SPRING Working Group                                            K. Salih
Internet-Draft                                                  S. Hegde
Intended status: Standards Track                               M. Rajesh
Expires: 3 September 2022                                      R. Bonica
                                                        Juniper Networks
                                                                 H. wang
                                                     Huawei Technologies
                                                            Shaofu. Peng
                                                         ZTE Corporation
                                                            2 March 2022


                     SRv6 inter-domain mapping SIDs
              draft-salih-spring-srv6-inter-domain-sids-02

Abstract

   This document describes three new SRv6 end-point behaviors, called
   END.REPLACE, END.REPLACEB6 and END.DB6.  These behaviors are used in
   distributed inter-domain solutions and are normally executed on
   border routers.  They also can be used to provide multiple intent-
   based paths across these domains.

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).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on 3 September 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.



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   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Usecases  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  usecase 1 . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  usecase 2 . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  SRv6 SID Behaviors  . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  END.REPLACE . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  END.REPLACEB6 . . . . . . . . . . . . . . . . . . . . . .   4
     4.3.  END.DB6 . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Interworking Procedures . . . . . . . . . . . . . . . . . . .   6
     5.1.  Option C Transport Interworking . . . . . . . . . . . . .   6
     5.2.  Option B service interworking . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Overview

   Segment Routing (SR) [RFC8402] allows source nodes to steer packets
   through SR paths.  It can be implemented over IPv6 [RFC8200] or MPLS
   [RFC3031].  When SR is implemented over IPv6, it is called SRv6
   [RFC8986].

   This document describes three new SRv6 end-point behaviors, called
   END.REPLACE, END.REPLACEB6 and END.DB6.  These behaviors are used to
   build paths across SRv6 domains.  They also facilitate end-to-end
   SRv6 intent-based path stitching.

2.  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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3.  Usecases

3.1.  usecase 1

   This use-case is mentioned in Section 4.1.1 of
   [I-D.hegde-spring-mpls-seamless-sr].

                     ---IBGP------EBGP----IBGP------EBGP-----IBGP---
                    |           |     |           |     |           |

                    +-----------+     +-----------+     +-----------+
                    |           |     |           |     |           |
                    |        ASBR1+--+ASBR2    ASBR3+--+ASBR4       |
                 PE1+     AS1   |  X  |     AS2   |  X  |     AS3   +PE2
                    |        ASBR5+--+ASBR6    ASBR7+--+ASBR8       |
                    |           |     |           |     |           |
                    +-----+-----+     +-----------+     +-----------+
                         PE3

                    |---SRv6---|      |---SRv6---|      |---SRv6---|



                Figure 1: Multiple ASes connected with E-BGP

   Figure 1 depicts three ASes (AS1, AS2 and AS3).  All the three
   domains deploy SRv6.  Inter-provider Option C[RFC4364] connectivity
   is maintained from PE1 to PE2.

3.2.  usecase 2



                  +-----------+   +------------+
                 /             \ /              \
                 |             ABR1             |
                 |              |               |
              PE1+    AS1       +     AS1       +PE2
                 |              |               |
                 |             ABR2             |
                 \              /\             /
                  +------------+  +-----------+



               Figure 2: Singe AS with different IGP domains





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   The above diagram Figure 2 shows two different SRv6 IGP domains.
   Services are running between PE1 and PE2 in option B [RFC4364] style.
   The requirement here is to avoid service route lookup on ABR1 and
   ABR2 to provide option B style end to end connectivity

4.  SRv6 SID Behaviors

4.1.  END.REPLACE

   The END.REPLACE behavior is applicable in the Multiple ASes Connected
   With E-BGP (Section 3.1) use-case.

   The End.REPLACE SID cannot be the last segment in SRH or SR Policy.

   Any SID instance of this behavior is associated with a set, J, of one
   or more L3 adjacencies of immediate BGP neighbors

   When Node N receives a packet destined to S and S is a locally
   instantiated End.REPLACE SID, Node N executes the following
   procedure:

   S01. When an SRH is processed {
   S02.   If (Segments Left == 0) {
   S03.     Stop processing the SRH, and proceed to process the next
            header in the packet, whose type is identified by
            the Next Header field in the routing header. Procedure is as
            per Section 4.1.1 of [RFC8986].
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.      Send an ICMP Time Exceeded message to the Source Address with Code 0
             (Hop limit exceeded in transit), interrupt packet processing, and discard packet
   S07.   }
   S08.   Decrement IPv6 Hop Limit by 1
   S09.   Update IPv6 DA with new destination address(SID) mapped with END.REPLACE SID.
   S10.   Submit the packet to the IPv6 module for transmission
             to the new destination via a member of J.
   S11. }


4.2.  END.REPLACEB6

   The END.REPLACEB6 behavior is applicable in the Multiple ASes
   Connected With E-BGP (Section 3.1) use-case.

