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Network Working Group                                      D. Voyer, Ed.
Internet-Draft                                               Bell Canada
Intended status: Standards Track                             C. Filsfils
Expires: January 28, 2021                                      R. Parekh
                                                     Cisco Systems, Inc.
                                                              H. Bidgoli
                                                                   Nokia
                                                                Z. Zhang
                                                        Juniper Networks
                                                           July 27, 2020


               Segment Routing Point-to-Multipoint Policy
                    draft-ietf-pim-sr-p2mp-policy-00

Abstract

   This document describes an architecture to construct a Point-to-
   Multipoint (P2MP) tree to deliver Multi-point services in a Segment
   Routing domain.  A SR P2MP tree is constructed by stitching a set of
   Replication segments together.  A SR Point-to-Multipoint (SR P2MP)
   Policy is used to define and instantiate a P2MP tree which is
   computed by a PCE.

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 January 28, 2021.





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Copyright Notice

   Copyright (c) 2020 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
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   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
   2.  P2MP Tree . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Sharing Replication segments across P2MP trees  . . . . .   4
   3.  SR P2MP Policy  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Using Controller to build a P2MP Tree . . . . . . . . . . . .   6
     4.1.  Provisioning SR P2MP Policy Creation  . . . . . . . . . .   6
       4.1.1.  API . . . . . . . . . . . . . . . . . . . . . . . . .   6
       4.1.2.  Invoking API  . . . . . . . . . . . . . . . . . . . .   7
     4.2.  P2MP Tree Computation . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Topology Discovery  . . . . . . . . . . . . . . . . .   8
       4.2.2.  Capability and Attribute Discovery  . . . . . . . . .   8
     4.3.  Instantiating P2MP tree on nodes  . . . . . . . . . . . .   8
       4.3.1.  PCEP  . . . . . . . . . . . . . . . . . . . . . . . .   8
       4.3.2.  BGP . . . . . . . . . . . . . . . . . . . . . . . . .   8
       4.3.3.  NetConf . . . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Protection  . . . . . . . . . . . . . . . . . . . . . . .   9
       4.4.1.  Local Protection  . . . . . . . . . . . . . . . . . .   9
       4.4.2.  Path Protection . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Illustration of SR P2MP Policy and P2MP Tree . . . .  11
     A.1.  P2MP Tree with non-adjacent Replication Segments  . . . .  12
     A.2.  P2MP Tree with adjacent Replication Segments  . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16





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1.  Introduction

   A Multi-point service delivery could be realized via P2MP trees in a
   Segment Routing domain [RFC8402].  A P2MP tree spans from a Root node
   to a set of Leaf nodes via intermediate Replication nodes.  It
   consists of a Replication segment
   [I-D.ietf-spring-sr-replication-segment] at the root node, one or
   more Replication segments at Leaf nodes and intermediate Replication
   nodes.  The Replication segments are stitched together.

   A Segment Routing P2MP policy, a variant of the SR Policy
   [I-D.ietf-spring-segment-routing-policy], is used to define a P2MP
   tree.  A PCE is used to compute the tree from the Root node to the
   set of Leaf nodes via a set of replication nodes.  The PCE then
   instantiates the P2MP tree in the SR domain by signaling Replication
   segments to Root, replication and Leaf nodes using various protocols
   (PCEP, BGP, NetConf etc.).

2.  P2MP Tree

   A P2MP tree in a SR domain connects a Root to a set of Leaf nodes via
   a set of intermediate Replication nodes.  It consists of a
   Replication segment at the root stitched to Replication segments at
   intermediate Replication nodes eventually reaching the Leaf nodes.

   The Replication SID of the Replication Segment at Root node is called
   Tree-SID.  The Tree-SID SHOULD also be used as Replication SID of
   Replication segments at Replication and Leaf nodes.  The Replication
   segments at Replication and Leaf nodes MAY use Replication SIDs that
   are not same as the Tree-SID.

