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RFC 8104

Proposed STD
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Pseudowire (PW) Endpoint Fast Failure Protection

Part 1 of 2, p. 1 to 29
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Internet Engineering Task Force (IETF)                           Y. Shen
Request for Comments: 8104                              Juniper Networks
Category: Standards Track                                    R. Aggarwal
ISSN: 2070-1721                                             Arktan, Inc.
                                                           W. Henderickx
                                                                   Nokia
                                                                Y. Jiang
                                           Huawei Technologies Co., Ltd.
                                                              March 2017


            Pseudowire (PW) Endpoint Fast Failure Protection

Abstract

   This document specifies a fast mechanism for protecting pseudowires
   (PWs) transported by IP/MPLS tunnels against egress endpoint
   failures, including egress attachment circuit (AC) failure, egress
   provider edge (PE) failure, multi-segment PW terminating PE failure,
   and multi-segment PW switching PE failure.  Operating on the basis of
   multihomed customer edge (CE), redundant PWs, upstream label
   assignment, and context-specific label switching, the mechanism
   enables local repair to be performed by the router upstream adjacent
   to a failure.  The router can restore a PW in the order of tens of
   milliseconds, by rerouting traffic around the failure to a protector
   through a pre-established bypass tunnel.  Therefore, the mechanism
   can be used to reduce traffic loss before global repair reacts to the
   failure and the network converges on the topology changes due to the
   failure.

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 7841.

   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/rfc8104.

[Page 2] 
Copyright Notice

   Copyright (c) 2017 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Specification of Requirements . . . . . . . . . . . . . . . .   5
   3.  Reference Models for Egress Endpoint Failures . . . . . . . .   5
     3.1.  Single-Segment PW . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Multi-Segment PW  . . . . . . . . . . . . . . . . . . . .   9
   4.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Local Repair  . . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  Context Identifier  . . . . . . . . . . . . . . . . . . .  14
       4.3.1.  Semantics . . . . . . . . . . . . . . . . . . . . . .  15
       4.3.2.  FEC . . . . . . . . . . . . . . . . . . . . . . . . .  16
       4.3.3.  IGP Advertisement and Path Computation  . . . . . . .  16
     4.4.  Protection Models . . . . . . . . . . . . . . . . . . . .  17
       4.4.1.  Co-located Protector  . . . . . . . . . . . . . . . .  17
       4.4.2.  Centralized Protector . . . . . . . . . . . . . . . .  19
     4.5.  Transport Tunnel  . . . . . . . . . . . . . . . . . . . .  20
     4.6.  Bypass Tunnel . . . . . . . . . . . . . . . . . . . . . .  21
     4.7.  Examples of Forwarding State  . . . . . . . . . . . . . .  22
       4.7.1.  Co-located Protector Model  . . . . . . . . . . . . .  22
       4.7.2.  Centralized Protector Model . . . . . . . . . . . . .  26
   5.  Restorative and Revertive Behaviors . . . . . . . . . . . . .  29
   6.  LDP Extensions  . . . . . . . . . . . . . . . . . . . . . . .  30
     6.1.  Egress Protection Capability TLV  . . . . . . . . . . . .  31
     6.2.  PW Label Distribution from Primary PE to Protector  . . .  32
     6.3.  PW Label Distribution from Backup PE to Protector . . . .  33
     6.4.  Protection FEC Element TLV  . . . . . . . . . . . . . . .  34
       6.4.1.  Encoding Format for PWid with IPv4 PE Addresses . . .  35
       6.4.2.  Encoding Format for Generalized PWid with IPv4 PE
               Addresses . . . . . . . . . . . . . . . . . . . . . .  36
       6.4.3.  Encoding Format for PWid with IPv6 PE Addresses . . .  37
       6.4.4.  Encoding Format for Generalized PWid with IPv6 PE
               Addresses . . . . . . . . . . . . . . . . . . . . . .  38
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  40
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  40
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  41
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

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1.  Introduction

   Per [RFC3985], [RFC8077], and [RFC5659], a pseudowire (PW) or PW
   segment can be thought of as a connection between a pair of
   forwarders hosted by two PEs, carrying an emulated Layer 2 service
   over a packet switched network (PSN).  In the single-segment PW
   (SS-PW) case, a forwarder binds a PW to an attachment circuit (AC).
   In the multi-segment PW (MS-PW) case, a forwarder on a terminating PE
   (T-PE) binds a PW segment to an AC, while a forwarder on a switching
   PE (S-PE) binds one PW segment to another PW segment.  In each
   direction between the PEs, PW packets are transported by a PSN
   tunnel, which is also called a transport tunnel.

   In order to protect the PW service against network failures, it is
   necessary to protect every link and node along the entire data path.
   For the traffic in a given direction, this includes ingress AC,
   ingress (T-)PE, intermediate routers of the transport tunnel, S-PEs,
   egress (T-)PE, and egress AC.  To minimize service disruption upon a
   failure, it is also desirable that each of these components is
   protected by a fast protection mechanism based on local repair.  Such
   mechanisms generally involve a bypass path that is pre-computed and
   pre-installed in the data plane on the router upstream adjacent to an
   anticipated failure.  This router is referred to as a "point of local
   repair" (PLR).  The bypass path has the property that it can guide
   traffic around the failure, while remaining unaffected by the
   topology changes resulting from the failure.  When the failure
   occurs, the PLR can invoke the bypass path to achieve fast
   restoration for the service.

   Today, fast protection against ingress AC failure and ingress (T-)PE
   failure can be achieved by using a multihomed CE and redundant ACs,
   such as a multi-chassis link aggregation group (MC-LAG).  Fast
   protection against the failure of an intermediate router of the
   transport tunnel can be achieved through RSVP fast reroute [RFC4090]
   or IP/LDP fast reroute [RFC5286] [RFC5714].  However, there is no
   equivalent mechanism that can be used against an egress AC failure,
   an egress (T-)PE failure, or an S-PE failure.  For these failures,
   service restoration has to rely on global repair or control-plane
   convergence.  Global repair normally involves the ingress CE or the
   ingress (T-)PE switching traffic to an alternative path, based on
   remote failure detection via PW status notification, end-to-end
   Operations, Administration, and Maintenance (OAM), and others.
   Control-plane convergence relies on control protocols to react on the
   topology changes due to a failure.  Compared to local repair, these
   mechanisms are relatively slow in reacting to a failure and restoring
   traffic.

