Network Working Group                                      D. Voyer, Ed.
Internet-Draft                                               Bell Canada
Intended status: Standards Track                             C. Filsfils
Expires: 24 February 2022                                      R. Parekh
                                                     Cisco Systems, Inc.
                                                              H. Bidgoli
                                                                   Nokia
                                                                Z. Zhang
                                                        Juniper Networks
                                                          23 August 2021


               Segment Routing Point-to-Multipoint Policy
                    draft-ietf-pim-sr-p2mp-policy-03

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 24 February 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
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   extracted from this document must include Simplified BSD License text
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   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 . . . . . . . . . . . . . . . . . . . . . . . . .   7
       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  . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.3.2.  BGP . . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.3.3.  NetConf . . . . . . . . . . . . . . . . . . . . . . .   9
     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  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Illustration of SR P2MP Policy and P2MP Tree . . . .  12
     A.1.  P2MP Tree with non-adjacent Replication Segments  . . . .  13
       A.1.1.  SR-MPLS . . . . . . . . . . . . . . . . . . . . . . .  14
       A.1.2.  SRv6  . . . . . . . . . . . . . . . . . . . . . . . .  15
     A.2.  P2MP Tree with adjacent Replication Segments  . . . . . .  17
       A.2.1.  SR-MPLS . . . . . . . . . . . . . . . . . . . . . . .  17
       A.2.2.  SRv6  . . . . . . . . . . . . . . . . . . . . . . . .  19



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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.).  Replication segments of a P2MP tree can
   be instantiated for SR-MPLS and SRv6 dataplanes.

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.

   For SR-MPLS, a PCE MAY decide not to 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



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   that is globally unique in SR domain. In this case, Replication Nodes
   connecting to Leaf nodes effectively does Penultimate-Hop Pop (PHP)
   behavior to pop Tree-SID from a packet.  A Multi-point service
   context assigned from "Domain-wide Common Block" (DCB) [I-D.ietf-
   bess-mvpn-evpn-aggregation-label] is an example of globally unique
   context.

   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:

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

   *  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 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 share
       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 for SR-MPLS.










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   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.  For SR-MPLS, this is similar to "tunnel stacking"
       concept.

   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:

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

   *  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:

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

   *  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.






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   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.

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).




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4.1.1.  API

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

   1.  Create SR P2MP policy: CreateSRP2MPPolicy<Root, Tree-ID>

   2.  Delete SR P2MP policy: DeleteSRP2MPPolicy<Root, Tree-ID>

   3.  Modify SR P2MP policy Leaf Set: SRP2MPPolicyLeafSetModify<Root,
       Tree-ID, {Leaf Set}>

   4.  Create a Candidate Path for SR P2MP policy:
       CreateSRP2MPCandidatePath<Root, Tree-ID, <CP-ID>>

   5.  Delete a Candidate Path for SR P2MP policy:
       DeleteSRP2MPCandidatePath<Root, Tree-ID, <CP-ID>>

   6.  Update a Candidate Path for SR P2MP policy:
       UpdateSRP2MPCandidatePath<Root, Tree-ID, <CP-ID>, Preference,
       [Constraints], [Optimization], ...>

   CP-ID is identifier of a Candidate Path within a SR P2MP policy.  One
   possible identifier is the tuple <Protocol-Origin, originator,
   discriminator> as specified in
   [I-D.ietf-spring-segment-routing-policy].

   Note these are conceptual APIs.  Actual implementations may offer
   different APIs as long as they provide same functionality.  For
   example, API might allow symbolic name to be assigned for a P2MP
   policy or APIs might allow individual Leaf nodes to be added or
   deleted from a policy instead of an update operation.

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.




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   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).

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.




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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

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.





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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 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

9.  References

9.1.  Normative References




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   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in
              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-13, 28 May 2021,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              segment-routing-policy-13.txt>.

   [I-D.ietf-spring-sr-replication-segment]
              (editor), D. V., Filsfils, C., Parekh, R., Bidgoli, H.,
              and Z. Zhang, "SR Replication Segment for Multi-point
              Service Delivery", Work in Progress, Internet-Draft,
              draft-ietf-spring-sr-replication-segment-05, 20 August
              2021, <https://www.ietf.org/archive/id/draft-ietf-spring-
              sr-replication-segment-05.txt>.

