Internet DRAFT - draft-lin-bfd-path-consistency-over-sr

draft-lin-bfd-path-consistency-over-sr



BFD Working Group                                               C. Lin
Internet Draft                                    New H3C Technologies
Intended status: Informational                                W. Cheng
Expires: May 8, 2024                                          W. Jiang
                                                          China Mobile
                                                               R. Chen
                                                       ZTE Corporation
                                                      November 8, 2023



                       BFD Path Consistency over SR
                 draft-lin-bfd-path-consistency-over-sr-02


Abstract

   Bidirectional Forwarding Detection (BFD) can be used to monitor
   paths between nodes.

   U-BFD defined in [I-D.ietf-bfd-unaffiliated-echo] can effectively
   reduce the device equipment.

   Seamless BFD (S-BFD) provides a simplified mechanism which is
   suitable for monitoring of paths that are setup dynamically and on a
   large scale network.

   In SR network, BFD can also be used to monitor SR paths. When a
   headend use BFD to monitor the segment list/CPath of SR Policy, the
   forward path of control packet is indicated by segment list, the
   reverse path of response control packet is via the shortest path
   from the reflector back to the initiator (headend) as determined by
   routing. The forward path and reverse path of control packet are
   likely inconsistent going through different intermediate nodes or
   links.

   This document describes a method to keep the forward path and
   reverse path consistent when using S-BFD or U-BFD to detect SR
   Policy

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.




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Table of Contents


   1. Introduction ................................................ 3
      1.1. Requirements Language .................................. 4
   2. Path consistency for BFD in SR network ...................... 4
      2.1. S-BFD .................................................. 5
      2.2. U-BFD .................................................. 6
   3. Path consistency for S-BFD .................................. 6
      3.1. Correlate bidirectional path using Path Segment ........ 6
      3.2. Procedure of S-BFD ..................................... 8
         3.2.1. S-BFD in SRv6 ..................................... 8
         3.2.2. S-BFD in SR-MPLS ................................. 11
   4. Path consistency for U-BFD ................................. 14
      4.1. Getting reverse segment list .......................... 14
      4.2. Procedure of U-BFD .................................... 15
         4.2.1. U-BFD in SRv6 .................................... 15
         4.2.2. U-BFD in SR-MPLS ................................. 18

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   5. IANA Considerations ........................................ 20
   6. Security Considerations .................................... 20
   7. References ................................................. 20
      7.1. Normative References .................................. 20
   Contributors .................................................. 22
   Authors' Addresses ............................................ 23

1. Introduction

   Segment Routing (SR) allows a headend node to steer a packet flow
   along any path. Per-path states of Intermediate nodes are eliminated
   thanks to source routing.  The headend node steers a flow into an SR
   Policy. The packets steered into an SR Policy carry an ordered list
   of segments associated with that SR Policy.

   SR can be instantiated on the MPLS data plane (MPLS-SR) and the IPv6
   data plane (SRv6). On the MPLS-SR data plane, a segment is encoded
   as an MPLS label, and an ordered list of segments is encoded as a
   stack of labels. On the SRv6 data plane, a segment is encoded as an
   IPv6 address (SRv6 SID) [RFC8986], and an ordered list of segments
   is encoded as an ordered list of SRv6 SIDs in the SR header (SRH)
   [RFC8754].

   BFD Echo function was originally defined in [RFC5880] and [RFC5881],
   where the remote system is required to loop the BFD Echo packets
   back to the local system. To support BFD Echo Function, some
   negotiations between the local system and remote system are needed,
   and both the local and remote system need to maintain the BFD
   session state.

   Unaffiliated BFD Echo Function (U-BFD) is defined in [I-D.ietf-bfd-
   unaffiliated-echo].Where the destination IP address of the BFD Echo
   packets is set to one of the IP addresses of the local system.
   Therefore, the Echo packets can be automatically looped back
   (through normal IP forwarding) by the remote system to the local
   system. With U-BFD, the remote system does not need to support any
   BFD related functions and maintain any session states. This further
   simplifies the BFD Echo Function process at the remote system hence
   greatly increases scalability.

