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Versions: 00 01 02                                                      
Network Working Group                                        Yisong Liu
Internet Draft                                                 W. Cheng
Intended status: Informational                             China Mobile
Expires: September 3, 2022                                       C. Lin
                                                   New H3C Technologies
                                                                X. Geng
                                                    Huawei Technologies
                                                                 Y. Liu
                                                                    ZTE
                                                          March 3, 2022



              Considerations for Protection of SRv6 Networks
             draft-liu-rtgwg-srv6-protection-considerations-01


Abstract

   This document describes the considerations for protection of SRv6
   network.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   This Internet-Draft will expire on September 3, 2022.





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Copyright Notice

   Copyright (c) 2022 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.

Table of Contents


   1. Introduction...................................................3
      1.1. Requirements Language.....................................3
      1.2. Terminology...............................................3
   2. Forwarding over SRv6 Network...................................3
      2.1. SRv6 BE Path..............................................3
      2.2. SRv6 TE Path..............................................4
   3. Protection Mechanisms..........................................5
      3.1. Path Protection...........................................5
         3.1.1. Local Proctection Mechanisms.........................5
         3.1.2. Liveness Check For Local Protection..................6
         3.1.3. Micro-Loop Avoidance.................................6
         3.1.4. End-to-End Protection Mechanisms.....................6
         3.1.5. Liveness Check For End-to-End Protection.............7
      3.2. Service Protection........................................8
         3.2.1. Local Repair.........................................8
         3.2.2. Ingress Node Switchover..............................8
   4. Implementation Recommendations.................................9
      4.1. SRv6 BE..................................................11
      4.2. SRv6 TE..................................................12
   5. Security Considerations.......................................15
   6. IANA Considerations...........................................15
   7. Contributors..................................................15
   8. References....................................................16
      8.1. Normative References.....................................16
      8.2. Informative References...................................17
   Authors' Addresses...............................................18



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1. Introduction

   Segment Routing [RFC8402] instantiated on the IPv6 dataplane (SRv6)
   provides network programming capability to create interoperable
   overlays with underlay optimization [RFC8986].

   This document describes the common failure scenarios and protection
   mechanisms in SRv6 networks. Then implementation recommendations for
   protection of SRv6 networks are proposed.

  1.1. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

  1.2. Terminology

   BE: Best Effort

   TE: Traffic Engineering

   G-SRv6: Generalized SRv6 Network Programming

2. Forwarding over SRv6 Network

   Segment Routing [RFC8402] leverages the source routing paradigm.
   Segment Routing instantiated on the IPv6 dataplane is referred to as
   SRv6. SRv6 provides network programming capability to create
   interoperable overlays with underlay optimization [RFC8986].

   In an SRv6 network, the ingress node encapsulates a received packet
   in an outer IPv6 header, followed by an optional Segment Routing
   Header (SRH) [RFC8754], which instructs the SRv6 network to forward
   the packet via a specific path to the egress node. The forwarding
   path is either an SRv6 BE path or an SRv6 TE path.

  2.1. SRv6 BE Path

   In the SRv6 BE path, the ingress PE encapsulates the payload in an
   outer IPv6 header where the destination address is the SRv6 Service
   SID provided by the egress PE. The underlay P nodes between the PEs
   only need to perform plain IPv6 shortest path forwarding.


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            -----------------------
           |      IPv6 Header      |
           |  DA = 2001:DB8:1:1::  |
            -----------------------
           |        Payload        |
            -----------------------

   Ingress PE ---> P nodes ---> Egress PE

   Figure 1: Forwarding over SRv6 BE

  2.2. SRv6 TE Path

   In the SRv6 TE path, the ingress PE steers the traffic flow into an
   SR Policy [I-D.ietf-spring-segment-routing-policy], and encapsulates
   the payload packet in an outer IPv6 header with the Segment Routing
   Header (SRH) carrying the segment list of the SR policy.  The
   underlay P nodes whose SRv6 SID's are part of the SRH segment list
   are called endpoint nodes. They will be involved in the forwarding
   path and execute the function associated with the SID.

