Network Working Group Praveen Muley
Internet Draft Mustapha Aissaoui
Expires: March 2007 Matthew Bocci
Alcatel
Jonathan Newton
Cable & Wireless
September 22, 2006
Pseudowire (PW) Redundancy
draft-muley-pwe3-redundancy-00.txt
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Abstract
This document describes a few scenarios 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 bit is needed to indicate the
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preferential forwarding status of active or standby for each PW in
the redundancy set. This draft specifies a new PW status bit for this
purpose.
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.................................................2
2. Introduction................................................3
3. Multi-homing Single SS-PW redundancy applications............4
3.1. One Multi-homed CE with Single SS-PW redundancy..........4
3.2. Multiple Multi-homed CEs with single SS-PW redundancy....5
4. Multi-homing MS-PW redundancy applications...................6
4.1. Multi-homed CE with MS-PW redundancy....................6
4.2. Single Homed CE with MS-PW redundancy...................7
5. Design considerations........................................8
6. Security Considerations......................................9
7. IANA Considerations.........................................9
7.1. Status Code for PW Preferential Forwarding Status........9
8. Acknowledgments.............................................9
9. References..................................................9
Author's Addresses............................................10
Intellectual Property Statement................................10
Disclaimer of Validity........................................11
Copyright Statement...........................................11
Acknowledgment................................................11
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 [4].
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.
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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.
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 multiple
secondary PWs in standby mode are configured in the network. The
paths of these PWs are diverse and are switched at different S-PE
nodes. In these applications, PW redundancy is important for the
service resilience.
This document describes these applications and specifies a new PW
status bit to indicate the preferential forwarding status of the PW
for the purpose of notifying the remote T-PE of the active/standby
state of each PW in the redundancy set. This status bit is different
from the operational status bits already defined in the PWE3 control
protocol [2]. The PW with both local and remote operational UP status
and local and remote preferential active status is selected to
forward traffic.
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3. Multi-homing Single SS-PW redundancy applications
3.1. One Multi-homed CE with single SS-PW redundancy
The following figure illustrates an application of single segment
pseudo-wire redundancy.
|<-------------- 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 1 Multi-homed CE with single SS-PW redundancy
In Figure 1, CE1 is dual homed to PE1 and to PE3 by attachment
circuits. The method for dual-homing of CE1 to PE1 and PE3 nodes and
the used protocols are outside the scope of this document. PE2 has an
attachment circuit from CE2. Two pseudo-wires pw1 and pw2 are
established, one connects PE1 to PE2 and the other one connects PE3
to PE2. On PE2, PW1 has a higher priority than PW2 by local
configuration.
In normal operation, PE1 and PE3 will advertise "Active" and
"Standby" preferential forwarding status (apart from operational
status) respectively to PE2. This status reflects the forwarding
state of the two AC's to CE1. PE2 advertises preferential status of
"Active" on both PW1 and PW2. As both the local and remote
operational and administrative status for PW1 are UP and Active,
traffic is forwarded over PW1 in both directions.
On failure of AC to PE1, PE1 sends a PW status notification to PE2
indicating that the AC operational status changed to DOWN. It will
also set the forwarding status of PW1 to "standby". PE3 AC will
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change preferential status to active and this status is also
communicated to PE2 using the newly proposed forwarding status bit in
the PW status TLV notification message. The changing of preferential
status on PE3 due to failure of AC at PE1 is achieved by various
methods depending of the used dual-homing protocol and is outside the
scope of this draft. On receipt of the status notifications, PE2
switches the path to the standby pseudo-wire PW2 as the newly changed
status turns PW2 as Active PW. Note in this example, the receipt of
the operational status of the AC on the CE1-PE1 link is normally
sufficient to have PE2 switch the path to PW2. However, the operator
may want to trigger the switchover of the path of the PW for
administrative reasons, i.e., maintenance, and thus the proposed PW
forwarding active/standby bit is required to notify PE2 to trigger
the switchover.
3.2. 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 2 Multiple Multi-homed CEs with single SS-PW redundancy
In the figure illustrated above the 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 advertises the preferential status "active" and operational
status "UP" for pseudo-wires PW1 and PW4 connected to PE3 and PE4.
This status reflects the forwarding state of the AC attached to PE1.
PE2 advertise preferential status "standby" where as operational
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status "UP" for pseudo-wires PW2 and PW3 to PE3 and PE4. PE3
advertises preferential status "standby" where as operational status
"UP" for pseudo-wires PW1 and PW3 to PE1 and PE2. PE4 advertise the
preferential status "active" and operational status "UP" for pseudo-
wires PW2 and PW4 to PE2 and PE1 respectively. Thus by matching the
local and remote preferential status "active" and operational status
"Up" of pseudo-wire the active pseudo-wire is selected. In this case
it is the PW4 that will be selected. On failure of AC between the CE1
and PE1 the preferential status on PE2 is changed. Different
mechanisms/protocols can be used to achieve this and these are beyond
the scope of this document. PE2 then announces the newly changed
preferential status "active" to PE3 and PE4. PE1 will advertise a PW
status notification message indicating that the AC between CE1 and
PE1 is operationally down. PE2 and PE4 checks the local and remote
preferential status "active" and operational status "Up" and selects
PW2 as the new active pseudo-wire to send 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.
