Network Working Group                                      D. Voyer, Ed.
Internet-Draft                                               Bell Canada
Intended status: Standards Track                             C. Filsfils
Expires: May 1, 2, 2021                                           R. Parekh
                                                     Cisco Systems, Inc.
                                                              H. Bidgoli
                                                                   Nokia
                                                                Z. Zhang
                                                        Juniper Networks
                                                        October 28, 29, 2020

        SR Replication Segment for Multi-point Service Delivery
              draft-ietf-spring-sr-replication-segment-01
              draft-ietf-spring-sr-replication-segment-02

Abstract

   This document describes the SR Replication segment for Multi-point
   service delivery.  A SR Replication segment allows a packet to be
   replicated from a replication node to downstream nodes.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 1, 2, 2021.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Replication Segment . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  SRv6  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  End.Replicate: Replicate and/or Decapsulate . . . . .   6
       2.1.2.  H.Encaps.Replicate: SR Headend encapsulation in
               Replication Segment . . . . . . . . . . . . . . . . .   7
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   7   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8   5
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10   7
   Appendix A.  Illustration of a Replication Segment  . . . . . . .  11
     A.1.  SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.2.  SRv6  . . . . . . . . . . . . . . . . . . . . . . . . . .  13   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15   9

1.  Introduction

   We define a new type of segment for Segment Routing [RFC8402], called
   Replication segment, which allows a node (henceforth called as
   Replication Node) to replicate packets to a set of other nodes
   (called Downstream Nodes) in a Segment Routing Domain.  Replication
   segments provide building blocks for Point-to-Multipoint Service
   delivery via SR Point-to-Multipoint (SR P2MP) policy.  A Replication
   segment can replicate packet to directly connected nodes or to
   downstream nodes (without need for state on the transit routers).
   Replication segments apply equally to both SR-MPLS and SRv6
   instantiations of Segment Routing.
   This document focuses on the Replication Segment building block.  The
   use of one or more stitched Replication Segments constructed for SR
   P2MP Policy tree is specified in [I-D.ietf-pim-sr-p2mp-policy]. [I-D.voyer-pim-sr-p2mp-policy].

2.  Replication Segment

   In a Segment Routing Domain, a Replication segment is a logical
   construct which connects a Replication Node to a set of Downstream
   Nodes.  A Replication segment is a local segment instantiated at a
   Replication node.  It can be either provisioned locally on a node or
   programmed by a PCE.  Replication segments apply equally to both SR-
   MPLS and SRv6 instantiations of Segment Routing.

   A Replication segment is identified by the tuple <Replication-ID,
   Node-ID>, where:

   o  Replication-ID: An identifier for a Replication segment that is
      unique in context of the Replication Node.

   o  Node-ID: The address of the Replication Node that the Replication
      segment is for.  Note that the root of a multi-point service is
      also a replication node.

   In simplest case, Replication-ID can be a 32-bit number, but it can
   be extended or modified as required based on specific use of a
   Replication segment.  When the PCE signals a Replication segment to
   its node, the <Replication-ID, Node-ID> tuple identifies the segment.
   Examples of such signaling and extension are described in
   [I-D.ietf-pim-sr-p2mp-policy].
   [I-D.voyer-pim-sr-p2mp-policy].

   A Replication segment includes the following elements:

   o  Replication SID: The Segment Identifier of a Replication segment.
      This is a SR-MPLS label or a SRv6 SID [RFC8402].

   o  Downstream Nodes: Set of nodes in Segment Routing domain to which
      a packet is replicated by the Replication segment.

   o  Replication State: See below.

   The Downstream Nodes and Replication State of a Replication segment
   can change over time, depending on the network state and leaf nodes
   of a multi-point service that the segment is part of.

   Replication State is a list of replication branches to the Downstream
   Nodes.  In this document, each branch is abstracted to a <Downstream
   Node, Downstream Replication SID> tuple.  A Downstream Node is
   represented by a SID-list or a Segment Routing Policy
   [I-D.ietf-spring-segment-routing-policy] that specifies the explicit
   path from the Replication Node to the Downstream Node, or even
   represented by another Replication segment.  The SID-list MAY just
   have one SID.  If a downstream node is adjacent to a Replication
   node, it MAY also be represented by an interface.