           ------------------------
          |      IPv6 Header       |
          |   DA = 2001:DB8:6:1::  |
           ------------------------
          |          SRH           |
          | Seg[0]= 2001:DB8:1:1:: |
          | Seg[1]= 2001:DB8:2:1:: |
          | Seg[2]= 2001:DB8:3:1:: |
          | Seg[3]= 2001:DB8:4:1:: |
          | Seg[4]= 2001:DB8:5:1:: |
          | Seg[5]= 2001:DB8:6:1:: |
           ------------------------
          |        Payload         |
           ------------------------

   Ingress PE ---> P nodes ---> Egress PE

   Figure 2: Forwarding over SRv6 TE

   If Compressed Segment List encoding is enabled in the SRv6 network
   [I-D.ietf-spring-srv6-srh-compression], the segment list in the SRH
   will be encoded in the compressed way. The compressed SRv6 Segment-
   List encoding can optimize the packet header length by avoiding the
   repetition of the Locator-Block and trailing bits with each
   individual SID.

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   The G-SRv6 mechanism will be used as an example for the encoding of
   SRv6 TE path in this document. Figure 3 shows the encapsulation of
   packet using the G-SRv6 mechanism.

           ------------------------
          |      IPv6 Header       |
          |   DA = 2001:DB8:6:1::  |
           ------------------------
          |          SRH           |
          |Seg[0]= 2001:DB8:1:1::  |
          |Seg[1]= 2:1|3:1|4:1|5:1 |
          |Seg[2]= 2001:DB8:6:1::  |
           ------------------------
          |        Payload         |
           ------------------------

   Ingress PE ---> P nodes ---> Egress PE

   Figure 3: Forwarding over G-SRv6 Encoded TE

3. Protection Mechanisms

  3.1. Path Protection

3.1.1. Local Proctection Mechanisms

   Local protection is performed by the node adjacent to the failed
   component using fast-reroute techniques [RFC5286] [RFC5714]. The
   common method of local repair is to provide a repair path for the
   destination avoiding the failed component.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa] describes the Topology
   Independent Loop-free Alternate Fast Re-route technology (TI-LFA)
   using Segment Routing, which is able to provide a loop free backup
   path irrespective of the topologies used in the network. For each
   destination in the network, TI-LFA pre-installs a backup forwarding
   entry for each protected destination ready to be activated upon
   detection of the failure of a link used to reach the destination.

   In SRv6 dataplane, the TI-LFA repair path is encoded as an SRv6 SID
   list, and encapsulated in the SRH along with an outer IPv6 header.
   If Compressed Segment List encoding is enabled, the repair node
   should check the G-SRv6 capability of nodes along the repair path
   and try to use G-SIDS to encode the repair path, which will help to
   optimize the packet header length.


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3.1.2. Liveness Check For Local Protection

   In order to perceive the failures of links and neighbors, a node
   should monitor the liveness of its adjacent components.

   [RFC5880] and [RFC7880] provide widely used mechanisms for liveness
   check, called Bidirectional Forwarding Detection (BFD) and Seamless
   Bidirectional Forwarding Detection (S-BFD).

   BFD can be associated with the interface state to detect the failure
   of directly-connected links. Two adjacent nodes may establish BFD or
   S-BFD sessions between each other, and send BFD control packets to
   monitor the liveness of each other. In another way, a node may send
   BFD echo packets to all the neighbors, and they will reflect the
   packets back, without establishing BFD sessions.

   Other OAM methods, such as Ping, TWAMP or STAMP, may also be used
   for liveness check for local protection, which will not be
   enumerated here in detail.

3.1.3. Micro-Loop Avoidance

   When a component fails or comes back up, the topology is changed.
   The routing convergence happens in each node at different times and
   during a different lapse of time. These transient routing
   inconsistencies may cause micro-loops.

   [I-D.bashandy-rtgwg-segment-routing-uloop] provides a mechanism
   leveraging segment routing to ensure loop-freeness during the IGP
   reconvergence process, which relies on the temporary use of SR
   policies ensuring loop-freeness over the post-convergence paths from
   the converging node to the destination.

   In SRv6 dataplane, the loop-free post-convergence path is encoded as
   an SRv6 SID list, and encapsulated in the SRH along with an outer
   IPv6 header. If Compressed Segment List encoding is enabled, the
   converging node should check the G-SRv6 capability of nodes along
   the post-convergence path and try to use G-SIDs to encode the path.

3.1.4. End-to-End Protection Mechanisms

   End-to-end protection lets the ingress PE node be in charge of the
   failure recovery. The ingress node should steer the flow from the
   failed path into another alive path.



