SPRING Working Group                                            S. Hegde
Internet-Draft                                               P. Kaneriya
Intended status: Standards Track                               R. Bonica
Expires: January 13, 2022                               Juniper Networks
                                                               P. Shaofu
                                                               G. Mirsky
                                                                Z. Zhang
                                                         ZTE Corporation
                                                             B. Decraene
                                                                  Orange
                                                           July 12, 2021


                 SR-MPLS / SRv6 Transport Interworking
                  draft-bonica-spring-srv6-end-dtm-06

Abstract

   This document describes procedures for interworking between an SR-
   MPLS transit domain and an SRv6 transit domain.  Each domain contains
   Provider Edge (PE) and Provider (P) routers.  Area Border Routers
   (ABR) provide connectivity between domains.

   The procedures described in this document require the ABR to carry a
   route to each PE router.  However, they do not required the ABR to
   carry service (i.e., customer) routes.  In that respect, these
   procedures resemble L3VPN Interprovider Option C.

   Procedures described in this document support interworking for global
   IPv4 and IPv6 service prefixes.  They do not support interworking for
   VPN services prefixes where the SR-MPLS domain uses MPLS service
   labels.

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."




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   This Internet-Draft will expire on January 13, 2022.

Copyright Notice

   Copyright (c) 2021 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.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Reference Topology  . . . . . . . . . . . . . . . . . . . . .   3
   4.  Forwarding Plane  . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  END.DM Processing . . . . . . . . . . . . . . . . . . . .   6
   5.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Signaling SR Paths That Originate In The SRv6 Domain  . .   7
     5.2.  Signaling SR Paths That Originate In The SR-MPLS Domain .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  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].  When SR is implemented over MPLS, it is called SR-MPLS
   [RFC8660].

   This document describes procedures for interworking between an SR-
   MPLS transit domain and an SRv6 transit domain.  Each domain contains
   Provider Edge (PE) and Provider (P) routers.  Area Border Routers
   (ABR) provide connectivity between domains.




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   The procedures described in this document require the ABR to carry a
   route to each PE router.  However, they do not required the ABR to
   carry service (i.e., customer) routes.  In that respect, these
   procedures resemble L3VPN Interprovider Option C [RFC4364].

   Procedures described in this document support interworking for global
   IPv4 and IPv6 service prefixes.  They do not support interworking for
   VPN services prefixes where the SR-MPLS domain uses MPLS service
   labels.

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.

3.  Reference Topology

       ----------------------- SR Path 1 -------------------------->

       <---------------------- SR Path 2 --------------------------

        ------       ------       ------       ------       ------
       | PE   |     |  P   |     | ABR  |     |  P   |     | PE   |
       |Node 1| --- |Node 2| --- |Node 3| --- |Node 4| --- |Node 5|
        ------       ------       ------       ------       ------

        Seg. A       Seg. B       Seg. C       Seg. D       Seg. E

       <---------- SRv6 Domain ---------->
                                <--------- SR-MPLS Domain --------->

                 Figure 1: Interworking Between SR Domains

   Figure 1 depicts interworking between an SR-MPLS domain and an SRv6
   domain.  The SRv6 domain contains PE Node 1 and P Node 2.  The SR-
   MPLS domain contains P Node 4 and PE node 5.  Both domains contain
   ABR Node 3.

   Nodes 1 and 2 MUST support SRv6 but are NOT REQUIRED to support SR-
   MPLS.  Nodes 4 and 5 MUST support SR-MPLS but are NOT required to
   support SRv6.  Node 3 MUST support both SRv6 and SR-MPLS.  It must
   also support interworking procedures.

   Network operators configure a loopback interface on Nodes 1 through
   5.  These are called Loopback1 through Loopback5.  They also



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   configure 2 additional loopback interfaces on PE Node 5.  These are
   called Loopback5.IPv4 and Loopback5.IPv6.

   Each node instantiates an SR Segment (i.e., Segment A through Segment
   E).  SR Path 1 begins on PE Node 1 and ends on PE Node 5.  It visits
   Nodes 2, 3, 4, and 5, executing the instructions associated with
   Segments B, C, and D.  SR Path 2 begins on PE Node 5 and ends on PE
   Node 1.  It visits Nodes 4, 3, 2, and 1, executing the instructions
   associated with Segments D, C, B and A.

