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Network Working Group                                      D. Voyer, Ed.
Internet-Draft                                               Bell Canada
Intended status: Standards Track                             C. Filsfils
Expires: February 3, 2020                                      R. Parekh
                                                     Cisco Systems, Inc.
                                                              H. Bidgoli
                                                                   Nokia
                                                                Z. Zhang
                                                        Juniper Networks
                                                            July 2, 2019


            SR Replication Policy for P2MP Service Delivery
                  draft-voyer-spring-sr-p2mp-policy-03

Abstract

   This document describes the SR policy architecture for P2MP service
   delivery.

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 November 15, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SR Replication Policy . . . . . . . . . . . . . . . . . . . .   3
   3.  SR P2MP Policy  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Using Controller to build a P2MP Segment  . . . . . . . . . .   5
     4.1.  SR P2MP Policy Creation . . . . . . . . . . . . . . . . .   6
       4.1.1.  API . . . . . . . . . . . . . . . . . . . . . . . . .   6
       4.1.2.  Invoking API  . . . . . . . . . . . . . . . . . . . .   6
     4.2.  P2MP Segment Computation  . . . . . . . . . . . . . . . .   7
       4.2.1.  Topology Discovery  . . . . . . . . . . . . . . . . .   7
       4.2.2.  Capability and Attribute Discovery  . . . . . . . . .   7
     4.3.  Instantiating P2MP segment nodes  . . . . . . . . . . . .   7
       4.3.1.  PCEP  . . . . . . . . . . . . . . . . . . . . . . . .   8
       4.3.2.  BGP . . . . . . . . . . . . . . . . . . . . . . . . .   8
       4.3.3.  NetConf . . . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Protection  . . . . . . . . . . . . . . . . . . . . . . .   8
       4.4.1.  Local Protection  . . . . . . . . . . . . . . . . . .   8
       4.4.2.  Path Protection . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This document defines variants of the SR Policy [I-D.  ietf-spring-
   segment-routing-policy] to support Point-to-Multipoint service
   delivery.

   We define a Point-to-Multipoint (P2MP) segment, which connects a Root
   node to a set of Leaf nodes in a Segment Routing Domain.

   We also define a Replication Segment, which corresponds to the state
   of a P2MP segment on a particular node.




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   A P2MP segment consists of replication segments for the root, leaves,
   and optionally intermediate replication nodes.  Note that a node may
   forward only one copy to a downstream node (be it a leaf or another
   intermediate node) or even just forward traffic off the p2mp segment
   (i.e. as a leaf), but we still call the forwarding behavior on the
   node a replication segment.

   For a P2MP segment, a controller may be used to compute paths from a
   Root node to a set of Leaf nodes, optionally via a set of replication
   nodes.  A packet is replicated at the root node and optionally on
   Replication nodes towards each Leaf node.

   A Point-to-Multipoint service delivery could be via Ingress
   Replication (aka Spray in some SR context), i.e., the root unicasts
   individual copies of traffic to each leaf.  The corresponding P2MP
   segment consists of replication segments only for the root and the
   leaves.

   A Point-to-Multipoint service delivery could also be via Downstream
   Replication (aka TreeSID in some SR context), i.e., the root and some
   downstream replication nodes replicate the traffic along the way as
   it traverses closer to the leaves.

   Notice that Spray is actually a special form of TreeSID.  Also notice
   that, the explicit path from the root or a replication node to a leaf
   or a downstream replication node can optionally be partially or
   completely specified by the controller or determined locally.

2.  SR Replication Policy

   An SR Replication policy is a variant of an SR policy [I-D.ietf-
   spring-segment-routing-policy].  A replication policy corresponds to
   a replication segment, which defines the forwarding behavior on a
   particular node on a particular P2MP segment.

   An SR Replication Policy can be either provisioned locally or
   programmed by a controller.

   An SR Replication Policy is identified through the tuple <Node-ID,
   Root, Tree-ID>.

   An SR Replication Policy is defined by following elements:

   o  Node-ID: The node that the replication segment is for.

   o  Root: The root of the P2MP segment that the replication segment is
      for.




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   o  Tree-ID: Tree that the replication segment is part of.

   o  Replication-SID: Segment ID for this Replication Segment.

   o  Candidate Paths: See below.

   The Replication-SID is instantiated into the forwarding plane at the
   node.  An incoming packet with the SID is forwarded according to the
   replication branches.  The Replication-SID may be the same on all
   nodes of the tree, and referred to as Tree-SID.

   A SR Replication Policy may comprise of multiple candidate paths.
   The active candidate path is selected based on the tie breaking rules
   amongst the valid candidate-paths.

