Network Working Group Praveen Muley
Internet Draft Mustapha Aissaoui
Intended Status: Informational Matthew Bocci
Expires: August 2008 Pranjal Kumar Dutta
Marc Lasserre
Alcatel
Jonathan Newton
Cable & Wireless
Olen Stokes
Extreme Networks
Hamid Ould-Brahim
Nortel
Dave Mcdysan
Verizon
Giles Heron
Thomas Nadeau
British Telecom
March 28, 2008
Pseudowire (PW) Redundancy
draft-ietf-pwe3-redundancy-00.txt
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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 path 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.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1].
Table of Contents
1. Terminology ......................................... 3
2. Introduction......................................... 4
3. Reference Model...................................... 4
3.1. Multiple Multi-homed CEs with single SS-PW redundancy... 5
3.2. Single Homed CE with MS-PW redundancy................ 6
3.3. PW redundancy between MTU-s and PEs in a multi-homed VPLS
application ......................................... 8
3.4. PW redundancy between n-PEs........................ 9
3.5. PW redundancy in Bridge Module Model................. 9
4. Generic PW redundancy requirements...................... 11
4.1. Protection switching requirements.................. 11
4.2. Operational requirements.......................... 11
5. Security Considerations............................... 12
6. Acknowledgments..................................... 12
7. References......................................... 12
7.1. Normative References............................. 12
7.2. Informative References........................... 12
Author's Addresses..................................... 13
Intellectual Property Statement .......................... 13
Disclaimer of Validity.................................. 14
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Acknowledgment ........................................ 14
1. Terminology
o PW Terminating Provider Edge (T-PE). A PE where the customer-
facing attachment circuits (ACs) are bound to a PW forwarder. A
Terminating PE is present in the first and last segments of a MS-
PW. This incorporates the functionality of a PE as defined in
RFC3985 [3].
o Single-Segment Pseudo Wire (SS-PW). A PW setup directly between
two T-PE devices. Each PW in one direction of a SS-PW traverses
one PSN tunnel that connects the two T-PEs.
o Multi-Segment Pseudo Wire (MS-PW). A static or dynamically
configured set of two or more contiguous PW segments that behave
and function as a single point-to-point PW. Each end of a MS-PW by
definition MUST terminate on a T-PE.
o PW Segment. A part of a single-segment or multi-segment PW, which
is set up between two PE devices, T-PEs and/or S-PEs.
o PW Switching Provider Edge (S-PE). A PE capable of switching the
control and data planes of the preceding and succeeding PW
segments in a MS-PW. The S-PE terminates the PSN tunnels of the
preceding and succeeding segments of the MS-PW.
o PW switching point for a MS-PW. A PW Switching Point is never the
S-PE and the T-PE for the same MS-PW. A PW switching point runs
necessary protocols to setup and manage PW segments with other PW
switching points and terminating PEs
o Active PW. A PW whose preferential status is set to Active and
Operational status is UP.
o Standby PW. A PW whose preferential status is set to Standby and
Operational status is UP.
o Primary Path. The configured path which is preferred when
revertive protection switching is used.
o Secondary Path. One or more configured paths that are used by
protection switching when current active PW path enters
Operational DOWN state.
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o Revertive protection switching. Traffic will be carried by primary
path if it is Operationally UP and the wait-to-restore timer
expires and primary path is made the Active PW.
o Non-revertive protection switching. Traffic will be carried by the
last PW path selected as a result of previous active path entering
Operationally DOWN state.
o Manual selection of PW path. Ability for the operator to manually
select the primary/secondary paths.
2. Introduction
In single-segment PW (SS-PW) applications, protection for the PW is
provided by the PSN layer. This may be an RSVP LSP with a FRR backup
and/or an end-to-end backup LSP. There are however applications 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].
The definition and operation of the preferential status is covered in
ref.[7]
3. Reference Model
Following figures shows the reference model for the PW redundancy and
its usage in different topologies and applications.
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3.1. Multiple Multi-homed CEs with single SS-PW redundancy
|<-------------- 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 1 Multiple Multi-homed CEs with single SS-PW redundancy
In the Figure 1 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 such as Multi-chassis Link
Aggregation Group (MC-LAG) 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 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 his 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. In case of MC-LAG it is derived by the Link Aggregation
Control protocol (LACP) negotiation.
A new algorithm needs to be developed using the preferential
forwarding state of PW and select only one PW to forward.
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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. For example the MC-
LAG control protocol changes the link status on PE2 to active. 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. 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 and 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.
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.
3.2. Single Homed CE with MS-PW redundancy
This is the main application of interest and the network setup is
shown in Figure 2
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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 2 Single homed CE with multi-segment pseudo-wire redundancy
In Figure 2, 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 that paths of the PW are synchronized because
for example of 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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3.3. PW redundancy between MTU-s and PEs in a multi-homed VPLS
application
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 3 Multi-homed MTU-s in H-VPLS core
In Figure 3, 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 requires
to communicate to PE2-rs 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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3.4. 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 4 Redundancy in Ring topology
In Figure 4, 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.
3.5. PW redundancy in Bridge Module Model
----------------------------+ Provider +------------------------
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. Core .
+------+ . . +------+
| n-PE |======================| n-PE |
Provider | (P) |---------\ /-------| (P) | Provider
Access +------+ ._ \ / . +------+ Access
Network . \/ . Network
(1) +------+ . /\ . +------+ (2)
| n-PE |----------/ \--------| n-PE |
| (B) |----------------------| (B) |_
+------+ . . +------+
. .
----------------------------+ +------------------------
Figure 5 Bridge Module Model
In Figure 5, 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.
In this scenario, Standby PW signaling defined in
[7] can be used among n-PEs that can disseminate the status of PWs
(active or blocked) among themselves and furthermore to have it tied
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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.
o (T-)PEs involved with protection switching SHOULD support the
local invocation of a lockout of protection switching.
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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. 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.
7. References
7.1. 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", March 2005
[4] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and B.
Thomas, "LDP Specification", RFC 3036, January 2001
[5] Kompella,V., Lasserrre, M. , et al., "Virtual Private LAN
Service (VPLS) Using LDP Signalling", RFC 4762, January 2007
7.2. Informative References
[6] Martini, L., et al., "Segmented Pseudo Wire", draft-ietf-pwe3-
segmented-pw-02.txt, March 2006.
[7] Muley, P. et al., "Preferential forwarding status bit", draft-
muley-dutta-pwe3-redundancy-bit-00.txt, August 2007.
[8] IEEE Std. 802.1D-2003-Media Access Control (MAC) Bridges.
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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
Matthew Bocci
Alcatel
Voyager Place, Shoppenhangers Rd
Maidenhead, Berks, UK SL6 2PJ
Email: matthew.bocci@alcatel-lucent.co.uk
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
Intellectual Property Statement
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