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Versions: 00 01 02 03 04 05 06                                          
SPRING                                                   S. Agrawal, Ed.
Internet-Draft                                                    Z. Ali
Intended status: Standards Track                             C. Filsfils
Expires: February 23, 2022                                 Cisco Systems
                                                                D. Voyer
                                                             Bell Canada
                                                                G. Dawra
                                                                LinkedIn
                                                                   Z. Li
                                                     Huawei Technologies
                                                         August 22, 2021


                       SRv6 and MPLS interworking
             draft-agrawal-spring-srv6-mpls-interworking-06

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 February 23, 2022.








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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.  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
   5.  SRv6 Policy Headend Behaviors . . . . . . . . . . . . . . . .   7
     5.1.  H.Encaps.M: H.Encaps applied to MPLS label stack  . . . .   7
     5.2.  H.Encaps.M.Red: H.Encaps.Red applied to MPLS label stack    7
   6.  Interworking Procedures . . . . . . . . . . . . . . . . . . .   8
     6.1.  Transport IW  . . . . . . . . . . . . . . . . . . . . . .   8
       6.1.1.  SR-PCE multi-domain On Demand Nexthop . . . . . . . .   9
       6.1.2.  BGP inter domain routing procedures . . . . . . . . .  11
     6.2.  Service IW  . . . . . . . . . . . . . . . . . . . . . . .  16
       6.2.1.  Gateway Interworking  . . . . . . . . . . . . . . . .  16
       6.2.2.  Translation between Service labels and SRv6 service
               SID . . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Migration and co-existence  . . . . . . . . . . . . . . . . .  18
   8.  Availability  . . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  BGP Prefix-SID TLV Types registry . . . . . . . . . . . .  18
     9.2.  SRv6 Endpoint Behaviors . . . . . . . . . . . . . . . . .  18
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  19
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21



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1.  Introduction

   Many of the deployments require SRv6 insertion in the brownfield
   networks.  The incremental deployment of SRv6 into existing networks
   require SRv6 to interwork and co-exist with SR-MPLS/MPLS.  This
   document discusses solutions for the various interworking scenarios.

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, document
   look 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

   Document assumes SR-MPLS-IPv4 for MPLS data plane.  Note: Procedures
   in the document equally work for SR-MPLS-IPv6, LDP-IPv4/IPv6 and
   RSVP-TE-MPLS.

2.1.  IW scenarios

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

   Below scenarios cover various cascading of SRv6/MPLS network, e.g.,
   SR-MPLS-IPv4 <-> SRv6 <-> SR-MPLS-IPv4 <-> SRv6 <-> SR-MPLS-IPv4,
   etc.

2.1.1.  Transport IW

   L3/L2 service continuity over a different intermediate transport.

   o  SRv6 over SR-MPLS-IPv4 (6oM)

      *  LI and LE domains are SRv6 data plane, C is SR-MPLS-IPv4 data
         plane




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      *  L3/L2 BGP SRv6 services [I-D.ietf-bess-srv6-services] between
         PEs.  The ingress PE encapsulates the payload in an outer IPv6
         header where the destination address(DA) is the SRv6 Service
         SID.

      *  Tunnel traffic destined to egress PE SRv6 locator over SR-MPLS-
         IPv4 C domain.

   o  SR-MPLS-IPv4 over SRv6 (Mo6)

      *  LI and LE domains are SR-MPLS-IPv4 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
         MPLS LSP to next hop.

      *  Tunnel MPLS LSP to egress PE next hop over SRv6 C domain.

2.1.2.  Service IW

   Service discontinuity over a different intermediate transport i.e.
   L2/L3 BGP SRv6 PE interworking with L2/L3 BGP MPLS PE for service
   connectivity.

   o  SRv6 to SR-MPLS-IPv4 (6toM): The ingress PE encapsulates the
      payload in an outer IPv6 header where the destination address is
      the SRv6 Service SID[I-D.ietf-bess-srv6-services].  Payload is
      delivered to egress PE with MPLS service label[RFC4364] that it
      advertised with service prefixes.

   o  SR-MPLS-IPv4 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.

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




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   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. }

   Note: IANA has allocated the Internet Protocol number 137 [RFC4023]
   for MPLS-in-IP.

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].







