One document matched: draft-ietf-pwe3-redundancy-05.txt
Differences from draft-ietf-pwe3-redundancy-04.txt
Network Working Group Praveen Muley, Ed.
Internet Draft Mustapha Aissaoui, Ed.
Intended Status: Informational Matthew Bocci, Ed.
Expires: March 2012 Alcatel-Lucent
September 22, 2011
Pseudowire Redundancy
draft-ietf-pwe3-redundancy-05.txt
Abstract
This document describes a framework comprised of a number of
scenarios and associated requirements for pseudowire (PW)
redundancy. A set of redundant PWs is configured between provider
edge (PE) nodes in single segment PW applications, or between
Terminating PE nodes in Multi-Segment PW applications. In order for
the PE/T-PE nodes to indicate the preferred PW to use for forwarding
PW packets to one another, a new PW status is required to indicate
the preferential forwarding status of active or standby for each PW
in the redundancy set.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 30, 2011
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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respect to this document. Code Components extracted from this
document must include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License.
Requirements Language
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 [1].
Table of Contents
Copyright Notice.....................................2
1. Introduction.........................................3
2. Terminology.........................................3
3. Reference Models.....................................5
3.1. PE Architecture..................................5
3.2. PW Redundancy Network Reference Scenarios.............5
3.2.1. Single Multi-Homed CE.........................5
3.2.2. Multiple Multi-homed CEs.......................7
3.3. Single Homed CE with MS-PW redundancy................9
3.4. PW redundancy between MTU-s in H-VPLS...............10
3.5. PW redundancy between VPLS n-PEs...................11
3.6. PW redundancy in VPLS Bridge Module Model............11
4. Generic PW redundancy requirements......................13
4.1. Protection switching requirements..................13
4.2. Operational requirements..........................13
5. Security Considerations...............................14
6. IANA considerations..................................14
7. Major Contributing Authors............................15
8. Acknowledgments.....................................15
9. References.........................................16
9.1. Normative References.............................16
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9.2. Informative References...........................16
Author's Addresses.....................................17
1. Introduction
The objective of PW redundancy is to provide sparing of attachment
circuits (ACs), Provider Edge nodes (PEs), and Pseudowires (PWs) to
eliminate single points of failure, while ensuring that only one
active path between a pair of Customer Edge nodes (CEs).
In single-segment PW (SS-PW) applications, protection for the PW is
provided by the PSN layer. This may be a Resource Reservation
Protocol with Traffic Engineering (RSVP-TE) labeled switched path
(LSP) with a fast-Reroute (FRR) backup or an end-to-end backup LSP.
PSN protection mechanisms cannot protect against failure of the a PE
node or the failure of the remote AC. Typically, this is supported
by dual-homing a Cutomer Edge (CE) node to different PE nodes which
provide a pseudowire emulated service across the PSN. A set of PW
mechanisms is theerfore required that enables a primary and one or
more backup backup PWs to terminate on different PE nodes.
In multi-segment PW (MS-PW) applications, PSN protection mechanisms
cannot protect against the failure of a switching PE (S-PE). A set
of mechanisms that support the operation of a primary and one or
more backup PWs via a diferent set of S-PEs is therefore required.
The paths of these PWs are diverse in the sense that they are
switched at different S-PE nodes.
In both of these applications, PW redundancy is important to
maximise the resiliency of the emulated service.
This document describes framework for these applications and its
associated operational requirements. The framework utilizes a new PW
status, called the Preferential Forwarding Status of the PW. This is
separate from the operational states defined in RFC4447 [2]. The
mechanisms for PW redundancy are modeled on general protection
switching principles.
2. Terminology
o UP PW: A PW which has been configured (label mapping exchanged
between PEs) and s not in any of the PW defect states specified
in [2]. Such a PW is available for forwarding traffic.