Top      ToC       Page 5 
   This document addresses the above need.  It specifies a fast
   protection mechanism based on local repair to protect PWs against the
   following endpoint failures:

   a.  Egress AC failure.

   b.  Egress PE failure: Link or node failure of an egress PE of an
       SS-PW or a T-PE of an MS-PW.

   c.  Switching PE failure: Link or node failure of an S-PE of an
       MS-PW.

   The mechanism is applicable to LDP-signaled PWs.  It is relevant to
   networks with redundant PWs and multihomed CEs.  It is designed on
   the basis of MPLS upstream label assignment and context-specific
   label switching [RFC5331].  Fast protection refers to its ability to
   restore traffic in the order of tens of milliseconds.  Compared with
   global repair and control-plane convergence, this mechanism can
   provide faster service restoration.  However, it is intended to
   complement these mechanisms, rather than replacing them, as networks
   rely on them to eventually move traffic to fully functional
   alternative paths.

2.  Specification of Requirements

   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 [RFC2119].

3.  Reference Models for Egress Endpoint Failures

   This document refers to the following topologies to describe egress
   endpoint failures and protection procedures.

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3.1.  Single-Segment PW

                  |<-------------- PW1 --------------->|

              - PE1 -------------- P1 ---------------- PE2 -
             /                                              \
            /                                                \
         CE1                                                  CE2
            \                                                /
             \                                              /
              - PE3 -------------- P2 ---------------- PE4 -

                  |<-------------- PW2 --------------->|

                                 Figure 1

   In Figure 1, the IP/MPLS network consists of PE and P routers.  It
   provides a PW service between CE1 and CE2.  Each CE is multihomed via
   two ACs to two PEs.  This forms two divergent paths between the CEs.
   The first path uses PW1 between PE1 and PE2, and the second path uses
   PW2 between PE3 and PE4.  For clarity, the transport tunnels of the
   PWs and other links between the routers are not shown in this figure.

   In general, a CE may operate the ACs in two modes when sending
   traffic to the remote CE, i.e., active-standby mode and active-active
   mode.

   o  In the active-standby mode, the CE chooses one AC as the active AC
      and the corresponding path as the active path, and it uses the
      other AC as the standby AC and the corresponding path as the
      standby path.  The CE only sends traffic on the active AC as long
      as the active path is operational.  The CE will only send traffic
      on the standby AC after it detects a failure of the active path.
      Note that the CE may receive traffic on the active or standby AC,
      depending on whether the remote CE chooses the same active path
      for the traffic of the reverse direction.  In this document, even
      if both CEs choose the same active path, each CE should still
      anticipate receiving traffic on a standby AC, because the traffic
      may be redirected to the standby path by the fast protection
      mechanism.

   o  In the active-active mode, the CE treats both ACs and their
      corresponding paths as active and sends traffic on both ACs in a
      load-balancing fashion.  In the reverse direction, the CE may
      receive traffic on both ACs.

   The above modes assume the traffic to be data traffic, which is not
   bound to a specific AC.  This does not include control-protocol

Top      ToC       Page 7 
   traffic between the CEs, when the CE-CE control-protocol sessions or
   adjacencies established on the two ACs are considered as distinct
   rather than having a primary and backup relationship.  In general, a
   dual-homed CE should not make any explicit or implicit assumptions
   regarding the specific AC from which it receives packets from the
   remote CE.

   For either mode, when considering the traffic flowing in a given
   direction over an active path, this document views the ACs, PEs, and
   PWs as serving primary or backup roles.  In particular, the ACs, PEs,
   and PWs along this active path have primary roles, while those along
   the other path have backup roles.  Note that in the active-active
   mode, each AC, PE, and PW on an active path has a primary role and
   also a backup role protecting the other path, which is also active.

   For Figure 1, the following roles are assumed for the traffic going
   from CE1 to CE2 via PW1.

      Primary ingress AC: CE1-PE1

      Primary ingress PE: PE1

      Primary PW: PW1

      Primary egress PE: PE2

      Primary egress AC: PE2-CE2

      Backup ingress AC: CE1-PE3

      Backup ingress PE: PE3

      Backup PW: PW2

      Backup egress PE: PE4

      Backup egress AC: PE4-CE2

   Based on this schema, this document describes egress endpoint
   failures and the fast protection mechanism on the per-active-path and
   per-direction basis.  In this case, an egress AC failure refers to
   the failure of the AC PE2-CE2, and an egress node failure refers to
   the failure of PE2.  The ultimate goal is that when a failure occurs,
   the traffic should be locally repaired, so that it can eventually
   reach CE2 via the backup egress PE (PE4) and the backup egress AC
   (PE4-CE2).

Top      ToC       Page 8 
   Subsequent to the local repair, either the current active path should
   heal after the control plane converges on the new topology or the
   ingress CE should switch traffic from the primary path to the backup
   path, depending on the failure scenario.  In the latter case, the
   ingress CE may perform the path switchover triggered by end-to-end
   OAM (in-band or out-band), PW status notification, CE-PE control
   protocols (e.g., the Link Aggregation Control Protocol (LACP)), and
   others.  In the active-standby mode, this will promote the standby
   path to a new active path.  In the active-active mode, it will make
   the other active path carry all the traffic between the two CEs.  In
   any case, this phase of restoration falls into the control-plane
   convergence and global repair category; hence, it is out of the scope
   of this document.  The purpose of the fast protection mechanism in
   this document is to reduce traffic loss before this phase of
   restoration takes place.

   Note that in Figure 1, if the traffic in the reverse direction (i.e.,
   from CE2 to CE1) traverses the AC CE2-PE2 and PE2 as an active path,
   the failure of PE2 and the failure of the AC PE2-CE2 will be
   considered as ingress failures of the traffic.  If CE2 can detect the
   failures, it may protect the traffic by switching it to the backup
   path via the AC CE2-PE4 and PE4.  However, this is categorized as
   ingress endpoint failure protection; hence, it is not handled by the
   mechanism described in this document.

