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SRv6 and MPLS interworking
draft-agrawal-spring-srv6-mpls-interworking-14

Document Type Active Internet-Draft (individual)
Authors Swadesh Agrawal , Clarence Filsfils , Daniel Voyer , Gaurav Dawra , Zhenbin Li , Shraddha Hegde
Last updated 2024-03-04
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draft-agrawal-spring-srv6-mpls-interworking-14
SPRING                                                   S. Agrawal, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                           Cisco Systems
Expires: 5 September 2024                                       D. Voyer
                                                             Bell Canada
                                                                G. Dawra
                                                                LinkedIn
                                                                   Z. Li
                                                     Huawei Technologies
                                                                S. Hegde
                                                        Juniper Networks
                                                            4 March 2024

                       SRv6 and MPLS interworking
             draft-agrawal-spring-srv6-mpls-interworking-14

Abstract

   This document describes SRv6 and MPLS/SR-MPLS interworking and co-
   existence procedures.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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 5 September 2024.

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Copyright Notice

   Copyright (c) 2024 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 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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Interworking(IW) scenarios  . . . . . . . . . . . . . . . . .   3
     2.1.  IW scenarios  . . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Transport IW  . . . . . . . . . . . . . . . . . . . .   4
       2.1.2.  Service IW  . . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  SRv6 SID behavior . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  End.DTM . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  End.DPM . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  SRv6 Policy Headend Behaviors . . . . . . . . . . . . . . . .   8
     5.1.  H.Encaps.M: H.Encaps applied to MPLS label stack  . . . .   8
     5.2.  H.Encaps.M.Red: H.Encaps.Red applied to MPLS label
           stack . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Interconnecting Binding SIDs  . . . . . . . . . . . . . . . .   8
   7.  Interworking Procedures . . . . . . . . . . . . . . . . . . .   8
     7.1.  Transport IW  . . . . . . . . . . . . . . . . . . . . . .   9
       7.1.1.  SR-PCE multi-domain On Demand Nexthop . . . . . . . .   9
       7.1.2.  BGP inter domain routing procedures . . . . . . . . .  12
     7.2.  Service IW  . . . . . . . . . . . . . . . . . . . . . . .  17
       7.2.1.  Gateway Interworking  . . . . . . . . . . . . . . . .  17
       7.2.2.  Translation between Service labels and SRv6 service
               SIDs  . . . . . . . . . . . . . . . . . . . . . . . .  18
   8.  Migration and co-existence  . . . . . . . . . . . . . . . . .  19
   9.  Availability  . . . . . . . . . . . . . . . . . . . . . . . .  20
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  SRv6 Endpoint Behaviors  . . . . . . . . . . . . . . . .  20
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  20
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  21

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     14.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   The incremental deployment of SRv6 into existing networks require
   SRv6 to interwork and co-exist with SR-MPLS/MPLS.  This document
   introduces interworking scenarios and building blocks for solutions
   to inter connect them.

   This document assumes SR-MPLS-IPv4 for MPLS domains but the design
   equally works for SR-MPLS-IPv6, LDP-IPv4/IPv6 and RSVP-TE-MPLS label
   binding protocols.

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.  Interworking(IW) scenarios

   A multi-domain network (Figure 1) can be generalized as a central
   domain C with many leaf domains around it.  Specifically, the
   document looks at a service flow from an ingress PE in an ingress
   leaf domain (LI), through the C domain and up to an egress PE of the
   egress leaf domain (LE).  Each domain runs its own IGP instance.  A
   domain has a single data plane type applicable both for its overlay
   and its underlay.

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          +-----+                +-----+  RD:V/v via 10   +-----+
   .......|S-RR1|<...............|S-RR2|<.................|S-RR3| <..
   :      +-----+                +-----+                  +-----+   :
   :                                                                :
   :                                                                :
+--:-------------------+----------------------+---------------------:-+
|  :      | 2 |        |        | 5 |         |         | 8 |       : |
|  :      +---+        |        +---+         |         +---+       : |
|  :                   |                      |                     : |
|  :                   |                      |                     : |
|  :                   |                      |                     : |
|----+    IGP1       +---+        IGP2      +---+      IGP3      +----|
| 1  |               | 4 |                  | 7 |                | 10 |
|----+               +---+                  +---+                +----|
|                      |                      |                       |
|                      |                      |                       |
|                      |                      |                       |
|         +---+        |        +---+         |         +---+         |
|         | 3 |        |        | 6 |         |         | 9 |         |
+----------------------+----------------------+-----------------------+
iPE                   iBR                    eBR                     ePE

<----------LI---------><----------C----------><-----------LE---------->

          Figure 1: Reference multi-domain network topology

   There are various SRv6 and SR-MPLS-IPv4 interworking scenarios
   possible.

   Below scenarios cover various cascading of SRv6 and MPLS networks,
   e.g., SR-MPLS-IPv4 <-> SRv6 <-> SR-MPLS-IPv4 <-> SRv6 <-> SR-MPLS-
   IPv4 etc, though not all combinations are described for brevity.