   Seamless BFD (S-BFD) defined in [RFC7880] provides a simplified
   mechanism which is suitable for monitoring of paths that are setup
   dynamically and on a large scale network.

   In the SR network, the headend node could use BFD(S-BFD or U-BFD) to
   monitor the connectivity of the SR path to implement path switching.
   When a headend use BFD to monitor the segment list/CPath of SR
   Policy, the forward path of control packet is indicated by segment

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   list, the reverse path of response control packet is via the
   shortest path from the reflector back to the initiator (headend) as
   determined by routing. The forward path and reverse path of control
   packet are likely inconsistent going through different intermediate
   nodes or links.

   The inconsistency impacts the detecting result. If the forward path
   is up and reverse path is down, then the BFD session will be down.
   If there are multiple path (segment list) in a SR Policy between a
   headend (local system) node and a tailend(remote system) node,
   multiple BFD session will be created for each path. Each BFD session
   uses corresponding path to send control packet, but the reverse path
   is identical for all BFD sessions. If the reverse path is down, all
   sessions will be down. Then the SR Policy is down.

   The consistency of forward and reverse path of the same BFD session
   should be guaranteed.

   This document describes how to ensure the consistency of the forward
   path and the reverse path when using BFD to detect SR Policy.

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.

2. Path consistency for BFD in SR network

   Monitor SR Policy using BFD is usually based on segment list. BFD
   session is created for each segment list and associated with the
   segment list.

   Referring to the following topology, there are two paths between
   Node A and D, and All nodes allocate end.x Segments on SRv6 data
   plane or adjacency SIDs on SR-MPLS data plane. Node A and D are
   headend and tailend nodes of each other, and SR policy is created on
   A and D respectively.








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                    SID-B1  SID-B2     SID-C1   SID-C2
                 +--------B-----------------C-----------+
          SID-A1/                                         \ SID-D1
               /                                           \
              A                                             D
               \                                           /
          SID-A2\           SID-E1      SID-E2            /SID-D2
                 +-------------------E-------------------+
                       Figure 1: reference topology


   Assuming that the deployed SR policy has one candidate path and each
   path has two segment lists. For ease of description, segment lists
   with the same number on Node A and D are forward and reverse paths
   to each other.



   Node A:                        Node D:

   SR Policy A-D                        SR Policy D-A
     Candidate Path1                      Candidate Path1
      Segment list1                         Segment list1
       SID-A1, SID-B2, SID-C2                  SID-D1, SID-C1, SID-B1
      Segment list2                         Segment list2
        SID-A2, SID-E2                         SID-D2, SID-E1


   Both Node A and Node D serve as head nodes and need to detect the
   connectivity of the segment list of a SR Policy. Regardless of
   whether S-BFD or U-BFD is used, there is a requirement for BFD
   packet path consistency.

2.1. S-BFD

   When node A is the S-BFD initiator, S-BFD sessions for segment list1
   and segment list2 could be created respectively. Node A will use the
   associated segment list to encapsulate IPv6 header and SRH of the
   control packet.

   As the S-BFD reflector, after Node D receives the S-BFD control
   packet, the response control packet should be able to return along
   the same path to avoid the false detection of the session caused by
   the inconsistency of the forward and reverse paths.

   The control packet of S-BFD session associated with the segment
   list1 is forwarded to node D according to the segment list1 of node
   A. The response control packet of node D needs to be returned to

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   node A according to the segment list1 of node D. Thus the forward
   and reverse paths of S-BFD packets are ensured to be consistent.

2.2. U-BFD

   The working mechanism of U-BFD is that the local system sends a bfd
   echo packet, and the destination address is the IP address of the
   local system. After the bfd echo packet reaches the remote system,
   the remote system returns the bfd echo packet to the local system in
   the data plane. So U-BFD usually works when there is only one hop
   between the local and remote systems.

   When deploying U-BFD in SR network, local system could creates a U-
   BFD session for each segment list under the SR Policy, and uses the
   segment list to encapsulate BFD echo packets. For SR-MPLS
   encapsulation is the label stack, for SRv6 it is the segment list in
   SRH. In this way, the U-BFD echo packet can reach the remote system
   through multiple hops.