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6.  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:

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

   o  1 and 10 are PEs.  They need to support single data plane type
      both for overlay and underlay.

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

   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

6.1.  Transport IW

   As described in Section 2.1.1, transport IW requires:

   o  Tunnel traffic destined to SRv6 Service SID bound to SRv6 locator
      of egress PE over SR-MPLS-IPv4 C domain.

   o  Tunnel MPLS LSP bound to IPv4 loopback address of egress PE over
      SRv6 C domain.

   This draft enhances two well-known solutions to achieve above
   tunneling: a controller(SR-PCE) and BGP inter domain routing based
   approach.  The SR-PCE based solution is applicable to both best
   effort as well as deployments where intents are required (e.g., On-
   Demand Next-hop like deployments scenarios) by L3/L2 services.  The
   BGP signaling covers the best effort case.

   Specifically, the draft proposes the following two ways:

   o  An SR-PCE [RFC8664] multi-domain On Demand Next-hop (ODN) SR
      policy [I-D.ietf-spring-segment-routing-policy] stitching end to
      end across different data plane domains.  These procedures can be
      used when overlay prefixes are signaled with a color extended
      community [I-D.ietf-idr-tunnel-encaps].




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   o  BGP Inter-Domain routing procedures advertising PE locator/IPv4
      Loopback address for best effort end to end connectivity.  These
      procedures can be used when overlay prefixes don't have color
      extended community.

6.1.1.  SR-PCE multi-domain On Demand Nexthop

   This procedure provides a best-effort as well as a path that
   satisfies the intent (e.g. low latency), across multiple domains.  A
   Color is a 32-bit numerical value that associates an SR Policy with
   an intent [I-D.ietf-spring-segment-routing-policy].  In this case,
   based on the intent, the PCE computes and programs end to end path
   using SR-Policy(C,PE).  The PCE is also aware of interworking
   requirement at border nodes, as each domain feeds topological
   information to the PCE through BGP LS feeds.  Intermediate domain of
   different data plane type is represented by Binding SID (BSID)
   [RFC8402] of ingress domain type in SID list.  In summary, an
   intermediate domain of different data plane is replaced by a BSID of
   the data plane nature of headend.

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

6.1.1.1.  6oM

   Refer Section 2.1.1 for 6oM scenario.  Service prefix (e.g.  VPN or
   EVPN) is received on head-end(node 1) with color extended
   community(C1) from egress PE(node 10) and SRv6 service SID.  Head-end
   does not know how to compute the traffic engineered path through the
   multi-domain network to node 10.  Node 1 requests SR-PCE to compute a
   path to node 10 providing intent (e.g. low latency).  The PCE
   computes low latency path via node 2, 5 and 8.  The PCE identifies
   the end-to-end path is not consistent data plane and kicks in
   interworking procedures at the border router(node 4).  It programs a
   SR policy with MPLS segment list at 4 along required SLA path(node 5
   and 7) bounded to an End.BM BSID
   [I-D.ietf-spring-srv6-network-programming].  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 described in the following:

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



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      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.

6.1.1.2.  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).  Head-end does not know how to
   compute the traffic engineered path through the multi-domain network
   to node 10.  Node 1 requests SR-PCE to compute a path to node 10
   providing intent(eg: low latency).  The PCE computes low latency path
   via node 2, 5 and 8.  The PCE identifies the end-to-end path is not
   consistent data plane and kicks in interworking procedures at the
   border router(node 4).  It programs a SRv6 policy bound to MPLS BSID
   at node 4 with SRv6 SID segment list along required SLA path with
   last segment of behavior End.DTM.  End.DTM behavior decapsulates the
   IPv6 header and looks up top MPLS label in MPLS table.  SR-PCE
   responds back to node 1 with MPLS segment list along required SLA
   path 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 described in the following:

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




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       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.

6.1.2.  BGP inter domain routing procedures

   BGP 3107 [I-D.ietf-mpls-seamless-mpls] like procedures to advertise
   PE locators and IPv4 loopbacks transport reachability in multi-domain
   network with next hop self on border routers.

