (ATM CC) with
the local ATM network and CE.
A PE exits the AC Forward Defect state when all defects it had
previously detected have disappeared. The exact conditions under
which a PE exits the AIS state, or declares that connectivity is
restored via ATM CC are defined in I.610 [I.610].
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7.2.2 AC Reverse Defect State Entry/Exit
A PE enters the AC reverse defect state if any of the following
conditions are met:
(i) It terminates an F4 RDI OAM flow, in the case of a
VPC, or an F5 RDI OAM flow, in the case of a VCC,
indicating that the ATM VPC or VCC is down in the
adjacent L2 ATM network (e.g., N1 for PE1). This is
applicable to the case of the out-of-band ATM OAM over
PW method only.
A PE exits the AC Reverse Defect state if the AC state transitions
to working or to the AC forward defect state. The criteria for
exiting the RDI state are described in I.610.
7.3 Ethernet AC State
PE1 enters the forward defect state 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 forward defect state when all defects
it had previously detected have disappeared.
8. PW Forward Defect Entry/Exit procedures
8.1 PW Forward Defect Entry Procedures
8.1.1 FR AC procedures
These procedures are applicable only if the transition from the
working state to the PW Forward defect state. A transition from PW
reverse defect state to the forward defect state does not require
any additional notification procedures to the FR AC as it has
already been told the peer is down.
(i) 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.
8.1.2 Ethernet AC Procedures
No procedures are currently defined.
8.1.3 ATM AC procedures
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The following text refers to AIS, RDI and CC without specifying
whether it it is an F4 (VP-level) flow or an F5 (VC-level)
flow, or whether it is an end-to-end or a segment flow.
Precise ATM OAM procedures are specified elsewhere (e.g. I.610)
and such references complement the descriptions below.
Note that it is a network operator option to support segment
OAM and to identify and provision PEs as segment end points.
On entry to the PW Forward Defect State
(i) PE1 MUST commence AIS insertion into the corresponding
AC.
(ii) PE1 MUST terminate any CC generation on the
corresponding AC.
8.1.4 Additional procedures for a FR PW, an ATM PW in the
out-of-band ATM OAM over PW method, and an Ethernet PW
If the PW failure was explicitly detected by PE1, it MUST assume
PE2 has no knowledge of the defect and MUST notify PE2 in the form
of a reverse defect notification:
For PW over MPLS PSN or MPLS-IP PSN
(i) A PW Status message indicating a reverse defect, or
(ii) A VCCV-BFD diagnostic code if the optional use of VCCV-BFD
notification has been negotiated
For PW over L2TP-IP PSN
(i) An L2TP Set-Link Info (LSI) message with a Circuit Status
AVP indicating "active" Or,
(ii) A VCCV-BFD diagnostic code if the optional use of VCCV-BFD
notification has been negotiated
Otherwise the entry to the defect state was the result of a
notification from PE2 (indicating that PE2 already had knowledge
of the fault) or loss of the control adjacency (similarly visible
to PE2).
8.2 PW Forward Defect Exit Procedures
8.2.1 FR AC procedures
On transition from the PW forward defect state to the reverse
defect state PE1 takes no action w.r.t. the AC.
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On exit from the PW Forward defect state
(i) PE1 MUST generate a full status report with the Active bit
= 1 (and optionally in the asynchronous status message),
as per Q.933 annex A, into N1 for the corresponding FR
ACs.
8.2.2 Ethernet AC Procedures
No procedures are currently defined
8.2.3 ATM AC procedures
On exit from the PW Forward Defect State
(i) PE1 MUST cease AIS insertion into the corresponding AC.
(ii) PE1 MUST resume any CC generation on the corresponding
AC.
8.2.4 Additional procedures for a FR PW, an ATM PW in the
out-of-band ATM OAM over PW method, and an Ethernet PW
If the PW failure was explicitly detected by PE1, it MUST notify
PE2 in the form of clearing the reverse defect notification:
For PW over MPLS PSN or MPLS-IP PSN
(i) A PW Status message with the reverse defect indication
clear, and the remaining indicators showing either working
or a transition to the forward defect state. Or,
(ii) A VCCV-BFD diagnostic code with the same attribute as (i)
if the optional use of VCCV-BFD notification has been
negotiated
For PW over L2TP-IP PSN
(i) An L2TP Set-Link Info (LSI) message with a Circuit Status
AVP indicating "active" Or,
(ii) A VCCV-BFD diagnostic code with the same attributes as (i)
if the optional use of VCCV-BFD notification has been
negotiated
8.3 PW Reverse Defect Entry Procedures
8.3.1 FR AC procedures
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On transition from the PW forward defect state to the reverse
defect state PE1 takes no action w.r.t. the AC.
On entry to the PW reverse defect state
(i) 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.
