Network Working Group Praveen Muley, Ed.
Internet Draft Mustapha Aissaoui, Ed.
Intended Status: Informational Alcatel-Lucent
Expires: November 2010
May 14, 2010
Pseudowire (PW) Redundancy
draft-ietf-pwe3-redundancy-03.txt
Abstract
This document describes a framework comprised of few scenarios and
associated requirements where PW redundancy is needed. A set of
redundant PWs is configured between PE nodes in SS-PW applications,
or between T-PE nodes in MS-PW applications. In order for the PE/T-PE
nodes to indicate the preferred PW to forward to one another, a new
status is needed to indicate the preferential forwarding status of
active or standby for each PW in the redundancy set.
Status of this Memo
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This Internet-Draft will expire on November 14, 2010.
Copyright Notice
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carefully, as they describe your rights and restrictions with respect
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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
1. Terminology .............................................. 2
2. Introduction.............................................. 3
3. Reference Model........................................... 4
3.1. PE Architecture...................................... 4
3.2. Multiple Multi-homed................................. 5
3.3. Single Homed CE with MS-PW redundancy................ 7
3.4. PW redundancy between MTU-s.......................... 8
3.5. PW redundancy between n-PEs.......................... 9
3.6. PW redundancy in Bridge Module Model................. 10
4. Generic PW redundancy requirements........................ 11
4.1. Protection switching requirements.................... 11
4.2. Operational requirements............................. 11
5. Security Considerations................................... 12
6. IANA considerations....................................... 12
7. Major Contributing Authors................................ 12
8. Acknowledgments........................................... 13
9. References................................................ 14
9.1. Normative References................................. 14
9.2. Informative References............................... 14
Author's Addresses........................................... 14
1. Terminology
o Active PW. A PW whose preferential status is set to Active and
Operational status is UP and is used for forwarding user and OAM
traffic.
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o Standby PW. A PW whose preferential status is set to Standby and
Operational status is UP and is not used for forwarding user
traffic but may forward OAM traffic.
o PW Endpoint: A PE where a PW terminates on a point where Native
Service Processing is performed, e.g., A SS-PW PE, an MS-PW T-PE,
or an H-VPLS MTU-s or PE-rs.
o Primary PW: the PW which a PW endpoint activates in preference to
any other PW when more than one PW qualify for active state. When
the primary PW comes back up after a failure and qualifies for
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 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 Operationally UP and the 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 . 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.
2. Introduction
In single-segment PW (SS-PW) applications, protection for the PW is
provided by the PSN layer. This may be an Resource Reservation
Protocol traffic engineered (RSVP-TE) labeled switch (LSP) with a
fast-Reroute (FRR) backup and/or an end-to-end backup LSP. There are
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applications however where the backup PW terminates on a different
target PE node. PSN protection mechanisms cannot protect against
failure of the target PE node or the failure of the remote AC.
In multi-segment PW (MS-PW) applications, a primary and one or more
secondary PWs in standby mode are configured in the network. The
paths of these PWs are diverse in the sense that they are switched at
different S-PE nodes. In these applications, PW redundancy is
important for the service resilience.
In some deployments, it is important for operators that particular PW
is preferred if it is available. For example, PW path with least
latency may be preferred.
This document describes framework for these applications and its
associated operational requirements. The framework comprises of new
required status called preferential status to PW apart from the
operational status already defined in the PWE3 control protocol [2].
3. Reference Model
Following figures shows the reference architecture of PE for the 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 using the criteria described in
this document.
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+----------------------------------------+
| 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. Multiple Multi-homed
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnels-->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | |....|.......PW1........|....| | +-----+
| |----------| PE1|...... .........| PE3|----------| |
| CE1 | +----+ \ / PW3 +----+ | CE2 |
| | +----+ X +----+ | |
| | | |....../ \..PW4....| | | |
| |----------| PE2| | PE4|--------- | |
+-----+ | |....|.....PW2..........|....| | +-----+
AC +----+ +----+ AC
Figure 2 Multiple Multi-homed CEs with single SS-PW redundancy
In the Figure 2 illustrated above both CEs, CE1 and CE2 are dual-
homed with PEs, PE1, PE2 and PE3, PE4 respectively. The method for
dual-homing and the used protocols 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.
PE1 has PW1 and PW4 service connecting PE3 and PE4 respectively.
Similarly PE2 has PW2 and Pw3 pseudo wire service connecting PE4 and
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PE3 respectively. PW1, PW2, PW3 and PW4 are all operationally UP. In
order to support N:1 or 1:1 only one PW is required to be selected to
forward the traffic. Thus the PW needs to reflect its new status
apart from the operational status. We call this as preferential
forwarding status with state representing 'active' the one carrying
traffic while the other 'standby' which is operationally UP but not
forwarding traffic. The method of deriving Active/Standby status of
the AC is outside the scope of this document.
A new algorithm needs to be developed using the preferential
forwarding state of PW and select only one PW to forward.
On failure of AC between the dual homed CE1 in this case lets say PE1
the preferential status on PE2 needs to be changed. Different
mechanisms/protocols can be used 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 the
traffic. In this application, because each dual-homing algorithm
running on the two node sets, i.e., {CE1, PE1, PE2} and {CE2, PE3,
PE4}, selects the active AC independently, there is a need to signal
the active status of the AC such that the PE nodes can select a
common active PW path for end-to-end forwarding between CE1 and CE2.
This helps in restricting the changes occurring on one side of
network due to failure to the other side of the network.