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o DOWN PW: A PW that has either not been fully configured, or has
been configured and is in any one of the PW defect states
specified in [2]. Such a PW is not available for forwarding
traffic.
o Active PW. An UP PW used for forwarding user, OAM and control
plane traffic.
o Standby PW. An UP PW that is not used for forwarding user traffic
but may forward OAM and specific control plane traffic.
o PW Endpoint: A PE where a PW terminates on a point where Native
Service Processing is performed, e.g., A Single Segment PW (SS-
PW) PE, a Multi-Segment Pseudowire (MS-PW) Terminating PE (T-PE),
or a Hierarchical VPLS MTU-s or PE-rs.
o Primary PW: the PW which a PW endpoint activates (i.e. uses for
forwarding) in preference to any other PW when more than one PW
qualifies for active state. When the primary PW comes back up
after a failure and qualifies for the active state, the PW
endpoint always reverts to it. The designation of Primary is
performed by local configuration for the PW at the PE.
o Secondary PW: when it qualifies for the active state, a Secondary
PW is only selected if no Primary PW is configured or if the
configured primary PW does not qualify for active state (e.g., is
DOWN). By default, a PW in a redundancy PW set is considered
secondary. There is no Revertive mechanism among secondary PWs.
o Revertive protection switching. Traffic will be carried by
primary PW if it is UP and a wait-to-restore timer expires and
primary PW is made the Active PW.
o Non-revertive protection switching. Traffic will be carried by
the last PW selected as a result of previous active PW entering
Operationally DOWN state.
o Manual selection of PW. The ability for the operator to manually
select the primary/secondary PWs.
This document uses the term 'PE' to be synonymous with both PEs as
per RFC3985 and T-PEs as per RFC5659.
This document uses the term 'PW' to be synonymous with both PWs as
per RFC3985 and SS-PWs, MS-PWs, S-PEs, PW-segment and PW
switching point as per RFC5659.
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3. Reference Models
Following sections describe show the reference architecture of the
PE for PW redundancy and its usage in different topologies and
applications.
3.1. PE Architecture
Figure 1 shows the PE architecture for PW redundancy, when more than
one PW in a redundant set is associated with a single AC. This is
based on the architecture in Figure 4b of RFC3985 [3]. The forwarder
selects which of the redundant PWs to use based on the criteria
described in this document.
+----------------------------------------+
| PE Device |
+----------------------------------------+
Single | | Single | PW Instance
AC | + PW Instance X<===========>
| | |
| |----------------------|
<------>o | Single | PW Instance
| Forwarder + PW Instance X<===========>
| | |
| |----------------------|
| | Single | PW Instance
| + PW Instance X<===========>
| | |
+----------------------------------------+
Figure 1 PE architecture for PW redundancy
3.2. PW Redundancy Network Reference Scenarios
This section presents a set of reference scenarios for PW
redundancy.
3.2.1. Single Multi-Homed CE
The following figure illustrates an application of single segment
pseudowire redundancy. This scenario is designed to protect the
emulated service against a failure of one of the PEs or ACs attached
to the multi-homed CE. Protection against failures of the PSN
tunnels is provided using PSN mechanisms such as MPLS Fast Reroute,
so that these failures do not impact the PW.
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CE1 is dual-homed to PE1 and PE3. A dual homing control protocol,
the details of which are outside the scope of this document, selects
which AC CE1 should use to forward towards the PSN, and which PE
(PE1 or PE3) should forward towards CE1.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| | | +-----+
| |----------|....|...PW1.(active)...|....|----------| |
| | | |==================| | | CE2 |
| CE1 | +----+ |PE2 | | |
| | +----+ | | +-----+
| | | |==================| |
| |----------|....|...PW2.(standby)..| |
+-----+ | | PE3|==================| |
AC +----+ +----+
Figure 2 PW Redundancy with one Multi-Homed CE
In this scenario, only one of the PWs should be used for forwarding
between PE1 / PE3, and PE2. PW redundancy determines which PW to
make active based on the forwarding state of the ACs so that only
one path is available from CE1 to CE2.