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   In the case of SRv6 TE path, the SR Policy itself allows for
   multiple candidate paths, of which at any point in time there is a
   single active candidate path that is provisioned in the forwarding
   plane and used for traffic steering [I-D.ietf-spring-segment-
   routing-policy]. The candidate path with highest preference is
   selected as the primary path, and the candidate path with second
   highest preference can be selected as the hot-standby backup. When
   the primary candidate path fails, switchover to the backup candidate
   path can be triggered by fast re-route mechanism.

   If all the candidate paths fail, the ingress node may use SRv6 BE
   path for best-effort forwarding.

3.1.5. Liveness Check For End-to-End Protection

   It is essential that the ingress PE node should check the end-to-end
   liveness of paths, including primary path and backup path. So that
   the ingress PE node can perceive the path failure and then trigger
   the switchover.

   In the case of SRv6 TE path, BFD or S-BFD can be used to monitor the
   liveness of SR Policy at the level of segment list. If all the BFD
   sessions associated with segment lists in a candidate path are down,
   the candidate path is deemed to be failed. If all the candidate
   paths is failed, the SR Policy is deemed to be failed.

   Moreover, If the SRv6 TE path is strict (every hop along the path
   appearing in the SID list), the reverse path of the BFD packets
   should be the same with the forward path. Otherwise, the failure in
   the reverse path may cause the misjudgement of the liveness of SR
   Policy. To achieve the consistence of forward path and reverse path,
   the egress node should be instructed to use specific path to send
   packets back to the ingress node.

   Other OAM methods, such as Ping, TWAMP or STAMP, may also be used
   for liveness check for end-to-end protection, which will not be
   enumerated here in detail.

   Local protection and end-to-end protection may both be used in the
   same SRv6 network. Since the speed of failure detection for local
   protection is faster than end-to-end protection, local protection
   usually performs the local repair in advance, which allows the path
   to remain alive. In this case, the ingress node will not perceive
   the failure and does not need to trigger end-to-end protection.



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  3.2. Service Protection

   If the failure occurs on the egress PE node, the service provided by
   that PE is not accessible anymore. TI-LFA or the hot-standby backup
   candidate path of SR Policy will not work under this circumstance.
   To provide protection, the packet should be forwarded to another
   backup Egress PE node of the same service, if it exists.

3.2.1. Local Repair

   In the case of egress PE node failure, the local repair node should
   forward packet to another Egress PE node.

   [I-D.ietf-rtgwg-srv6-egress-protection] provides a method to use
   Mirror SID for egress protection. The Mirror SID is configured on
   the backup egress PE to protect the primary egress PE, and it will
   be used by the repair node to encode the segment list of repair
   path.

3.2.2. Ingress Node Switchover

   If there are multiple egress PE nodes, the ingress PE node receives
   all their advertisements of the same service, and builds paths for
   each of them respectively. The ingress PE node may use Fast Reroute
   (FRR) for these different paths. When the primary egress PE node
   fails, the ingress node steers the flow to the path belonging to
   another egress PE node for protection.

   BFD can be used to monitor the liveness of the service SID, locator
   or interface address of the egress PE node. If the BFD session is
   down, the egress PE node is deemed to be unreachable.

   Service protection and path protection may both be used in the same
   SRv6 network. Among the different paths to the same egress PE node
   and the paths to different egress PE nodes, one is selected as the
   primary path and others are used as backup. The priorities of
   multiple backup paths may be decided by the egress-node-first
   strategy or the TE-first strategy.

   By the Egress-node-first strategy, paths to the primary egress PE
   nodes are prioritized. For example, if a failure occurs on the
   primary path, the ingress PE node will select another path still
   leading to the primary egress PE nodes. Unless all the paths to the
   primary egress PE node are failed, the ingress PE node would use the
   path to the backup egress PE node.


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   By the TE-first strategy, SRv6 TE paths to any egress PE node have
   higher priorities than SRv6 BE paths. For example, if a failure
   occurs on the primary path and there is no other alive SRv6 TE paths
   to the primary egress PE node, the ingress node will select an SRv6
   TE path to the backup egress PE node, rather than an SRv6 BE path
   still leading to the primary egress PE node.