   Figure 2 shows another possible scenario, where CE1 is single-homed
   to PE1, while CE2 remains multihomed to PE2 and PE4.  From the
   perspective of egress endpoint protection for the traffic going from
   CE1 to CE2 over PW1, this scenario is the same as the scenario shown
   in Figure 1.

                   |<-------------- PW1 --------------->|

                      ------------- P1 ---------------- PE2 -
                     /                                       \
                    /                                         \
          CE1 -- PE1                                          CE2
                    \                                         /
                     \                                       /
                      ------------- P2 ---------------- PE4 -

                   |<-------------- PW2 --------------->|

                                 Figure 2

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   For clarity, primary egress AC, primary egress PE, backup egress AC,
   and backup egress PE may simply be referred to as primary AC, primary
   PE, backup AC, and backup PE, respectively, when the context of a
   discussion is egress endpoint.

3.2.  Multi-Segment PW

                  |<--------------- PW1 --------------->|
                  |<----- SEG1 ----->|<----- SEG2 ----->|

             - TPE1 -------------- SPE1 --------------- TPE2 -
            /                                                 \
           /                                                   \
        CE1                                                     CE2
           \                                                   /
            \                                                 /
             - TPE3 -------------- SPE2 --------------- TPE4 -

                  |<----- SEG3 ----->|<----- SEG4 ----->|
                  |<--------------- PW2 --------------->|

                                 Figure 3

   Figure 3 shows a topology that is similar to Figure 1 but in an MS-PW
   environment.  PW1 and PW2 are both MS-PWs.  PW1 is established
   between TPE1 and TPE2 and switched between segments SEG1 and SEG2 at
   SPE1.  PW2 is established between TPE3 and TPE4 and switched between
   segments SEG3 and SEG4 at SPE2.  CE1 is multihomed to TPE1 and TPE3.
   CE2 is multihomed to TPE2 and TPE4.  For clarity, the transport
   tunnels of the PW segments are not shown in this figure.

   In this document, the following primary and backup roles are assigned
   for the traffic going from CE1 to CE2:

      Primary ingress AC: CE1-TPE1

      Primary ingress T-PE: TPE1

      Primary PW: PW1

      Primary S-PE: SPE1

      Primary egress T-PE: TPE2

      Primary egress AC: TPE2-CE2

      Backup ingress AC: CE1-TPE3

Top      ToC       Page 10 
      Backup ingress T-PE: TPE3

      Backup PW: PW2

      Backup S-PE: SPE2

      Backup egress T-PE: TPE4

      Backup egress AC: TPE4-CE2

   In this case, an egress AC failure refers to the failure of the AC
   TPE2-CE2.  An egress node failure refers to the failure of TPE2.  An
   S-PE failure refers to the failure of SPE1.

   For consistency with the SS-PW scenario, primary T-PEs and primary
   S-PEs may simply be referred to as primary PEs in this document,
   where specifics are not required.  Similarly, backup T-PEs and backup
   S-PEs may be referred to as backup PEs.

4.  Theory of Operation

   The fast protection mechanism in this document provides three types
   of protection for PWs, corresponding to the three types of failures
   described in Section 1:

   a.  Egress AC protection

   b.  Egress (T-)PE node protection

   c.  S-PE node protection

4.1.  Applicability

   The mechanism is applicable to LDP-signaled PWs in an environment
   where an egress CE is multihomed to a primary PE and a backup PE and
   there exists a backup PW, as described in Section 3.  The procedure
   for S-PE node protection is applicable when there exists a backup
   S-PE on the backup PW.

   The mechanism assumes IP/MPLS transport tunnels and is applicable to
   tunnels with single path and equal-cost multipaths (ECMPs).  As an
   example of ECMPs, imagine a tunnel carrying one or multiple PWs and
   traversing a router with ECMPs to a primary PE.  The ECMPs consist of
   at least one direct link to the PE and some multi-hop paths to the
   PE.  Due to the direct link, the router is considered as a
   penultimate hop of the tunnel and can perform local detection and
   repair for an egress node failure.  The router normally uses a
   hashing algorithm to distribute PW packets over the ECMPs, on a

Top      ToC       Page 11 
   per-PW or per-flow basis.  Upon a failure of the direct link, i.e.,
   transit link failure, the router removes the link from the hashing
   algorithm, which automatically redistributes the traffic of the link
   to the other paths of the ECMPs, achieving local repair.  This
   scenario is not the focus of this document.  Upon a failure of the
   PE, i.e., egress node failure, the router SHOULD perform local repair
   by rerouting the entire traffic of the ECMPs, as the failure will
   affect every path.  If the router does not have a fast or reliable
   mechanism to detect the egress node failure, it is RECOMMENDED that
   the router SHOULD treat the failure of the direct link as an egress
   node failure.

   The mechanism is applicable to both best-effort and traffic
   engineering (TE) transport tunnels.  For TE transport tunnels that
   require bandwidth protection, TE bypass tunnels with reserved
   bandwidth MAY be used to avoid congestion for rerouted traffic.

   It is also RECOMMENDED that the mechanism SHOULD be used in
   conjunction with global repair and control-plane convergence, in such
   a manner that the mechanism temporarily repairs a failed path by
   using a bypass tunnel, and global repair and control-plane
   convergence eventually move traffic to a fully functional alternative
   path.

4.2.  Local Repair

   The fast protection ability of the mechanism comes from local repair
   performed by routers upstream adjacent to failures.  Each of these
   routers is referred to as a PLR.  A PLR MUST be able to detect a
   failure by using a rapid mechanism, such as physical-layer failure
   detection, Bidirectional Forwarding Detection (BFD) [RFC5880],
   Seamless BFD (S-BFD) [RFC7880], and others.  In anticipation of the
   failure, the PLR MUST also pre-establish a bypass tunnel to a
   "protector" and pre-install a bypass route for the bypass tunnel in
   the data plane.  The protector is either a backup PE or a router that
   will forward traffic to a backup PE.  The bypass tunnel MUST have the
   property that it will not be affected by the topology changes due to
   the failure.  Specifically, it MUST NOT traverse the primary PE or
   the penultimate link of the protected transport tunnel or share any
   shared risk link groups (SRLGs) with the penultimate link.  Upon
   detecting the failure, the PLR invokes the bypass route in the data
   plane and reroutes PW traffic to the protector through the bypass
   tunnel.  The protector in turn sends the traffic to the target CE.
   This procedure is referred to as local repair.