4.  Forwarding Plane

        ----------------------- SR Path 1 -------------------------->

          ------       ------       ------       ------       ------
         | PE   |     |  P   |     | ABR  |     |  P   |     | PE   |
         |Node 1| --- |Node 2| --- |Node 3| --- |Node 4| --- |Node 5|
          ------       ------       ------       ------       ------

           Seg. A       Seg. B       Seg. C       Seg. D       Seg. E

            IPv6:          IPv6:           SR-MPLS:         SR-MPLS:
             SA: Node 1     SA: Node 1      Seg. D           Exp. Null
             DA: Seg. B     DA: Seg. C      Exp. Null       Payload
            SRH:           SRH:            Payload
             SL:  1         SL: 0
             SID: Seg. C    SID: Seg. C
            Payload       Payload


                 Figure 2: Encapsulation: SRv6 To SR-MPLS

   Figure 2 depicts the forwarding plane as a packet traverses SR Path
   1, from Node 1 to Node 5.  In this example, PE Node 1 receives an
   IPv4 packet.

   PE Node 1 encapsulates the IPv4 packet in an SRv6 header.  The SRv6
   header contains an IPv6 header and a Segment Routing Header (SRH)
   [RFC8754].  The Destination Address in the IPv6 header is a Segment
   Identifier (SID) that represents Segment B.  Segment B is an END
   instruction instantiated on P Node 2.  The SRH contains a Segments
   Left field and one SID.  The Segments Left field is equal to 1 and
   the SID represents Segment C, and END.DM (Section 4.1) instruction
   instantiated on ABR Node 3.

   PE Node 1 forwards the packet to P Node 2.  When P Node 2 receives
   the packet, it processed the END instruction.  It decrements the
   Segments Left field in the SRH and copies the SID from the SRH to the



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   Destination Address field of the IPv6 header.  It then forwards the
   packet to ABR Node 3.

   When ABR Node 3 receives the packet, it processes the END.DM
   instruction.  It removes the SRv6 header and replaces it with an SR-
   MPLS label stack that contains two entries.  The top entry represents
   a prefix SID instantiated on P Node 4.  The bottom entry is an
   Explicit Null instruction (i.e., MPLS label 0), instantiated on PE
   Node 5.

   ABR Node 3 then forwards the packet to P Node 4.  P Node 4 processes
   the prefix SID, removing the top entry from the SR-MPLS label stack
   and forwarding the packet to PE Node 5.  PE Node 5 processes the
   Explicit Null instruction, removing the remaining SR-MPLS label stack
   entry and processing the payload.

        <----------------------- SR Path 2 --------------------------

          ------       ------       ------       ------       ------
         | PE   |     |  P   |     | ABR  |     |  P   |     | PE   |
         |Node 1| --- |Node 2| --- |Node 3| --- |Node 4| --- |Node 5|
          ------       ------       ------       ------       ------

           Seg. A       Seg. B       Seg. C       Seg. D       Seg. E

            IPv6:          IPv6:           SR-MPLS:         SR-MPLS:
             SA: Node 3     SA: Node 3      Seg. C           Seg. D
             DA: Seg. A      DA: Seg. B    Payload           Seg. C
            SRH:           SRH:                             Payload
             SL:  0         SL: 1
             SID: Seg. A    SID: Seg. A
            Payload       Payload


                 Figure 3: Encapsulation: SR-MPLS to IPv6

   Figure 3 depicts the forwarding plane as a packet traverses SR Path
   2, from Node 5 to Node 1.  In this example, PE Node 5 receives an
   IPv4 packet.

   PE Node 5 encapsulates the IPv4 packet in an SR-MPLS label stack that
   contains two entries.  The top entry represents a prefix SID
   instantiated on P Node 4.  The bottom entry is a binding SID
   instantiated on ABR Node 3.

   PE Node 5 then forwards the packet to P Node 4.  P Node 4 processes
   the prefix SID, removing the top entry from the SR-MPLS label stack
   and forwarding the packet to ABR Node 3.  ABR Node 3 processes



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   binding SID, removing the remaining SR-MPLS label stack entry and
   replacing it with an SRv6 header.  The SRv6 header contains an IPv6
   header and an SRH.  The Destination Address in the IPv6 header is a
   Segment Identifier (SID) that represents Segment B.  Segment B is an
   END instruction instantiated on P Node 2.  The SRH contains a
   Segments Left field and one SID.  The Segments Left field is equal to
   1 and the SID represents Segment A, an END.DT46 instruction
   instantiated on PE Node 1.  That instruction causes the packet to be
   forwarded using the main IP forwarding table, not a VPN forwarding
   table.