4. Implementation Recommendations

   This section will introduce the implementation recommendations of
   protection for SRv6 BE and SRv6 TE scenarios in SRv6 network:

   Figure 5 is used as a reference topology in this section. PE1 and
   PE3 are primary PE nodes for VPN service access. PE2 and PE4 are
   used as backup. The prefix of CE2, along with VPN service SID, is
   advertised by BGP routes from PE3 and PE4 to PE1 and PE2. The VPN
   traffic is from CE1 to CE2.

        PE1-----P1-----P3-----P5-----P7----PE3
       /   \  / | \  / | \  / | \  / | \  /   \
      /     \/  |  \/  |  \/  |  \/  |  \/     \
    CE1     /\  |  /\  |  /\  |  /\  |  /\     CE2
      \    /  \ | /  \ | /  \ | /  \ | /  \    /
       \  /    \|/    \|/    \|/    \|/    \  /
        PE2-----P2-----P4-----P6-----P8----PE4

   Figure 5: Reference Topology

   The link metrics are configured as follows:

   o Metrics of PE1-P2, PE2-P1, P1-P4, P2-P3, P3-P6, P4-P5, P5-P8, P6-
      P7, P7-PE4, and P8-PE3 links are 11.

   o Metrics of all other links are 5.

   o Link metrics are bidirectional.

   All P and PE nodes are capable of G-SRv6 compression. The SRv6 SIDs
   are configured as follows:







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       Locator           End.DT
   PE1 2001:DB8:A1::/48  2001:DB8:A1:100::
   PE2 2001:DB8:A2::/48  2001:DB8:A2:100::
   PE3 2001:DB8:A3::/48  2001:DB8:A3:100::
   PE4 2001:DB8:A4::/48  2001:DB8:A4:100::
   P1  2001:DB8:B1::/48  -
   P2  2001:DB8:B2::/48  -
   P3  2001:DB8:B3::/48  -
   P4  2001:DB8:B4::/48  -
   P5  2001:DB8:B5::/48  -
   P6  2001:DB8:B6::/48  -
   P7  2001:DB8:B7::/48  -
   P8  2001:DB8:B8::/48  -

       End               End with COC
   PE1 2001:DB8:A1:1::   2001:DB8:A1:2::
   PE2 2001:DB8:A2:1::   2001:DB8:A2:2::
   PE3 2001:DB8:A3:1::   2001:DB8:A3:2::
   PE4 2001:DB8:A4:1::   2001:DB8:A4:2::
   P1  2001:DB8:B1:1::   2001:DB8:B1:2::
   P2  2001:DB8:B2:1::   2001:DB8:B2:2::
   P3  2001:DB8:B3:1::   2001:DB8:B3:2::
   P4  2001:DB8:B4:1::   2001:DB8:B4:2::
   P1  2001:DB8:B5:1::   2001:DB8:B5:2::
   P2  2001:DB8:B6:1::   2001:DB8:B6:2::
   P3  2001:DB8:B7:1::   2001:DB8:B7:2::
   P4  2001:DB8:B8:1::   2001:DB8:B8:2::

   The SR Policies on PE1 are configured as follows:

















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   SR Policy 1 (Strict Path to PE3)
     Candidate Path 1
       Preference: 20
       Segment List: 2001:DB8:B1:2::, 2001:DB8:B3:2::, 2001:DB8:B5:2::,
   2001:DB8:B7:2::, 2001:DB8:A3:1::
     Candidate Path 2
       Preference: 10
       Segment List: 2001:DB8:B2:2::, 2001:DB8:B4:2::, 2001:DB8:B6:2::,
   2001:DB8:B8:2::,2001:DB8:A3:1::

   SR Policy 2 (Loose Path to PE4)
     Candidate Path 1
       Preference: 20
       Segment List: 2001:DB8:B4:2::, 2001:DB8:B8:2::,2001:DB8:A4:1::

  4.1. SRv6 BE

   In this scenario, SRv6 BE paths are used to steer the VPN service.
   The deployments of protection are as follows:

   o All nodes enable TI-LFA for local protection.

   o All nodes enable BFD for links and neighbors.

   o Ingress PE node enables FRR of SRv6 BE path to backup egress PE
      node for service protection.

   o Ingress PE node enables BFD for locator of egress PE node to
      monitor the liveness of SRv6 BE path.

   PE1 installs the SRv6 BE path to PE3 with destination address
   2001:DB8:A3:100:: as the primary next-hop for the VPN flow.
   Meanwhile, PE1 also installs the SRv6 BE path to PE4 with
   destination address 2001:DB8:A4:100:: as the backup next-hop.