   When the U-BFD echo packet reaches the remote system, the
   destination address of the packet has been updated to the IP address
   of the local system, so the remote system sends the U-BFD echo
   packet back to the local system on the data plane.

   The U-BFD echo packet returned from remote system to local system
   should follow the same path from local system to remote system.

3. Path consistency for S-BFD

   This draft proposes to forward S-BFD control packets and response
   control packets through the consistent path by path segment.

3.1. Correlate bidirectional path using Path Segment

   A Path Segment is defined to identify an SR path. In SR for MPLS
   data plane (SR-MPLS), Path Segment is defined in [draft-ietf-spring-
   mpls-path-segment]. In SR for IPv6 data plane (SRv6), Path Segment
   is defined in [I-D.ietf-spring-srv6-path-segment].

   SR(SR-MPLS or SRv6) Path segments can be used to correlate the two
   unidirectional SR paths at both ends of the paths.

   [I-D.ietf-idr-sr-policy-path-segment] proposes an extension to BGP
   SR Policy distribute SR policies carrying Path Segment and
   bidirectional path information.




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   Through this extension, when distributing SR policy to the headend
   node, reverse path information and path segment of segment list can
   be carried together.

   Node A                       Node D

   SR Policy A-D                      SR Policy D-A
     Candidate Path1                     Candidate Path1
      Segment list1                       Segment list1
          SID-A1, SID-B2, SID-C2           SID-D1, SID-C1, SID-B1
          Path Segment: SID-Path-1        Path Segment: SID-Path-2
          Reverse Path Segment:            Reverse Path Segment:
             SID-Path-2                      SID-Path-1
      Segment list2                       Segment list2
          SID-A2, SID-E2                    SID-D2, SID-E1
          Path Segment: SID-Path-3        Path Segment: SID-Path-4
          Reverse Path Segment:             Reverse Path Segment:
            SID-Path-4                          SID-Path-3


   In this way, on the headend node in both directions of the forward
   and reverse paths, the path segment of the paths in both directions
   can be obtained, and the paths in both directions use the same
   intermediate links.

   The headend node can use path segment in two directions to establish
   a mapping table. Using this mapping table, the headend node can get
   the reverse path through the path segment of the forward path.

   The mapping table of Node A and Node D is shown below:

   Node A:
            +-----------------+          +--------------------+
            |  Path Segment   |          |Reverse Path Segment|
            +-----------------+          +--------------------+
            |  SID-Path-1     |-+        | SID-Path-2         |--+
            +-----------------+ |        +--------------------+  |
            |  SID-Path-3     | |        | SID-Path-4         |--|-+
            +-----------------+ |        +--------------------+  | |
                   |            |                                | |
                   |            |  +-----------------------+     | |
                   |            |  | segment List          |     | |
                   |            |  +-----------------------+     | |
                   |            +->|SID-A1, SID-B2, SID-C2 |<----+ |
                   |               +-----------------------+       |
                   +-------------->|SID-A2, SID-E2         |<------+
                                   +-----------------------+


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   Node D:
            +-----------------+          +--------------------+
            |  Path Segment   |          |Reverse Path Segment|
            +-----------------+          +--------------------+
            | SID-Path-2      |-+        | SID-Path-1         |--+
            +-----------------+ |        +--------------------+  |
            | SID-Path-4      | |        | SID-Path-3         |--|-+
            +-----------------+ |        +--------------------+  | |
                   |            |                                | |
                   |            |  +-----------------------+     | |
                   |            |  | segment List          |     | |
                   |            |  +-----------------------+     | |
                   |            +->|SID-D1, SID-C1, SID-B1 |<----+ |
                   |               +-----------------------+       |
                   +-------------->|SID-D2, SID-E1         |<------+
                                   +-----------------------+
                     Figure 2: mapping table


   For instance, the S-BFD initiator is Node A in Figure 1, and the S-
   BFD session is bounded with Segment List1 of Policy A-D. The
   following sub-section describes the processing of S-BFD in SR-MPLS
   and SRv6 networks respectively.