2.1.  IW scenarios

2.1.1.  Transport IW

   Provider edge devices run MPLS based [RFC4364] or SRv6 Service SID
   based [RFC9252] BGP L3(e.g.VPN) or L2(e.g.EVPN) services through
   service Route Reflectors.  Service endpoint signaling through borders
   routers and corresponding forwarding state provide interworking over
   intermediate transport domain.

   *  SRv6 over MPLS (6oM)

      -  LI and LE domains are SRv6 data plane, C is MPLS data plane

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      -  L3/L2 BGP SRv6 services [RFC9252] extend between PEs.  The
         ingress PE encapsulates the payload in an outer IPv6 header
         where the SRv6 Service SID is the last segment or destination
         address(DA).

      -  Transport IW border nodes forward SRv6 encapsulated traffic
         destined to egress PE over MPLS C domain.

   *  MPLS over SRv6 (Mo6)

      -  LI and LE domains are MPLS data plane, C is SRv6 data plane

      -  L3/L2 BGP MPLS services [RFC4364], [RFC7432].  The ingress PE
         encapsulates the payload in an MPLS service label and sends it
         through MPLS LSP to egress PE.

      -  Transport IW nodes forward encapsulated label stack to egress
         PE over SRv6 C domain.

   Note: Easiest and most probable deployment is ships in the night i.e.
   supporting dual stack and IPv4 MPLS in each domain.

2.1.2.  Service IW

   L3/L2 service signaling discontinuity i.e. SRv6 service SID based PE
   interworks with BGP MPLS based PE for service connectivity.  L3/L2
   service BGP signaling and forwarding state provide interworking over
   intermediate domain.

   *  SRv6 to MPLS(6toM): The ingress PE encapsulates the payload in an
      outer IPv6 header where the destination address is the SRv6
      Service SID[RFC9252].  Payload is delivered to egress PE with MPLS
      service label[RFC4364] that it advertised with service prefixes.

   *  MPLS to SRv6 (Mto6): The ingress PE encapsulates the payload in an
      MPLS service label.  Payload is delivered to egress PE with IPv6
      header with destination address as SRv6 service SID that it
      advertised with service prefixes.

3.  Terminology

   The following terms used within this document are defined in
   [RFC8402]: Segment Routing, SR-MPLS, SRv6, SR Domain, Segment ID
   (SID), SRv6 SID, Prefix-SID.

   Domain: Without loss of the generality, domain is assumed to be
   instantiated by a single IGP instance or a network within IGP if
   there is clear separation of data plane.

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   Node k has a classic IPv6 loopback address Ak::1/128.

   A SID at node k with locator block B and function F is represented by
   B:k:F::

   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.

   (SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with:

   IPv6 header with source address SA, destination addresses DA and SRH
   as next-header

   SRH with SID list <S1, S2, S3> with SegmentsLeft = SL

   Note the difference between the <> and () symbols: <S1, S2, S3>
   represents a SID list where S1 is the first SID and S3 is the last
   SID to traverse.  (S3, S2, S1; SL) represents the same SID list but
   encoded in the SRH format where the rightmost SID in the SRH is the
   first SID and the leftmost SID in the SRH is the last SID.  When
   referring to an SR policy in a high-level use-case, it is simpler to
   use the <S1, S2, S3> notation.  When referring to an illustration of
   the detailed packet behavior, the (S3, S2, S1; SL) notation is more
   convenient.

4.  SRv6 SID behavior

   This document introduces a new SRv6 SID behavior.  This behavior is
   executed on border routers between the SRv6 and MPLS domain.

4.1.  End.DTM

   The "Endpoint with decapsulation and MPLS table lookup" behavior.

   The End.DTM SID MUST be the last segment in a SR Policy, and a SID
   instance is associated with an MPLS table.

   When N receives a packet destined to S and S is a local End.DTM SID,
   N does:

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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.   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.DTM SID, N does:

S01. If (Upper-Layer Header type == 137(MPLS) ) {
S02.    Remove the outer IPv6 Header with all its extension headers
S03.    Set the packet's associated FIB table to T
S04.    Submit the packet to the MPLS FIB lookup for
        transmission according to the lookup result.
S05. } Else {
S06.    Process as per [ietf-spring-srv6-network-programming] section 4.1.1
S07. }

4.2.  End.DPM

   The "Endpoint with decapsulation and MPLS label push" behavior.

   The End.DPM SID MUST be the last segment and a SID instance is
   associated with label stack.