8.3.2 Ethernet AC Procedures
No procedures are currently defined
8.3.3 ATM AC procedures
On entry to the PW Reverse Defect State
(i) PE1 MUST commence RDI insertion into the corresponding
AC. This applies to the case of an ATM PW in the out-of-
band ATM OAM over PW method only.
8.4 PW Reverse Defect Exit Procedures
8.4.1 FR AC procedures
On transition from the PW reverse defect state to the PW forward
defect state PE1 takes no action with respect to the AC.
On exit from the PW Reverse defect state
(i) PE1 MUST generate a full status report with the Active bit
= 1 (and optionally in the asynchronous status message),
as per Q.933 annex A, into N1 for the corresponding FR
ACs.
8.4.2 Ethernet AC Procedures
No procedures are currently defined
8.4.3 ATM AC procedures
On exit from the PW Reverse Defect State
(i) PE1 MUST cease RDI insertion into the corresponding AC.
This applies to the case of an ATM PW in the out-of-band ATM OAM
over PW method only.
8.5 Procedures in FR Port Mode
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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.
8.6 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 cannot relay PW defect state over these VCCs and VPCs.
If ATM CC is run on the VCCs and VPCs end-to-end (CE1 to CE2), or
on a segment originating and terminating in the ATM network and
spanning the PSN network, it will timeout and cause the CE or ATM
switch to enter the ATM AIS state.
9 AC Defect Entry/Exit Procedures
9.1 AC Forward defect entry:
On entry to the forward defect state, PE1 may need to perform
procedures on both the PW and the AC.
9.1.1 Procedures for a FR PW, an ATM PW in the out-of-band ATM OAM
over PW method, or an Ethernet PW
On entry to the AC forward defect state, PE1 notifies PE2 of a
forward defect:
For PW over MPLS PSN or MPLS-IP PSN
(i) A PW Status message indicating forward defect, or
(ii) A VCCV-BFD diagnostic code of forward defect if the
optional use of VCCV-BFD notification has been negotiated.
For PW over L2TP-IP PSN
(i) An L2TP Set-Link Info (LSI) message with a Circuit Status
AVP indicating "inactive", or
(ii) A VCCV-BFD diagnostic code of forward defect if the
optional use of VCCV-BFD notification has been negotiated.
9.1.2 Procedures for a ATM PW in the inband ATM OAM over PW
method
On entry to the AC forward defect state, PE1 MUST:
a. Commence insertion of ATM AIS cells into the corresponding
PW.
b. If PE1 is originating F4 or F5 I.610 CC cells, PE1 will
suspend CC generation for the duration of the defect
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state.
9.1.3 Additional procedures for ATM ACs
On entry to the AC forward defect state PE1 will commence RDI
insertion into the AC as per I.610. This procedure is applicable
to the out-of-band ATM OAM over PW method only.
9.2 AC Reverse defect entry
9.2.1 Procedures for a FR PW, an ATM PW in the out-of-band ATM OAM
over PW method, or an Ethernet PW
On entry to the AC reverse defect state, PE1 notifies PE2 of a
reverse defect:
For PW over MPLS PSN or MPLS-IP PSN
(iii) A PW Status message indicating reverse defect,or
(iv) A VCCV-BFD diagnostic code of reverse defect if the
optional use of VCCV-BFD notification has been negotiated.
For PW over L2TP-IP PSN
(iii) An L2TP Set-Link Info (LSI) message with a Circuit Status
AVP indicating "inactive", or
(iv) A VCCV-BFD diagnostic code of reverse defect if the
optional use of VCCV-BFD notification has been negotiated.
9.2.2 Procedures for a ATM PW in the inband ATM OAM over PW
method
There are no procedures in this case as the AC reverse defect
state is not valid for PE1 operating in this method.
9.3 AC Forward Defect Exit
9.3.1 Procedures for a FR PW, an ATM PW in the out-of-band ATM OAM
over PW method, or an Ethernet PW
On exit from the AC forward defect state PE1 notifies PE2 that the
forward defect state has cleared (note that this may be a direct
state transition to either the working state or the reverse defect
state):
For PW over MPLS PSN or MPLS-IP PSN
(i) A PW Status message with forward defect clear and the
remaining indicators showing either working or reverse
defect state, or
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(ii) A VCCV-BFD diagnostic code with the same attributes as (i)
if the optional use of VCCV-BFD notification has been
negotiated.
For PW over L2TP-IP PSN
(i) An L2TP Set-Link Info (LSI) message with a Circuit Status
AVP indicating "active", or
(ii) A VCCV-BFD diagnostic code with the same attributes as (i)
if the optional use of VCCV-BFD notification has been
negotiated.
9.3.2 Procedures for a ATM PW in the inband ATM OAM over PW
method
On exit from the AC forward defect state, PE1 MUST:
(i) Cease insertion of ATM AIS cells into the corresponding
PW.
(ii) If PE1 is originating F4 or F5 I.610 CC cells, PE1 will
resume CC generation for the duration of the defect state.