   Each candidate path includes a list of replication branches.  In this
   document, each branch is abstracted to a <Downstream Node, Downstream
   Replication-SID> tuple.  For the signaling from a controller to a
   tree node, the Downstream Node in the tuple could be represented by
   its Node-SID (i.e. it does not matter how traffic gets to the
   downstream node, whether it's directly connected or not), or in case
   of a directly connected Downstream Node it could be represented by
   one of this node's Adjacency-SIDs (for the interface connecting to
   the directly connected Downstream Node).  Alternatively, the
   Downstream Node could also be expanded to a SID-list that partially/
   fully specify the explicit path to it.  In all cases, the node
   converts the signaled SIDs to its local forwarding representation
   (e.g., a Node/Adjacency-SID of a directly connected Downstream Node
   is translated to a local interface).

   Each replication branch may also include one or more backup branches
   for protection purpose.  Details will be added in a future revision.

3.  SR P2MP Policy

   The SR P2MP policy is a variant of an SR policy [I-D.ietf-spring-
   segment-routing-policy].  It correspond to an SR P2MP Segment.

   A SR P2MP Policy is defined by following elements:

   o  Root node: This is the headend of the P2MP segment.

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

   o  Constraints/Objectives: Optional set of topological/resource
      constraints and optimization objectives to be satisfied by the
      P2MP segment.




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   A SR P2MP Policy is identified through the tuple <Root node, Tree-
   ID>.

   An SR P2MP Policy has a BSID [I-D.ietf-spring-segment-routing-policy]
   instantiated into the forwarding plane.  The BSID is applicable only
   at the Root node.

   An SR P2MP policy can be either provisioned locally or programmed by
   a controller onto the root node of the segment, for the purpose of
   steering traffic into the segment.  A controller calculates the tree
   and program corresponding replication segments on root, leaves and
   optional replication nodes.

   Traffic is steered into a SR P2MP Policy in two ways:

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

   o  Based on remote classification and steering via the BSID of the SR
      P2MP Policy at the Root node.

   Traffic is then forwarded toward the leaves following the replication
   segments.

4.  Using Controller to build a P2MP Segment

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























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                         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.  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 P2MP SR policy

   2.  Delete P2MP SR policy

   3.  Update P2MP SR policy

4.1.2.  Invoking API

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




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4.2.  P2MP Segment Computation

   Network operator passes the addresses of the root (R) and set of
   leaves {L} as well as Traffic Engineering (TE) attributes (e.g.,
   constraints such as link color, optimization criteria such as
   latency) of the P2MP segment to PCE via a suitable North-Bound API.
   The PCE computes the tree instantiates the P2MP segment on Root,
   Replication, and Leaf nodes.

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

   Ideally, same P2MP SID SHOULD be used for forwarding entries at Root,
   Mid, and Leaf nodes.  Different P2MP SIDs MAY be used at different
   node(s) if it is not feasible to use same P2MP SID.  SIDs (BSID as
   well as P2MP SID) can also be assigned by operator.

   A PCE can modify a P2MP segment 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 tree and remove the
   old tree from the network in order to minimize traffic loss.  As
   such, a separate P2MP SID can be used for the new tree.

   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 operators 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 TreeSID capability via
   IGP and/or BGP-LS.  Similarly, a PCE can also advertise its TreeSID
   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 TreeSID capable nodes.

4.3.  Instantiating P2MP segment nodes

   Once a PCE computes a tree for P2MP segment, it needs to instantiate
   the segment on the relevant network nodes.  The PCE can use various




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   protocols to program the forwarding entries, and these protocols are
   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 forwarding entries in a stateful fashion.

4.3.2.  BGP

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

4.3.3.  NetConf

   TBD

4.4.  Protection

4.4.1.  Local Protection

   A network link/node on the tree of a P2MP segment 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.

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.








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6.  Security Considerations

   There are no additional security risks introduced by this design.

7.  Acknowledgements

   The authors would like to acknowledge Siva Sivabalan.

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



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   Email: jayant.kotalwar@nokia.com

   Tanmoy Kundu
   Nokia
   Mountain View
   US

   Email: tanmoy.kundu@nokia.com

9.  Normative References

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
              bogdanov@google.com, b., and P. Mattes, "Segment Routing
              Policy Architecture", draft-ietf-spring-segment-routing-
              policy-03 (work in progress), May 2019.

   [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>.

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





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   Hooman Bidgoli
   Nokia
   Ottawa
   CA

   Email: hooman.bidgoli@nokia.com


   Zhaohui Zhang
   Juniper Networks

   Email: zzhang@juniper.net







































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