   When N receives a packet destined to S and S is a local End.DPM SID,
   N does:

   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.   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.DPM SID, N does:

   S01. Remove the outer IPv6 Header with all its extension headers
   S02. Push the MPLS label stack associated with S
   S03. Submit the packet to the MPLS engine for transmission

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5.  SRv6 Policy Headend Behaviors

5.1.  H.Encaps.M: H.Encaps applied to MPLS label stack

   The H.Encaps.M behavior encapsulates a received MPLS Label stack
   [RFC3032] packet in an IPv6 header with an SRH.  Together MPLS label
   stack and its payload becomes the payload of the new IPv6 packet.
   The Next Header field of the SRH MUST be set to 137 [RFC4023].

5.2.  H.Encaps.M.Red: H.Encaps.Red applied to MPLS label stack

   The H.Encaps.M.Red behavior is an optimization of the H.Encaps.M
   behavior.  H.Encaps.M.Red reduces the length of the SRH by excluding
   the first SID in the SRH of the pushed IPv6 header.  The first SID is
   only placed in the Destination Address field of the pushed IPv6
   header.  The push of the SRH MAY be omitted when the SRv6 Policy only
   contains one segment and there is no need to use any flag, tag or
   TLV.  In such case, the Next Header field of the IPv6 header MUST be
   set to 137 [RFC4023].

6.  Interconnecting Binding SIDs

   Binding Segment (BSID) is bound to SR policy [RFC8402].  Further an
   SR-MPLS label can be bound to an SRv6 Policy and an SRv6 SID can be
   bound to an SR-MPLS Policy.  The IW SR-PCE solution Section 7.1.1
   leverage these BSIDs as segments of SR policy on headend domain to
   represent intermediate domain of different dataplane type.  In
   summary, an intermediate domain of different data plane type is
   represented by BSID of ingress domain data plane type in SID list.

7.  Interworking Procedures

   Figure 1 shows reference multi-domain network topology and Section 2
   its description.  The procedure in this section are illustrated using
   the topology.

   Following is assumed for data plane support of various nodes:

   *  Nodes 2,3,5,6,8,9 are provider(P) routers which need to support
      single data plane type.

   *  1 and 10 are PEs.  They support single data plane type in overlay
      and underlay.

   *  Border routers 4 and 7 need to support both the SRv6 and SR-MPLS-
      IPv4 data plane.

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   A VPN route is advertised via service RRs (S-RR) between an egress
   PE(node 10) and an ingress PE (node 1).

   For illustrations, the SRGB range starts from 16000 and prefix SID of
   a node is 16000 plus node number

7.1.  Transport IW

   As described in Section 2.1.1, transport IW requires:

   *  For 6oM, tunnel traffic destined to SRv6 Service SID of egress PE
      over MPLS C domain.

   *  For Mo6, tunnel MPLS label stack bound to IPv4 loopback address of
      egress PE over SRv6 C domain.

   This draft enhances two well-known solutions to achieve above:

   *  An SR-PCE [RFC8664] multi-domain On Demand Next-hop (ODN) SR
      policy [RFC9256] stitching end to end across different data plane
      domains using interconnecting binding SIDs.  These procedures can
      be used when overlay prefixes are signaled with a color extended
      community [RFC9012].

   *  BGP Inter-Domain routing procedures advertising PE locator or IPv4
      Loopback address for best effort or intent aware end to end
      connectivity.

7.1.1.  SR-PCE multi-domain On Demand Nexthop

   This procedure provides a best-effort path as well as a path that
   satisfies the intent (e.g. low latency), across multiple domains.
   Service routes (VPN/EVPN) are received on ingress PE with color
   extended community from egress PE.  A Color is a 32-bit numerical
   value that associates an SR Policy with an intent [RFC9256].  Ingress
   PE does not know how to compute the traffic engineered path through
   the multi-domain network to egress PE and requests SR-PCE for it.
   The SR-PCE is aware of interworking requirement at border nodes as
   its fed with BGP-LS topological information from each domain.  It
   programs intermediate domain data plane specific policy on border
   nodes for the given intent and represents it in end to end path SID
   list on ingress PE leveraging Section 6.

   Below sections describe 6oM and Mo6 IW with SR-PCE

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7.1.1.1.  6oM

   Service prefix (e.g.  VPN or EVPN) is received on head-end (node 1)
   with color extended community (C1) from egress PE (node 10) with SRv6
   service SID.  The PCE computes (C1,10) path via node 2, 5 and 8.  It
   programs an SR policy at border node 4 with segment list node 5 and 7
   bounded to an End.BM BSID [RFC8986].  SR-PCE responds back to node 1
   with SRv6 segments along required SLA including End.BM at node 4 to
   traverse SR-MPLS-IPv4 C domain.