   The Replication segment at Root of a P2MP tree MUST be associated
   with that P2MP tree (i.e. <Root, Tree-ID> identifier in SR P2MP
   policy section below) to map a Multi-point service to the tree.  A
   Replication segment that terminates a P2MP tree at a Leaf node MUST
   be associated with the P2MP tree to determine the context for a
   Multi-point service.  The The information that can be used to derive
   this association is specific to encoding of the protocol (PCEP, BGP,
   NetConf etc.) used to instantiate the Replication segment for a P2MP
   tree.  Replication segments at intermediate Replication nodes of a
   tree are also associated with that tree.

   A PCE MAY decide not instantiate Replication segments at Leaf nodes
   of a P2MP tree if it is known a priori that Multi-point services
   mapped to the P2MP tree can be identified using a context that is
   globally unique in SR domain.  Multi-point service contexts assigned
   from "Domain-wide Common Block" (DCB)
   [I-D.ietf-bess-mvpn-evpn-aggregation-label] are an example of such



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   globally unique contexts.  A Segment Routing Global Block (SRGB)
   [RFC8402] MAY be used to allocate globally unique Multi-point service
   contexts, but it is NOT RECOMMENDED to do so as the service contexts
   only need to be unique at service edge nodes.  In this case,
   Replication nodes connecting to Leaf nodes SHOULD use Penultimate-Hop
   Pop (PHP) behavior to pop Tree-SID from a packet.

   A packet steered into a P2MP tree is replicated by the Replication
   segment at Root node to each downstream node in the Replication
   segment, with the Replication SID of the Replication Segment at the
   downstream node.  A downstream node could be a Leaf node or an
   intermediate Replication node.  In the latter case, replication
   continues with the Replication segments until all Leaf nodes are
   reached.  A packet is steered into a P2MP tree in two ways:

   o  Based on a local policy-based routing at the Root node.

   o  Based on steering via the Tree-SID at the Root node.

2.1.  Sharing Replication segments across P2MP trees

   Two or more P2MP trees MAY share a Replication segment at Root or
   Replication nodes if at minimum as the first condition below is
   satisfied.  A tree always has its own Replication segment at its root
   even if shares another Replication segment.  A tree that shares
   another Replication segment may or may not have its own Replication
   segment on its Leaf nodes.  If not, the second and third conditions
   apply to such situations.

   1.  The Leaf nodes reached via a shared Replication segment must be
       subset of Leaf or Replication nodes of the P2MP trees that shares
       this segment.  Note if a Replication segment is shared, all its
       downstream Replication segments are also shared.

   2.  Some Multi-point services realized by the P2MP trees may need
       service context (e.g. packets are for certain VPNs, and/or from
       certain nodes).  If the trees do not have their own Replication
       segments at their Leaf nodes then the packets transported on the
       P2MP trees MUST carry a service context that does not rely on the
       tree or root identification, e.g. a service label assigned from
       Domain-wide Common Block or common SRGB.

   3.  For some Multi-point services using P2MP trees that share
       Replication segments, packets transported on these trees MAY
       require a Tree context (e.g.  MVPN Extranet [RFC7900] to avoid
       certain ambiguities - see Section 2.3.1 of RFC 7900).  In this
       case, the trees MUST have their own Replication segments on the
       Leaf nodes.  This is similar to "tunnel stacking" concept.



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   Sharing of a Replication segment for P2MP trees is OPTIONAL.  Exact
   procedures to ensure validity of above conditions across PM2P
   services on nodes of a Segment Routing domain are outside the scope
   of this document.

3.  SR P2MP Policy

   The SR P2MP policy is a variant of an SR policy
   [I-D.ietf-spring-segment-routing-policy] and is used to instantiate
   SR P2MP trees.