   Different routers may serve as PLR and protector in different
   scenarios.

Top      ToC       Page 12 
   o  In egress AC protection, the PLR is the primary PE, and the
      protector is the backup PE (Figure 4).

                  |<-------------- PW1 --------------->|

              - PE1 -------------- P1 ---------------- PE2 -
             /                                         PLR  \
            /                                           |    \
         CE1                                      bypass|     CE2
            \                                           |    /
             \                                          |   /
              - PE3 -------------- P2 ---------------- PE4 -
                                                    protector

                  |<-------------- PW2 --------------->|

                                 Figure 4

   o  In egress PE node protection, the PLR is the penultimate hop
      router of the transport tunnel of the primary PW, and the
      protector is the backup PE (Figure 5).

                  |<-------------- PW1 --------------->|

              - PE1 -------------- P1 ------- P3 ----- PE2 -
             /                               PLR \          \
            /                                     \          \
         CE1                                 bypass\          CE2
            \                                       \        /
             \                                       \      /
              - PE3 -------------- P2 ---------------- PE4 -
                                                    protector

                  |<-------------- PW2 --------------->|

                                 Figure 5

Top      ToC       Page 13 
   o  In S-PE node protection, the PLR is the penultimate hop router of
      the transport tunnel of the primary PW segment, and the protector
      is the backup S-PE (Figure 6).

                  |<--------------- PW1 --------------->|
                  |<----- SEG1 ----->|<----- SEG2 ----->|

             - TPE1 ----- P1  ----- SPE1 -------------- TPE2 -
            /             PLR \                               \
           /                   \                               \
        CE1               bypass\                               CE2
           \                     \                             /
            \                     \                           /
             - TPE3 --------------- SPE2 -------------- TPE4 -
                                 protector

                  |<----- SEG3 ----->|<----- SEG4 ----->|
                  |<--------------- PW2 --------------->|

                                 Figure 6

   In egress AC protection, a PLR realizes its role based on
   configuration of a "context identifier", which is introduced in this
   document (Section 4.3).  The PLR establishes a bypass tunnel to the
   protector in the same fashion as a normal PSN tunnel.

   In egress PE and S-PE node protection, a PLR is a transit router on
   the transport tunnel, and it normally does not have knowledge of the
   PW(s) carried by the transport tunnel.  In this document, the PLR
   simply computes and establishes a node-protection bypass tunnel in
   the same fashion as the normal IP/MPLS node protection, except that
   with the notion of the context identifier, the bypass tunnel will be
   established from the PLR to the protector (Section 4.6).  Conversely,
   when the router is no longer a PLR for egress PE or S-PE node
   protection due to a change in network topology or the transport
   tunnel's path, the router should revert to the role of regular
   transit router, including PLR for transit link and node protection.

   In local repair, a PLR simply switches all the traffic received on
   the transport tunnel to the bypass tunnel.  This requires that the
   protector given by the bypass tunnel MUST be intended for all the PWs
   carried by the transport tunnel.  This is achieved by the ingress PE
   using a context identifier to associate a PW with the specific pair
   of {primary PE, protector} and map the PW to a transport tunnel
   destined for the same {primary PE, protector}.  The ingress PE MAY
   map multiple PWs to the transport tunnel, if they share the {primary
   PE, protector} in common.

Top      ToC       Page 14 
   In local repair, the PLR keeps the PW label intact in packets.  This
   obviates the need for the PLR to maintain bypass routes on a per-PW
   basis and allows bypass tunnel sharing between PWs.  On the other
   hand, this imposes a requirement on the protector that it MUST be
   able to forward the packets based on a PW label that is assigned by
   the primary PE and ensure that the traffic MUST reach the target CE
   via a backup path.  From the protector's perspective, this PW label
   is an upstream assigned label [RFC5331].  To achieve this, the
   protector MUST learn the PW label from the primary PE prior to the
   failure and install a proper forwarding state for the PW label in a
   dedicated label space associated with the primary PE.  During local
   repair, the protector MUST perform PW label lookup in this label
   space.

   The previous examples have shown the scenarios where the protectors
   are backup (T-/S-)PEs.  It is also possible that a protector is a
   dedicated router to serve such a role, separate from the backup
   (T-/S-)PE.  During local repair, the PLR still reroutes traffic to
   the protector through a bypass tunnel.  The protector then forwards
   the traffic to the backup (T-/S-)PE, which further forwards the
   traffic to the target CE via a backup AC or a backup PW segment.
   More detail is included in Section 4.4.

4.3.  Context Identifier

   A protector may protect multiple primary PEs.  The protector MUST
   maintain a separate label space for each primary PE.  Likewise, the
   PWs terminated on a primary PE may be protected by multiple
   protectors, each for a subset of the PWs.  In any case, a given PW
   MUST be associated with one and only one pair of {primary PE,
   protector}.

   This document introduces the notion of a context identifier to
   facilitate protection establishment.  A context identifier is an
   IPv4/v6 address assigned to each ordered pair of {primary PE,
   protector}.  The address MUST be globally unique or unique in the
   address space of the network where the primary PE and the protector
   reside.

Top      ToC       Page 15 
4.3.1.  Semantics

   The semantics of a context identifier is twofold:

   o  A context identifier identifies a primary PE and an associated
      protector.  It represents the primary PE as the PW destination on
      a per-protector basis.  A given primary PE may be protected by
      multiple protectors, each for a subset of the PWs terminated on
      the primary PE.  A distinct context identifier MUST be assigned to
      each {primary PE, protector} pair.