   Replication SID identifies the Replication segment in the forwarding
   plane.  At a Replication node, the Replication SID is the equivalent
   of Binding SID [I-D.ietf-spring-segment-routing-policy] of a Segment
   Routing Policy.

   A packet steered into a Replication segment at a Replication node is
   replicated to each Downstream Node with the Downstream Replication
   SID that is relevant at that node.  A packet is steered into a
   Replication Segment in two ways:

   o  When the Active Segment [RFC8402] is the Replication SID.  In this
      case, the operation is NEXT followed by a PUSH for a replicated
      copy. copy is CONTINUE.

   o  On the root of a multi-point service, based on local policy-based
      routing.  In this case, the operation for a replicated copy is
      PUSH.

   If a Downstream Node is an egress (aka leaf) of the multi-point
   service, i.e. no further replication is needed, then that leaf node's
   Replication segment will not have any Replication State and the
   operation is NEXT.  At an egress node, the Replication SID MAY be
   used to identify that portion of the multi-point service.  Notice
   that the segment on the leaf node is still referred to as a
   Replication segment for the purpose of generalization.

   A node can be a bud node, i.e. it is a replication node and a leaf
   node of a multi-point service at the same time
   [I-D.ietf-pim-sr-p2mp-policy].
   [I-D.voyer-pim-sr-p2mp-policy].  In this case, the Replication
   segment's Replication State includes a branch with the Downstream
   Node being itself and the operation for the replicated copy is NEXT.

   The Replication SID MUST be the last SID (at the bottom of stack for
   SR-MPLS) in a packet that is steered out from a Replication node of a
   Replication Segment.  The behavior at Downstream nodes of a
   Replication Segment is undefined If there are any SIDs after the
   Replication SID and is outside the scope of this document.

2.1.  SRv6

   SRv6 network programming [I-D.ietf-spring-srv6-network-programming]
   introduces concept of functions.  A function defines local behavior
   on a node and is identified by opaque function part of a SRv6 SID.
   Familiarity with SRv6 Network Programming is expected.

3.  Use Cases

   In SRv6, the simplest use case, a single Replication Segment can be realized by defining segment includes the
   root node of a SRv6
   Segment Endpoint behavior for replication.  End.Replicate is an
   Endpoint function for replicating packets and, if required,
   decapsulation multi-point service and processing the egress/leaf nodes of next header.  This function is bound
   to a local SRv6 Replication SID at the Replication Node and
   the service as all the Downstream Nodes of a Nodes.  This achieves Ingress
   Replication segment.  FUNCT part of a [RFC7988] that has been widely used for MVPN [RFC6513]
   and EVPN [RFC7432] BUM (Broadcast, Unknown and Multicast) traffic.

   Replication SID segments can represent both replication function also be used as well the
   Replication State of a specific Replication Segment, or the
   Replication state MAY be represented by ARG part of Replication SID.
   For example, assuming two Replication Segments, RS1 and RS2 at a
   node, the node can bind two functions 0x00F1 and 0x00F2 (F=16, A=0)
   to End.Replicate function on Replication Segments RS1 and RS2
   respectively.  The node can also choose to bind one function 0x00FA
   with End.Replicate and ARGs 0x0001 and 0x0002 (F=16, A=16) to RS1 and
   RS2 respectively.

   A Replication Node will replicate packet matching local SRv6
   Replication SID to all Downstream Nodes.  Each building blocks for
   replication is
   equivalent to pushing segment list of an SRv6 policy to a Downstream
   Node, If there is only one SID, the Downstream trees when Replication SID and
   there is no need to use any Flag, Tag or TLV, segments on the SRH MAY be omitted root, intermediate
   replication nodes and the Downstream Replication SID is set as IPv6 DA in replicated
   copy of packet.  In this case, the LOC part of routed Downstream
   Replication SID takes packet from Replication Node leaf nodes are stitched together to the Downstream
   Node.  If an SRH achieve
   efficient replication.  That is inserted in a replicated copy of packet, the
   Downstream Replication SID MUST be the last Segment specified in SRH i.e at
   Segment List index 0.