   [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.filsfils-spring-srv6-net-pgm-illustration]
              Filsfils, C., Garvia, P. C., Li, Z., Matsushima, S.,
              Decraene, B., Steinberg, D., Lebrun, D., Raszuk, R., and
              J. Leddy, "Illustrations for SRv6 Network Programming",
              Work in Progress, Internet-Draft, draft-filsfils-spring-
              srv6-net-pgm-illustration-04, 30 March 2021,
              <https://www.ietf.org/archive/id/draft-filsfils-spring-
              srv6-net-pgm-illustration-04.txt>.

   [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", Work in
              Progress, Internet-Draft, draft-ietf-bess-mvpn-evpn-
              aggregation-label-06, 19 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-bess-mvpn-
              evpn-aggregation-label-06.txt>.







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   [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>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

Appendix A.  Illustration of SR P2MP Policy and P2MP Tree

   Consider the following topology:

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

                             Figure 2: 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.

   For SRv6, the reader is expected to be familiar with SRv6 Network
   Programming [RFC8986] to follow the examples.  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 in 64 bits and a function expressed
      in 16 bits

   *  Node k has 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 in its IGP



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   *  Function :1:: (function 1, for short) represents the End function
      with PSP support

   *  Function :Cn:: (function Cn, for short) represents the End.X
      function to Node n

   *  Function :C1n: (function C1n for short) represents the End.X
      function to Node n with USD

   Each 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 SID instantiation 2001:db8:cccc:k:C1j::/128 bound to
      an End.X function to neighbor J with additional support for USD

   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 Candidate-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.  For
   SRv6, assume the Replication SID at node k, bound to an End.Replcate
   function, is 2001:db8:cccc:k:FA::/128.

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.




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A.1.1.  SR-MPLS

   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>

   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:






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   *  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.

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

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

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

A.1.2.  SRv6

   For SRv6, the replicated packet from R2 to R7 has to traverse R4
   using a SR-TE policy, Policy27.  The policy has one SID in segment
   list: End.X function with USD of R4 to R7 . The Replication segment
   state at nodes R1, R2, R6 and R7 is shown below.

   Policy27: <2001:db8:cccc:4:C17::>

   Replication segment at R1:

   Replication segment <R1,T-ID,R1>:
    Replication SID: 2001:db8:cccc:1:FA::
    Replication State:
      R2: <2001:db8:cccc:2:FA::->L12>

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

   Replication segment at R2:








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   Replication segment <R1,T-ID,R2>:
    Replication SID: 2001:db8:cccc:2:FA::
    Replication State:
      R2: <Leaf>
      R6: <2001:db8:cccc:6:FA::>
      R7: <2001:db8:cccc:7:FA:: -> Policy27>

   R2 is a Bud-Node.  It performs role of Leaf as well as a transit node
   replicating to R6 and R7.  Replication to R6, steers packet via IGP
   shortest path to that node.  Replication to R7, via SR-TE policy,
   first encapsulates the packet using H.Encaps and then steers the
   outer packet to R4.  End.X USD on R4 decapsulates outer header and
   sends the original inner packet to R7.

   Replication segment at R6:

   Replication segment <R1,T-ID,R6>:
    Replication SID: 2001:db8:cccc:6:FA::
    Replication State:
      R6: <Leaf>

   Replication segment at R7:

   Replication segment <R1,T-ID,R7>:
    Replication SID: 2001:db8:cccc:7:FA::
    Replication State:
      R7: <Leaf>

   When a packet (A,B2) is steered into the SR P2MP Policy at R1 using
   H.Encaps.Replicate behavior:

   *  Since R1 is directly connected to R2, R1 sends replicated copy
      (2001:db8::1, 2001:db8:cccc:2:FA::) (A,B2) to R2 on interface L12.

   *  R2, as Leaf removes outer IPv6 header and delivers the payload.
      R2, as a bud node, also replicates the packet.

      -  For replication to R6, R2 sends (2001:db8::1,
         2001:db8:cccc:6:FA::) (A,B2) to R3.  R3 forwards the packet
         using 2001:db8:cccc:6::/64 packet to R6.