      The ingress PE of a PW learns the context identifier of the PW's
      {primary PE, protector} from the primary PE via the Interface_ID
      TLV [RFC3471] [RFC3472] in the LDP Label Mapping message of the
      PW.  The ingress PE then sets up or resolves a transport tunnel
      with the context identifier, rather than a private IP address of
      the primary PE, as the destination.  This destination not only
      makes the transport tunnel reach the primary PE but also conveys
      the identity of the protector to the PLR, which MUST use the
      context identifier as the destination for the bypass tunnel to the
      protector.  The ingress PE MUST map only the PWs terminated by the
      exact primary PE and protected by the exact protector to the
      transport tunnel.

   o  A context identifier indicates the primary PE's label space on the
      protector.  The protector may protect PWs for multiple primary
      PEs.  For each primary PE, it MUST maintain a separate label space
      to store the PW labels assigned by that primary PE.  It associates
      a PW label with a label space via the context identifier of the
      {primary PE, protector}, as below.

      In addition to the normal LDP PW signaling, the primary PE MUST
      have a targeted LDP session with the protector and advertise PW
      labels to the protector via LDP Label Mapping messages
      (Section 6).  The primary PE MUST attach the context identifier to
      each message.  Upon receiving the message, the protector MUST
      install the advertised PW label in the label space identified by
      the context identifier.

      When a PLR sets up or resolves a bypass tunnel to the protector,
      it MUST use the context identifier rather than a private IP
      address of the protector as the destination.  The protector MUST
      use the bypass tunnel, either the MPLS tunnel label or the IP
      tunnel destination address, as the pointer to the corresponding
      label space.  The protector MUST forward PW packets received on
      the bypass tunnel based on label lookup in that label space.

Top      ToC       Page 16 
4.3.2.  FEC

   In an MPLS network, a context identifier represents a Forwarding
   Equivalence Class (FEC) for transport tunnels and bypass tunnels
   destined for it.  For example, it may be encoded in an LDP Prefix FEC
   Element or in the "tunnel endpoint address" of an RSVP Session
   object.  The FEC is associated with a unique forwarding state on PLRs
   and the protector, which cannot be shared with other FECs.  Some MPLS
   protocols (e.g., LDP) support FEC aggregation [RFC3031].  In this
   case, FEC aggregation MUST NOT be applied to a context identifier's
   FEC, and every router MUST assign a unique label to the FEC.

4.3.3.  IGP Advertisement and Path Computation

   Using a context identifier as the destination for both the transport
   tunnel and bypass tunnel requires coordination between the primary PE
   and the protector in IGP advertisement of the context identifier in
   the routing domain and TE domain.  The context identifier should be
   advertised in such a way that all the routers on the tunnels MUST be
   able to independently reach the following common view of paths:

   o  The transport tunnel MUST have the primary PE as the path
      endpoint.

   o  The bypass tunnel MUST have the protector as the path endpoint.
      In egress PE and S-PE node protection, the path MUST avoid the
      primary PE.

   There are generally two categories of approaches to achieve the
   above:

   o  The first category does not require an ingress PE or a PLR to have
      knowledge of the PW egress endpoint protection schema.  It does
      not require any IGP extension for context identifier
      advertisement.  A context identifier is advertised by the primary
      PE and the protector as an address reachable via both routers.
      The ingress PE and the PLR can compute paths by using a normal
      method, such as Dijkstra, constrained shortest path first (CSPF),
      Loop-Free Alternate (LFA) [RFC5286], and Maximally Redundant Tree
      (MRT) [RFC7812].  One example is to advertise a context identifier
      as a virtual proxy node connected to the primary PE and the
      protector, with the link between the proxy node and the primary PE
      having a more preferable IGP and TE metric than the link between
      the proxy node and the protector.  The transport tunnel will
      follow the shortest path or a TE path to the primary PE and be
      terminated by the primary PE.  The PLR will no longer view itself
      as a penultimate hop of the transport tunnel, but rather two hops
      away from the proxy node, via the primary PE.  Hence, a node-

Top      ToC       Page 17 
      protection bypass tunnel will be available via the protector to
      the proxy node, but it will actually be terminated by the
      protector.

   o  The second category requires a PLR to have knowledge of the PW
      egress endpoint protection schema.  The primary PE advertises the
      context identifier as a regular IP address, while the protector
      advertises it by using an explicit "context identifier object",
      which MUST be understood by the PLR.  The context identifier
      object requires an IGP extension.  In both the routing domain and
      the TE domain, the context identifier is only reachable via the
      primary PE.  This ensures that the transport tunnel is terminated
      by the primary PE.  The PLR views itself as the penultimate hop of
      the transport tunnel, and based on the IGP context identifier
      object, it establishes or resolves a bypass tunnel to the
      advertiser (i.e., the protector), while avoiding the primary PE.

   The mechanism in this document intends to be flexible on the approach
   used by a network, as long as it satisfies the above requirements for
   the transport tunnel path and bypass tunnel path.  In theory, the
   network can use one approach for context ID X and another approach
   for context ID Y.  For a given context ID, all relevant routers,
   including primary PE, protector, and PLR, must support and agree on
   the chosen approach.  The coordination between the routers can be
   achieved by configuration.

4.4.  Protection Models

   There are two protection models based on the location of a protector:
   the co-located protector model and the centralized protector model.
   A network MAY use either model or both.

4.4.1.  Co-located Protector

   In this model, the protector is a backup PE that is directly
   connected to the target CE via a backup AC, or it is a backup S-PE on
   a backup PW.  That is, the protector is co-located with the backup
   (S-)PE.  Examples of this model are shown in Figures 4, 5, and 6 in
   Section 4.2.

   In egress AC protection and egress PE node protection, when a
   protector receives traffic from the PLR, it forwards the traffic to
   the CE via the backup AC.  This is shown in Figure 7, where PE2 is
   the PLR for egress AC failure, P3 is the PLR for PE2 failure, and PE4
   (backup PE) is the protector.