   For example, SR-PCE create SR-MPLS policy (C1,7) at node 4 with
   segments <16005,16007>.  It is bound to End.BM behavior with SRv6
   BSID as B:4:BM-C1-7::

   The data plane operations for the above-mentioned interworking
   example are:

   *  Node 1 performs SRv6 function H.Encaps.Red with VPN service SID
      and SRv6 Policy (C1,10):

      Packet leaving node 1 IPv6 ((A:1::, B:2:E::) (B:10::DT4, B:8:E::,
      B:4:BM-C1-7:: ; SL=3))

   *  Node 2 performs End function

      Packet leaving node 2 IPv6 ((A:1::, B:4:BM-C1-7::) (B:10::DT4,
      B:8:E::, B:4:BM-C1-7:: ; SL=2))

   *  Node 4(border rout4er) performs End.BM function

      Packet leaving node 4 MPLS (16005,16007,2)((A:1::, B:8:E::)
      (B:10::DT4, B:8:E::, B:4:BM-C1-7-:: ; SL=1)).

   *  Node 7 performs a native IPv6 lookup on due PHP behavior for 16007

      Packet leaving node 7 IPv6 ((A:1::, B:8:E::) (B:10::DT4, B:8:E::,
      B:4:BM-C1-7:: ; SL=1))

   *  Node 8 performs End(PSP) function

      Packet leaving node 8 IPv6 ((A:1::, B:10::DT4))

   *  Node 10 performs End.DT function and lookups IP in VRF and send
      traffic to CE.

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7.1.1.2.  Mo6

   Refer Section 2.1.1 for Mo6 scenario.  MPLS Service prefix (e.g.  VPN
   or EVPN) is received on head-end(node 1) with color extended
   community(C1) from egress PE(node 10).  The PCE computes color-aware
   C1 path via node 2, 5 and 8.  It programs a SRv6 policy bound to MPLS
   BSID at border node 4 with SRv6 segment list along required color-
   aware path with last segment of behavior End.DTM Section 4.1.  SR-PCE
   responds back to node 1 with MPLS segment list including MPLS BSID of
   SRv6 policy at node 4 to traverse SRv6 core domain.

   For example, SR-PCE create SRv6 policy (C1,7) at node 4 with segments
   <B:5:E::,B:7:DTM::>.  It is bound to MPLS BSID 24407.

   The data plan operations for the above-mentioned interworking example
   are:

   1.  Node 1 performs MPLS label stack encapsulation with VPN label and
       SR-MPLS Policy (C1,10):

       Packet leaving node 1 towards 2 (Note: PHP of node 2 prefix SID):
       MPLS packet (16004,24407,16008,16010,vpn_label)

   2.  Node 2 forwards traffic towards 4 (PHP of 16004)

       Packet leaving node 2 MPLS packet (24407,16008,16010,vpn_label)

   3.  Node 4 steers MPLS traffic into SRv6 policy bound to 24407

       Packet leaving node 4 IPv6(A:4::, B:5:E::) (B:7:DTM:: ;
       SL=1)NH=137) MPLS((16008,16010,vpn_label)

   4.  Node 7 receive IPv6 packet with DA=B:7:DTM::. It performs DTM
       behavior to remove IPv6 header and perform 16008 lookup in MPLS
       table.

       Packet leaves node 7 towards node 8(PHP of 16008) MPLS packet
       (16010,vpn_label)

   5.  Node 8 forwards traffic towards 10 (PHP of 16010)

       Packet leaving node 8 MPLS packet (vpn_label)

   6.  Node 10 performs vpn_label lookup and send traffic to CE.

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7.1.2.  BGP inter domain routing procedures

   Procedures described below build upon BGP 3107
   [I-D.ietf-mpls-seamless-mpls] and [RFC4798] to advertise transport
   reachability for PE IPv4 loopbacks or SRv6 locators across a multi-
   domain network.  The procedures leverage existing SAFIs ( for
   example, BGP-LU(1/4, 2/4)) and IPv6 (2/1)).  Nexthop self on border
   routers provide independence of intra domain tunnel technology in
   different domains.

   The sections below describe 6oM and Mo6 IW with BGP procedures for
   best effort paths to a locator or loopback prefix.  The procedures
   are equally applicable to intent aware paths, i.e., locator assigned
   for a given intent, for instance from an IGP-FlexAlgo.  They are also
   applicable to color-aware routes [I-D.ietf-idr-bgp-car] recursing
   over intent aware intra-domain paths.

7.1.2.1.  6oM

   Refer Section 2.1.1 for 6oM scenario.  SRv6 based L3/L2 BGP services
   are signaled with SRv6 Service SID allocated from egress PE locator
   prefix and with no BGP Color Extended community.  Ingress PE learns
   the service routes and need to resolve SRv6 Service SID over egress
   PE locator or its summary.  Below describes propagation of locators
   or its summary to create end to end underlay path.

   *  Egress border router learns LE domain PE locators through IGP.
      These are redistributed in BGP like any IPv6 global prefixes.
      Alternatively, locator is originated by egress PE in the BGP IPv6
      unicast address family (AFI=2,SAFI=1) to border nodes.

   *  Egress border router advertise these locators in BGP IPv6 LU
      [AFI=2/SAFI=4] with locally allocated label or explicit NULL to
      ingress border router with IPv4 next hop.  Next hop has SR MPLS
      IPv4 LSP paths built in C domain.  Note: Egress border router may
      advertise summary prefix covering all PE locators in LE domain.