   If a Downstream Node is an egress (aka leaf) of the multi-point
   service, i.e.
   [I-D.voyer-pim-sr-p2mp-policy].

4.  IANA Considerations

   This document makes no further replication is needed, then that leaf node's
   Replication segment will not have any Replication State and the
   operation on packet wtih local Replication SID is decapsulation with
   processing of next header equivalent to End.DT46.

   A bud node performs both the replication and decapsulation part of
   End.Replicate function on a packet with local Replication SID.

   H.Encaps.Replicate is behavior on the root request of a multipoint service to
   steer a packet into a SRv6 Replication Segment. IANA.

5.  Security Considerations of SRv6 Small SID/Compresion SID for SRv6 Replication
   SID will be addressed in future revision of

   There are no additional security risks introduced by this document.

2.1.1.  End.Replicate: Replicate and/or Decapsulate

   The "Endpoint with replication and/or decapsulate behavior
   (End.Replicate for short) is variant of End behavior.

   We define a generic Replicate function on a packet for Replication
   State (RS).

S01. Replicate(RS, packet)
S02. {
S03.    For each Replication R with Downstream Replication SID, R-SID {
S04.       Make copy of packet
S05.       If (NumSID(R)== 1) {
S06.          Set IPv6 DA = R-SID
S07.          Set NH-Header in copy to Next-Header value of packet
S08.       } Else {
S09.         Insert SRH with R-SID at SID List[0] followed by other SIDS
S10.         Set NH-Header of SRH to Next-Header value of packet
S10.         Set IPv6 DA = First SID of R
S11.         Set NH-Header in copy to SRH
S12.       }
S13.       Submit the packet to the egress IPv6 FIB lookup and
           transmission to the new destination
S14. }

   When N receives a packet whose IPv6 DA is S and S is a local
   End.Replicate SID, N does:

S01.   Lookup FUNCT OR (FUNCT,ARG) portion of S to get Replication State RS
S02.   Call Replicate(RS, packet)
S03.   If NH==SRH and SL != 0 {
S04.      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.
S05.   } Else If "decap check" success: {
S06.      Process packet according to End.DT46 behavior in SRv6 Network Programming
S07.   } Else {
S08.      Drop packet
S09.   }

   Notes:
   The "decap check" would succeed on egress or bud node.  The SRv6
   Replication SID is bound to a specific tenant table at these nodes.

2.1.2.  H.Encaps.Replicate: SR Headend encapsulation in Replication
        Segment

   Node N receives two packets P1=(A, B2) and P2=(A,B2)(B3, B2, B1;
   SL=1).  B2 is neither a local address nor SID of N.

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

   N steers the transit packets P1 and P2 into an SRv6 Replication
   Segment, R, with a Source Address T and Replication State RS..

   The H.Encaps.Replicate encapsulation behavior is defined as follows:

   S01.   Push an IPv6 header
   S02.   Set outer IPv6 SA = T
   S03.   Set outer Payload Length, Traffic Class, Hop Limit and
             Flow Label fields
   S04.   Set the outer Next-Header value
   S05.   Decrement inner IPv6 Hop Limit or IPv4 TTL
   S06.   Call Replicate(RS, Outer packet)

   After the H.Encaps behavior, assuming a directly adjacent Downstream
   Node with Downstream Replication SID, D-RSID, P1' and P2'
   respectively look like:

   - (T, D-RSID) (A, B2)

   - (T, D-RSID) (A, B2) (B3, B2, B1; SL=1)

   After the H.Encaps behavior, assuming a non-adjacent Downstream Node
   with Downstream Replication SID, D-RSID and a Segment list <S1, S2>
   to reach Downstream Node, P1' and P2' respectively look like:

   - (T, S1) (D-RSID, S2, S1; SL=2) (A, B2)

   - (T, S1) (D-RSID, S2, S1; SL=2) (A, B2) (B3, B2, B1; SL=1)

3.  Use Cases

   In the simplest use case, a single Replication segment includes the
   root node of a multi-point service and the egress/leaf nodes of the
   the service as all the Downstream Nodes.  This achieves Ingress
   Replication [RFC7988] that has been widely used for MVPN [RFC6513]
   and EVPN [RFC7432] BUM (Broadcast, Unknown and Multicast) traffic.