Top      ToC       Page 18 
                 |<-------------- PW1 --------------->|

             - PE1 -------------- P1 ------- P3 ----- PE2 ----
            /                               PLR \     PLR     \
           /                                     \     |       \
        CE1                                 bypass\    |bypass  CE2
           \                                       \   |       /
            \                                       \  |      /
             - PE3 -------------- P2 ---------------- PE4 ----
                                                   protector

                 |<-------------- PW2 --------------->|

                                 Figure 7

   In S-PE node protection, when a protector receives traffic from the
   PLR, it forwards the traffic over the next segment of the backup PW.
   The T-PE of the backup PW in turn forwards the traffic to the CE via
   a backup AC.  This is shown in Figure 8, where P1 is the PLR for SPE1
   failure, and SPE2 (backup S-PE) is the protector for SPE1.  SPE2
   receives traffic from P1, swaps SEG1's label to SEG4's label, and
   forwards the traffic over a transport tunnel to TPE4.

                  |<--------------- PW1 --------------->|
                  |<----- SEG1 ----->|<----- SEG2 ----->|

             - TPE1 ----- P1  ----- SPE1 -------------- TPE2 -
            /             PLR \                               \
           /                   \                               \
        CE1               bypass\                               CE2
           \                     \                             /
            \                     \                           /
             - TPE3 --------------- SPE2 -------------- TPE4 -
                                 protector

                  |<----- SEG3 ----->|<----- SEG4 ----->|
                  |<--------------- PW2 --------------->|

                                 Figure 8

   In the co-located protector model, the number of context identifiers
   needed by a network is the number of distinct {primary PE, backup PE}
   pairs.  From the perspective of scalability, the model is suitable
   for networks where the number of primary PEs and the average number
   of backup PEs per primary PE are both relatively low.

Top      ToC       Page 19 
4.4.2.  Centralized Protector

   In this model, the protector is a dedicated P router or PE router
   that serves the role.  In egress AC protection and egress PE node
   protection, the protector may or may not be a backup PE directly
   connected to the target CE.  In S-PE node protection, the protector
   may or may not be a backup S-PE on the backup PW.

   In egress AC protection and egress PE node protection, if the
   protector is not directly connected to the CE, it forwards the
   traffic to a backup PE, which in turn forwards the traffic to the CE
   via a backup AC.  This is shown in Figure 9, where the protector
   receives traffic from P3 (PLR for egress PE failure) or PE2 (PLR for
   egress AC failure), swaps PW1's label to PW2's label, and forwards
   the traffic via a transport tunnel to PE4 (backup PE).  The protector
   may be protecting other PWs and other primary PEs as well; for
   clarity, this is not shown in the figure.

                  |<------------- PW1 --------------->|

              - PE1 ------------- P1 ------- P3 ----- PE2 --
             /                              PLR \     PLR   \
            /                                    \     /     \
           /                                bypass\   /bypass \
          /                                        \ /         \
       CE1                                      protector       CE2
          \                                         \          /
           \                                transport\        /
            \                                  tunnel \      /
             \                                         \    /
              - PE3 ------------- P2 -----------------PE4 --

                  |<------------- PW2 --------------->|

                                 Figure 9

   In S-PE node protection, if the protector is not a backup S-PE, it
   forwards the traffic to the backup S-PE, which in turn forwards the
   traffic over the next segment of the backup PW.  Finally, the T-PE of
   the backup PW forwards the traffic to the CE via the backup AC.  This
   is shown in Figure 10, where the protector receives traffic from P1
   (PLR), swaps SEG1's label to SEG3's label, and forwards the traffic
   via a transport tunnel to SPE2 (backup S-PE).  SPE2 in turn performs
   MS-PW switching from SEG3's label to SEG4's label and forwards the
   traffic over a transport tunnel to TPE4 (backup T-PE).  The protector
   may be protecting other PW segments and other primary S-PEs as well;
   for clarity, this is not shown in the figure.

Top      ToC       Page 20 
                  |<--------------- PW1 --------------->|
                  |<----- SEG1 ----->|<----- SEG2 ----->|

             - TPE1 ----- P1  ----- SPE1 -------------- TPE2 -
            /             PLR \                               \
           /                   \                               \
          /               bypass\                               \
         /                       \                               \
      CE1                     protector                           CE2
         \                        \                              /
          \               transport\                            /
           \                 tunnel \                          /
            \                        \                        /
             - TPE3 --------------- SPE2 -------------- TPE4 -

                  |<----- SEG3 ----->|<----- SEG4 ----->|
                  |<--------------- PW2 --------------->|

                                 Figure 10

   The centralized protector model allows multiple primary PEs to share
   one protector.  Each primary PE may need only one protector.
   Therefore, the number of context identifiers needed by a network may
   be bound to the number of primary PEs.

4.5.  Transport Tunnel

   A PW is associated with a pair of {primary PE, protector}, which is
   represented by a unique context identifier.  The ingress PE of the PW
   sets up or resolves a transport tunnel by using the context
   identifier rather than a private IP address of the primary PE as the
   destination.  This not only ensures that the PW is transported to the
   primary PE but also facilitates bypass tunnel establishment at PLR,
   because the context identifier contains the identity of the protector
   as well.  This is also the case for a multi-segment PW, where the
   ingress PE and egress PE are T-/S-PEs.

   An ingress PE learns the association between a PW and a context
   identifier from the primary PE, which MUST advertise the context
   identifier as a "third-party next hop" via the IPv4/v6 Interface_ID
   TLV [RFC3471] [RFC3472] in the LDP Label Mapping message of the PW.

   In an ECMP scenario, a transport tunnel may have multiple penultimate
   hop routers.  Each of them SHOULD act as a PLR independently.  Also
   in an ECMP scenario, a penultimate hop router may have ECMPs to the
   primary PE.  At least one path of the ECMPs must be a direct link to
   the primary PE, qualifying the router as a penultimate hop.  The
   other paths of the ECMPs may be direct links or indirect paths to the

Top      ToC       Page 21 
   primary PE.  In egress PE node protection and S-PE node protection,
   when a node failure is detected, or a link failure is detected on a
   direct link and treated as a node failure, the penultimate hop router
   SHOULD act as a PLR and reroute the entire traffic of the ECMPs to
   the protector.

4.6.  Bypass Tunnel

   A PLR may protect multiple PWs associated with one or multiple pairs
   of {primary PE, protector}.  The PLR MUST establish a bypass tunnel
   to each protector for each context identifier associated with that
   protector.  The destination of the bypass tunnel MUST be the context
   identifier (Section 4.3.1).  Since the PLR is a transit router of the
   transport tunnel, it SHOULD derive the context identifier from the
   destination of the transport tunnel.