   A SR P2MP Policy is identified by the tuple <Root, Tree-ID>, where:

   o  Root: The address of Root node of P2MP tree instantiated by the SR
      P2MP Policy

   o  Tree-ID: A identifier that is unique in context of the Root.  This
      is an unsigned 32-bit number.

   A SR P2MP Policy is defined by following elements:

   o  Leaf nodes: A set of nodes that terminate the P2MP trees.

   o  Candidate Paths: See below.

   A SR P2MP policy is provisioned on a PCE to instantiate the P2MP
   tree.  The Tree-SID SHOULD be used as Binding SID of the P2MP policy.
   A PCE computes the P2MP tree and instantiates Replication segments at
   Root, Replication and Leaf nodes.  When Replication segments are not
   shared across P2MP trees, the Root and Tree-ID of the SR P2MP policy
   are mapped to Replication-ID element of the Replication segment
   identifier i.e the SR Replication segment identifier is <Root, Tree-
   ID, Node-ID>.  A shared Replication segment MAY be identified with
   zero Root-ID address (0.0.0.0 for IPv4 and :: for IPv6) and a
   Replication-ID that is unique in context of Node address where the
   Replication segment is instantiated when it is not associated a
   particular tree.

   A SR P2MP Policy has one or more Candidate paths.  The active
   Candidate path is selected based on the tie breaking rules amongst
   the candidate-paths as specified
   in[I-D.ietf-spring-segment-routing-policy].  Each candidate path has
   a set of topological/resource constraints and/or optimization
   objectives which determine the P2MP tree for that Candidate path.
   Tree-SID is an identifier of the P2MP tree of the candidate path in
   the forwarding plane.  It is instantiated in the forwarding plane at
   Root node, intermediate Replication nodes and Leaf nodes.  The Tree-
   SID MAY be different at Replication and Leaf nodes.



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4.  Using Controller to build a P2MP Tree

   A P2MP tree can be built using a Path Computation Element (PCE).
   This section outlines a high-level architecture for such an approach.


                         North Bound                South Bound
                         Programming          ..... Programming
                         Interface                  Interface
                              |
                              |
                              v
                           +-----+ ..........................
              .............| PCE | .............             .
              .            +-----+             .             .
              .               .                .             .
              .               .                .             .
              .               .                .             .
              .               .                V             .
              .               .              +----+          .
              .               .              | N3 |          .
              .               .              +----+          .
              .               .                 | Leaf (L2)   .
              .               .                 |            .
              .               .                 |            .
              V               V                 |            V
            +----+          +----+ --------------          +----+
            | N1 |----------| N2 |-------------------------| N4 |
            +----+          +----+                         +----+
           Root (R)         Replication node (M)           Leaf (L1)


                 Figure 1: Centralized Control Plane Model

4.1.  Provisioning SR P2MP Policy Creation

   A SR P2MP policy can be instantiated and maintained in a centralized
   fashion using a Path Computation Element (PCE).

4.1.1.  API

   North-bound APIs on a PCE can be used to:

   1.  Create SR P2MP policy

   2.  Delete SR P2MP policy

   3.  Update SR P2MP policy



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   4.  Create a Candidate Path for SR P2MP policy

   5.  Update a Candidate Path for SR P2MP policy

   6.  Delete a Candidate Path for SR P2MP policy

4.1.2.  Invoking API

   Interaction with a PCE can be via PCEP, REST, Netconf, gRPC, CLI.
   Yang model shall be be developed for this purpose as well.

4.2.  P2MP Tree Computation

   An entity (an operator, a network node or a machine) provisions a SR
   P2MP policy by specifying the addresses of the root (R) and set of
   leaves {L} as well as Traffic Engineering (TE) attributes of
   Candidate paths via a suitable North-Bound API.  The PCE computes the
   tree of Active candidate path.  The PCE MAY compute P2MP trees for
   all Candidate paths., If tree computation is successful, PCE
   instantiates the P2MP tree(s) using Replication segments on Root,
   Replication, and Leaf nodes.

