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

        SR Replication Segment for Multi-point Service Delivery


   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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Replication Segment . . . . . . . . . . . . . . . . . . . . .   3
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Illustration of a Replication Segment  . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   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).
   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.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

   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 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

   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.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.

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

4.  IANA Considerations

   This document makes no request of IANA.

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


   Kurtis Gillis
   Bell Canada


   Arvind Venkateswaran
   Cisco Systems, Inc.
   San Jose


   Zafar Ali
   Cisco Systems, Inc.

   Swadesh Agrawal
   Cisco Systems, Inc.
   San Jose


   Jayant Kotalwar
   Mountain View


   Tanmoy Kundu
   Mountain View


   Andrew Stone


   Tarek Saad
   Juniper Networks

8.  References

8.1.  Normative References

              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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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, <>.

8.2.  Informative References

              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
              Zhang, "Segment Routing Point-to-Multipoint Policy",
              draft-voyer-pim-sr-p2mp-policy-02 (work in progress), July

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <>.

   [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, <>.

   [RFC7988]  Rosen, E., Ed., Subramanian, K., and Z. Zhang, "Ingress
              Replication Tunnels in Multicast VPN", RFC 7988,
              DOI 10.17487/RFC7988, October 2016,

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.voyer-pim-sr-p2mp-policy].

   Consider the following topology:

                                 /         \
                                 \         /

                                 Figure 1

   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 to R6.  R2 performs CONTINUE operation on N-SID6 and
      forwards it to R3.  R3 is the penultimate hop for N-SID6; it
      performs penultimate hop popping, which corresponds to the NEXT
      operation and the packet is then sent to R6 with <R-SID6> 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 for
      N-SID4; it performs penultimate hop popping, which corresponds to
      the NEXT operation and the packet is then sent to R4 with
      <A-SID47, R-SID1> in the label stack.  R4 performs NEXT operation,
      pops A-SID47, and delivers packet to R7 with <R-SID7> in the label
      stack.  R7, as Leaf, performs NEXT operation, pops R-SID7 label
      and delivers the payload.

Authors' Addresses

   Daniel Voyer (editor)
   Bell Canada


   Clarence Filsfils
   Cisco Systems, Inc.


   Rishabh Parekh
   Cisco Systems, Inc.
   San Jose

   Hooman Bidgoli


   Zhaohui Zhang
   Juniper Networks