   PE1 enables BFD for locator 2001:DB8:A3::/48 and 2001:DB8:A4::/48 to
   monitor the liveness of SRv6 BE paths.

   TI-LFA is enabled on all nodes. Take P5 for example. The shortest
   path from P5 to PE3 is via neighbor P7. In order to provide local
   protection for P7 node failure, P5 computes and installs the repair
   path P5->P6->P8->PE3, using [2001:DB8:B6:2::, B8:2, A3:1] as the G-
   SRv6 SID list.

   All nodes use BFD to monitor the liveness of links and adjacent
   nodes.

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   Under normal circumstances, PE1 encapsulates the VPN payload in an
   outer IPv6 header where the destination address is
   2001:DB8:A3:100::.

   Assume that a failure occurs on P7. The fail-timer of BFD echo from
   P5 to P7 expires, so P5 perceives the failure. When P5 forwards the
   VPN packet, the TI-LFA repair path is used. Then, P5 encapsulates
   the packet in an outer IPv6 Header with SRH carrying a compressed
   segment-list of [2001:DB8:B6:2::, B8:2, A3:1], as shown in the
   following figure. The packet is forwarded in the repair path P5->P6-
   >P8->PE3 according to the outer IPv6 Header and SRH. So the failure
   is repaired by local protection.

                                  ------------------------
                                 |      IPv6 Header       |
                                 |  DA = 2001:DB8:B6:2::  |
                                  ------------------------
                                 |          SRH           |
                                 |Seg[0]= B8:2|A3:1       |
                                 |Seg[1]= 2001:DB8:B6:2:: |
   ------------------------      ------------------------
   |      IPv6 Header       |    |      IPv6 Header       |
   | DA = 2001:DB8:A3:100:: |    | DA = 2001:DB8:A3:100:: |
    ------------------------      ------------------------
   |      VPN Payload       |    |      VPN Payload       |
    ------------------------      ------------------------

   P3         --->            P5            --->         P6

   Assume that a failure occurs on PE3. TI-LFA does not work and the
   packets along the SRv6 BE path are dropped. Then the BFD session
   from PE1 to locator 2001:DB8:A3::/48 is down, so PE1 triggers the
   switchover to the SRv6 BE path to PE4 and encapsulates the VPN
   payload in an outer IPv6 header where the destination address is
   2001:DB8:A4:100::. After that, the VPN traffic from CE1 to CE2 is
   recovered.

  4.2. SRv6 TE

   In this scenario, the SRv6 TE strict path with G-SRv6 compression is
   used to steer the VPN traffic flows to the primary egress node PE3,
   and the SRv6 TE loose path with G-SRv6 compression is used for the
   backup egress node PE4.

   The deployments of protection are as follows:


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   o In the SR Policy of SRv6 TE strict path, disjoint backup
      candidate path is used as hot standby for end-to-end protection.

   o Ingress PE node uses SRv6 BE paths as backup for end-to-end
      protection of SRv6 TE paths.

   o Ingress PE node enables BFD for SR Policy. In the case of SRv6 TE
      strict path, the reverse path of BFD packet keeps consistent with
      forward path.

   o Ingress PE node enables BFD for locator of egress PE node to
      monitor the liveness of SRv6 BE path.

   o Ingress PE node enables FRR of paths to backup egress PE node for
      service protection.

   o All nodes enable TI-LFA for local protection. All nodes enable
      BFD for links and neighbors.

   PE1 installs SR Policy 1, which is the SRv6 TE strict path to PE3,
   as the primary next-hop for the VPN flow. SR Policy 1 has two
   disjoint candidate paths. The candidate path with higher preference
   is selected as the primary candidate path, and the candidate path
   with lower preference is selected as hot standby backup.

   Meanwhile, the SRv6 BE path to PE3, the SRv6 TE loose path to PE4
   (SR Policy 2), and the SRv6 BE path to PE4 are also installed as
   backup next-hops. The priorities of multiple backup paths may be
   decided by either of the egress-node-first strategy or the TE-first
   strategy.