   ABR Node 3 forwards the packet to P Node 2.  When P Node 2 receives
   the packet, it processed the END instruction.  It decrements the
   Segments Left field in the SRH and copies the SID from the SRH to the
   Destination Address field of the IPv6 header.  It then forwards the
   packet to PE Node 1.  PE Node 1 processes its END.DT46 instruction,
   removing the SRv6 header and processing the payload.

4.1.  END.DM Processing

   The End.DM SID MUST be the last segment in a SR Policy.  Its
   arguments are associated with an SR-MPLS label stack.

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

   S01. When an IPv6 Routing Header 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.   Proceed to process the next header in the packet
   S06. }


   When processing the Upper-layer header of a packet matching a FIB
   entry locally instantiated as an End.DM SID, N executes the following
   procedure:

   S01. Decapsulate the packet (i.e., remove the outer IPv6 Header and all
        its extension headers)
   S02. Push the SR-MPLS label stack that is associated with the END.DM
        arguments. Set the MPLS Traffic Class and TTL values to reflect
        the Traffic Class and Hop count values received in the IPv6 header.
   S03. Submit the packet to the MPLS FIB lookup for transmission to the
        new destination



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5.  Control Plane

       <------------------- Customer Routes (iBGP) -------------->

       <-- PE and ABR Routes (iBGP) ->
                                    <-- PE and ABR Routes(BGP-LU) ->
        ------       ------       ------       ------       ------
       | PE   |     |  P   |     | ABR  |     |   P  |     | PE   |
       |Node 1| --- |Node 2| --- |Node 3| --- |Node 4| --- |Node 5|
        ------       ------       ------       ------       ------

       <---------- SRv6 Domain ---------->
                                <--------- SR-MPLS Domain --------->

                        Figure 4: BGP NLRI Exchange

   In the Figure 4, PE Node 1 and PE Node 5 exchange customer Network
   Layer Reachability Information (NLRI) [RFC4271] using either a direct
   BGP session or a route reflector [RFC4456].  All customer routes
   exchanged between PE Node 1 and PE Node 5 belong to the general
   routing instance.  They cannot belong to a VPN.

   PE Node 1 exchanges loopback routes with ABR Node 3, using either a
   direct BGP session or a route reflector.  Likewise, ABR Node 3
   exchanges loopback with PE Node 5, using either a direct BGP session
   or a route reflector.

   PE Node 1 and ABR Node 3 bind SIDs to the loopback routes that they
   exchange, as described in [I-D.dawra-bess-srv6-services].  PE Node 5
   and ABR Node 3 bind labels to the loopback routes that they exchange,
   as described in [RFC8277].

   Both domains use an IGP to distribute link state information and
   establish connectivity within the domain.

5.1.  Signaling SR Paths That Originate In The SRv6 Domain

   PE Node 5 advertises an IPv4 customer route to PE Node 1 using BGP as
   follows:

   o  IPv4 Prefix

   o  Next-hop: Loopback5.IPv4

   This causes PE Node 1 to resolve the customer route through its route
   to Loopback5.IPv4.  The following paragraphs describe how PE Node 1
   acquires a route to Loopback5.IPv4.




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   PE Node 5 advertises Loopback5.IPv4 to ABR Node 3 using BGP Labeled
   Unicast (BGP-LU) as follows:

   o  Prefix: Loopback5.IPv4

   o  Next-hop: Loopback5

   o  Color Community: Color to distinguish between paths between ABR
      Node 3 and PE Node 5

   o  MPLS Label: Explicit Null (0)

   Now, ABR Node 3 resolves its route to Loopback5.IPv4 through its IGP
   route to Loopback5.  Therefore, when forwarding traffic bound for
   Loopback5.IPv4, it imposes:

   o  An SR-MPLS label stack associated with the IGP route to Loopback5

   o  An additional Explicit Null label

   ABR Node 3 advertises Loopback5.IPv4 to PE Node 1 using BGP as
   follows:

   o  Prefix: Loopback5.IPv4

   o  Next-hop: Loopback3

   o  Color Community: Color to distinguish between paths between ABR
      Node 3 and PE Node 1

   o  SID: SID C (i.e., an END.DM SID instantiated on ABR Node 3)