   *  Ingress border router advertise these remote locators in LI
      domain.  Options to advertise are:

      -  To ingress PE in BGP address family (AFI=2,SAFI=1) with SRv6
         transport SID of local End behavior in Prefix-SID attribute TLV
         type 5 [RFC9252].  This option will result in additional SRv6
         encapsulation at ingress PE.

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      -  Alternatively, leak remote locators or their summary in LI IGP
         or propagate using BGP hop by hop routing model to each of P
         and PE routers through infrastructure route reflector.  This
         option will avoid additional SRv6 transport SID encapsulation.

   *  Ingress PE learn remote locators or their summary with or without
      SRv6 SID encapsulation.  When learnt with SRv6 SID, it builds the
      packet encapsulation that contains the SRv6 Service SID and SRv6
      transport SID in the SID list and tunnels traffic to ingress
      border node in LI domain (P routers (node 2 and 3) does not need
      remote prefix).  When learnt without SRv6 SID, the packet
      encapsulation contains the SRv6 Service SID and forwarded hop by
      hop based on remote locator IPv6 prefix lookup.

   Control plane example:

   1.  Routing Protocol(RP) @10:

       *  In ISIS advertise locator B:10::/48

       *  BGP AFI=1,SAFI=128 originates a VPN route RD:V/v via B:10::1
          and Prefix-SID attribute B:10:DT4::. This route is advertised
          to service RR.

   2.  RP @ 7:

       *  ISIS redistribute B:10::/48 into BGP

       *  BGP Originates B:10::/48 in AFI=2/SAFI=4 with next hop node 7
          and label explicit null among border routers.

   3.  RP @ 4:

       *  BGP learns B:10::/48 with next hop node 7 and outgoing label.

       *  BGP advertise B:10::/48 in AFI=2/SAFI=1 with next hop B:4::1
          and Prefix-SID attribute tlv type 5 carrying local End
          behavior function B:4:END:: to node 1

       *  Alternatively, BGP redistributes remote locator or summary
          route in LI domain IGP.

   4.  RP @ 1:

       *  BGP learns B:10::/48 via B:4::1 and Prefix-SID attribute TLV
          type 5 with SRv6 SID B:4:END::

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       *  Alternatively, B:10::/48 or summary route reachability is
          learned through ISIS

       *  BGP AFI=1, SAFI=128 learn service prefix RD:V/v, next hop
          B:10::1 and PrefixSID attribute TLV type 5 with SRv6 SID
          B:10:DT4

   FIB state

@1: IPv4 VRF V/v => H.Encaps.red <B:4:END::, B:10:DT4::> with SRH, SRH.NH=IPv4
@4: IPv6 Table: B:4:END:: => Update DA with B:10:DT4::,set IPv6.NH=IPv4, pop the SRH
@4: IPv6 Table: B:10::/48 => push MPLS label 2 (Explicit NULL), push MPLS Label 16007
@7: MPLS label 2 => pop and lookup next IPv6 DA
@7: IPv6 Table B:10::/48 => forward via ISIS path to 10
@10: IPv6 Table B:10:DT4:: => pop the outer header and lookup the inner IPv4 DA in the VRF

7.1.2.2.  Mo6

   Refer Section 2.1.1 for Mo6 scenario.  MPLS based L3/L2 BGP services
   are signaled with IPv4 next-hop of egress PE through Service RRs with
   no color extended community.  Ingress PE need labelled reachability
   to remote PE IPv4 loopback address advertised as next hop with
   service routes.

   BGP LU [RFC8277] advertise IPv4 PE loopbacks.  Next hop self is
   performed on the border routers.

   Following are options and protocol extensions to tunnel IPv4 PE
   loopback LSP through SRv6 C domain

7.1.2.2.1.  Tunnel BGP LU LSP across SRv6 C domain

   Intuitive solution for an MPLS-minded operator

   *  Existing BGP-LU label cross-connect on border routers for each PE
      IPv4 loopback address.

   *  The lookups at the ingress border router are based on BGP3107
      label as usual

   *  Just the SR-MPLS IGP label to next hop is replaced by an IPv6
      tunnel with DA = SRv6 SID associated with DTM behavior in C
      domain.

   *  Ingress border router forwarding perform 3107 label swap and
      H.Encaps.M with DA = SRv6 SID associated with DTM behavior

   *  Similar to MPLS-over-IP

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   Existing BGP LU updates between border routers need to signal SRv6
   SID associated with DTM behavior.
   [I-D.agrawal-bess-bgp-srv6-mpls-interworking] proposes "SRv6 tunnel
   for label route" TLV of the BGP Prefix-SID Attribute to signal SRv6
   SID to tunnel MPLS packet with label in NLRI at the top of its label
   stack through SRv6/IPv6 domain.  Below describes the control plane
   and corresponding FIB state to achieve such tunneling:

   Control plane example

   1.  Routing Protocol(RP) @10:

       *  ISIS originates its IPv4 PE loopback with Node SID 16010

       *  BGP AFI=1,SAFI=4 originate IPv4 loopback address with next hop
          node 10 and optionally label index=10 in Label-Index TLV of
          Prefix-SID attribute.