   Egress-node-first strategy:

   o primary: SRv6 TE path to primary egress node PE3 (SR Policy 1)

   o backup(1st priority): SRv6 BE path to primary egress node PE3

   o backup(2nd priority): SRv6 TE path to backup egress node PE4 (SR
      Policy 2)

   o backup(3rd priority): SRv6 BE path to backup egress node PE4

   TE-first strategy:

   o primary: SRv6 TE path to primary egress node PE3 (SR Policy 1)


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   o backup(1st priority): SRv6 TE path to backup egress node PE4 (SR
      Policy 2)

   o backup(2nd priority): SRv6 BE path to primary egress node PE3

   o backup(3rd priority): SRv6 BE path to backup egress node PE4

   Egress-node-first strategy is used as an example below.

   PE1 enables BFD for SR Policy 1 and SR Policy 2 to monitor the
   liveness of SRv6 TE paths. For SR Policy 1 which is the strict path,
   the forward and reverse paths of BFD packet should be the same. For
   example, the primary path of SR Policy 1 is PE1->P1->P3->P5->P7-
   >PE3, so the reverse path should be PE3->P7->P5->P3->P1->PE1. A
   segment list of such reverse path is installed on PE3, and the BSID
   is 2001:DB8:A3:200. PE1 may send BFD echo packet with the segment
   list of SR Policy 1 along with the BSID of reverse path, which is
   [2001:DB8:B1:2::, B3:2, B5:2, B7:2, A3:1, 2001:DB8:A3:200]. When the
   BFD echo packet is forwarded along the strict path to PE3, PE3 will
   add an outer IPv6 header with SRH carrying the segment list of
   [2001:DB8:B7:2::, B5:2, B3:2, B1:2, A1:1], which instructs the
   packet to be forwarded along the same strict path back to PE1.

   PE1 enables BFD for locator 2001:DB8:A3::/48 and 2001:DB8:A4::/48 to
   monitor the liveness of SRv6 BE paths.

   TI-LFA is enabled on all nodes. BFD are used to monitor the liveness
   of links and adjacent nodes.

   Under normal circumstances, PE1 encapsulates the VPN payload in an
   outer IPv6 header with SRH carrying the segment list of primary
   candidate path of SR Policy 1 along with the VPN SID advertised by
   PE3. Using G-SRv6 compression, the segment list will be encoded as
   [2001:DB8:B1:2::, B3:2, B5:2, B7:2, A3:1, 2001:DB8:A3:100::].

   Assume that a failure occurs on P3. The packets are dropped since
   the failed P3 is on the path. The BFD session of the segment list in
   the primary candidate path of SR Policy 1 is down, so PE1 triggers
   the switchover to the backup candidate path of SR Policy 1. Then PE1
   encapsulates the VPN payload in an outer IPv6 header with SRH
   carrying the segment list of [2001:DB8:B2:2::, B4:2, B6:2, B8:2,
   A3:1, 2001:DB8:A3:100::].

   Before the recovery of P3, assume that P6 also fails. The BFD
   session of the segment list in the backup candidate path of SR
   Policy 1 is also down. Then PE1 triggers the switchover to the 1st

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   priority backup next-hop which is the SRv6 BE path to PE3. PE1
   encapsulates the VPN payload in an outer IPv6 header where the
   destination address is 2001:DB8:A3:100::.

   Assume that a failure occurs on PE3. Both the BFD sessions of SR
   Policy 1 and locator 2001:DB8:A3::/48 are down, which means the
   primary next-hop and the 1st priority backup next-hop are down. So
   PE1 triggers the switchover to the 2nd priority backup next-hop,
   which is the SRv6 TE loose path to PE4. Then PE1 encapsulates the
   VPN payload in an outer IPv6 header with SRH carrying the segment
   list of [2001:DB8:B4:2::, B8:2, A4:1, 2001:DB8:A4:100::].

   Before the recovery of PE3, assume that a failure occurs on P6. The
   fail-timer of BFD echo from P4 to P6 expires, so P4 perceives the
   failure. When P4 forwards the VPN packet, the TI-LFA repair path is
   used. Then, P4 encapsulates the packet in an outer IPv6 Header with
   SRH carrying a compressed segment-list of [2001:DB8:B3:2::, B5:2,
   A8:1]. The packet is forwarded in the repair path P4->P3->P5->P8
   according to the outer IPv6 Header and SRH. So the failure is
   repaired by local protection.