   Candidate path constraints shall include link color affinity,
   bandwidth, disjointness (link, node, SRLG), delay bound, link loss,
   etc.  Candidate path shall be optimized based on IGP or TE metric or
   link latency.

   The Tree SID of Candidate path of a SR P2MP policy can be either
   dynamically allocated by the PCE or statically assigned by entity
   provisioning the SR P2MP policy.  Ideally, same Tree-SID SHOULD be
   used for Replication segments at Root, Replication, and Leaf nodes.
   Different Tree-SIDs MAY be used at replication node(s) if it is not
   feasible to use same Tree SID.

   A PCE can modify a P2MP tree following network element failure or in
   case a better path can be found based on the new network state.  In
   this case, the PCE may want to setup the new instance of the tree and
   remove the old instance of the tree from the network in order to
   minimize traffic loss.  In this case, the instances of trees for all
   the Candidate paths of a P2MP policy can be identified by an
   Instance-ID which is unique in context of the P2MP policy.  As such,
   the identifier of non-shared Replication segments used to instantiate
   these trees becomes <Root-ID, Tree-ID, Node-ID, Instance-ID>.

   A PCE shall be capable of computing paths across multiple IGP areas
   or levels as well as Autonomous Systems (ASs).





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4.2.1.  Topology Discovery

   A PCE shall learn network topology, TE attributes of link/node as
   well as SIDs via dynamic routing protocols (IGP and/or BGP-LS).  It
   may be possible for entities to pass topology information to PCE via
   north-bound API.

4.2.2.  Capability and Attribute Discovery

   It shall be possible for a node to advertise SR P2MP tree capability
   via IGP and/or BGP-LS.  Similarly, a PCE can also advertise its P2MP
   tree computation capability via IGP and/or BGP-LS.  Capability
   advertisement allows a network node to dynamically choose one or more
   PCE(s) to obtain services pertaining to SR P2MP policies, as well a
   PCE to dynamically identify SR P2MP tree capable nodes.

4.3.  Instantiating P2MP tree on nodes

   Once a PCE computes a P2MP tree for Candidate path of SR P2MP policy,
   it needs to instantiate the tree on the relevant network nodes via
   Replication segments.  The PCE can use various protocols to program
   the Replication segments as described below.

4.3.1.  PCEP

   PCE Protocol (PCEP)has been traditionally used:

   1.  For a head-end to obtain paths from a PCE.

   2.  A PCE to instantiate SR policies.

   PCEP protocol can be stateful in that a PCE can have a stateful
   control of an SR policy on a head-end which has delegated the control
   of the SR policy to the PCE.  PCEP shall be extended to provision and
   maintain SR P2MP trees in a stateful fashion.

4.3.2.  BGP

   BGP has been extended to instantiate and report SR policies.  It
   shall be extended to instantiate and maintain P2MP trees for SR P2MP
   policies.

4.3.3.  NetConf

   TBD






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4.4.  Protection

4.4.1.  Local Protection

   A network link, node or path on the tree of a P2MP tree can be
   protected using SR policies computed by PCE.  The backup SR policies
   shall be programmed in forwarding plane in order to minimize traffic
   loss when the protected link/node fails.  It is also possible to use
   node local Fast Re-Route protection mechanisms (LFA) to protect link/
   nodes of P2MP tree.

4.4.2.  Path Protection

   It is possible for PCE create a disjoint backup tree for providing
   end-to-end path protection.

5.  IANA Considerations

   This document makes no request of IANA.

6.  Security Considerations

   There are no additional security risks introduced by this design.

7.  Acknowledgements

   The authors would like to acknowledge Siva Sivabalan, Mike Koldychev
   and Vishnu Pavan Beeram for their valuable inputs..

8.  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



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   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








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9.  References

9.1.  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-sr-replication-segment]
              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
              Zhang, "SR Replication Segment for Multi-point Service
              Delivery", draft-ietf-spring-sr-replication-segment-00
              (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,
              <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>.