   Below sections describe 6oM and Mo6 IW with BGP procedures

6.1.2.1.  6oM

   Refer Section 2.1.1 for 6oM scenario.  SRv6 based L3/L2 BGP services
   are signaled with SRv6 Service SID between PEs through Service RRs
   with no color extended community.  Ingress PEs need reachability to
   remote locator to send traffic to SRv6 service SID.

   o  Egress border router learns local PE locators through IGP.  These
      should be redistributed in BGP like any IPv6 global prefixes.
      Alternatively, locator is advertised by PE in the BGP ipv6 unicast
      address family (AFI=2,SAFI=1) to border nodes.

   o  Egress border router advertise LE domain PE locators in BGP IPv6
      LU[AFI=2/SAFI=4] with local label (explicit NULL) to ingress
      border router with IPv4 next hops.  These next hops have SR-MPLS-
      IPv4 LSP paths built in C domain.  It may advertise summary prefix
      covering all locators in LE domain.





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   o  If ingress border router advertise remote locators in LI domain to
      ingress PE in BGP address family (AFI=2,SAFI=1), it attaches local
      End behavior as SRv6 SID in Prefix-SID attribute TLV type 5
      [I-D.ietf-bess-srv6-services].  Alternatively, it may leak remote
      locators in LI IGP domain such that P routers also have
      reachability

   o  Ingress PE learn remote locator over BGP ipv6 address family
      AFI=2, SAFI=1 or through LI IGP.  When learnt through BGP, SRv6
      SID carried in Prefix-SID attribute TLV 5 tunnels traffic to
      ingress border node in LI domain as P routers(node 2 and 3) will
      not be aware of remote locator

   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::

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



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       *  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

6.1.2.2.  Mo6

   Refer Section 2.1.1 for Mo6 scenario.  MPLS based L3/L2 BGP services
   are signaled with IPv4 next-hop of 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-
   performed on border routers.

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

6.1.2.2.1.  Tunnel BGP LU LSP across SRv6 C domain

   Intuitive solution for an MPLS-minded operator

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

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

   o  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.

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

   o  Similar to MPLS-over-IP






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   Following section describes how existing BGP LU updates between
   border routers may carry SRv6 SID associated with DTM behavior to
   tunnel LSP across SRv6 C domain

6.1.2.2.1.1.  SRv6 label route tunnel TLV

   This document introduces a new TLV called "SRv6 label route tunnel"
   TLV of the BGP Prefix-SID Attribute to achieve signaling of SRv6 SIDs
   to tunnel MPLS packet with label in NLRI at the top of its label
   stack through SRv6/IPv6 domain.  Behavior which may be encoded but
   not limited to is End.DTM.  SRv6 label route tunnel TLV signals "AND"
   semantics i.e.  push label signaled in NLRI and perform H.Encaps.M
   with DA as SRv6 SID signaled in TLV.

   o  Reminder: RFC 8669 introduced Prefix-SID attribute with TLV type 1
      for label index and TLV type 3 for Originator SRGB for AFI=1/2 and
      SAFI 4 (BGP LU)

   o  This document extends the BGP Prefix-SID attribute [RFC8669] to
      carry new "SRv6 label route tunnel" TLV.  This document limits the
      usage of this new TLV to AFI=1/2 SAFI 4.  The usage of this TLV
      for other AFI/SAFI is out of scope of this document.

   o  "SRv6 label route tunnel" TLV is encoded exactly like SRv6 Service
      TLVs in Prefix-SID Attribute [I-D.ietf-bess-srv6-services] with
      following modification:

      1.  TLV Type (1 octet): This field is assigned values from the
          IANA registry "BGP Prefix-SID TLV Types".  It is set to 7 for
          "SRv6 label route tunnel" TLV.

      2.  No transposition scheme is allowed i.e. transposition length
          MUST be 0 in SRv6 SID Structure Sub-Sub-TLV

   o  Possibility of label encapsulation when dataplane has LSP to next
      hop irrespective of SRv6 SID signaled in "SRv6 label route tunnel"
      of Prefix-SID attribute.  This allows existing implementation to
      keep operating(legacy ingress border routers).

   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.



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       *  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
          label route tunnel" 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
          label route tunnel" TLV of Prefix-Sid attribute i.e.
          B:7:DTM::.

