BESS                                                         Yisong Liu
Internet Draft                                             China Mobile
Intended status: Standards Track                                 C. Lin
Expires: August 28, 2024                                        M. Chen
                                                   New H3C Technologies
                                                                 Y. Liu
                                                                    ZTE
                                                          March 1, 2024


                       SRv6 Service SID Anycast Flag
              draft-liu-bess-srv6-service-sid-anycast-flag-00


Abstract

   In some multihoming SRv6 L3VPN and EVPN scenarios, there are
   requirements for the egress PE to advertise both unicast and anycast
   SRv6 Service SIDs for the same service. This document defines the
   Anycast-flag for SRv6 Service SIDs carried in BGP messages.

Status of this Memo

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   This Internet-Draft will expire on August 28, 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors. All rights reserved.




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   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
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Table of Contents


   1. Introduction...................................................2
      1.1. Requirements Language.....................................2
   2. Anycast Service SID............................................3
   3. Extensions for BGP.............................................4
   4. Backward Compatibility.........................................5
   5. Security Considerations........................................5
   6. IANA Considerations............................................6
   7. References.....................................................6
      7.1. Normative References......................................6
   Authors' Addresses................................................6

1. Introduction

   [RFC9252] defines procedures and messages for SRv6-based BGP
   services, including Layer 3 Virtual Private Network (L3VPN),
   Ethernet VPN (EVPN), and Internet services. In some multihoming
   scenarios, there are requirements for the egress PE to advertise
   both unicast and anycast SRv6 Service SIDs for the same service. And
   those anycast SIDs need to be identified in the BGP messages.

   This document defines the Anycast-flag for SRv6 Service SIDs carried
   in BGP messages.

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.






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2. Anycast Service SID

   In the multihoming SRv6 L3VPN and EVPN scenarios, anycast Service
   SID may be used to advertise the same service at different egress
   PEs, which can improve service reliability and load balancing.

                +-----+             +-----+
                | CE1 |             | CE2 |
                +-----+             +-----+
                   |                   |
                +-----+             +-----+
     ---------- | PE1 |             | PE2 |
         ^      +-----+             +-----+
         |             *           *
         |              *         *
       SRv6              +-------+
     L3VPN/EVPN          |BGP-RR |
         |               +-------+
         |              *         *
         |             *           *
         v      +-----+             +-----+
     ---------- | PE3 |             | PE4 |
                +-----+             +-----+
         1. Anycast    \           /  1. Anycast
          Service SID   \         /    Service SID
         2. Unicast      \       /    2. Unicast
          Service SID-1   +-----+      Service SID-2
                          | CE3 |
                          +-----+

     PE1:
       VPN Traffic Policy:
         PE3 & PE4 Load Balancing
       FIB Entry for VPN Traffic:
         Next-hop: Anycast Service SID

     PE2:
       VPN Traffic Policy:
         PE3 Active, PE4 Backup
       FIB Entry for VPN Traffic:
         Primary Next-hop: Unicast Service SID-1
         Backup Next-hop: Unicast Service SID-2

                          Figure 1

   As shown in Figure 1, PE3 and PE4 use the same anycast SRv6 Service
   SID for the VPN service of CE3. The ingress PE1 encapsulates the
   payload in an outer IPv6 header where the destination address is

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   that anycast SRv6 Service SID. The packets from CE1 can reach CE3
   through either PE3 or PE4. Assume that the path from PE1 to PE3 and
   the path from PE1 to PE4 have the same cost. The traffic flows will
   be load balanced between PE3 and PE4.

   PE3 and PE4 also have unicast SRv6 Service SIDs, which are SID-1 and
   SID-2, for the VPN service of CE3. The ingress PE2 uses SID-1 as the
   primary SRv6 Service SID, and SID-2 as backup. The packets from CE2
   will be forwarded to CE3 through PE3. If any failure occurs on the
   path to PE3, service will be switched to PE4.

   Since ingress PE1 and PE2 have different strategies for the control
   of VPN traffics, egress PE3 and PE4 each need to advertise two SRv6
   Service SIDs, an anycast SID for ingress PE1 and a unicast SID for
   ingress PE2. Local export policy may be used by egress PE3 and PE4
   to control which SID is advertised to ingress PE1 and which is
   advertised to ingress PE2. However, if BGP Route Reflector is
   deployed, both the anycast Service SID and the unicast Service SID
   will be advertised to RR and reflected to ingress PEs, and the
   receiver has to choose which Service SID to use.

   It is required to identify which Service SID is anycast and which
   Service SID is unicast, when both two SIDs are advertised in BGP
   messages.

   IGP has Anycast-flag for SRv6 locator, but the IGP Anycast-flag can
   be lost due to summarization. This document defines the Anycast-flag
   for SRv6 Service SIDs carried in BGP messages.

3. Extensions for BGP

   This document defines a new flag in the SRv6 Service SID Flags field
   of the SRv6 SID Information Sub-TLV [RFC9252]:















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SRv6 Service  |    SRv6 Service               |               |
   | Sub-TLV       |    Sub-TLV                    |               |
   | Type=1        |    Length                     |  RESERVED1    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  SRv6 SID Value (16 octets)                                  //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Svc SID Flags |   SRv6 Endpoint Behavior      |   RESERVED2   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  SRv6 Service Data Sub-Sub-TLVs                              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Svc SID Flags:

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     | |A|           |
     +-+-+-+-+-+-+-+-+

   o A-flag: Anycast flag. When set, the associated SID is anycast.

   The new-defined flag can be used for the SRv6 Service SIDs of L3 and
   L2 services, such as End.DX4, End.DT4, End.DX6, End.DT6, End.DT46.
   End.DX2, End.DX2V, End.DT2U, etc.

4. Backward Compatibility

   According to [RFC9252],

   o Any unknown flags in the SRv6 Service SID Flags field MUST be
      ignored by the receiver.

   o When multiple SRv6 SID Information Sub-TLVs are present, the
      ingress PE SHOULD use the SRv6 SID from the first instance of the
      Sub-TLV.

   When the egress PE advertises multiple service SIDs, the unicast
   service SID SHOULD be carried in the first instance of Sub-TLV. If
   there are PE routers not supporting the flag defined in this
   document, the egress PE MAY expect those routers to use the first
   SID and ignore the new-defined flag.

5. Security Considerations

   TBD.


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6. IANA Considerations

   This document defines the following bit in the SRv6 Service SID
   Flags field of SRv6 SID Information Sub-TLV:

   TLV Code Point    Value
   --------------------------------------------------------
   TBD               A-flag

7. References

7.1. 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

   [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>.

Authors' Addresses

   Yisong Liu
   China Mobile
   China
   Email: liuyisong@chinamobile.com


   Changwang Lin
   New H3C Technologies
   China
   Email: linchangwang.04414@h3c.com


   Mengxiao Chen
   New H3C Technologies
   China
   Email: chen.mengxiao@h3c.com






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   Yao Liu
   ZTE
   China
   Email: liu.yao71@zte.com.cn












































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