   Replication segments can also be used as building blocks for
   replication trees when Replication segments on the root, intermediate
   replication nodes and leaf nodes are stitched together to achieve
   efficient replication.  That is specified in
   [I-D.ietf-pim-sr-p2mp-policy].

4.  IANA Considerations

   This document requires registration of End.Replicate behavior in
   "SRv6 Endpoint Behaviors" sub-registry of "Segment Routing
   Parameters" top-level registry.

           +-------+-----+------------------------+-----------+
           | Value | Hex |   Endpoint behavior    | Reference |
           +-------+-----+------------------------+-----------+
           | TBD   | TBD |     End.Replicate      | [This.ID] |
           | TBD   | TBD | End.Replicate with ARG | [This.ID] |
           +-------+-----+------------------------+-----------+

                  Table 1: IETF - SRv6 Endpoint Behaviors

5.  Security Considerations

   There are no additional security risks introduced by this design.

6.  Acknowledgements

   The authors would like to acknowledge Siva Sivabalan, Mike Koldychev,
   Vishnu Pavan Beeram, Alexander Vainshtein, Bruno Decraene and Joel
   Halpern for their valuable inputs.

7.  Contributors

   Clayton Hassen
   Bell Canada
   Vancouver
   Canada

   Email: clayton.hassen@bell.ca

   Kurtis Gillis
   Bell Canada
   Halifax
   Canada

   Email: kurtis.gillis@bell.ca

   Arvind Venkateswaran
   Cisco Systems, Inc.
   San Jose
   US

   Email: arvvenka@cisco.com

   Zafar Ali
   Cisco Systems, Inc.
   US

   Email: zali@cisco.com

   Swadesh Agrawal
   Cisco Systems, Inc.
   San Jose
   US

   Email: swaagraw@cisco.com

   Jayant Kotalwar
   Nokia
   Mountain View
   US

   Email: jayant.kotalwar@nokia.com

   Tanmoy Kundu
   Nokia
   Mountain View
   US

   Email: tanmoy.kundu@nokia.com

   Andrew Stone
   Nokia
   Ottawa
   Canada

   Email: andrew.stone@nokia.com

   Tarek Saad
   Juniper Networks
   Canada

   Email:tsaad@juniper.net

8.  References

8.1.  Normative References

   [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-08 (work in progress),
              July 2020.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming-24 (work in
              progress), October 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>.

   [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>.

8.2.  Informative References

   [I-D.filsfils-spring-srv6-net-pgm-illustration]
              Filsfils, C., Camarillo, P., Li, Z., Matsushima, S.,
              Decraene, B., Steinberg, D., Lebrun, D., Raszuk, R., and
              J. Leddy, "Illustrations for SRv6 Network Programming",
              draft-filsfils-spring-srv6-net-pgm-illustration-03 (work
              in progress), September 2020.

   [I-D.ietf-pim-sr-p2mp-policy]
              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
              Zhang, "Segment Routing Point-to-Multipoint Policy",
              draft-ietf-pim-sr-p2mp-policy-00 (work in progress), July
              2020.

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <https://www.rfc-editor.org/info/rfc6513>.

   [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>.

   [RFC7988]  Rosen, E., Ed., Subramanian, K., and Z. Zhang, "Ingress
              Replication Tunnels in Multicast VPN", RFC 7988,
              DOI 10.17487/RFC7988, October 2016,
              <https://www.rfc-editor.org/info/rfc7988>.

Appendix A.  Illustration of a Replication Segment

   This section illustrates an example of a single Replication Segment.
   Examples showing Replication Segment stitched together to form P2MP
   tree (based on SR P2MP policy) are in [I-D.ietf-pim-sr-p2mp-policy].