       *  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 label route tunnel" 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:






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@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


6.1.2.2.2.  Label and SRv6 SID cross connect for BGP LU route

   o  Allocate SRv6 SID associated with behavior that is decap variant
      of End.BM in [I-D.ietf-spring-srv6-network-programming] for each
      BGP LU route(IPv4 loopback address of PE) received from LE domain
      on egress border router

   o  Lookup of SRv6 SID result in decaps of IPv6 header and push of BGP
      LU outgoing label and MPLS LSP to next hop

   o  Advertise BGP LU route with SRv6 SID to ingress border router

   o  Ingress border router allocate local label and performs pop and
      H.Encaps.M.Red with DA=per PE SRv6 SID on receiving packet with
      local label

   BGP protocol extension will be detailed in future version.

6.2.  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.

6.2.1.  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.

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






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   o  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.

   o  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
      [I-D.ietf-bess-srv6-services].

   o  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].

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

   o  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.

   o  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) [I-D.ietf-spring-srv6-network-programming].
      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.

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

6.2.2.  Translation between Service labels and SRv6 service SID

   This is similar to inter-as option B control plane procedures
   described in [RFC4364].

   This would be described in future version of draft.






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7.  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.

   These procedures would be detailed in a future revision

8.  Availability

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

   o  Procedures listed in [I-D.ietf-spring-segment-routing-policy]
      provides protection in SR-PCE multi-domain On Demand Nexthop (ODN)
      SR policy based approach.

   o  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.

9.  IANA Considerations

9.1.  BGP Prefix-SID TLV Types registry

   This document introduce a new TLV Type of the BGP Prefix-SID
   attribute.  IANA is requested to assign Type value in the registry
   "BGP Prefix-SID TLV Types" as follows

       Value     Type                             Reference
       ----------------------------------------------------------
       TBD       SRv6 label route tunnel TLV     <this document>

9.2.  SRv6 Endpoint Behaviors

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




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

10.  Security Considerations

11.  Acknowledgements

   The authors would like to acknowledge Kamran Raza, Dhananjaya Rao,
   Stephane Litkowski, Pablo Camarillo, Ketan Talaulikar

12.  References

12.1.  Normative References

   [I-D.ietf-bess-srv6-services]
              Dawra, G., Filsfils, C., Talaulikar, K., Raszuk, R.,
              Decraene, B., Zhuang, S., and J. Rabadan, "SRv6 BGP based
              Overlay Services", draft-ietf-bess-srv6-services-07 (work
              in progress), April 2021.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-13 (work in progress),
              May 2021.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Garvia, P. C., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", draft-ietf-spring-srv6-
              network-programming-28 (work in progress), December 2020.

   [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>.







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   [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>.

   [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>.

   [RFC8669]  Previdi, S., Filsfils, C., Lindem, A., Ed., Sreekantiah,
              A., and H. Gredler, "Segment Routing Prefix Segment
              Identifier Extensions for BGP", RFC 8669,
              DOI 10.17487/RFC8669, December 2019,
              <https://www.rfc-editor.org/info/rfc8669>.

12.2.  Informative References

   [I-D.ietf-idr-tunnel-encaps]
              Patel, K., Velde, G. V. D., Sangli, S. R., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", draft-ietf-idr-
              tunnel-encaps-22 (work in progress), January 2021.






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   [I-D.ietf-mpls-seamless-mpls]
              Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
              M., and D. Steinberg, "Seamless MPLS Architecture", draft-
              ietf-mpls-seamless-mpls-07 (work in progress), June 2014.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
              Decraene, B., and D. Voyer, "Topology Independent Fast
              Reroute using Segment Routing", draft-ietf-rtgwg-segment-
              routing-ti-lfa-07 (work in progress), June 2021.

Authors' Addresses

   Swadesh Agrawal (editor)
   Cisco Systems

   Email: swaagraw@cisco.com


   Zafar ALI
   Cisco Systems

   Email: zali@cisco.com


   Clarence Filsfils
   Cisco Systems

   Email: cfilsfil@cisco.com


   Daniel Voyer
   Bell Canada
   Canada

   Email: daniel.voyer@bell.ca


   Gaurav dawra
   LinkedIn
   USA

   Email: gdawra.ietf@gmail.com








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   Zhenbin Li
   Huawei Technologies
   China

   Email: lizhenbin@huawei.com














































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