   For example, in Figures 7 and 9, a bypass tunnel is established from
   PE2 (PLR for egress AC failure) to the protector, and another bypass
   tunnel is established from P3 (PLR for egress node failure) to the
   protector.  In Figures 8 and 10, a bypass tunnel is established from
   P1 (PLR for S-PE failure) to the protector.

   In local repair, a PLR reroutes traffic to the protector through a
   bypass tunnel, with the PW label intact in the packets.  This
   normally involves pushing a label to the label stack, if the bypass
   tunnel is an MPLS tunnel, or pushing an IP header to the packets, if
   the bypass tunnel is an IP tunnel.  Upon receipt of the packets, the
   protector forwards them based on the PW label.  Specifically, the
   protector uses the bypass tunnel as a context to determine the
   primary PE's label space.  If the bypass tunnel is an MPLS tunnel,
   the protector should have assigned a non-reserved label to the bypass
   tunnel; hence, this label can serve as the context.  This label is
   also called a "context label", as it is actually bound to the context
   identifier.  If the bypass tunnel is an IP tunnel, the context
   identifier should be the destination address of the IP header.

   To be useful for local repair, a bypass tunnel MUST have the property
   that it is not affected by any topology changes caused by the
   failure.  It MUST NOT traverse the primary PE or the penultimate link
   of the transport tunnel, or share any SRLG with the penultimate link.
   A bypass tunnel may be a TE tunnel with reserved bandwidth to avoid
   traffic congestion for rerouted traffic.  A bypass tunnel should
   remain effective during local repair, until the traffic is moved to
   an alternative path, i.e., either the same PW over a fully functional
   transport tunnel or another fully functional PW.

Top      ToC       Page 22 
   There is little or no benefit to protect a bypass tunnel.  Therefore,
   a bypass tunnel SHOULD NOT be protected against a transit link
   failure, transit node failure, or egress node failure.

4.7.  Examples of Forwarding State

   This section provides some detailed examples of forwarding state on
   the PLR, protector, and other relevant routers.

   A protector learns PW labels from all the primary PEs that it
   protects (Section 6.2) and maintains the PW labels in separate label
   spaces on a per-primary-PE basis.  In the control plane, each label
   space is identified by the context identifier of the corresponding
   {primary PE, protector}.  In the forwarding plane, the label space is
   indicated by the bypass tunnel(s) destined for the context
   identifier.

4.7.1.  Co-located Protector Model

   In Figure 11, PE4 is a co-located protector that protects PW1 against
   egress AC failure and egress node failure.  It maintains a label
   space for PE2, which is identified by the context identifier of {PE2,
   PE4}.  It learns PW1's label from PE2 and installs a forwarding entry
   for the label in that label space.  The next hop of the forwarding
   entry indicates a label pop with an outgoing interface pointing to
   the backup AC PE4-CE2.

Top      ToC       Page 23 
             |<-------------- PW1 --------------->|

         - PE1 -------------- P1 ------- P3 ----- PE2 ------
        /                               PLR \     PLR       \
       /                                     \     |         \
      /                                       \    |          \
   CE1                                 bypass P4   P5 bypass   CE2
      \                                        \   |          /
       \                                        \  |         /
        \                                        \ |        /
         - PE3 -------------- P2 ---------------- PE4 ------
                                               protector

             |<-------------- PW2 --------------->|

            PW1's label assigned by PE2: 100
            PW2's label assigned by PE4: 200
            On P3: </t>
                Incoming label of transport tunnel to PE2: 1000
                Outgoing label of transport tunnel to PE2: implicit null
                Outgoing label of bypass tunnel to PE4: 2000
            On PE2:
                Outgoing label of bypass tunnel to PE4: 3000
            On PE4:
                Context label (incoming label of bypass tunnels): 999

            Forwarding state on P3:
            label 1000 -- primary next hop: pop, to PE2
                          backup next hop:  swap 2000, to P4

            Forwarding state on PE2:
            label 100 -- primary next hop: pop, to CE2
                         backup next hop:  push 3000, to P5

            Forwarding state on P4:
            label 2000 -- next hop: swap 999, to PE4

            Forwarding state on P5:
            label 3000 -- next hop: swap 999, to PE4

            Forwarding state on PE4:
            label 200 -- next hop: pop, to CE2
            label 999 -- next hop: label table of PE2's label space

            Label table of PE2's label space on PE4:
            label 100 -- next hop: pop, to CE2

                                 Figure 11

Top      ToC       Page 24 
   In Figure 12, SPE2 is a co-located protector that protects PW1
   against S-PE failure.  It maintains a label space for SPE1, which is
   identified by the context identifier of {SPE1, SPE2}.  It learns
   SEG1's label from SPE1 and installs a forwarding entry in the label
   space.  The next hop of the forwarding entry indicates a label swap
   to SEG4's label and a label push with the label of a transport tunnel
   to TPE4.

Top      ToC       Page 25 
             |<--------------- PW1 --------------->|
             |<----- SEG1 ----->|<----- SEG2 ----->|

        - TPE1 ----- P1  ----- SPE1 --- P3 ------- TPE2 -
       /             PLR \                               \
      /                   \                               \
   CE1              bypass P2                              CE2
      \                     \                             /
       \                     \                           /
        - TPE3 --------------- SPE2 --- P4 ------- TPE4 -
                            protector

             |<----- SEG3 ----->|<----- SEG4 ----->|
             |<--------------- PW2 --------------->|

            SEG1's label assigned by SPE1: 100
            SEG2's label assigned by TPE2: 200
            SEG3's label assigned by SPE2: 300
            SEG4's label assigned by TPE4: 400
            On P1:
               Incoming label of transport tunnel to SPE1: 1000
               Outgoing label of transport tunnel to SPE1: implicit null
               Outgoing label of bypass tunnel to SPE2: 2000
            On SPE1:
               Outgoing label of transport tunnel to TPE2: 3000
            On SPE2:
               Outgoing label of transport tunnel to TPE4: 4000
               Context label (incoming label of bypass tunnel): 999