   Consider the following topology:

                                  R3------R6
                                 /         \
                         R1----R2----R5-----R7
                                 \         /
                                  +--R4---+

                                 Figure 1

A.1.  SR-MPLS

   In this example, the Node-SID of a node Rn is N-SIDn and Adjacency-
   SID from node Rm to node Rn is A-SIDmn.  Interface between Rm and Rn
   is Lmn.

   Assume a Replication Segment identified with R-ID at replication node
   R1 and downstream Nodes R2, R6 and R7.  The Replication SID at node n
   is R-SIDn.  A packet replicated from R1 to R7 has to traverse R4.

   The Replication Segment state at nodes R1, R2, R6 and R7 is shown
   below.  Note nodes R3, R4 and R5 do not have state for the
   Replication Segment.

   Replication Segment at R1:

   Replication Segment <R-ID,R1>:
    Replication SID: R-SID1
    Replication State:
      R2: <R-SID2->L12>
      R6: <N-SID6, R-SID6>
      R7: <N-SID4, A-SID47, R-SID7>

   Replication to R2 steers packet directly to R2 on interface L12.
   Replication to R6, using N-SID6, steers packet via IGP shortest path
   to that node.  Replication to R7 is steered via R4, using N-SID4 and
   then adjacency SID A-SID47 to R7.

   Replication Segment at R2:

   Replication Segment <R-ID,R2>:
    Replication SID: R-SID2
    Replication State:
      R2: <Leaf>

   Replication Segment at R6:

   Replication Segment <R-ID,R6>:
    Replication SID: R-SID6
    Replication State:
      R6: <Leaf>

   Replication Segment at R7:

   Replication Segment <R-ID,R7>:
    Replication SID: R-SID7
    Replication State:
      R7: <Leaf>

   When a packet is steered into the replication segment at R1:

   o  Since R1 is directly connected to R2, R1 performs PUSH operation
      with just <R-SID2> label for the replicated copy and sends it to
      R2 on interface L12.  R2, as Leaf, performs NEXT operation, pops
      R-SID2 label and delivers the payload.

   o  R1 performs PUSH operation with <N-SID6, R-SID6> label stack for
      the replicated copy to R6 and sends it to R2, the nexthop on IGP
      shortest path design.

6.  Acknowledgements

   The authors would like to R6.  R2 performs CONTINUE operation on N-SID6 acknowledge Siva Sivabalan, Mike Koldychev,
   Vishnu Pavan Beeram, Alexander Vainshtein, Bruno Decraene and
      forwards it to R3.  R3 is the penultimate hop Joel
   Halpern for N-SID6; it
      performs penultimate hop popping, which corresponds to the NEXT
      operation their valuable inputs.

7.  Contributors

   Clayton Hassen
   Bell Canada
   Vancouver
   Canada

   Email: clayton.hassen@bell.ca

   Kurtis Gillis
   Bell Canada
   Halifax
   Canada

   Email: kurtis.gillis@bell.ca

   Arvind Venkateswaran
   Cisco Systems, Inc.
   San Jose
   US

   Email: arvvenka@cisco.com

   Zafar Ali
   Cisco Systems, Inc.
   US

   Email: zali@cisco.com
   Swadesh Agrawal
   Cisco Systems, Inc.
   San Jose
   US

   Email: swaagraw@cisco.com

   Jayant Kotalwar
   Nokia
   Mountain View
   US

   Email: jayant.kotalwar@nokia.com

   Tanmoy Kundu
   Nokia
   Mountain View
   US

   Email: tanmoy.kundu@nokia.com

   Andrew Stone
   Nokia
   Ottawa
   Canada

   Email: andrew.stone@nokia.com

   Tarek Saad
   Juniper Networks
   Canada

   Email:tsaad@juniper.net

8.  References

8.1.  Normative References

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and the packet is then sent to R6 with <R-SID6>
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-08 (work in the
      label stack.  R6, as Leaf, performs NEXT operation, pops R-SID6
      label and delivers the payload.

   o  R1 performs PUSH operation with <N-SID4, A-SID47, R-SID7> label
      stack for the replicated copy to R7 and sends it to R2, the
      nexthop on IGP shortest path to R4.  R2 is the penultimate hop progress),
              July 2020.