Consider the example where the AC from CE1 to PE1 is initially
active and the AC from CE1 to PE3 is initially standby. PW1 is made
active and PW2 is made standby in order to complete the path to CE2.
On failure of the AC between CE1 and PE1, the forwarding state of
the AC on PE3 transitions to Active. The preferential forwarding
state of PW2 therefore needs to become active, and PW1 standby, in
order to reestablish connectivity between CE1 and CE2. PE3 therefore
uses PW2 to forward towards CE2, and PE2 uses PW2 instead of PW1 to
forward towards CE1. PW redundancy in this scenario requires that
the forwarding status of the ACs at PE1 and PE3 be signaled to PE2
so that PE2 can choose which PW to make active.
Changes occurring on the dual homed side of network due to a failure
of the AC or PE are not propagated to the ACs on the other side of
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the network. Furthermore, failures in the PSN are not be propagated
to the attached CEs.
3.2.2. Multiple Multi-homed CEs
This scenario, illustrated in Figure 3, is also designed to protect
the emulated service against failures of the ACs and failures of the
PEs. Here, both CEs, CE1 and CE2, are dual-homed to their respective
PEs, PE1 and PE2, and PE3 and PE4. The method used by the CEs to
choose which AC to use to forward traffic towards the PSN is
determined by a dual-homing control protocol. The details of this
protocol are outside the scope of this document.
Note that the PSN tunnels are not shown in this figure for clarity.
However, it can be assumed that each of the PWs shown is
encapsulated in a separate PSN tunnel. Protection against failures
of the PSN tunnels is provided using PSN mechanisms such as MPLS
Fast Reroute, so that these failures do not impact the PW.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudowire ------->| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | |....|.......PW1........|....| | +-----+
| |----------| PE1|...... .........| PE3|----------| |
| CE1 | +----+ \ / PW3 +----+ | CE2 |
| | +----+ X +----+ | |
| | | |....../ \..PW4....| | | |
| |----------| PE2| | PE4|--------- | |
+-----+ | |....|.....PW2..........|....| | +-----+
AC +----+ +----+ AC
Figure 3 Multiple Multi-homed CEs with SS-PW redundancy
PW1 and PW4 connect PE1 to PE3 and PE4, respectively. Similarly, PE2
has PW2 and PW3 connect PE2 to PE4 and PE3. PW1, PW2, PW3 and PW4
are all UP. In order to support N:1 or 1:1 protection, only one PW
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MUST be selected to forward traffic. This document defines an
additional PW that reflects this forwarding state, which is separate
from the operational status of the PW. This is the 'Preferential
Forwarding Status'.
If a PW has a preferential forwarding status of 'active', it can be
used for forwarding traffic. The actual UP PW chosen by the combined
set of PEs that interconnect the CEs is determined by considering
the preferential forwarding status of each PW at each PE. The
mechanisms for achieving this selection are outside the scope of
this document. Only one PW is used for forwarding.
The following failure scenario illustrates the operation of PW
redundancy in Figure 2. In the initial steady state, when there are
no failures of the ACs, one of the PWs is chosen as the active PW,
and all others are chosen as standby. The dual-homing protocol
between CE1 and PE1/PE2 chooses to use the AC to PE2, while the
protocol between CE2 and PE3/PE4 chooses to use the AC to PE4.
Therefore the PW between PE2 and PE4 is chosen as the active PW to
complete the path between CE1 and CE2.
On failure of the AC between the dual-homed CE1 and PE2, the
preferential forwarding status of the PWs at PE1, PE2, PE3 and PE4
needs to change so as to re-establish a path from CE1 to CE2.
Different mechanisms can beused to achieve this and these are beyond
the scope of this document. After the change in status the algorithm
for selection of PW needs to revaluate and select PW to forward
traffic. In this application each dual-homing algorithm, i.e., {CE1,
PE1, PE2} and {CE2, PE3, PE4}, selects the active AC independently.
There is therefore a need to signal the active status of each AC
such that the PEs can select a common active PW for forwarding
between CE1 and CE2.