            Forwarding state on P1:
            label 1000 -- primary next hop: pop, to SPE1
                          backup next hop:  swap 2000, to P2

            Forwarding state on SPE1:
            label 100 -- next hop: swap 200, push 3000, to P3

            Forwarding state on P2:
            label 2000 -- next hop: swap 999, to SPE2

            Forwarding state on SPE2:
            label 300 -- next hop: swap 400, push 4000, to P4
            label 999 -- next hop: label table of SPE1's label space

            Label table of SPE1's label space on SPE2:
            label 100 -- next hop: swap 400, push 4000, to P4

                                 Figure 12

Top      ToC       Page 26 
4.7.2.  Centralized Protector Model

   In the centralized protector model, for each primary PW of which the
   protector is not a backup (S-)PE, the protector MUST also learn the
   label of the backup PW from the backup (S-)PE (Section 6.3).  This is
   the backup (S-)PE that the protector will forward traffic to.  The
   protector MUST install a forwarding entry with a label swap from the
   primary PW's label to the backup PW's label and a label push with the
   label of a transport tunnel to the backup (S-)PE.

   In Figure 13, the protector is a centralized protector that protects
   PW1 against egress AC failure and egress node failure.  It maintains
   a label space for PE2, which is identified by the context identifier
   of {PE2, protector}.  It learns PW1's label from PE2 and PW2's label
   from PE4.  It installs a forwarding entry for PW1's label in the
   label space.  The next hop of the forwarding entry indicates a label
   swap to PW2's label and a label push with the label of a transport
   tunnel to PE4.

               |<-------------- PW1 --------------->|

           - PE1 ------------- P1 ------- P3 ------ PE2 ----
          /                              PLR \      PLR     \
         /                                    \      /       \
        /                              bypass P5    P6 bypass \
       /                                        \  /           \
      /                                          \/             \
   CE1                                      protector            CE2
      \                                           \             /
       \                                transport  \           /
        \                                  tunnel  P7         /
         \                                          \        /
          \                                          \      /
           - PE3 ------------- P2 ----------------- PE4 ----

               |<-------------- PW2 --------------->|

            PW1's label assigned by PE2: 100
            PW2's label assigned by PE4: 200
            On P3:
                Incoming label of transport tunnel to PE2: 1000
                Outgoing label of transport tunnel to PE2: implicit null
                Outgoing label of bypass tunnel to protector: 2000
            On PE2:
                Outgoing label of bypass tunnel to protector: 3000
            On protector:
                Context label (incoming label of bypass tunnels): 999
                Outgoing label of transport tunnel to PE4: 4000

Top      ToC       Page 27 
            Forwarding state on P3:
            label 1000 -- primary next hop: pop, to PE2
                          backup next hop:  swap 2000, to P5

            Forwarding state on PE2:
            label 100 -- primary next hop: pop, to CE2
                         backup next hop:  push 3000, to P6

            Forwarding state on P5:
            label 2000 -- next hop: swap 999, to protector

            Forwarding state on P6:
            label 3000 -- next hop: swap 999, to protector

            Forwarding state on P7:
            label 4000 -- next hop: pop, to PE4

            Forwarding state on PE4:
            label 200 -- next hop: pop, to CE2

            Forwarding state on protector:
            label 999 -- next hop: label table of PE2's label space

            Label table of PE2's label space on protector:
            label 100 -- next hop: swap 200, push 4000, to P7

                                 Figure 13

   In Figure 14, the protector is a centralized protector that protects
   the PW segment SEG1 of PW1 against the node failure of SPE1.  It
   maintains a label space for SPE1, which is identified by the context
   identifier of {SPE1, protector}.  It learns SEG1's label from SPE1
   and SEG3's label from SPE2.  It installs a forwarding entry for
   SEG1's label in the label space.  The next hop of the forwarding
   entry indicates a label swap to SEG3's label and a label push with
   the label of a transport tunnel to TPE4.

Top      ToC       Page 28 
                |<--------------- PW1 --------------->|
                |<----- SEG1 ----->|<----- SEG2 ----->|

           - TPE1 ----- P1 ----- SPE1 --- P2 -------- TPE2 -
          /            PLR \                                \
         /                  \                                \
        /            bypass P4                                \
       /                     \                                 \
      /                       \                                 \
   CE1                     protector                             CE2
      \                        \                                /
       \                        \                              /
        \             transport P5                            /
         \                tunnel  \                          /
          \                        \                        /
           - TPE3 -------------- SPE2 --- P3 -------- TPE4 -

                |<----- SEG3 ----->|<----- SEG4 ----->|
                |<--------------- PW2 --------------->|

            SEG1's label assigned by SPE1: 100
            SEG2's label assigned by TPE2: 200
            SEG3's label assigned by SPE2: 300
            SEG4's label assigned by TPE4: 400
            On P1:
               Incoming label of transport tunnel to SPE1: 1000
               Outgoing label of transport tunnel to SPE1: implicit null
               Outgoing label of bypass tunnel to protector: 2000
            On SPE1:
               Outgoing label of transport tunnel to TPE2: 3000
            On SPE2:
               Outgoing label of transport tunnel to TPE4: 4000
            On protector:
               Context label (incoming label of bypass tunnel): 999
               Outgoing label of transport tunnel to SPE2: 5000

            Forwarding state on P1:
            label 1000 -- primary next hop: pop, to SPE1
                          backup next hop:  swap 2000, to P4

            Forwarding state on SPE1:
            label 100 -- next hop: swap 200, push 3000, to P2

            Forwarding state on P4:
            label 2000 -- next hop: swap 999, to protector

            Forwarding state on P5:
            label 5000 -- next hop: pop, to SPE2

Top      ToC       Page 29 
            Forwarding state on SPE2:
            label 300 -- next hop: swap 400, push 4000, to P3

            Forwarding state on protector:
            label 999 -- next hop: label table of SPE1's label space

            Label table of SPE1's label space on protector:
            label 100 -- next hop: swap 300, push 5000, to P5

                                 Figure 14



(page 29 continued on part 2)

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