   [RFC2119]  Bradner, S., "Key words for
      N-SID4; it performs penultimate hop popping, which corresponds use in RFCs to
      the NEXT operation Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and the packet is then sent to R4 with
      <A-SID47, R-SID1> in the label stack.  R4 performs NEXT operation,
      pops A-SID47, R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

8.2.  Informative References

   [I-D.voyer-pim-sr-p2mp-policy]
              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and delivers packet to R7 with <R-SID7> Z.
              Zhang, "Segment Routing Point-to-Multipoint Policy",
              draft-voyer-pim-sr-p2mp-policy-02 (work in the label
      stack.  R7, as Leaf, performs NEXT operation, pops R-SID7 label progress), July
              2020.

   [RFC6513]  Rosen, E., Ed. and delivers the payload.

A.2.  SRv6

   For SRv6 , we use SID allocation scheme, reproduced below, from
   Illustrations for SRv6 Network Programming
   [I-D.filsfils-spring-srv6-net-pgm-illustration]

      2001:db8::/32 is an IPv6 block allocated by a RIR to the operator

      2001:db8:0::/48 is dedicated to the internal address space

      2001:db8:cccc::/48 is dedicated to the internal SRv6 SID space

      We assume a location expressed R. Aggarwal, Ed., "Multicast in 64 bits MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <https://www.rfc-editor.org/info/rfc6513>.

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

   [RFC7988]  Rosen, E., Ed., Subramanian, K., and Z. Zhang, "Ingress
              Replication Tunnels in 16 bits

      Node k has Multicast VPN", RFC 7988,
              DOI 10.17487/RFC7988, October 2016,
              <https://www.rfc-editor.org/info/rfc7988>.

Appendix A.  Illustration of a classic IPv6 loopback address 2001:db8::k/128 which
      is advertised in the IGP

      Node k has 2001:db8:cccc:k::/64 for its local SID space.  Its SIDs
      will be explicitly assigned from that block

      Node k advertises 2001:db8:cccc:k::/64 Replication Segment

   This section illustrates an example of a single Replication Segment.
   Examples showing Replication Segment stitched together to form P2MP
   tree (based on SR P2MP policy) are in its IGP

      Function :1:: (function 1, for short) represents [I-D.voyer-pim-sr-p2mp-policy].

   Consider the End function
      with PSP support

      Function :Cn:: (function Cn, for short) represents following topology:

                                  R3------R6
                                 /         \
                         R1----R2----R5-----R7
                                 \         /
                                  +--R4---+

                                 Figure 1

   In this example, the End.X
      function from to Node n

   Each Node-SID of a node k has:

      An explicit SID instantiation 2001:db8:cccc:k:1::/128 bound to an
      End function with additional support for PSP

      An explicit SID instantiation 2001:db8:cccc:k:Cj::/128 bound to an
      End.X function to neighbor J with additional support for PSP

      An explicit Rn is N-SIDn and Adjacency-
   SID instantiation 2001:db8:cccc:k:Fk::/128 bound from node Rm to an
      End.Replcate function node Rn is A-SIDmn.  Interface between Rm and Rn
   is Lmn.

   Assume a Replication Segment identified with R-ID at replication node
   R1 and downstream Nodes R2, R6 and R7.  The Replication SID at node
   k, bound to an End.Replcate function, n
   is 2001:db8:cccc:k:Fk::/128
   with ARG value 0. R-SIDn.  A packet replicated from R1 to R7 has to traverse R4.

   The Replication Segment state at nodes R1, R2, R6 and R7 is shown
   below.  Note nodes R3, R4 and R5 do not have state for the
   Replication Segment.

   Replication Segment at R1:

   Replication Segment <R-ID,R1>:
    Replication SID: 2001:db8:cccc:1:F1::0 R-SID1
    Replication State:
      R2: <2001:db8:cccc:2:F2::0->L12> <R-SID2->L12>
      R6: <2001:db8:cccc:6:F6::0> <N-SID6, R-SID6>
      R7: <2001:db8:cccc:4:C7::0, 2001:db8:cccc:7:F7::0> <N-SID4, A-SID47, R-SID7>

   Replication to R2 steers packet directly to R2 on interface L12.
   Replication to R6, using 2001:db8:cccc:6:F6::0, N-SID6, steers packet via IGP shortest path
   to that node.  Replication to R7 is steered via R4, using End.X N-SID4 and
   then adjacency SID 2001:db8:cccc:4:C7::0 at R4 A-sID47 to R7.