       *  BGP AFI=1, SAFI=128 originates a VPN route RD:V/v next hop
          node 10.  This route is advertised to service RR.

   2.  RP @ 7:

       *  ISIS v6, advertise locator B:7::/48 in C domain

       *  BGP learns node 10 IPv4 loopback address with outgoing label.
          It allocates local label (based on label index if present) and
          programs label swap to outgoing label and MPLS LSP to next
          hop.

       *  BGP AFI=1, SAFI=4 advertise IPv4 loopback address of node 10
          to node 4.  NLRI label is set to local label and SRv6 SID
          B:7:DTM:: carried in SRv6 SID Information Sub-TLV of "SRv6
          tunnel for label route" TLV in Prefix-Sid attribute.  If
          received, label index=10 in Label-Index TLV of Prefix-SID
          attribute is also signaled.

   3.  RP @ 4:

       *  ISIS v4 originates its IPv4 loopback with prefix SID 16004 in
          LI domain.

       *  BGP learns node10 IPv4 loopback address from node 7 with
          outgoing label.  It allocate local label (based on label index
          if present) and programs label swap and H.Encaps.M.red with
          IPv6 header destination address as SRv6 SID received in "SRv6
          tunnel for label route" TLV of Prefix-Sid attribute i.e.
          B:7:DTM::.

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       *  BGP AFI=1, SAFI=4 advertise IPv4 Loopback address of node 10
          to node 1.  NLRI label is set to local label and do not signal
          "SRv6 tunnel for label route" TLV in Prefix-SID attribute.

   4.  RP @ 1:

       *  BGP learns IPv4 loopback address of node 10 from node 4 with
          outgoing label.  It programs route to push outgoing label and
          MPLS LSP to next hop i.e. node 4

       *  BGP AFI=1, SAFI=128 learn service prefix RD:V/v, next hop IPv4
          loopback address of node 10 and service label.

   Forwarding state at different nodes:

@1: IPv4 VRF: V/v => out label=vpn_label, next hop=IPv4 address of node 10
@1: IPv4 table: IPv4 address of node 10 => out label=16010, next hop=node4
@1: IPv4 table: IPv4 address of node 4 => out label=16004, next hop=interface to reach 2
@4: MPLS Table: 16010 => out label=16010, H.Encaps.M.red with DA=B:7:DTM::
@4: IPv6 table: B:7::/48 => next hop=interface to reach 5
@7: SRv6 My SID table: B:7:DTM:: => decaps IPv6 header and lookup top label.
@7: MPLS table: 16010 => out label=16010, next hop=interface to reach 8
@10: MPLS table: vpn label => pop label and lookup the inner IPv4 DA in the VRF

   During transition when MPLS data plane is still enabled in C domain,
   an ABR that does not understand "SRv6 tunnel for label route" TLV in
   BGP Prefix-SID Attribute or based on operator configured local policy
   can continue MPLS encapsulation using label in NLRI and LSP to next
   hop.

7.1.2.2.2.  Label and SRv6 SID translation per BGP LU route

   For each PE IPv4 loopback address, existing BGP 3107 label cross-
   connect on area border router is replaced by label to SRv6 SID cross-
   connect or vice versa.  In effect, it creates a translation between
   from 3107 label to SRv6 SID at ingress of SRv6 domain and SRv6 SID to
   3107 label on egress.

   *  For each BGP LU route (IPv4 loopback address of PE) received from
      LE domain on egress border router, allocate SRv6 SID of DPM
      behavior bound to the PE address.  Lookup of SRv6 SID result in
      decapsulation of IPv6 header and push of BGP LU outgoing label and
      MPLS LSP to next hop.

   *  Advertise BGP route to PE address with SRv6 SID to ingress border
      router.

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   *  Ingress border router allocate local label and advertise to LI
      domain.

   *  The lookups at the ingress border router are based on BGP 3107
      label as usual.  Lookup results SRv6 SID of DPM behavior signaled
      by egress border node.  Decap BGP3107 label and perform H.Encaps.M
      with DA = SRv6 SID.

   Section 2.2 of [I-D.agrawal-bess-bgp-srv6-mpls-interworking]
   describes how existing BGP advertisement can signal SRv6 SID
   associated with DPM behavior from egress to ingress border router.

7.2.  Service IW

   As described in Section 2.1.2 Service IW need BGP SRv6 based L2/L3 PE
   interworking with BGP MPLS based L2/L3 PE.

   There are a number of different ways of handling this scenario as
   detailed below.

