





Internet Engineering Task Force (IETF)                           J. Dong
Request for Comments: 7795                                       H. Wang
Category: Standards Track                            Huawei Technologies
ISSN: 2070-1721                                            February 2016


      Pseudowire Redundancy on the Switching Provider Edge (S-PE)

Abstract

   This document describes Multi-Segment Pseudowire (MS-PW) protection
   scenarios in which pseudowire redundancy is provided on the Switching
   Provider Edge (S-PE) as defined in RFC 5659.  Operations of the S-PEs
   that provide PW redundancy are specified in this document.  Signaling
   of the Preferential Forwarding status as defined in RFCs 6870 and
   6478 is reused.  This document does not require any change to the
   Terminating Provider Edges (T-PEs) of MS-PW.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7795.

Copyright Notice

   Copyright (c) 2016 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
   carefully, as they describe your rights and restrictions with 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.





Dong & Wang                  Standards Track                    [Page 1]

RFC 7795                  PW Redundancy on S-PE            February 2016


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Typical Scenarios of PW Redundancy on S-PE  . . . . . . . . .   3
     2.1.  MS-PW Redundancy on S-PE  . . . . . . . . . . . . . . . .   3
     2.2.  MS-PW Redundancy on S-PE with S-PE Protection . . . . . .   4
   3.  S-PE Operations . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Applications of PW Redundancy on S-PE . . . . . . . . . . . .   5
     4.1.  Applications in Scenario 1  . . . . . . . . . . . . . . .   5
     4.2.  Applications in Scenario 2  . . . . . . . . . . . . . . .   6
   5.  VCCV Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   [RFC6718] describes the framework and requirements for pseudowire
   (PW) redundancy, and [RFC6870] specifies a PW redundancy mechanism
   for scenarios where a set of redundant PWs are configured between
   Provider Edge (PE) nodes in Single-Segment Pseudowire (SS-PW)
   [RFC3985] applications, or between Terminating Provider Edge (T-PE)
   nodes in Multi-Segment Pseudowire (MS-PW) [RFC5659] applications.

   In some MS-PW scenarios, there are benefits of providing PW
   redundancy on Switching Provider Edges (S-PEs), such as reducing the
   burden on the access T-PE nodes and enabling faster protection
   switching compared to the end-to-end MS-PW protection mechanisms.

   This document describes some scenarios in which PW redundancy is
   provided on S-PEs and specifies the operations of the S-PEs.  The
   S-PEs connect to the neighboring T-PEs or S-PEs with PW segments.
   For the S-PE that provides PW redundancy for an MS-PW, there is a
   single PW segment on one side, which is called the single-homed side,
   and there are multiple PW segments on the other side, which is called
   the multi-homed side.  The scenario in which the S-PE has two multi-
   homed sides is out of scope.  Signaling of the Preferential
   Forwarding status as defined in [RFC6870] and [RFC6478] is reused.
   This document does not require any change to the T-PEs of MS-PW.

   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 [RFC2119].





Dong & Wang                  Standards Track                    [Page 2]

RFC 7795                  PW Redundancy on S-PE            February 2016


2.  Typical Scenarios of PW Redundancy on S-PE

   In some MS-PW deployment scenarios, there are benefits of providing
   PW redundancy on S-PEs.  This section describes typical scenarios of
   PW redundancy on S-PE.

2.1.  MS-PW Redundancy on S-PE

                                               +-----+  AC
           +---+                  +-----+      |     |  |  +---+
           |   |                  |     |------|T-PE2|-----|   |
           |   |  AC +-----+      |  ..PW-Seg2.......|     |   |
           |   |  |  |....PW-Seg1.....  |      +-----+     |   |
           |CE1|-----|T-PE1|------|S-PE1|                  |CE2|
           |   |     |     |      |  .  |      +-----+     |   |
           |   |     +-----+      |  ..PW-Seg3.......|     |   |
           |   |                  |     |------|T-PE3|-----|   |
           +---+                  +-----+      |     |  |  +---+
                                               +-----+  AC

                    Figure 1: MS-PW Redundancy on S-PE

   As illustrated in Figure 1, Customer Edge (CE) node CE1 is connected
   to T-PE1 while CE2 is dual-homed to T-PE2 and T-PE3.  T-PE1 is
   connected to S-PE1 only, and S-PE1 is connected to both T-PE2 and
   T-PE3.  The MS-PW is switched on S-PE1, and PW segments PW-Seg2 and
   PW-Seg3 provide resiliency on S-PE1 for the failure of T-PE2, T-PE3,
   or the connected Attachment Circuits (ACs).  PW-Seg2 is selected as
   the primary PW segment, and PW-Seg3 is the secondary PW segment.