4. Multi-homing MS-PW redundancy applications
4.1. Multi-homed CE with MS-PW redundancy
The following figure illustrates an application of multi-segment
pseudo-wire redundancy.
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|=========|T-PE4| | +----+
+-----+ +-----+ AC
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Figure 3 Multi-homed CE with MS-PW redundancy
In Figure 3, the PEs that provide PWE3 to CE1 and CE2 are
Terminating-PE1 (T-PE1) and Terminating-PE2 (T-PE2) respectively. A
PSN tunnel extends from T-PE1 to switching-PE1 (S-PE1) across PSN1,
and a second PSN tunnel extends from S-PE1 to T-PE2 across PSN2. PW1
and PW2 are used to connect the attachment circuits (ACs) between T-
PE1 and T-PE2. Each PW segment on the tunnel across PSN1 is switched
to a PW segment in the tunnel across PSN2 at S-PE1 to complete the
multi-segment PW (MS-PW) between T-PE1 and T-PE2. S-PE1 is therefore
the PW switching point. PW1 has two segments and is active pseudo-
wire while PW2 has two segments and is a standby pseudo-wire. This
application requires support for MS-PW with segments of the same type
as described in [3]. The operation in this case is the same as in the
case of SS-PW. The only difference is that the S-PW nodes need to
relay the PW status notification containing both the operational and
forwarding status to the T-PE nodes.
4.2. 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 4 Single homed CE with multi-segment pseudo-wire redundancy
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In Figure 4, CE1 is connected to PE1 in provider Edge 1 and CE2 to
PE2 in provider edge 2 respectively. There are three segmented PWs. A
primary PW, PW1, is switched at S-PE1. A standby PW, PW2, which is
switched at S-PE2 and has a priority of 1. Finally, another standby
PW, PW3, is switched at S-PE3 and has a priority of 2. This means T-
PE1 and T-PE2 will select PW1 over PW2, and PW2 over PW3 if all of
them are in the UP state. Moreover, a T-PE node will revert back to
the primary PW, PW1, whenever it comes back up.
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 will transmit over the PW segment which is
switched by the same S-PE. This is desirable for ease of operation
and troubleshooting.
Since there is no multi-homing running on the AC, the T-PE nodes
would advertise 'Active" for the forwarding status. However, this
does not guarantee that the paths of the PW are synchronized because
for example of mismatch of the configuration of the PW priority in
each T-PE. Thus, there is a need to devise an augmented mechanism to
achieve the desirable synchronization of the PW paths and to add the
ability to have a T-PE instruct the remote T-PE to perform a
coordinated switchover to a common Active path.
The solution required for this specific scenario is left for further
study.
5. Design considerations
While using the pseudo-wire redundancy application, the T-LDP peers
MUST negotiate the usage of PW status TLV. The status code defined
below carries the active/standby preferential forwarding status of
the pseudo-wire. The pseudo-wire is only considered active pseudo-
wire only when both the local PW status and the remote PW status
indicate preferential status "active" and operational status as Up.
Any other status combination keeps the pseudo-wire in standby mode.
The pseudo-wires are given different preference level. In case of
network failure, the PE/T-PE will first switch to the standby PW with
a higher preference. Although the configuration of the pseudo-wire
preference is matter of local policy matter and is outside the scope
of this, it is desirable to have the preferences configured on both
end points be similar. In mis-configuration, a method to force the
synchronization of the PW paths is required is for further study.
While in standby status, a pseudo-wire can still receive packets in
order to avoid black holing of the in-flight packets during
switchover.
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6. Security Considerations
This document specifies the LDP extensions that are needed for
protecting pseudo-wires. It will have the same security properties as
in LDP [5] and the PW control protocol [2].
7. IANA Considerations
We have defined the following codes for the pseudo-wire redundancy
application.
7.1. Status Code for PW Preferential Forwarding Status
The T-PE nodes need to indicate to each other the preferential
forwarding status of active/inactive of the pseudo-wire.
0x00000020 When the bit is set it represents "PW forwarding
standby".
When the bit is cleared, it represents "PW forwarding
"active".
8. Acknowledgments
The authors would like to thank Vach Kompella, Kendall Harvey,
Tiberiu Grigoriu, Neil Hart, Kajal Saha, and Philippe Niger for their
valuable comments and suggestions.
9. 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] Martini, L., et al., "Segmented Pseudo Wire", draft-ietf-pwe3-
segmented-pw-02.txt, March 2006.
[4] Bryant, S., et al., " Pseudo Wire Emulation Edge-to-Edge (PWE3)
Architecture", March 2005
[5] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and B.
Thomas, "LDP Specification", RFC 3036, January 2001
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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.co.uk
Jonathan Newton
Cable & Wireless
Email: Jonathan.Newton@cwmsg.cwplc.com
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