Changes occurring on one side of network due to a failure of the AC
or PE are not propagated to the ACs on the other side of the
network. Furthermore, failures in the PSN are not be propagated to
the attached CEs.
Note that End-to-end native service protection switching can also be
used to protect the emulated service in this scenario. In this case,
PW3 and PW4 are not necessary.
If the CEs do not perform native service protection switching, they
may instead may use load balancing across the paths between the CEs.
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3.3. Single Homed CE with MS-PW redundancy
This application is shown in Figure 4. The main objective is to
protect the emulated service against failures of the S-PEs.
Native |<----------- Pseudowires ----------->| Native
Service | | Service
(AC) | |<-PSN1-->| |<-PSN2-->| | (AC)
| V V V V V V |
| +-----+ +-----+ +-----+ |
+----+ | |T-PE1|=========|S-PE1|=========|T-PE2| | +----+
| |-------|......PW1-Seg1.......|.PW1-Seg2......|-------| |
| CE1| | |=========| |=========| | | CE2|
| | +-----+ +-----+ +-----+ | |
+----+ |.||.| |.||.| +----+
|.||.| +-----+ |.||.|
|.||.|=========| |========== .||.|
|.||...PW2-Seg1......|.PW2-Seg2...||.|
|.| ===========|S-PE2|============ |.|
|.| +-----+ |.|
|.|============+-----+============= .|
|.....PW3-Seg1.| | PW3-Seg2......|
==============|S-PE3|===============
| |
+-----+
Figure 4 Single homed CE with multi-segment pseudowire redundancy
CE1 is connected to PE1 and CE2 to PE2, respectively. There are
three multi-segment PWs. PW1 is switched at S-PE1, PW2 is switched
at S-PE2, and PW3 is switched at S-PE3.
Since there is no multi-homing running on the ACs, the T-PE nodes
would advertise 'Active' for the forwarding status based on a
priority for the PW. Priorities associate meaning of 'primary PW'
and 'secondary PW'. These priorities MUST be used in revertive mode
as well and paths must be switched accordingly. The priority can be
configuration or derivation from the PWid. Lower the PWid higher the
priority. However, this does not guarantee selection of same PW by
the T-PEs because, for example, mismatch of the configuration of the
PW priority in each T-PE. The intent of this application is to have
T-PE1 and T-PE2 synchronize the transmit and receive paths of the PW
over the network. In other words, both T-PE nodes are required to
transmit over the PW segment which is switched by the same S-PE.
This is desirable for ease of operation and troubleshooting.
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3.4. PW redundancy between MTU-s in H-VPLS
Following figure illustrates the application of use of PW redundancy
to Hierarchical VPLS (H-VPLS). Here, and MTU-s is dual-homed to two
PE-rs.
|<-PSN1-->| |<-PSN2-->|
V V V V
+-----+ +-----+
|MTU-s|=========|PE1 |========
|..Active PW group....| H-VPLS-core
| |=========| |=========
+-----+ +-----+
|.|
|.| +-----+
|.|===========| |==========
|...Standby PW group|.H-VPLS-core
=============| PE2|==========
+-----+
Figure 5 Multi-homed MTU-s in H-VPLS core
In Figure 5, the MTU-s is dual homed to PE1 and PE2 and has spoke
PWs to each of them. The MTU-s needs to choose only one of the spoke
PWs ( the active PW) to one of the PE to forward the traffic and the
other to standby status. The MTU-s can derive the status of the PWs
based on local policy configuration. PE1 and PE2 are connected to
the H-VPLS core on the other side of network. The MTU-s communicates
the status of its member PWs for a set of VSIs having common status
of Active or Standby. Here the MTU-s controls the selection of PWs
to forward the traffic. Signaling using PW grouping with a common
group-id in PWid FEC Element or Grouping TLV in Generalized PWid FEC
Element as defined in [2] to PE1 and PE2 respectively, is
recommended to scale better.