   MS-PW redundancy on S-PE is beneficial for the scenario in Figure 1
   since T-PE1 as an access node may not support PW redundancy.
   Besides, with PW redundancy on S-PE, the number of PW segments
   required between T-PE1 and S-PE1 is only half of the number of PW
   segments needed when end-to-end MS-PW redundancy is used.  In
   addition, in this scenario, PW redundancy on S-PE could provide
   faster protection switching, compared with end-to-end protection
   switching of MS-PW.













Dong & Wang                  Standards Track                    [Page 3]

RFC 7795                  PW Redundancy on S-PE            February 2016


2.2.  MS-PW Redundancy on S-PE with S-PE Protection

         +---+     +-----+      +-----+           +-----+
         |   |     |     |      |     |           |     |
         |   |  AC |......PW1-Seg1......PW1-Seg2........|
         |   |  |  |   . |      |  .  |           |     |
         |CE1|-----|T-PE1|------|S-PE1|-----------|T-PE2|  AC
         |   |     |   . |      |  .  | PW1-Seg3  |     |  |  +---+
         |   |     |   . |      |  .........      ......|-----|   |
         |   |     |   . |      |     |    .     .|     |     |   |
         +---+     +---.-+      +-----+     .   . +-----+     |   |
                      |.                     . .              |CE2|
                      |.                      ..              |   |
                      |.        +-----+      .  . +-----+     |   |
                      |.        |     |     .    .|     |-----|   |
                      |...PW2-Seg1..........      ......|  |  +---+
                      |         |  .  | PW2-Seg2  |     |  AC
                      ----------|S-PE2|-----------|T-PE3|
                                |  .  |           |     |
                                |  .....PW2-Seg3........|
                                |     |           |     |
                                +-----+           +-----+

          Figure 2: MS-PW Redundancy on S-PE with S-PE Protection

   As illustrated in Figure 2, CE1 is connected to T-PE1 while CE2 is
   dual-homed to T-PE2 and T-PE3.  T-PE1 is connected to both S-PE1 and
   S-PE2, and both S-PE1 and S-PE2 are connected to both T-PE2 and
   T-PE3.  There are two MS-PWs that are switched at S-PE1 and S-PE2,
   respectively, to provide S-PE node protection.  For PW1, S-PE1
   provides resiliency using PW1-Seg2 and PW1-Seg3.  For PW2, S-PE2
   provides resiliency using PW2-Seg2 and PW2-Seg3.  PW1 is the primary
   MS-PW, and PW1-Seg2 between S-PE1 and T-PE2 is the primary PW
   segment.  PW2 is the secondary MS-PW.

   MS-PW redundancy on S-PE is beneficial for this scenario because it
   reduces the number of end-to-end MS-PWs required for both T-PE and
   S-PE protection.  In addition, PW redundancy on S-PE could provide
   faster protection switching, compared with end-to-end protection
   switching of MS-PW.

3.  S-PE Operations

   For an S-PE that provides PW redundancy for MS-PW, it is important to
   advertise the proper preferential forwarding status to the PW
   segments on both sides and perform protection switching according to
   the received status information.  Note that when PW redundancy for
   MS-PW is provided on S-PE, the optional S-PE Bypass mode as defined



Dong & Wang                  Standards Track                    [Page 4]

RFC 7795                  PW Redundancy on S-PE            February 2016


   in [RFC6478] MUST NOT be used; otherwise, the S-PE will not receive
   the PW status messages originated by T-PEs.  This section specifies
   the operations of S-PEs on which PW redundancy is provisioned.  This
   section does not make any change to the T-PEs of MS-PW.

   The S-PEs connect to the neighboring T-PEs or other S-PEs on two
   sides with PW segments.  For the S-PE that provides PW redundancy for
   an MS-PW, on one side there is a single PW segment, which is called
   the single-homed side, and on the other side there are multiple PW
   segments, which is called the multi-homed side.  The scenario in
   which the S-PE has two multi-homed sides is out of scope.

   The S-PE that provides PW redundancy MUST work in Slave mode for the
   single-homed side, and MUST work in Independent mode for the multi-
   homed side.  Consequently, the T-PE on the single-homed side MUST
   work in the Master mode, and the T-PEs on the multi-homed side MUST
   work in the Independent mode.  The signaling of the Preferential
   Forwarding bit as defined in [RFC6870] and [RFC6478] is reused.