7.2.1.  Gateway Interworking

   Gateway is router which supports both BGP SRv6 based L2/L3 services
   and BGP MPLS based L2/L3 services for a service instance (e.g.  L3
   VRF, EVPN EVI).  It terminates service encapsulation and perform L2/
   L3 destination lookup in service instance.

   *  A border router between SRv6 domain and SR-MPLS-IPv4 domain is
      suitable for Gateway role.

   *  Transport reachability to SRv6 PE and gateway locators in SRv6
      domain or MPLS LSP to PE/gateway IPv4 Loopbacks can be exchanged
      in IGP or through mechanism detailed in Section 2.1.1.

   *  Gateway exchange BGP L2/L3 service prefix with SRv6 based Service
      PEs via set of service RRs.  This session will learn/advertise L3/
      L2 service prefixes with SRv6 service SID in prefix SID attribute
      [RFC9252].

   *  Gateway exchange BGP L2/L3 service prefix with MPLS based Service
      PEs via set of distinct service RRs.  This session will learn/
      advertise L3/L2 service prefixes with service labels [RFC4364]
      [RFC7432].

   *  L2/L3 prefix received from a domain is locally installed in
      service instance and re advertised to other domain with modified
      service encapsulation information.

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   *  Prefix learned with SRv6 service SID from SRv6 PE is installed in
      service instance with instruction to perform H.Encaps.  It is
      advertised to MPLS service PE with service label.  When gateway
      receives traffic with service label from MPLS service PE, it
      perform destination lookup in service instance.  Lookup result in
      instruction to perform H.Encaps with DA being SRv6 Service SID
      learnt with prefix from SRv6 PE.

   *  Prefix learned with MPLS service label from MPLS service PE is
      installed in service instance with instruction to perform service
      label encapsulation and send to MPLS LSP to nexthop.  It is
      advertised to SRv6 service PE with SRv6 service SID of behavior
      (e.g.  DT4/DT6/DT2U) [RFC8986].  When gateway receives traffic
      with SRv6 Service SID as DA of IPv6 header from SRv6 service PE,
      it perform destination lookup in service instance after decaps of
      IPv6 header.  Lookup result in instruction to push service label
      and send it to nexthop.

        +-------------------+                             +-------------------+
        |   ....|S-RR|....  |                             |  ....|S-RR|.....  |
        |   :   +----+   :  |                             |  :   +----+    :  |
        |   :            :  |                             |  :             :  |
        |----+          +-------------------------------------+          +----|
        |PE1 |          |             IW border node          |          | PE2|
        |----+          | VPN Label<->L2/L3 lookup<->SRv6 SID |          +----|
        |               |                                     |               |
        |               +-------------------------------------+               |
        |      MPLS         |                             |       SRv6        |
        +-------------------+                             +-------------------+
        <------MPLS VPN----->                             <------SRv6 VPN----->

                         Figure 2: Gateway IW

   Couple of border routers can act as gateway for redundancy.  It can
   scale horizontally by distributing service instance among them.

7.2.2.  Translation between Service labels and SRv6 service SIDs

   This is similar to inter-as option B procedures described in
   [RFC4364] just that service label cross-connect on border router is
   replaced with service label to SRv6 service SID or vice verse
   translation on IW node.

   *  IW node does not need service instance like VRF or EVI.

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   *  IW node exchange BGP L2/L3 service prefix with SRv6 based Service
      PEs via set of service RRs.  This BGP session will learn/advertise
      L3/L2 service prefixes with SRv6 service SID in prefix SID
      attribute [RFC9252].

   *  IW node exchange BGP L2/L3 service prefix with MPLS based service
      PEs via set of distinct service RRs.  This BGP session will learn/
      advertise L3/L2 service prefixes with service labels [RFC4364]
      [RFC7432].

   *  IW node allocates SRv6 SID of behavior End.DPM that result in
      pushing service label and MPLS label stack to service nexthop for
      BGP L2/L3 service learnt from MPLS PE.  It advertises the service
      to SRv6 domain.

   *  IW node allocates service label that results in H.Encaps with IPv6
      header DA set to SRv6 SID signaled in BGP L2/L3 service learnt
      from SRv6 PE.  Advertises the service to MPLS domain with
      allocated service label.

        +-------------------+                             +-------------------+
        |   ....|S-RR|....  |                             |  ....|S-RR|.....  |
        |   :   +----+   :  |                             |  :   +----+    :  |
        |   :            :  |                             |  :             :  |
        |----+          +-------------------------------------+          +----|
        |PE1 |          |             IW border node          |          | PE2|
        |----+          |          VPN Label -> SRv6 SID      |          +----|
        |               | VPN label, LSP PE1 <- SRv6 SID      |               |
        |               +-------------------------------------+               |
        |      MPLS         |                             |       SRv6        |
        +-------------------+                             +-------------------+
        <------MPLS VPN----->                             <------SRv6 VPN----->

                    Figure 3: Service translation

   Certain L2 service specific information (eg. control word)
   translation is out of the scope.  It will be covered in separate
   document.