   Before the recovery of PE3, assume that a failure occurs on P8. When
   P6 forwards the VPN packet to destination address 2001:DB8:B8:2::
   which is one of the segments in the segment list of SRH, the TI-LFA
   on P6 does not work, since the failed node P8 is the destination. So
   the packets are dropped. The BFD session of SR Policy 2 is down, and
   PE1 triggers the switchover to the 3rd priority backup next-hop
   which is the SRv6 BE path to PE4. Then PE1 encapsulates the VPN
   payload in an outer IPv6 header where the destination address is
   2001:DB8:A4:100::. If the routing convergence is not completed at
   the moment, P6 will use TI-LFA repair path P6->P5->P7->PE4 to
   forward the packet. After the routing convergence is done, P nodes
   will forward the packet along new shortest path excluding P8.

5. Security Considerations

   TBD.

6. IANA Considerations

   This document has no IANA actions.

7. Contributors

   In addition to the authors listed on the front page, the following
   co-authors have also contributed to this document:

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   Mengxiao Chen
   H3C
   Email: chen.mengxiao@h3c.com

8. References

  8.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, May 2017

   [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
             Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
             (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
             <https://www.rfc-editor.org/info/rfc8754>.

   [I-D.ietf-spring-segment-routing-policy] Filsfils, C., Talaulikar,
             K., Voyer, D., Bogdanov, A., and P. Mattes, "Segment
             Routing Policy Architecture", draft-ietf-spring-segment-
             routing-policy-18 (work in progress), February 2022.

   [I-D.ietf-spring-srv6-srh-compression] Cheng, W., Filsfils, C., Li,
             Z., Decraene, B., Cai, D., Clad, F., Zadok, S., Guichard,
             J., Aihua, L., Raszuk, R. and C. Li, " Compressed SRv6
             Segment List Encoding in SRH", draft-ietf-spring-srv6-srh-
             compression-00 (work in progress), February 2022.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa] Litkowski, S., Bashandy, A.,
             Filsfils, C., Francois, P., Decraene, B., and D. Voyer,
             "Topology Independent Fast Reroute using Segment Routing",
             draft-ietf-rtgwg-segment-routing-ti-lfa-08 (work in
             progress), January 2022.

   [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
             (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
             <https://www.rfc-editor.org/info/rfc5880>.

   [RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
             Pallagatti, "Seamless Bidirectional Forwarding Detection
             (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
             <https://www.rfc-editor.org/info/rfc7880>.



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  8.2. Informative References

   [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
             Decraene, B., Litkowski, S., and R. Shakir, "Segment
             Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
             July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
             D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
             (SRv6) Network Programming", RFC 8986, DOI
             10.17487/RFC8986, February 2021, <https://www.rfc-
             editor.org/info/rfc8986>.

   [RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
             IP Fast Reroute: Loop-Free Alternates", RFC 5286, DOI
             10.17487/RFC5286, September 2008, <https://www.rfc-
             editor.org/info/rfc5286>.

   [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
             5714, DOI 10.17487/RFC5714, January 2010,
             <https://www.rfc-editor.org/info/rfc5714>.

   [I-D.bashandy-rtgwg-segment-routing-uloop] Bashandy, A., Filsfils,
             C., Litkowski, S., Decraene, B., Francois, P. and P.,
             Psenak, "Loop avoidance using Segment Routing", draft-
             bashandy-rtgwg-segment-routing-uloop-12 (work in
             progress), December 2021.

   [I-D.ietf-rtgwg-srv6-egress-protection] Hu, Z., Chen, H., Chen, H.,
             Wu, P., Toy, M., Cao, C., He, T., Liu, L., and X. Liu,
             "SRv6 Path Egress Protection", Work in Progress, Internet-
             Draft, draft-ietf-rtgwg-srv6- egress-protection-04, 17
             October 2021, <https://www.ietf.org/archive/id/draft-ietf-
             rtgwg-srv6-egress-protection-04.txt>.












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Authors' Addresses

   Yisong Liu
   China Mobile
   China

   Email: liuyisong@chinamobile.com


   Weiqiang Cheng
   China Mobile
   China

   Email: chengweiqiang@chinamobile.com


   Changwang Lin
   New H3C Technologies
   China

   Email: linchangwang.04414@h3c.com


   Xuesong Geng
   Huawei Technologies
   China

   Email: gengxuesong@huawei.com


   Yao Liu
   ZTE Corp.
   China

   Email: liu.yao71@zte.com.cn











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