   The S-PE MUST pass the Preferential Forwarding status received from
   the single-homed side unchanged to all the PW segments on the multi-
   homed side.  The S-PE MUST advertise the Standby Preferential
   Forwarding status to the single-homed side if it receives Standby
   status from all the PW segments on the multi-homed side, and it MUST
   advertise the Active Preferential Forwarding status to the single-
   homed side if it receives Active status from any of the PW segments
   on the multi-homed side.  For the single-homed side, the active PW
   segment is determined by the T-PE on this side, which works in the
   Master mode.  On the multi-homed side, since both the S-PE and T-PEs
   work in the Independent mode, the PW segment which has both the local
   and remote Up/Down status as Up and both the local and remote
   Preferential Forwarding status as Active MUST be selected for traffic
   forwarding.  When a switchover happens on the S-PE, if the S-PE
   supports the SP-PE TLV processing as defined in [RFC6073], it SHOULD
   advertise the updated SP-PE TLVs by sending a Label Mapping message
   to the T-PEs.

4.  Applications of PW Redundancy on S-PE

4.1.  Applications in Scenario 1

   For the scenario in Figure 1, assume the AC from CE2 to T-PE2 is
   active.  In normal operation, S-PE1 would receive the Active
   Preferential Forwarding status bit on the single-homed side from
   T-PE1, then it would advertise the Active Preferential Forwarding
   status bit on both PW-Seg2 and PW-Seg3.  T-PE2 and T-PE3 would
   advertise the Active and Standby Preferential Forwarding status bit
   to S-PE1, respectively, reflecting the forwarding state of the two



Dong & Wang                  Standards Track                    [Page 5]

RFC 7795                  PW Redundancy on S-PE            February 2016


   ACs connected to CE2.  By matching the local and remote Up/Down
   status and Preferential Forwarding status, PW-Seg2 would be used for
   traffic forwarding.

   On failure of the AC between CE2 and T-PE2, the forwarding state of
   AC on T-PE3 is changed to Active.  T-PE3 then advertises the Active
   Preferential Forwarding status to S-PE1, and T-PE2 would advertise a
   PW status Notification message to S-PE1, indicating that the AC
   between CE2 and T-PE2 is down.  S-PE1 would perform the switchover
   according to the updated local and remote Preferential Forwarding
   status and the status of "Pseudowire forwarding", and select PW-Seg3
   as the new PW segment for traffic forwarding.  Since S-PE1 still
   connects to an Active PW segment on the multi-homed side, it will not
   advertise any change of the PW status to T-PE1.  If S-PE1 supports
   the SP-PE TLV processing as defined in [RFC6073], it would advertise
   the updated SP-PE TLVs by sending a Label Mapping message to T-PE1.

4.2.  Applications in Scenario 2

   For the scenario of Figure 2, assume the AC from CE2 to T-PE2 is
   active.  T-PE1 works in Master mode and it would advertise the Active
   and Standby Preferential Forwarding status bit to S-PE1 and S-PE2
   respectively according to configuration.  According to the received
   Preferential Forwarding status bit, S-PE1 would advertise the Active
   Preferential Forwarding status bit to both T-PE2 and T-PE3, and S-PE2
   would advertise the Standby Preferential Forwarding status bit to
   both T-PE2 and T-PE3.  T-PE2 would advertise the Active Preferential
   Forwarding status bit to both S-PE1 and S-PE2, and T-PE3 would
   advertise the Standby Preferential Forwarding status bit to both
   S-PE1 and S-PE2, reflecting the forwarding state of the two ACs
   connected to CE2.  By matching the local and remote Up/Down Status
   and Preferential Forwarding status, PW1-Seg2 from S-PE1 to T-PE2
   would be used for traffic forwarding.  Since S-PE1 connects to the
   Active PW segment on the multi-homed side, it would advertise the
   Active Preferential Forwarding status bit to T-PE1, and S-PE2 would
   advertise the Standby Preferential Forwarding status bit to T-PE1
   because it does not have any Active PW segment on the multi-homed
   side.

   On failure of the AC between CE2 and T-PE2, the forwarding state of
   AC on T-PE3 is changed to Active.  T-PE3 would then advertise the
   Active Preferential Forwarding status bit to both S-PE1 and S-PE2,
   and T-PE2 would advertise a PW status Notification message to both
   S-PE1 and S-PE2, indicating that the AC between CE2 and T-PE2 is
   down.  S-PE1 would perform the switchover according to the updated
   local and remote Preferential Forwarding status and the status of
   "Pseudowire forwarding", and select PW1-Seg3 for traffic forwarding.
   Since S-PE1 still has an Active PW segment on the multi-homed side,



Dong & Wang                  Standards Track                    [Page 6]

RFC 7795                  PW Redundancy on S-PE            February 2016


   it would not advertise any change of the PW status to T-PE1.  If
   S-PE1 supports the SP-PE TLV processing as defined in [RFC6073], it
   would advertise the updated SP-PE TLVs by sending a Label Mapping
   message to T-PE1.