9.2.  Informative References

   [I-D.ietf-bess-mvpn-evpn-aggregation-label]
              Zhang, Z., Rosen, E., Lin, W., Li, Z., and I. Wijnands,
              "MVPN/EVPN Tunnel Aggregation with Common Labels", draft-
              ietf-bess-mvpn-evpn-aggregation-label-03 (work in
              progress), October 2019.

   [RFC7900]  Rekhter, Y., Ed., Rosen, E., Ed., Aggarwal, R., Cai, Y.,
              and T. Morin, "Extranet Multicast in BGP/IP MPLS VPNs",
              RFC 7900, DOI 10.17487/RFC7900, June 2016,
              <https://www.rfc-editor.org/info/rfc7900>.

Appendix A.  Illustration of SR P2MP Policy and P2MP Tree

   Consider the following topology:









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

                                 Figure 1

   In these examples, 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 PCE is provisioned following SR P2MP policy at Root R1 with
   Tree-ID T-ID:

   SR P2MP Policy <R1,T-ID>:
    Leaf Nodes: {R2, R6, R7}
    Candidate-path 1:
      Optimize: IGP metric
      Tree-SID: T-SID1

   The PCE is responsible for P2MP tree computation.  Assume PCE
   instantiates P2MP trees by signalling non-shared Replication segments
   i.e. Replication-ID of these Replication Segments is <Root, Tree-ID>.
   If a Candidat-path can have multiple instances of P2MP trees, the
   Replication-ID is <Root, Tree-ID, Instance-ID>.  In this example, we
   assume one instance of P2MP tree for a candidate-path.  All
   Replication Segments use the Tree-SID T-SID1 as Replication-SID.

A.1.  P2MP Tree with non-adjacent Replication Segments

   Assume PCE computes a P2MP tree with Root node R1, Intermediate and
   Leaf node R2, and Leaf nodes R6 and R7.  The PCE instantiates the
   P2MP tree by stitching Replication Segments at R1, R2, R6 and R7.
   Replication Segment at R1 replicates to R2.  Replication Segment at
   R2 replicates to R6 and R7.  Note nodes R3, R4 and R5 do not have any
   Replication Segment state for the tree.

   The Replication Segment state at nodes R1, R2, R6 and R7 is shown
   below.

   Replication Segment at R1:

   Replication Segment <R1,T-ID,R1>:
    Replication SID: T-SID1
    Replication State:
      R2: <T-SID1->L12>




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   Replication to R2 steers packet directly to the node on interface
   L12.

   Replication Segment at R2:

   Replication Segment <R1,T-ID,R2>:
    Replication SID: T-SID1
    Replication State:
      R2: <Leaf>
      R6: <N-SID6, T-SID1>
      R7: <N-SID7, T-SID1>

   R2 is a Bud-Node.  It performs role of Leaf as well as a transit node
   replicating to R6 and R7.  Replication to R6, using N-SID6, steers
   packet via IGP shortest path to that node.  Replication to R7, using
   N-SID7, steers packet via IGP shortest path to R7 via either R5 or R4
   based on ECMP hashing.

   Replication Segment at R6:

   Replication Segment <R1,T-ID,R6>:
    Replication SID: T-SID1
    Replication State:
      R6: <Leaf>

   Replication Segment at R7:

   Replication Segment <R1,T-ID,R7>:
    Replication SID: T-SID1
    Replication State:
      R7: <Leaf>

   When a packet is steered into the SR P2MP Policy at R1:

   o  Since R1 is directly connected to R2, R1 performs PUSH operation
      with just <T-SID1> label for the replicated copy and sends it to
      R2 on interface L12.