3.2. Procedure of S-BFD

3.2.1. S-BFD in SRv6

   o S-BFD Initiator procedure

   Refer to [I-D.draft-liu-spring-bfd-srv6-policy-encap] for the
   description of how to encapsulate S-BFD packet with SRv6 Policy.
   When path segment is used, the encapsulation format of S-BFD control
   packet is as follows:














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       +-----------------------------------------------+
       | IPv6 Header                                   |
       .  Source IP Address = Initiator's IPv6 Address .
       .  Destination IP Address = SegmentList[SL]     .
       .  Next-Header = SRH (43)                       .
       .                                               .
       +-----------------------------------------------+
       | SRH as specified in RFC 8754                  |
       .  Next-Header = IPv6                           .
       .  <P-Flag=1, PathSegment, Segment List>        .
       .                                               .
       +-----------------------------------------------+
       |                                               |
       .  sbfd-payload                                 .
       |                                               |
       +-----------------------------------------------+


   NodeA Encapsulates the path segment of segment list1 in SRH, and set
   SRH.P-Flag.

   The S-BFD control packet is encapsulated and forwarded as follows:

                A------------->B------------>C---------->D

        +-----------------+                      +-----------------+
        | SA=A's Ipv6Addr |                      | SA=A's Ipv6Addr |
        +-----------------+                      +-----------------+
        | DA=SID-B1       |                      | DA=D's ipv6Addr |
        +-----------------+                      +-----------------+
        | SL=2 | P-Flag=1 |                      | SL=0 | P-Flag=1 |
        +-----------------+                      +-----------------+
        | D's ipv6Addr    |                      | D's ipv6Addr    |
        +-----------------+                      +-----------------+
        | SID-C2          |                      | SID-C2          |
        +-----------------+                      +-----------------+
        | SID-B2          |                      | SID-B2          |
        +-----------------+                      +-----------------+
        | SID-A1          |                      | SID-A1          |
        +-----------------+                      +-----------------+
        | SID-Path-1      |                      | SID-Path-1      |
        +-----------------+                      +-----------------+
        | sbfd-payload    |                      | sbfd-payload    |
        |                 |                      |                 |
        +-----------------+                      +-----------------+
              Figure 3: Example of S-BFD control packet



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   o S-BFD Reflector procedure

   S-BFD control packet is forwarded along the path A->B->C-D. While
   packet arrives at Node D, SRH.SL is 0 and the destination address is
   IPv6 address of Node D. Packet is delivered up to the S-BFD module
   in control plane.

   S-BFD module detects SRH.P-flag is set, extracts the path segment of
   the forward path from SRH, gets the path segment of the reverse path
   through the mapping table. When responding to S-BFD control packet,
   S-BFD module uses the segment list associated with path segment of
   the reverse path to encapsulate SRH.

   The encapsulation format of S-BFD response control packet is as
   follows:

       +-----------------------------------------------+
       | IPv6 Header                                   |
       .  Source IP Address = Reflector's IPv6 Address .
       .  Destination IP Address = SegmentList[SL]     .
       .  Next-Header = SRH (43)                       .
       .                                               .
       +-----------------------------------------------+
       | SRH as specified in RFC 8754                  |
       .  Next-Header = IPv6                           .
       .  <Segment List>                               .
       .                                               .
       +-----------------------------------------------+
       |                                               |
       . sbfd-payload                                  .
       |                                               |
       +-----------------------------------------------+


   The S-BFD response packet is encapsulated and forwarded as follows:













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                D------------->C------------>B---------->A
        +-----------------+                      +-----------------+
        | SA=D's Ipv6Addr |                      | SA=D's Ipv6Addr |
        +-----------------+                      +-----------------+
        | DA=SID-C1       |                      | DA=A's ipv6Addr |
        +-----------------+                      +-----------------+
        | SL=2 | P-Flag=0 |                      | SL=0 | P-Flag=0 |
        +-----------------+                      +-----------------+
        | A's ipv6Addr    |                      | A's ipv6Addr    |
        +-----------------+                      +-----------------+
        | SID-B1          |                      | SID-B1          |
        +-----------------+                      +-----------------+
        | SID-C1          |                      | SID-C1          |
        +-----------------+                      +-----------------+
        | SID-D1          |                      | SID-D1          |
        +-----------------+                      +-----------------+
        | sbfd-payload    |                      | sbfd-payload    |
        |                 |                      |                 |
        +-----------------+                      +-----------------+


   The S-BFD response control packet will be forward along the path D-
   >C->B->A. In this way, the forward and reverse paths of S-BFD are
   guaranteed to be consistent.