Also the failures in the carrier core network MUST NOT be propagated
to customer network. Hence network operator should take this
consideration while designing the network. For ex. if there is
failure of LSP tunnel, operator should have rely on FRR or an
alternate LSP path/tunnel which will be seamless to the PW service.
Note this method also protects against any single PE failure or some
dual PE failures.
One Multi-homed CE with single SS-PW redundancy application is a
subset of above. Only PW1 and PW3 exist in this case. This helps
against AC failure and PE failure of dual homed AC. Similar
requirements applies in usage MS-PW redundancy as well. An additional
requirement applicable to MS-PW is forwarding of status notification
through S-PE. In general from customer view, SS-PW and MS-PW has
similar resiliency requirement.
There is also a 1:1 protection switching case that is a subset of the
above where PW3 and PW4 are not present.
o If the CEs do not perform native service protection switching, but
instead may use load balancing. This protects against AC failures
and can use the native service to indicate active/failed state.
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o If each CE homes to different PEs, then the CEs can implement
native service protection switching, without any PW redundancy
functions. All that the PW needs to do is detect AC, PE, or PSN
tunnel failures and convey that information to both PEs at the end
of the PW. This is applicable to MS-PW as well.
3.3. Single Homed CE with MS-PW redundancy
This is the main application of interest and the network setup is
shown in Figure 3
Native |<------------Pseudo Wire------------>| 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 3 Single homed CE with multi-segment pseudo-wire redundancy
In Figure 3, CE1 is connected to PE1 in provider Edge 1 and CE2 to
PE2 in provider edge 2 respectively. There are three segmented PWs. A
PW1, is switched at S-PE1, PW2, which is switched at S-PE2 and PW3,
is switched at S-PE3.
Since there is no multi-homing running on the AC, the T-PE nodes
would advertise 'Active' for the forwarding status based on the
priority. 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
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priority in each T-PE. The intent of this application is to have T-
PE1 and T-PE2 synchronize the transmit and receive path 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.
3.4. PW redundancy between MTU-s
Following figure illustrates the application of use of PW redundancy
in spoke PW by dual homed MTU-s to PEs.
|<-PSN1-->| |<-PSN2-->|
V V V V
+-----+ +-----+
|MTU-s|=========|PE1 |========
|..Active PW group....| H-VPLS-core
| |=========| |=========
+-----+ +-----+
|.|
|.| +-----+
|.|===========| |==========
|...Standby PW group|.H-VPLS-core
=============| PE2|==========
+-----+
Figure 4 Multi-homed MTU-s in H-VPLS core
In Figure 4, MTU-s is dual homed to PE1 and PE2 and has spoke PWs to
each of them. MTU-s needs to choose only one of the spoke PW (active
PW) to one of the PE to forward the traffic and the other to standby
status. MTU-s can derive the status of the PWs based on local policy
configuration. PE1 and PE2 are connected to H-VPLS core on the other
side of network. MTU-s communicates the status of its member PWs for
a set of VSIs having common status Active/Standby. Here MTU-s
controls the selection of PWs to forward the traffic. Signaling
using PW grouping with 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 encouraged 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
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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.
3.5. PW redundancy between 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 5 Redundancy in Ring topology
In Figure 5, 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 PE in
respective domain is connected to each other as well to form the ring
topology. Such scenarios may arise in inter-domain H-VPLS deployments
where RSTP or other mechanisms may be used to maintain loop free
connectivity of PW groups.
Ref.[5] outlines about multi-domain VPLS service without specifying
how redundant border PEs per domain per VPLS instance can be
supported. In the example above, PW group1 may be blocked at PE1 by
RSTP and it is desirable to block the group at PE2 by virtue of
exchanging the PW preferential status as Standby. How the PW grouping
should be done here is again deployment specific and is out of scope
of the solution.
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3.6. PW redundancy in Bridge Module Model
----------------------------+ Provider +------------------------
. Core .
+------+ . . +------+
| n-PE |======================| n-PE |
Provider | (P) |---------\ /-------| (P) | Provider
Access +------+ ._ \ / . +------+ Access
Network . \/ . Network
(1) +------+ . /\ . +------+ (2)
| n-PE |----------/ \--------| n-PE |
| (B) |----------------------| (B) |_
+------+ . . +------+
. .
----------------------------+ +------------------------
Figure 6 Bridge Module Model
In Figure 6, 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
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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.
In this scenario, n-PEs can disseminate the status of PWs
active/standby among themselves 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.
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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.
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.
7. Major Contributing Authors
The editors would like to thank Matthew Bocci, 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.
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Matthew Bocci
Alcatel
Voyager Place, Shoppenhangers Rd
Maidenhead, Berks, UK SL6 2PJ
Email: matthew.bocci@alcatel.com
Pranjal Kumar Dutta
Alcatel-Lucent
Email: pdutta@alcatel-lucent.com
Marc Lasserre
Alcatel-Lucent
Email: mlasserre@alcatel-lucent.com
Jonathan Newton
Cable & Wireless
Email: Jonathan.Newton@cwmsg.cwplc.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
BT
Email: giles.heron@gmail.com
Thomas Nadeau
BT
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", draft-ietf-pwe3-
segmented-pw-14.txt, October 2010.
Author's Addresses
Praveen Muley
Alcatel
701 E. Middlefiled Road
Mountain View, CA, USA
Email: Praveen.muley@alcatel.com
Mustapha Aissaoui
Alcatel
600 March Rd
Kanata, ON, Canada K2K 2E6
Email: mustapha.aissaoui@alcatel.com
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