      -  For replication to R7 using Policy27, R2 encapsulates and sends
         (2001:db8::2, 2001:db8:cccc:4:C17::) (2001:db8::1,
         2001:db8:cccc:7:FA::) (A,B2) to R4.  R4 performs End.X USD
         behavior, decapsulates outer IPv6 header and sends
         (2001:db8::1, 2001:db8:cccc:7:FA::) (A,B2) to R7.

   *  R6, as Leaf, removes outer IPv6 header and delivers the payload.



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   *  R7, as Leaf, removes outer IPv6 header 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.

A.2.1.  SR-MPLS

   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 the 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:

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



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   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:

   *  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.

   *  R2, as Leaf, performs NEXT operation, pops T-SID1 label and
      delivers the payload.  It also performs PUSH 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.

   *  R3 performs NEXT operation on T-SID1 and performs a PUSH operation
      for replication to R6 and sends <T-SID1> label stack to R6 on
      interface L36.

   *  R5 performs NEXT operation on T-SID1 and performs a PUSH operation
      for replication to R7 and sends <T-SID1> label stack to R7 on
      interface L57.

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




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

A.2.2.  SRv6

   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: 2001:db8:cccc:1:FA::
    Replication State:
      R2: <2001:db8:cccc:2:FA::->L12>

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

   Replication segment at R2:

   Replication segment <R1,T-ID,R2>:
    Replication SID: 2001:db8:cccc:2:FA::
    Replication State:
      R2: <Leaf>
      R3: <2001:db8:cccc:3:FA::->L23>
      R5: <2001:db8:cccc:5:FA::->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:

   Replication segment <R1,T-ID,R3>:
    Replication SID: 2001:db8:cccc:3:FA::
    Replication State:
      R6: <2001:db8:cccc:6:FA::->L36>

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

   Replication segment at R5:

   Replication segment <R1,T-ID,R5>:
    Replication SID: 2001:db8:cccc:5:FA::
    Replication State:
      R7: <2001:db8:cccc:7:FA::->L57>




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

   Replication segment at R6:

   Replication segment <R1,T-ID,R6>:
    Replication SID: 2001:db8:cccc:6:FA::
    Replication State:
      R6: <Leaf>

   Replication segment at R7:

   Replication segment <R1,T-ID,R7>:
    Replication SID: 2001:db8:cccc:7:FA::
    Replication State:
      R7: <Leaf>

   When a packet (A,B2) is steered into the SR P2MP Policy at R1 using
   H.Encaps.Replicate behavior:

   *  Since R1 is directly connected to R2, R1 sends replicated copy
      (2001:db8::1, 2001:db8:cccc:2:FA::) (A,B2) to R2 on interface L12.

   *  R2, as Leaf, removes outer IPv6 header and delivers the payload.
      R2, as a bud node, also replicates the packet.  For replication to
      R3, R2 sends (2001:db8::1, 2001:db8:cccc:3:FA::) (A,B2) to R3 on
      interface L23.  For replication to R5, R2 sends (2001:db8::1,
      2001:db8:cccc:5:FA::) (A,B2) to R5 on interface L25.

   *  R3 replicates and sends (2001:db8::1, 2001:db8:cccc:6:FA::) (A,B2)
      to R6 on interface L36.

   *  R5 replicates and sends (2001:db8::1, 2001:db8:cccc:7:FA::) (A,B2)
      to R7 on interface L57.

   *  R6, as Leaf, removes outer IPv6 header and delivers the payload.

   *  R7, as Leaf, removes outer IPv6 header and delivers the payload.

Authors' Addresses

   Daniel Voyer (editor)
   Bell Canada
   Montreal
   Canada

   Email: daniel.voyer@bell.ca





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   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com


   Rishabh Parekh
   Cisco Systems, Inc.
   San Jose,
   United States of America

   Email: riparekh@cisco.com


   Hooman Bidgoli
   Nokia
   Ottawa
   Canada

   Email: hooman.bidgoli@nokia.com


   Zhaohui Zhang
   Juniper Networks

   Email: zzhang@juniper.net























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