   The End.REPLACEB6 SID cannot be the last segment in a SRH or SR
   Policy.





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   Node N is configured with an IPv6 address T (e.g., assigned to its
   loopback).

   When Node N receives a packet destined to S and S is a locally
   instantiated End.REPLACEB6 SID, Node N executes the following
   procedure:

   S01. When an SRH is processed {
   S02.   If (Segments Left == 0) {
   S03.     Stop processing the SRH, and proceed to process the next
            header in the packet, whose type is identified by
            the Next Header field in the routing header. Procedure is as
            per Section 4.1.1 of [RFC8986].
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.      Send an ICMP Time Exceeded message to the Source Address with Code 0
             (Hop limit exceeded in transit), interrupt packet processing, and discard packet
   S07.   }
   S08.   Decrement IPv6 Hop Limit by 1
   S09.   Update IPv6 DA with new destination address(SID) mapped with END.REPLACEB6.
   S10.   Push an IPv6 header with an SRH.
   S11.   Set outer IPv6 SA = T and outer IPv6 DA to the first SID in the segment list
   S12.   Set outer Payload Length, Traffic Class, Hop Limit, and Flow Label fields
   S13.   Set the outer Next Header value
   S14.   Submit the packet to the IPv6 module for transmission to the First SID.
   S15. }

Note :
       S10 - S13. Implemetation may choose to avoid outer encapsulation for flex-algo and best effort based SRv6 transport tunnels.
       S12. The Payload Length, Traffic Class, Hop Limit, and Next Header fields are set as per [RFC2473]. The Flow Label is
           computed as per [RFC6437].


4.3.  END.DB6

   For the use-case mentioned under Section 3.2 END.DB6 SID is
   applicable.

   The End.DB6 SID MUST be the last segment in SRH or SR Policy.

   Node N is configured with an IPv6 address T (e.g., assigned to its
   loopback).

   When Node N receives a packet destined to S and S is a locally
   instantiated End.DB6 SID, Node N executes the following procedure:






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   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.     Send an ICMP Parameter Problem to the Source Address,
            Code 0 (Erroneous header field encountered),
            Pointer set to the Segments Left field,
            interrupt packet processing and discard the packet.
   S04.   }
   S05.   If (Upper-Layer header type == 4(IPv4) OR  Upper-Layer header type == 41(IPv6) OR
                      Upper-Layer header type == 143(Ethernet)) {
   S06.     Remove the outer IPv6 header with all its extension headers.
   S07.     Push the new IPv6 header with the SRv6 SIDs associated with the END.DB6 sid in an SRH.
   S08.     Set outer IPv6 SA = T and outer IPv6 DA to the first SID in the segment list.
   S09.     Set outer Payload Length, Traffic Class, Hop Limit, and Flow Label fields
   S10.     Set the outer Next Header value
   S11.     Submit the packet to the IPv6 module for transmission to First SID.
   S12.   } else {
   S13.     Process as per Section 4.1.1 of [RFC8986].
   S14.   }
   S15. }
Note :
       S09. The Payload Length, Traffic Class, Hop Limit, and Next Header fields are set as per [RFC2473]. The Flow Label is
           computed as per [RFC6437].


5.  Interworking Procedures

   Here we will describe the control plane and data plane procedures by
   taking examples.

   Node n has a classic IPv6 loopback address An::1/128.  One of the SID
   at node n with locator block B and function F is represented by
   B:n:F::sid_num.

   A SID list is represented as

   <S1, S2, S3>

   where S1 is the first SID to visit, S2 is the second SID to visit and
   S3 is the last SID to visit along the SR path.

5.1.  Option C Transport Interworking

   Here we will discuss the use-case mentioned under Section 3.1








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                  ---IBGP----------EBGP--------IBGP--------EBGP-------IBGP-------
                 |              |         |             |        |               |

                 +-----[2]------+         +-----[8]-----+        +------[14]-----+
                 |              |         |             |        |               |
                 |             [4] +---+ [6]          [10]+----+[12]             |
                [1]     AS1     |    X    |    AS2      |    X   |    AS3       [16]
                 |             [5] +---+ [7]          [11]+----+[13]             |
                 |              |         |             |        |               |
                 +-----[3]-----+          +-----[9]-----+        +------[15]-----+
                      PE3

                 |---SRv6---|               |---SRv6---|           |---SRv6---|


                Figure 3: Option C Style Interworking

   Node [1] acts as ingress PE and Node [16] acts as egress PE.

