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+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>.
+
+
+
+
+
+
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+
+
+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
+
+
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+Dong & Wang                  Standards Track                    [Page 9]
+