3.2.2. S-BFD in SR-MPLS

   o S-BFD Initiator procedure

   The encapsulation format using SR Policy with path segment of S-BFD
   control packet is as follows:















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                  +--------------------+
                  |       ...          |
                  +--------------------+
                  |      Label 1       |
                  +--------------------+
                  |      Label 2       |
                  +--------------------+
                  |       ...          |
                  +--------------------+
                  |      Label n       |
                  +--------------------+
                  |     Path Segment   |
                  +--------------------+
                  | IPv6 Header:       |
                  | Source IP          |
                  | Destination IP     |
                  ~                    ~
                  +--------------------+
                  ~   sbfd-Payload     ~
                  +--------------------+


   Node A Encapsulates the segment list1 and path segment in label
   stack. The source IP is the IPv6 address of Node A, and the
   destination IP is the IPv6 address of Node D.

   The S-BFD control packet is encapsulated and forwarded as follows:

             A------------->B------------>C---------->D
   +-----------------+
   | SID-A1          |
   +-----------------+
   | SID-B2          |
   +-----------------+
   | SID-C2          |
   +-----------------+                      +-----------------+
   | SID-Path-1      |                      | SID-Path-1      |
   +-----------------+                      +-----------------+
   | SA=A's Ipv6Addr |                      | SA=A's Ipv6Addr |
   +-----------------+                      +-----------------+
   | DA=D's ipv6Addr |                      | DA=D's ipv6Addr |
   +-----------------+                      +-----------------+
   | sbfd-payload    |                      | sbfd-payload    |
   |                 |                      |                 |
   +-----------------+                      +-----------------+


   o S-BFD Reflector procedure

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   S-BFD control packet is forwarded along the path A->B->C-D. When the
   packet arrives at node D, the top-level label is path segment.
   Packet with path segment is delivered up to the S-BFD module in
   control plane.

   When responding to S-BFD control packet, the S-BFD module uses the
   mapping table to find the label stack of the reverse path through
   the path segment to encapsulate the response control packet.

   The encapsulation format of S-BFD response control packet is as
   follows:

   The source IP is the IPv6 address of Node D, and the destination IP
   is the IPv6 address of Node A.


                  +--------------------+
                  |       ...          |
                  +--------------------+
                  |      Label 1       |
                  +--------------------+
                  |      Label 2       |
                  +--------------------+
                  |       ...          |
                  +--------------------+
                  |      Label n       |
                  +--------------------+
                  | IPv6 Header:       |
                  | Source IP          |
                  | Destination IP     |
                  ~                    ~
                  +--------------------+
                  ~   sbfd-Payload     ~
                  +--------------------+

   The S-BFD response packet is encapsulated and forwarded as follows:












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          D------------->C------------>B---------->A
   +-----------------+
   | SID-D1          |
   +-----------------+
   | SID-C1          |
   +-----------------+
   | SID-B1          |
   +-----------------+                 +-----------------+
   | SA=D's Ipv6Addr |                 | SA=D's Ipv6Addr |
   +-----------------+                 +-----------------+
   | DA=A's Ipv6Addr |                 | DA=A's Ipv6Addr |
   +-----------------+                 +-----------------+
   | sbfd-payload    |                 | sbfd-payload    |
   |                 |                 |                 |
   +-----------------+                 +-----------------+


4. Path consistency for U-BFD

   This document proposes to encapsulate the segment list of the return
   path in the U-BFD echo packet to guide the packet from the remote
   system to the local system along the same path.

4.1. Getting reverse segment list

   [I-D.ietf-idr-sr-policy-path-segment] proposes an extension to BGP
   SR Policy distribute SR policies carrying reverse path information.