8.  Migration and co-existence

   In addition, the draft also addresses migration and coexistence of
   the SRv6 and SR-MPLS-IPv4.  Co-existence means a network that
   supports both SRv6 and MPLS in a given domain.  This may be a
   transient state when brownfield SR-MPLS-IPv4 network upgrades to SRv6
   (migration) or permanent state when some devices are not capable of
   SRv6 but supports native IPv6 and SR-MPLS-IPv4.

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   These procedures would be detailed in a future revision

9.  Availability

   *  Failure within domain are taken care by existing FRR mechanisms
      [I-D.ietf-rtgwg-segment-routing-ti-lfa].

   *  Procedures listed in [RFC9256] provides protection in SR-PCE
      multi-domain On Demand Nexthop (ODN) SR policy based approach.

   *  Convergence on failure of border routers can be achieved by well
      known methods for BGP inter domain routing approach:

      -  BGP Add Path provide diverse path visibility

      -  BGP backup path pre-programming

      -  Sub-second convergence on border router failure notified by
         local IGP.

10.  IANA Considerations

10.1.  SRv6 Endpoint Behaviors

   This document introduces a new SRv6 Endpoint behaviors "End.DTM" and
   "End.DPM".  IANA is requested to assign identifier value in the "SRv6
   Endpoint Behaviors" sub-registry under "Segment Routing Parameters"
   registry.

   +-------------+--------+-------------------------+------------------+
   | Value       |  Hex   |    Endpoint behavior    |    Reference     |
   +-------------+--------+-------------------------+------------------+
   | TBD         |  TBD   |    End.DTM              | <this document>  |
   +-------------+--------+-------------------------+------------------+
   | TBD         |  TBD   |    End.DPM              | <this document>  |
   +-------------+--------+-------------------------+------------------+

11.  Security Considerations

12.  Contributors

   Zafar Ali
   Cisco Systems
   Email: zali@cisco.com

   Srihari Sangli
   Juniper Networks
   Email: ssangli@juniper.net

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13.  Acknowledgements

   The authors would like to acknowledge Kamran Raza, Dhananjaya Rao,
   Stephane Litkowski, Pablo Camarillo, Ketan Talaulikar, Jorge Rabadan
   for their comments and review.

14.  References

14.1.  Normative References

   [I-D.agrawal-bess-bgp-srv6-mpls-interworking]
              Agrawal, S., Rao, D., Ali, Z., Filsfils, C., Voyer, D.,
              and Z. Li, "BGP extensions for SRv6 and MPLS
              interworking", Work in Progress, Internet-Draft, draft-
              agrawal-bess-bgp-srv6-mpls-interworking-00, 24 October
              2022, <https://datatracker.ietf.org/doc/html/draft-
              agrawal-bess-bgp-srv6-mpls-interworking-00>.

   [I-D.ietf-idr-bgp-car]
              Rao, D., Agrawal, S., and Co-authors, "BGP Color-Aware
              Routing (CAR)", Work in Progress, Internet-Draft, draft-
              ietf-idr-bgp-car-06, 2 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
              car-06>.

   [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>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
              "Encapsulating MPLS in IP or Generic Routing Encapsulation
              (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
              <https://www.rfc-editor.org/info/rfc4023>.

   [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>.

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   [RFC4798]  De Clercq, J., Ooms, D., Prevost, S., and F. Le Faucheur,
              "Connecting IPv6 Islands over IPv4 MPLS Using IPv6
              Provider Edge Routers (6PE)", RFC 4798,
              DOI 10.17487/RFC4798, February 2007,
              <https://www.rfc-editor.org/info/rfc4798>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [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>.

   [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>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [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>.

   [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>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

14.2.  Informative References

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   [I-D.ietf-mpls-seamless-mpls]
              Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
              M., and D. Steinberg, "Seamless MPLS Architecture", Work
              in Progress, Internet-Draft, draft-ietf-mpls-seamless-
              mpls-07, 28 June 2014,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
              seamless-mpls-07>.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
              Decraene, B., and D. Voyer, "Topology Independent Fast
              Reroute using Segment Routing", Work in Progress,
              Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
              13, 16 January 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
              segment-routing-ti-lfa-13>.

   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,
              <https://www.rfc-editor.org/info/rfc9012>.

Authors' Addresses

   Swadesh Agrawal (editor)
   Cisco Systems
   Email: swaagraw@cisco.com

   Clarence Filsfils
   Cisco Systems
   Email: cfilsfil@cisco.com

   Daniel Voyer
   Bell Canada
   Canada
   Email: daniel.voyer@bell.ca

   Gaurav dawra
   LinkedIn
   United States of America
   Email: gdawra.ietf@gmail.com

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   Zhenbin Li
   Huawei Technologies
   China
   Email: lizhenbin@huawei.com

   Shraddha Hegde
   Juniper Networks
   Email: shraddha@juniper.net

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