   o  R2, as Leaf, performs NEXT operation, pops T-SID1 label and
      delivers the payload.  For replication to R6, R2 performs a PUSH
      operation of N-SID6, to send <N-SID6,T-SID1> label stack 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 <T-SID1> in the label stack.  For replication
      to R7, R2 performs a PUSH operation of N-SID7, to send
      <N-SID7,T-SID1> label stack to R4, one of IGP ECMP nexthops
      towards R7.  R4 is the penultimate hop for N-SID6; it performs
      penultimate hop popping, which corresponds to the NEXT operation



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      and the packet is then sent to R7 with <T-SID1> in the label
      stack.

   o  R6, as Leaf, performs NEXT operation, pops T-SID1 label and
      delivers the payload.

   o  R7, as Leaf, performs NEXT operation, pops R-SID7 label and
      delivers the payload.

A.2.  P2MP Tree with adjacent Replication Segments

   Assume PCE computes a P2MP tree with Root node R1, Intermediate and
   Leaf node R2, Intermediate nodes R3 and R5, and Leaf nodes R6 and R7.
   The PCE instantiates the P2MP tree by stitching Replication Segments
   at R1, R2, R3, R5, R6 and R7.  Replication Segment at R1 replicates
   to R2.  Replication Segment at R2 replicates to R3 and R5.
   Replication segment at R3 replicates to R6.  Replication segment at
   R5 replicates to R7.  Note node R4 does not have any Replication
   Segment state for the tree.

   The Replication Segment state at nodes R1, R2, R3, R5, R6 and R7 is
   shown below.

   Replication Segment at R1:

   Replication Segment <R1,T-ID,R1>:
    Replication SID: T-SID1
    Replication State:
      R2: <T-SID1->L12>

   Replication to R2 steers packet directly to tje node on interface
   L12.

   Replication Segment at R2:

   Replication Segment <R1,T-ID,R2>:
    Replication SID: T-SID1
    Replication State:
      R2: <Leaf>
      R3: <T-SID1->L23>
      R5: <T-SID1->L25>

   R2 is a Bud-Node.  It performs role of Leaf as well as a transit node
   replicating to R3 and R5.  Replication to R3, steers packet directly
   to the node on L23.  Replication to R5, steers packet directly to the
   node on L25.

   Replication Segment at R3:



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   Replication Segment <R1,T-ID,R3>:
    Replication SID: T-SID1
    Replication State:
      R6: <T-SID1->L36>

   Replication to R6, steers packet directly to the node on L36.

   Replication Segment at R5:

   Replication Segment <R1,T-ID,R5>:
    Replication SID: T-SID1
    Replication State:
      R7: <T-SID1->L57>

   Replication to R7, steers packet directly to the node on L57.

   Replication Segment at R6:

   Replication Segment <R1,T-ID,R6>:
    Replication SID: T-SID1
    Replication State:
      R6: <Leaf>

   Replication Segment at R7:

   Replication Segment <R1,T-ID,R7>:
    Replication SID: T-SID1
    Replication State:
      R7: <Leaf>

   When a packet is steered into the SR P2MP Policy at R1:

   o  Since R1 is directly connected to R2, R1 performs PUSH operation
      with just <T-SID1> label for the replicated copy and sends it to
      R2 on interface L12.

   o  R2, as Leaf, performs NEXT operation, pops T-SID1 label and
      delivers the payload.  It also performs CONTINUE operation on
      T-SID1 for replication to R3 and R5.  For replication to R6, R2
      sends <T-SID1> label stack to R3 on interface L23.  For
      replication to R5, R2 sends <T-SID1> label stack to R5 on
      interface L25.

   o  R3 performs CONTINUE operation on T-SID1 for replication to R6 and
      sends <T-SID1> label stack to R6 on interface L36.

   o  R5 performs CONTINUE operation on T-SID1 for replication to R7 and
      sends <T-SID1> label stack to R7 on interface L57.



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   o  R6, as Leaf, performs NEXT operation, pops T-SID1 label and
      delivers the payload.

   o  R7, as Leaf, 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







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