9.3.3 Additional procedures for ATM ACs
On exit from the AC forward defect state PE1 will cease RDI
insertion into the AC as per I.610. This procedure is applicable
to the out-of-band ATM OAM over PW method only.
9.4 AC Reverse Defect Exit
9.4.1 Procedures for a FR PW, an ATM PW in the out-of-band ATM OAM
over PW method, or an Ethernet PW
On exit from the AC reverse defect state, PE1 notifies PE2 that
the reverse defect state has cleared (note that this may be a
direct state transition to either the working state or the forward
defect state):
For PW over MPLS PSN or MPLS-IP PSN
(i) A PW Status message with the reverse defect indicator
cleared and the remaining indicators showing either
working or a transition to the forward defect state, or
(ii) A VCCV-BFD diagnostic code with the same information as
(i) if the optional use of VCCV-BFD notification has been
negotiated.
For PW over L2TP-IP PSN
(i) An L2TP Set-Link Info (LSI) message with a Circuit Status
AVP indicating "active", or
(ii) A VCCV-BFD diagnostic code with the same information as
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(i) if the optional use of VCCV-BFD notification has been
negotiated.
9.4.2 Procedures for a ATM PW in the inband ATM OAM over PW
method
There are no procedures in this case as the AC reverse defect
state is not valid for PE1 operating in this method.
10 SONET Encapsulation (CEP)
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)
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
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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 Appendix A: Native Service Management
12.1 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]
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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. TO achieve commonality
with other technologies, down is represented as a forward
defect.
Frame relay connection management is not implemented over the PW
using either of the techniques native to FR, therefore PW
mechanisms are used to synchronize the view each PE has of the
remote NS/AC. A PE will treat a remote NS/AC failure in the same
way it would treat a PW or PSN failure, that is using AC facing FR
connection management to notify the CE that FR is down.
12.2 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
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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 to instrument virtual paths
and virtual channels respectively with regard to their performance
and availability. 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.
12.3 Ethernet Management
At this point in time, inband Ethernet OAM standards are being
specified in the International Telecommunications Union
Telecommunications (ITU-T) and the Institute of Electrical and
Electronics Engineers (IEEE). However, it will take some time
before they are widely deployed. Therefore, this document
specifies only the procedures for mapping a defect due to a
Ethernet physical layer fault. Defects on a remote Ethernet AC or
defects in a PW cannot be mapped back to the local Ethernet
network.
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, Chris Metz, Ben Washam, Tiberiu
Grigoriu.
15. IANA Considerations
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None at this time.
16. References
16.1 Normative References
[BFD] Katz, D., Ward, D., "Bidirectional Forwarding Detection",
Internet Draft <draft-ietf-bfd-base-05.txt>, June 2006.
[FRF.19] Frame Relay Forum, Frame Relay Operations,
Administration, and Maintenance Implementation Agreement,
March 2001.
[RFC792] Postel, J. "Internet Control Message Protocol",
RFC792
[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
[RFC3931] Lau, J., et.al. " Layer Two Tunneling Protocol (Version
3", RFC 3931, March 2005
[RFC4379] Kompella, K., Pan, P., Sheth, N., Cooper, D., Swallow,
G., Wadhwa, S., Bonica, R., " Detecting MPLS Data Plane
Failures", RFC4379, February 2006.
[RFC4023] Worster. T., et al., Encapsulating MPLS in IP or
Generic Routing Encapsulation (GRE), RFC 4023,
March 2005.
16.2 Informative References
[RFC3985] Bryant, S., "Pseudo Wire Emulation Edge-to-Edge (PWE3)
Architecture", RFC 3985, March 2005.
[RFC4377] Nadeau, T. et.al., "OAM Requirements for MPLS Networks",
RFC4377, February 2006.
[RFC4447] Martini, L., Rosen, E., Smith, T., "Pseudowire
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Setup and Maintenance using LDP", RFC4447, April 2006.
[RFC4446] Martini, L., et al., "IANA Allocations for pseudo
Wire Edge to Edge Emulation (PWE3)", RFC4446,
April 2006.
[RFC4714] Martini, L., et al., "Encapsulation Methods for Transport
of ATM Cells/Frame Over IP and MPLS Networks", RFC4717,
December 2006
[RFC3916] Xiao, X., McPherson, D., Pate, P., "Requirements for
Pseudo Wire Emulation Edge to-Edge (PWE3)", RFC 3916,
September 2004
[RFC3209] 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-
13.txt>, March 2007.
17. 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
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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
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
Simon Delord
France Telecom
2 av, Pierre Marzin
22300 LANNION, France
E-mail: simon.delord@francetelecom.com
Vasile Radoaca
West Ridge Networks
Littleton, MA 01460
Email: vradoaca@westridgenetworks.com
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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 that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Nadeau et al. Expires September 2007 [Page 30]