   Now, PE Node 1 resolves its route to Loopback5.IPv4 through its IGP
   route to Loopback3.  Therefore, when forwarding traffic bound for
   Loopback5.IPv4, it imposes an SRv6 header that includes the following
   SIDs:

   o  SIDS associated with the IGP route to Loopback3

   o  SID C

5.2.  Signaling SR Paths That Originate In The SR-MPLS Domain

   PE Node 1 advertises an IPv4 customer route to PE Node 5 using BGP as
   follows:

   o  IPv4 Prefix




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   o  Next-hop: Loopback1

   This causes PE Node 5 to resolve the customer route through its route
   to Loopback1.  The following paragraphs describe how PE Node 5
   acquires a route to Loopback1.

   PE Node 1 advertises Loopback1 to ABR Node 3 using BGP as follows:

   o  Prefix: Loopback1

   o  Next-hop: Loopback1

   o  Color Community: Color to distinguish between paths between ABR
      Node 3 and PE Node 1

   o  SID: SID A (i.e., An END.DT46 SID instantiation on PE Node 1.
      This instruction causes a packet to be forwarded using the main IP
      forwarding table, not a VPN forwarding table.)

   Now, ABR Node 3 resolves its route to Loopback1 through its IGP route
   to Loopback1.  Therefore, when forwarding traffic bound for
   Loopback1, it imposes an SRv6 header that includes:

   o  SIDS associated with the IGP route to Loopback1

   o  SID A

   ABR Node 3 advertises Loopback1 to PE Node 5 using BGP-LU as follows:

   o  Prefix: Loopback1

   o  Next-hop: Loopback3

   o  Color Community: Color to distinguish between paths between ABR
      Node 3 and PE Node 5

   o  MPLS Label: A binding label that represents the SRv6 path between
      ABR Node 3 and PE Node 5

   Now, PE Node 5 resolves its route to Loopback1 through its IGP route
   to Loopback3.  Therefore, when forwarding traffic bound for
   Loopback1, it imposes:

   o  An SR-MPLS label stack associated with the IGP route to ABR3

   o  An additional label representing a binding SID.  The binding SID
      maps to the SRv6 path between ABR Node 3 and PE Node 5




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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.  Acknowledgements

   Thanks to Melchior Aelmans, Takuya Miyasaka and Jeff Tantsura for
   their comments.

9.  References

9.1.  Normative References

   [I-D.dawra-bess-srv6-services]
              Dawra, G., Filsfils, C., Brissette, P., Agrawal, S.,
              Leddy, J., Voyer, D., Bernier, D., Steinberg, D., Raszuk,
              R., Decraene, B., Matsushima, S., Zhuang, S., and J.
              Rabadan, "SRv6 BGP based Overlay services", draft-dawra-
              bess-srv6-services-02 (work in progress), July 2019.

   [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>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [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>.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <https://www.rfc-editor.org/info/rfc4456>.




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   [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>.

   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
              <https://www.rfc-editor.org/info/rfc8277>.

   [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>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [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>.

9.2.  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.
              Contreras, "Seamless SR Problem Statement", draft-hegde-
              spring-mpls-seamless-sr-05 (work in progress), February
              2021.

   [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>.



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Authors' Addresses

   Shraddha Hegde
   Juniper Networks
   Embassy Business Park
   Bangalore, KA  560093
   India

   Email: shraddha@juniper.net


   Parag Kaneriya
   Juniper Networks
   Elnath-Exora Business Park Survey
   Bangalore, Karnataka  560103
   India

   Email: pkaneria@juniper.net


   Ron Bonica
   Juniper Networks
   Herndon, Virginia  20171
   USA

   Email: rbonica@juniper.net


   Peng Shaofu
   ZTE Corporation
   Peoples Republic of China

   Email: peng.shaofu@zte.com.cn


   Greg Mirsky
   ZTE Corporation
   USA

   Email: gregimirsky@gmail.com


   Zheng Zhang
   ZTE Corporation
   Peoples Republic of China

   Email: zhang.zheng@zte.com.cn




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   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com














































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