   Nodes [2], [3], [8], [9], [14] and [15] are P routers.

   Nodes [4], [5], [6], [7], [10], [11], [12] and [13] are ASBR routers.

   A VPN route is advertised via service RRs between an egress PE(node
   16) and an ingress PE (node 1).  The example below shows IBGP-CT
   connection between border routers in each domain and single hop EBGP-
   CT for inter-domain connections.  However the forwarding procedure
   for the sids remains the same irrespective of the the various inter-
   domain protocol extensions used to advertise the sids.  AS1, AS2 and
   AS3 has SRTE policy for the required intent paths.





















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   Control plane example:

    For simplicity only one path is tracked.

    For a route if the next hop is one hop away then while advertising use END.REPLACE SID. For a route if the
    next hop is multi hop away then while advertising use END.REPLACEB6 SID. For single hop neighbor case, no encap
    required as it is just replace and forward on specific link while in multihop case one encap will be required.

    Routing Protocol(RP) @16:
          * In ISIS advertise locator B:16::/48 and an END SID B:16::END::1.
          * BGP AFI=1,SAFI=128 originates a VPN route RD:V/v via A:16::1 and Prefix-SID attribute B:16:DT4::1.
            This route is advertised to service RR with color extended community red.
          * BGP originates prefix A:16::1 with color red to ASBR [12] with SRv6 SID B:16:END::1 since its the egress node.
    RP @12:
          * BGP receives the route A:16::1 over the ibgp session and readvertises with nexthop self to ASBR [10].
            it advertises the SRv6 SID B:12:REPLACEB6::1 in the protocol extensions. As the advertisement was received on a
            multihop i-bgp session this node allocates a REPLACEB6 sid.
    RP @10:
          * BGP receives the route A:16::1 over the ebgp session and readvertises with nexthop self to ASBR [6].
            it advertises the SRv6 SID B:10:REPLACE::1 in the protocol extensions. As the advertisement was received on a
            single hop e-bgp session this node allocates a REPLACE sid.
    RP @6:
          * BGP receives the route A:16::1 over the ibgp session and readvertises with nexthop self to ASBR [4].
            it advertises the SRv6 SID B:6:REPLACEB6::1 in the protocol extensions. As the advertisement was received on a
            multihop i-bgp session this node allocates a REPLACEB6 sid.
    RP @4:
          * BGP receives the route A:16::1 over the ebgp session and readvertises with nexthop self to PE [1].
            it advertises the SRv6 SID B:4:REPLACE::1 in the protocol extensions. As the advertisement was received on a
            single hop e-bgp session this node allocates a REPLACE sid.
    RP @1:
          * BGP receives the route A:16::1 with color red over the ibgp session.
          * BGP AFI=1, SAFI=128 learn service prefix RD:V/v, next hop A:16::1 and PrefixSID attribute TLV type 5
            with SRv6 SID B:16:DT4


















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   FIB State:

        @1: IPv4 VRF V/v => H.Encaps.red <B:2:END::1, B:4:REPLACE::1, B:16:DT4::1> with SRH, SRH NextHeader=IPv4 where the first
            sid B:2:END::1 belongs to the SR-policy in AS1.
        @2: IPv6 Table: B:2:END::1 => Update DA with B:4:REPLACE::1, decrement SL and forward towards the ASBR [4].
        @4: IPv6 Table: B:4:REPLACE::1 => Update DA with B:6:REPLACEB6::1 and forward on the interface/interfaces identified by the
            ebgp neigbhor; the SL remains at 1.
        @6: IPv6 Table: B:6:REPLACEB6::1 => Update DA with B:10:REPLACE::1 AND do a fresh H.Encaps.red <B:8:END::1, B:10:END::1>
            with SRH where the new SRH SIDs belong to SR policy in AS2.
        @8: IPv6 Table: B:8:END::1 => Update outer IPv6 packet DA with B:10:END::1 and forward towards ASBR [10]
        @10: IPv6 table: B:10:END::1 => Decap Outer IPv6 header and lookup next IPv6 DA B:10:REPLACE::1 => Update DA to B:12:REPLACEB6::1
             and forward on the interface/interfaces identified by the ebgp neigbour. SL remains at 1.
        @12: IPv6 Table B:12:REPLACEB6::1 => Update DA with B:16:END::1 and do a fresh H.Encaps.red <B:15:END::1, B:16:END::1> with SRH
             where the new SIDs belong to the SR policy in AS3.
        @15: IPv6 Table B:15:END::1 => Update outer IPv6 packet DA with B:16:END::1 and forward towards [16].
        @16: IPv6 Table B:16:END::1 => Decap the outer header and lookup the inner DA which results in B:16:DT4::1 lookup. DT4 lookup
             results in Decap and inner IPv4 packet DA lookup in the corresponding VRF.

