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Versions: 00 01                                                         
BFD Working Group                                               C. Lin
Internet Draft                                    New H3C Technologies
Intended status: Informational                                W. Cheng
Expires: Oct 12, 2022                                          W. Jiang
                                                          China Mobile
                                                         April 12, 2022



                     S-BFD Path Consistency over SRv6
               draft-lin-sbfd-path-consistency-over-srv6-01


Abstract

   Bidirectional Forwarding Detection (BFD) can be used to monitor
   paths between nodes. 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 SRv6, when a headend
   use S-BFD to monitor the segment list/CPath of SRv6 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 of S-BFD consistent when detecting SRv6 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
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   http://www.ietf.org/shadow.html



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   This Internet-Draft will expire on October 12 2022.

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
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   publication of this document. Please review these documents
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   warranty as described in the Simplified BSD License.



Table of Contents


   1. Introduction ................................................ 2
      1.1. Requirements Language .................................. 3
   2. Requirement for S-BFD in SRv6 ............................... 3
   3. Correlate bidirectional path using Path Segment ............. 4
   4. S-BFD Procedure with Path segment ........................... 6
      4.1. S-BFD Initiator procedure .............................. 6
      4.2. S-BFD Reflector procedure .............................. 8
   5. IANA Considerations ........................................ 10
   6. Security Considerations .................................... 10
   7. References ................................................. 10
      7.1. Normative References .................................. 10
   Contributors .................................................. 11
   Authors' Addresses ............................................ 12

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.

   S-BFD is used to monitor different kinds of paths between nodes. In
   SRv6, when a headend use S-BFD to monitor the segment list/CPath of
   SRv6 Policy, the forward and reverse path of S-BFD packet are
   inconsistent with high probability because the reverse path is via

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   IPv6 forwarding and forward path is via SRv6 segment list (loose
   path or explicit path).

   The inconsistency impacts the detecting result. If the forward path
   is up and reverse path is down, then the S-BFD session will be down.
   If there are multiple path (segment list) in a SRv6 Policy between a
   headend (initiator) router and a tailend(reflector) router, multiple
   S-BFD session will be created for each path. Each S-BFD session uses
   corresponding path to send control packet, but the reverse path is
   identical for all S-BFD sessions. If the reverse path is down, all
   sessions will be down. Then the SRv6 Policy is down.

   The consistency of forward and reverse path of the same S-BFD
   session should be guaranteed. This document describes a method to
   keep the forward path and reverse path of S-BFD consistent using
   path segment when detecting SRv6 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. Requirement for S-BFD in SRv6

   Monitor SRv6 Policy using S-BFD is usually based on segment list S-
   BFD creates session for each segment list and associates the session
   with segment list.

   When S-BFD initiator detects the continuity of an S-BFD session, it
   will use the associated segment list to encapsulate IPv6 header and
   SRH of the control packet.

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

   Referring to the following topology, there are two paths between
   Node A and D, and All nodes allocate end.x Segments. Node A and D
   are headend and tailend nodes of each other, and SRv6 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-D2
          SID-A2\           SID-E1      SID-E2            /
                 +-------------------E-------------------+
                       Figure 1: reference topology


   Assuming that the deployed SRv6 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:

   SRv6 Policy A-D                     SRv6 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


   When node A is the S-BFD initiator, S-BFD sessions for segment list1
   and segment list2 could be created respectively.

   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
   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.

3. Correlate bidirectional path using Path Segment

   A Path Segment is defined to identify an SR path in [draft-ietf-
   spring-srv6-path-segment]. SRv6 Path segments can be used to
   correlate the two unidirectional SRv6 paths at both ends of the
   paths.

   [draft-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 SRv6 policy to the
   headend, reverse path information and path segment of segment list
   can be carried together.

   Node A                       Node D

   SRv6 Policy A-D                    SRv6 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 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 link.