Whenever MTU-s performs a switchover, it needs to communicate to PE2
for the Standby PW group the changed status of active.
In this scenario, PE devices are aware of switchovers at MTU-s and
could generate MAC Withdraw Messages to trigger MAC flushing within
the H-VPLS full mesh. By default, MTU-s devices should still trigger
MAC Withdraw messages as currently defined in [5] to prevent two
copies of MAC withdraws to be sent (one by MTU-s and another one by
PEs). Mechanisms to disable MAC Withdraw trigger in certain devices
is out of the scope of this document.
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3.5. PW redundancy between VPLS n-PEs
Following figure illustrates the application of use of PW redundancy
for dual homed connectivity between PE devices in a ring topology.
+-------+ +-------+
| PE1 |=====================| PE2 |====...
+-------+ PW Group 1 +-------+
|| ||
VPLS Domain A || || VPLS Domain B
|| ||
+-------+ +-------+
| PE3 |=====================| PE4 |==...
+-------+ PW Group 2 +-------+
Figure 6 Redundancy in Ring topology
In Figure 6, PE1 and PE3 from VPLS domain A are connected to PE2 and
PE4 in VPLS domain B via PW group 1 and group 2. Each of the PEs in
the respective domains is connected to each other as well as forming
the ring topology. Such scenarios may arise in inter-domain H-VPLS
deployments where rapid spanning tree (RSTP) or other mechanisms may
be used to maintain loop free connectivity of PW groups.
[5] outlines multi-domain VPLS services without specifying how
multiple redundant border PEs per domain per VPLS instance can be
supported. In the example above, PW group 1 may be blocked at PE1 by
RSTP and it is desirable to block the group at PE2 by virtue of
exchanging the PW preferential forwarding status of Standby. How the
PW grouping should be done here is again deployment specific and is
out of scope of the solution.
3.6. PW redundancy in VPLS Bridge Module Model
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----------------------------+ Provider +------------------------
. Core .
+------+ . . +------+
| n-PE |======================| n-PE |
Provider | (P) |---------\ /-------| (P) | Provider
Access +------+ ._ \ / . +------+ Access
Network . \/ . Network
(1) +------+ . /\ . +------+ (2)
| n-PE |----------/ \--------| n-PE |
| (B) |----------------------| (B) |_
+------+ . . +------+
. .
----------------------------+ +------------------------
Figure 7 Bridge Module Model
In Figure 7, two provider access networks, each having two n-PEs,
where the n-PEs are connected via a full mesh of PWs for a given
VPLS instance. As shown in the figure, only one n-PE in each access
network is serving as a Primary PE (P) for that VPLS instance and
the other n-PE is serving as the backup PE (B). In this figure, each
primary PE has two active PWs originating from it. Therefore, when a
multicast, broadcast, and unknown unicast frame arrives at the
primary n-PE from the access network side, the n-PE replicates the
frame over both PWs in the core even though it only needs to send
the frames over a single PW (shown with == in the figure) to the
primary n-PE on the other side. This is an unnecessary replication
of the customer frames that consumes core-network bandwidth (half of
the frames get discarded at the receiving n-PE). This issue gets
aggravated when there is three or more n-PEs per provider, access
network. For example if there are three n-PEs or four n-PEs per
access network, then 67% or 75% of core-BW for multicast, broadcast
and unknown unicast are respectively wasted.
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In this scenario, n-PEs can disseminate the status of PWs
active/standby among them and furthermore to have it tied up with
the redundancy mechanism such that per VPLS instance the status of
active/backup n-PE gets reflected on the corresponding PWs emanating
from that n-PE.
4. Generic PW redundancy requirements
4.1. Protection switching requirements
o Protection architecture such as N:1,1:1 or 1+1 can be used. N:1
protection case is somewhat inefficient in terms of capacity
consumption hence implementations SHOULD support this method
while 1:1 being subset and efficient MUST be supported. 1+1
protection architecture can be supported but is left for further
study.
o Non-revertive mode MUST be supported, while revertive mode is an
optional one.
o Protection switchover can be operator driven like Manual
lockout/force switchover or due to signal failure. Both methods
MUST be supported and signal failure MUST be given higher
priority than any local or far end request.