        Note: At [16] its possible to optimize the lookups required with minor control plane extensions.

5.2.  Option B service interworking

   Here we will discuss the use-case mentioned under Section 3.2

                     ---MP-IBGP/---- ---MP-IBGP/--
                    |      EBGP    |       EBGP  |

                    +-----[2]------+-----[5]-----+
                    |              |             |
                    |              |             |
                   [1]     AS1    [4]   AS1     [7]
                    |              |             |
                    |              |             |
                    +-----[3]------+-----[6]----+

                    |---SRv6---|    |---SRv6---|

               Figure 4: Option B style Service Interworking

   Nodes [1] and [7] are PE routers.  Node [4] is an option B style
   configured ABR/RR.










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   Control Plane example:

    Routing Protocol(RP) @7:
      *  BGP AFI=1,SAFI=128 originates a VPN route RD:V/v via A:7::1 and Prefix-SID
         attribute B:7:DT4::1. This route is advertised to service RR [4].
    RP @4:
      *  BGP receives the route over MP-IBGP/MP-EBGP session and readvertises with nexthop self to PE [1].
         it advertises the SRv6 SID B:4:DB6::1 in the Prefix-SID attribute TLV along with it. For all prefixes
         having SRv6 service SID B:7:DT4::1; the same DB6 SID B:4:DB6::1 will be reused. if a different service sid
         B:7:DT4::2 comes then a different DB6 SID B:4:DB6::2 will be allocated. This ensures that if the egress allocates
         per CE sid; the translation at border also ensure per CE sid.

    RP @1:
      *  BGP AFI=1, SAFI=128 learn service prefix RD:V/v, next hop A:4::1 and PrefixSID attribute TLV type 5
         with SRv6 SID B:4:DB6::1

   FIB State:

    @1: IPv4 VRF V/v => H.Encaps.red <B:4:DB6::1> with SRH, SRH NextHeader=IPv4 where the first sid belongs to the SR-policy in AS1
    @4: IPv6 Table: B:4:DB6::1 => Decapsulate the incoming IPv6 header and H.Encaps <B:7:DT4::1>
    @7: IPv6 Table: B:7:DT4::1  => Decapsulate the  header and lookup the inner IPv4 packet DA in the VRF

6.  IANA Considerations

   This document requires no IANA action.

   The authors will request an early allocation from the "SRv6 Endpoint
   Behaviors" sub-registry of the "Segment Routing Parameters" registry.

7.  Security Considerations

   Because SR inter-working requires co-operation between inter-working
   domains, this document introduces no security consideration beyond
   those addressed in [RFC8402], [RFC8754] and [RFC8986].

8.  Contributors

       Jie Dong
       Huawei Technologies
       Email: jie.dong@huawei.com

       Swamy SRK
       Juniper Networks
       Email: swamys@juniper.net

       G. Sri Karthik Goud
       Juniper Networks
       Email: gkarthik@juniper.net



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9.  Acknowledgements

   Thanks to Ram Santhanakrishnan, Srihari Sangli, Rajendra Prasad
   Bollam and Kiran Kushalad for their valuable comments.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

10.2.  Informative References

   [I-D.hegde-spring-mpls-seamless-sr]
              Hegde, S., Bowers, C., Xu, X., Gulko, A., Bogdanov, A.,
              Uttaro, J., Jalil, L., Khaddam, M., Alston, A., and L. M.



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              Contreras, "Seamless SR Problem Statement", Work in
              Progress, Internet-Draft, draft-hegde-spring-mpls-
              seamless-sr-06, 24 September 2021,
              <https://www.ietf.org/archive/id/draft-hegde-spring-mpls-
              seamless-sr-06.txt>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

Authors' Addresses

   Salih K A
   Juniper Networks
   Embassy Business Park
   Bangalore 560093
   KA
   India
   Email: salih@juniper.net


   Shraddha Hegde
   Juniper Networks
   Embassy Business Park
   Bangalore 560093
   KA
   India
   Email: shraddha@juniper.net


   Rajesh
   Juniper Networks
   Embassy Business Park
   Bangalore 560093
   KA
   India
   Email: mrajesh@juniper.net


   Ron Bonica
   Juniper Networks
   Herndon, Virginia 20171
   United States of America
   Email: rbonica@juniper.net






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   Haibo Wang
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Road
   Beijing
   100095
   China
   Email: rainsword.wang@huawei.com


   Peng Shaofu
   ZTE Corporation
   China
   Email: peng.shaofu@zte.com.cn






































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