   The headend can use path segment in two directions to establish a
   mapping table. Using this mapping table, the headend can index 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


4. S-BFD Procedure with Path segment

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

4.1. S-BFD Initiator procedure

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



















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       +-----------------------------------------------------------+
       | IPv6 Header                                               |
       .  Source IP Address = S-BFD Initiator IPv6 Address         .
       .  Destination IP Address = SegmentList[SL]                 .
       .  Next-Header = SRH (43)                                   .
       .                                                           .
       +-----------------------------------------------------------+
       | SRH as specified in RFC 8754                              |
       .  Next-Header = IPv6                                       .
       .  <PathSegment, Segment List>                              .
       .                                                           .
       +-----------------------------------------------------------+
       | IPv6 Header                                               |
       .  Source IP Address = S-BFD Initiator IPv6 Address         .
       .  Destination IP Address = S-BFD Reflector IPv6 Address    .
       .  Next-Header = UDP                                        .
       .                                                           .
       +-----------------------------------------------------------+
       | UDP Header                                                |
       .                                                           .
       +-----------------------------------------------------------+
       | Payload                                                   |
       .                                                           .
       +-----------------------------------------------------------+
          Figure 3: Encapsulation format of S-BFD control packet


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

   The S-BFD control packet is as follows:

















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

        +-----------------+                      +-----------------+
        | SA=A's Ipv6Addr |                      | SA=A's Ipv6Addr |
        +-----------------+                      +-----------------+
        | DA=SID-A1       |                      | DA=D's ipv6Addr |
        +-----------------+                      +-----------------+
        | SL=3 | 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-A1     |                      | SID-Path-A1     |
        +-----------------+                      +-----------------+
        | sbfd-payload    |                      | sbfd-payload    |
        |                 |                      |                 |
        +-----------------+                      +-----------------+
              Figure 4: Example of S-BFD control packet


4.2. S-BFD Reflector procedure

   S-BFD control packet is forwarded along the path A->B->C-D. While
   packet arrives at Node D, RH.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:









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       +----------------------------------------------------------+
       | IPv6 Header                                              |
       .  Source IP Address = S-BFD Reflector IPv6 Address        .
       .  Destination IP Address = SegmentList[SL]                .
       .  Next-Header = SRH (43)                                  .
       .                                                          .
       +----------------------------------------------------------+
       | SRH as specified in RFC 8754                             |
       .  Next-Header = IPv6                                      .
       .  <Segment List>                                          .
       .                                                          .
       +----------------------------------------------------------+
       | IPv6 Header                                              |
       .  Source IP Address = S-BFD Reflector IPv6 Address        .
       .  Destination IP Address = S-BFD Sender IPv6 Address      .
       .  Next-Header = UDP                                       .
       .                                                          .
       +----------------------------------------------------------+
       | UDP Header                                               |
       .                                                          .
       +----------------------------------------------------------+
       | Payload                                                  |
        .                                                         .
       +----------------------------------------------------------+
        Figure 5: Encapsulation format of S-BFD response control packet


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




















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                D------------->C------------>B---------->A
        +-----------------+                      +-----------------+
        | SA=D's Ipv6Addr |                      | SA=D's Ipv6Addr |
        +-----------------+                      +-----------------+
        | DA=SID-D1       |                      | DA=A's ipv6Addr |
        +-----------------+                      +-----------------+
        | SL=3 | 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    |
        |                 |                      |                 |
        +-----------------+                      +-----------------+
            Figure 6: Example of S-BFD response control packet


   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.

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.ietf-idr-segment-routing-te-policy] Previdi, S., Filsfils, C.,
             Talaulikar, K., Mattes, P.,Rosen, E., Jain, D., and S.
             Lin, "Advertising Segment Routing Policies in BGP", draft-
             ietf-idr-segment-routing-te-policy-14 (work in progress),
             November 2021


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   [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-07 (work in progress), December 2021.

   [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-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-03
             (work in progress),November 2021.

   [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-05(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>.

   [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>.

   [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>.

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

























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