   Replication Segment at R2:

   Replication Segment <R-ID,R2>:
    Replication SID: 2001:db8:cccc:2:F2::0 R-SID2
    Replication State:
      R2: <Leaf>

   Replication Segment at R6:

   Replication Segment <R-ID,R6>:
    Replication SID: 2001:db8:cccc:6:F6::0 R-SID6
    Replication State:
      R6: <Leaf>

   Replication Segment at R7:

   Replication Segment <R-ID,R7>:
    Replication SID: 2001:db8:cccc:7:F7::0 R-SID7
    Replication State:
      R7: <Leaf>

   At R1, a H.Encaps.Replicate behavior is associated with the
   replication segment.

   When a packet, (A,B2), packet is steered into the replication segment at R1:

   o  Since R1 is directly connected to R2, R1 creates encapsulated performs PUSH operation
      with just <R-SID2> label for the replicated copy (2001:db8::1, 2001:db8:cccc:2:F2::0) (A, B2), and sends it to
      R2 on interface L12.  R2, as Leaf, executes
      decapsulation operation of End.Replicate, removes outer IPv6
      header performs NEXT operation, pops
      R-SID2 label and delivers the payload.

   o  R1 creates encapsulated performs PUSH operation with <N-SID6, R-SID6> label stack for
      the replicated copy (2001:db8::1,
      2001:db8:cccc:6:F6::0) (A, B2) then forwards to R6 and sends it to R2, the resulting packet nexthop on the IGP
      shortest path to 2001:db8:cccc:6::/64. R6.  R2 performs CONTINUE operation on N-SID6 and
      forwards it to R3.  R3 forward is the penultimate hop for N-SID6; it
      performs penultimate hop popping, which corresponds to the NEXT
      operation and the packet using 2001:db8:cccc:6::/64. is then sent to R6 with <R-SID6> in the
      label stack.  R6, as Leaf, executes
      decapsulation operation of End.Replicate, removes outer IPv6
      header performs NEXT operation, pops R-SID6
      label and delivers the payload.

   o  R1 created encapsulated performs PUSH operation with <N-SID4, A-SID47, R-SID7> label
      stack for the replicated copy (2001:db8::1,
      2001:db8:cccc:4:C7::0) (2001:db8:cccc:7:F7::0; SL=1) (A, B2) to R7 and sends it to R2, the
      nexthop on IGP shortest path to
      2001:db8:cccc:4::/64. R4.  R2 forwards is the penultimate hop for
      N-SID4; it performs penultimate hop popping, which corresponds to
      the NEXT operation and the packet is then sent to R4 using
      2001:db8:cccc:4::/64. with
      <A-SID47, R-SID1> in the label stack.  R4 executes End.X function on
      2001:db8:cccc:4:C7::0, performs PSP action, removes SRH NEXT operation,
      pops A-SID47, and sends
      resulting delivers packet (2001:db8::1, 2001:db8:cccc:7:F7::0) (A, B2) to
      R4. R7 with <R-SID7> in the label
      stack.  R7, as Leaf, executes decapsulation operation of
      End.Replicate, removes outer IPv6 header performs NEXT operation, pops R-SID7 label
      and delivers the payload.

Authors' Addresses

   Daniel Voyer (editor)
   Bell Canada
   Montreal
   CA

   Email: daniel.voyer@bell.ca

   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com

   Rishabh Parekh
   Cisco Systems, Inc.
   San Jose
   US

   Email: riparekh@cisco.com
   Hooman Bidgoli
   Nokia
   Ottawa
   CA

   Email: hooman.bidgoli@nokia.com

   Zhaohui Zhang
   Juniper Networks

   Email: zzhang@juniper.net