   The reverse path information includes reverse segment list and
   reverse path segment. The reverse path segment can be used for S-BFD
   path consistency, and the reverse segment list can be used for U-BFD
   path consistency.

   Referring to the example topology, the SR Policy on nodes A and D
   that contains complete reverse path information is as follows



   Node A                       Node D

   SR Policy A-D                      SR Policy D-A
     Candidate Path1                     Candidate Path1
      Segment list1                       Segment list1
          SID-A1, SID-B2, SID-C2              SID-D1, SID-C1, SID-B1
          Path Segment: SID-Path-1            Path Segment: SID-Path-2
       Reverse segment list                 Reverse Segment list
          SID-D1, SID-C1, SID-B1              SID-A1, SID-B2, SID-C2
          Reverse Path Segment:               Reverse Path Segment:

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             SID-Path-2                           SID-Path-1
      Segment list2                       Segment list2
          SID-A2, SID-E2                      SID-D2, SID-E1
          Path Segment: SID-Path-3            Path Segment: SID-Path-4
        Reverse segment list                 Reverse segment list
           SID-D2, SID-E1                       SID-A2, SID-E2
          Reverse Path Segment:               Reverse Path Segment:
            SID-Path-4                          SID-Path-3


4.2. Procedure of U-BFD

   The headend node uses U-BFD to detect a segment list of SR-Policy.
   In order to achieve path consistency, the reverse segment list can
   be encapsulated in the U-BFD echo packet at the same time. When the
   U-BFD echo packet reaches the tailend node of SR-Policy, it will be
   looped back to the headend node according to the path specified by
   the reverse segment list.

   According to this method, when the segment list in the SR-Policy and
   its corresponding reverse segment list are planned to pass through
   the same intermediate link, the U-BFD echo packet's round-trip path
   will be consistent.

4.2.1. U-BFD in SRv6

   In SRv6, the reverse segment list can be encapsulated in one SRH
   with the forward segment list, or it can be encapsulated in an
   independent SRH

   When the forward and reverse segment lists are in the same SRH, the
   encapsulation is as follows

















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       +--------------------------------------------+
       | IPv6 Header                                |
       .  Source IP Address = Node A's IPv6 Address .
       .  Destination IP Address = SegmentList[SL]  .
       .  Next-Header = SRH (43)                    .
       .                                            .
       +--------------------------------------------+
       | SRH as specified in RFC 8754               |
       .  Next-Header = IPv6                        .
       .  Node A's IPv6 Address                     .
       .  <ReverseSegment List>                     .
       .  <Segment List>                            .
       .                                            .
       +--------------------------------------------+
       |                                            |
       .  ubfd-payload                              .
       |                                            |
       +--------------------------------------------+


   When the forward and reverse segment lists are in different SRHs,
   the encapsulation is as follows

       +--------------------------------------------+
       | IPv6 Header                                |
       .  Source IP Address = Node A's IPv6 Address .
       .  Destination IP Address = SegmentList[SL]  .
       .  Next-Header = SRH (43)                    .
       .                                            .
       +--------------------------------------------+
       | SRH as specified in RFC 8754               |
       .  Next-Header = SRH (43)                    .
       .  <Segment List>                            .
       .                                            .
       +--------------------------------------------+
       | SRH as specified in RFC 8754               |
       .  Next-Header = IPv6                        .
       .  Node A's IPv6 Address                     .
       .  <ReverseSegment List>                     .
       .                                            .
       +--------------------------------------------+
       |                                            |
       .  ubfd-payload                              .
       |                                            |
       +--------------------------------------------+




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   Referring to the sample topology, take node A as the head node and D
   as the tail node as an example. Node A uses U-BFD to detect segment
   list1. The forward segment list and reverse segment list and the
   address of node A are encapsulated in one SRH.