   If S-PE1 fails, T-PE1 would notice this through some detection
   mechanism and then advertise the Active Preferential Forwarding
   status bit to S-PE2, and PW2-Seg1 would be selected by T-PE1 for
   traffic forwarding.  On receipt of the newly changed Preferential
   Forwarding status, S-PE2 would advertise the Active Preferential
   Forwarding status to both T-PE2 and T-PE3.  T-PE2 and T-PE3 would
   also notice the failure of S-PE1 by some detection mechanism.  Then
   by matching the local and remote Up/Down and Preferential Forwarding
   status, PW2-Seg2 would be selected for traffic forwarding.

5.  VCCV Considerations

   For PW Virtual Circuit Connectivity Verification (VCCV) [RFC5085],
   the Control Channel (CC) type 1 "PW ACH" can be used with the S-PE
   redundancy mechanism.  VCCV CC type 2 "Router Alert Label" is not
   supported for MS-PW as specified in [RFC6073].  If VCCV CC type 3
   "TTL Expiry" is to be used, the PW label TTL MUST be set to the
   appropriate value to reach the target PE.  The hop count from one
   T-PE to the target PE can be obtained via SP-PE TLVs, through MS-PW
   path trace, or based on management-plane information.

6.  Security Considerations

   Since PW redundancy is provided on the S-PE nodes of MS-PWs, it is
   important that the security mechanisms as defined in [RFC4447],
   [RFC6073], and [RFC6478] be implemented to ensure that the S-PE nodes
   and the messages sent and received by the S-PE nodes are not
   compromised.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4447]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
              G. Heron, "Pseudowire Setup and Maintenance Using the
              Label Distribution Protocol (LDP)", RFC 4447,
              DOI 10.17487/RFC4447, April 2006,
              <http://www.rfc-editor.org/info/rfc4447>.



Dong & Wang                  Standards Track                    [Page 7]

RFC 7795                  PW Redundancy on S-PE            February 2016


   [RFC6073]  Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
              Aissaoui, "Segmented Pseudowire", RFC 6073,
              DOI 10.17487/RFC6073, January 2011,
              <http://www.rfc-editor.org/info/rfc6073>.

   [RFC6478]  Martini, L., Swallow, G., Heron, G., and M. Bocci,
              "Pseudowire Status for Static Pseudowires", RFC 6478,
              DOI 10.17487/RFC6478, May 2012,
              <http://www.rfc-editor.org/info/rfc6478>.

   [RFC6870]  Muley, P., Ed. and M. Aissaoui, Ed., "Pseudowire
              Preferential Forwarding Status Bit", RFC 6870,
              DOI 10.17487/RFC6870, February 2013,
              <http://www.rfc-editor.org/info/rfc6870>.

7.2.  Informative References

   [RFC3985]  Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,
              <http://www.rfc-editor.org/info/rfc3985>.

   [RFC5085]  Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
              Circuit Connectivity Verification (VCCV): A Control
              Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
              December 2007, <http://www.rfc-editor.org/info/rfc5085>.

   [RFC5659]  Bocci, M. and S. Bryant, "An Architecture for Multi-
              Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
              DOI 10.17487/RFC5659, October 2009,
              <http://www.rfc-editor.org/info/rfc5659>.

   [RFC6718]  Muley, P., Aissaoui, M., and M. Bocci, "Pseudowire
              Redundancy", RFC 6718, DOI 10.17487/RFC6718, August 2012,
              <http://www.rfc-editor.org/info/rfc6718>.
















Dong & Wang                  Standards Track                    [Page 8]

RFC 7795                  PW Redundancy on S-PE            February 2016


Acknowledgements

   The authors would like to thank Mach Chen, Lizhong Jin, Mustapha
   Aissaoui, Luca Martini, Matthew Bocci, and Stewart Bryant for their
   valuable comments and discussions.

Authors' Addresses

   Jie Dong
   Huawei Technologies
   Huawei Building, No.156 Beiqing Rd.
   Beijing  100095
   China

   Email: jie.dong@huawei.com


   Haibo Wang
   Huawei Technologies
   Huawei Building, No.156 Beiqing Rd.
   Beijing  100095
   China

   Email: rainsword.wang@huawei.com



























Dong & Wang                  Standards Track                    [Page 9]