4.2. Operational requirements
o (T-)PEs involved in protecting a PW SHOULD automatically discover
and attempt to resolve inconsistencies in the configuration of
primary/secondary PW.
o (T-)PEs involved in protecting a PW SHOULD automatically discover
and attempt to resolve inconsistencies in the configuration of
revertive/non-revertive protection switching mode.
o (T-)PEs that do not automatically discover or resolve
inconsistencies in the configuration of primary/secondary,
revertive/non-revertive, or other parameters MUST generate an
alarm upon detection of an inconsistent configuration.
o (T-)PEs involved with protection switching MUST support the
configuration of revertive or non-revertive protection switching
mode.
o (T-)PEs involved with protection switching SHOULD support the
local invocation of protection switching.
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o (T-)PEs involved with protection switching SHOULD support the
local invocation of a lockout of protection switching.
o In standby status PW can still receive packets in order to avoid
black holing of in-flight packets during switchover. However in
case of use of VPLS application packets are dropped in standby
status except for the OAM packets.
5. Security Considerations
This document expects extensions to LDP that are needed for
protecting pseudo-wires. It will have the same security properties
as in LDP [4] and the PW control protocol [2].
6. IANA considerations
This document has no actions for IANA.
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7. Major Contributing Authors
The editors would like to thank Pranjal Kumar Dutta, Marc Lasserre,
Jonathan Newton, Hamid Ould-Brahim, Olen Stokes, Dave Mcdysan, Giles
Heron and Thomas Nadeau who made a major contribution to the
development of this document.
Pranjal Dutta
Alcatel-Lucent
Email: pranjal.dutta@alcatel-lucent.com
Marc Lasserre
Alcatel-Lucent
Email: marc.lasserre@alcatel-lucent.com
Jonathan Newton
Cable & Wireless
Email: Jonathan.Newton@cw.com
Olen Stokes
Extreme Networks
Email: ostokes@extremenetworks.com
Hamid Ould-Brahim
Nortel
Email: hbrahim@nortel.com
Dave McDysan
Verizon
Email: dave.mcdysan@verizon.com
Giles Heron
Cisco Systems
Email: giles.heron@gmail.com Formatted: Field Code Thomas Nadeau Formatted: Computer Associates Formatted: Email: tnadeau@lucidvision.com
8. Acknowledgments
The authors would like to thank Vach Kompella, Kendall Harvey,
Tiberiu Grigoriu, Neil Hart, Kajal Saha, Florin Balus and Philippe
Niger for their valuable comments and suggestions.
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9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Martini, L., et al., "Pseudowire Setup and Maintenance using
LDP", RFC 4447, April 2006.
[3] Bryant, S., et al., " Pseudo Wire Emulation Edge-to-Edge
(PWE3) Architecture", RFC 3985 March 2005
[4] Andersson, L., Minei, I., and B. Thomas, "LDP Specification",
RFC 5036, January 2001
[5] Kompella,V., Lasserrre, M. , et al., "Virtual Private LAN
Service (VPLS) Using LDP Signalling", RFC 4762, January 2007
9.2. Informative References
[6] Martini, L., et al., "Segmented Pseudo Wire", RFC6073, January
2011.
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Author's Addresses
Praveen Muley
Alcatel-Lucent
701 E. Middlefiled Road
Mountain View, CA, USA
Email: Praveen.muley@alcatel-lucent.com
Mustapha Aissaoui
Alcatel-Lucent
600 March Rd
Kanata, ON, Canada K2K 2E6
Email: mustapha.aissaoui@alcatel-lucent.com
Matthew Bocci
Alcatel-Lucent
Voyager Place
Shoppenhangers Rd,
Maidenhead, Berks, UK
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Muley et al. Expires March 22, 2012 [Page 17]
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