   The U-BFD echo packet is encapsulated and forwarded as follows:

         A------------->B------------>C---------->D

   +-----------------+                      +-----------------+
   | SA=A's Ipv6Addr |                      | SA=A's Ipv6Addr |
   +-----------------+                      +-----------------+
   | DA=SID-B2       |                      | DA=SID-D1       |
   +-----------------+                      +-----------------+
   | SL=5            |                      | SL=3            |
   +-----------------+                      +-----------------+
   | A's ipv6Addr    |                      | A's ipv6Addr    |
   +-----------------+                      +-----------------+
   | SID-B1          |                      | SID-B1          |
   +-----------------+                      +-----------------+
   | SID-C1          |                      | SID-C1          |
   +-----------------+                      +-----------------+
   | SID-D1          |                      | SID-D1          |
   +-----------------+                      +-----------------+
   | SID-C2          |                      | SID-C2          |
   +-----------------+                      +-----------------+
   | SID-B2          |                      | SID-B2          |
   +-----------------+                      +-----------------+
   | SID-A1          |                      | SID-A1          |
   +-----------------+                      +-----------------+
   | ubfd-payload    |                      | ubfd-payload    |
   |                 |                      |                 |
   +-----------------+                      +-----------------+
        Figure 7: Example of U-BFD echo packet in SRv6


   After the u-BFD packet reaches Node D, Node D continues to forward
   the u-BFD packet according to the SRH, and returns the U-BFD echo
   packet to Node A.









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         D------------->C------------>B---------->A
   +-----------------+                      +-----------------+
   | SA=A's Ipv6Addr |                      | SA=A's Ipv6Addr |
   +-----------------+                      +-----------------+
   | DA=SID-C1       |                      | DA=A's ipv6Addr |
   +-----------------+                      +-----------------+
   | SL=2            |                      | SL=0            |
   +-----------------+                      +-----------------+
   | A's ipv6Addr    |                      | A's ipv6Addr    |
   +-----------------+                      +-----------------+
   | SID-B1          |                      | SID-B1          |
   +-----------------+                      +-----------------+
   | SID-C1          |                      | SID-C1          |
   +-----------------+                      +-----------------+
   | SID-D1          |                      | SID-D1          |
   +-----------------+                      +-----------------+
   | SID-C2          |                      | SID-C2          |
   +-----------------+                      +-----------------+
   | SID-B2          |                      | SID-B2          |
   +-----------------+                      +-----------------+
   | SID-A1          |                      | SID-A1          |
   +-----------------+                      +-----------------+
   | sbfd-payload    |                      | sbfd-payload    |
   |                 |                      |                 |
   +-----------------+                      +-----------------+


   The U-BFD echo packet will be forward along the path D->C->B->A. In
   this way, the forward and reverse paths of U-BFD are guaranteed to
   be consistent.



4.2.2. U-BFD in SR-MPLS

   In SR-MPLS, The segment list and the reverse segment list can be
   encapsulated in the label stack at the same time.











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                  +--------------------+
                  |       ...          |
                  +--------------------+
                  |                    |
                  .     label stack    .
                  |                    |
                  +--------------------+
                  |                    |
                  . reverse label stack.
                  |                    |
                  +--------------------+
                  | IPv6 Header:       |
                  | Source IP          |
                  | Destination IP     |
                  ~                    ~
                  +--------------------+
                  ~   ubfd-Payload     ~
                  +--------------------+


   Take node A as the headend node and D as the tailend node as an
   example, Node A uses U-BFD to detect segment list1. In order to
   achieve consistent paths, the encapsulation and processing of U-BFD
   echo packets are as follows

             A------------->B------------>C---------->D
   +-----------------+
   | SID-A1          |
   +-----------------+
   | SID-B2          |
   +-----------------+
   | SID-C2          |
   +-----------------+                      +-----------------+
   | SID-D1          |                      | SID-D1          |
   +-----------------+                      +-----------------+
   | SID-C1          |                      | SID-C1          |
   +-----------------+                      +-----------------+
   | SID-B1          |                      | SID-B1          |
   +-----------------+                      +-----------------+
   | SA=A's Ipv6Addr |                      | SA=A's Ipv6Addr |
   +-----------------+                      +-----------------+
   | DA=A's ipv6Addr |                      | DA=A's ipv6Addr |
   +-----------------+                      +-----------------+
   | ubfd-payload    |                      | ubfd-payload    |
   |                 |                      |                 |
   +-----------------+                      +-----------------+



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   After the u-BFD packet reaches Node D, Node D continues to forward
   the u-BFD packet according to the label stack, and returns the U-BFD
   echo packet to Node A.

          D------------->C------------>B---------->A
   +-----------------+
   | SID-D1          |
   +-----------------+
   | SID-C1          |
   +-----------------+
   | SID-B1          |
   +-----------------+                 +-----------------+
   | SA=A's Ipv6Addr |                 | SA=A's Ipv6Addr |
   +-----------------+                 +-----------------+
   | DA=A's Ipv6Addr |                 | DA=A's Ipv6Addr |
   +-----------------+                 +-----------------+
   | ubfd-payload    |                 | ubfd-payload    |
   |                 |                 |                 |
   +-----------------+                 +-----------------+


5. IANA Considerations

   This document has no IANA actions.

6. Security Considerations

   The security requirements and mechanisms described in [RFC8402] and
   [RFC8754] also apply to this document.

   This document does not introduce any new security consideration.

7. References

7.1. Normative References

   [I-D.draft-liu-spring-bfd-srv6-policy-encap] Liu, Y., Cheng, W.,
             Lin, C., Chen, M., "Encapsulation of BFD for SRv6 Policy",
             draft-liu-spring-bfd-srv6-policy-encap-00(work in
             progress), October 2022

   [I-D.ietf-bfd-unaffiliated-echo] Cheng, W., Wang, R., Min, X.,
             Rahman, R., and R. C. Boddireddy, "Unaffiliated BFD Echo",
             Work in Progress, Internet-Draft, draft-ietf-bfd-
             unaffiliated-echo-06, 25 March 2023,
             <https://datatracker.ietf.org/doc/html/draft-ietf-bfd-
             unaffiliated-echo-06>.


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   [I-D.ietf-idr-segment-routing-te-policy] Previdi, S., Filsfils, C.,
             Talaulikar, K., Mattes, P., Jain, D., and S. Lin,
             "Advertising Segment Routing Policies in BGP", draft-ietf-
             idr-segment-routing-te-policy-20 (work in progress), July
             2022

   [I-D.ietf-spring-mpls-path-segment] Cheng, W., Li, H., Chen, M.,
             Gandhi, R., and R. Zigler, "Path Segment in MPLS Based
             Segment Routing Network",draft-ietf-spring-mpls-path-
             segment-08 (work in progress), September 2022.

   [I-D.ietf-spring-srv6-path-segment] Li, C., Cheng, W., Chen, M.,
             Dhody, D., and Y. Zhu, "Path Segment for SRv6 (Segment
             Routing in IPv6)", draft-ietf-spring-srv6-path-segment-05
             (work in progress),October 2022.

   [I-D.ietf-idr-sr-policy-path-segment] Li, C., Li, Z., Yin, Y.,
             Cheng, W., Talaulikar, K., "SR Policy Extensions for Path
             Segment and Bidirectional Path", draft-ietf-idr-sr-policy-
             path-segment-07(work in progress), February 2023.

   [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>.

   [RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
             (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, DOI
             10.17487/RFC5881, June 2010, <https://www.rfc-
             editor.org/info/rfc5881>.

   [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>.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

   [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>.

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   [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
             0.17487/RFC8986, February 2021, <https://www.rfc-
             editor.org/info/rfc8986>.

   [RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
             B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
             Based on Segment Routing over IPv6 (SRv6)", RFC 9252, DOI
             10.17487/RFC9252, July 2022, <https://www.rfc-
             editor.org/info/rfc9252>.

Contributors

   Yisong Liu contributed to the content of this document.

































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Authors' Addresses

   Changwang Lin
   New H3C Technologies
   Beijing
   China

   Email: linchangwang.04414@h3c.com


   Weiqiang Cheng
   China Mobile
   Beijing
   CN

   Email: chengweiqiang@chinamobile.com

   Wenying Jiang
   China Mobile
   Beijing
   CN

   Email: jiangwenying@chinamobile.com
   
   
   Ran Chen
   ZTE Corporation
   China
   Email: chen.ran@zte.com.cn



















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