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Network Working Group                                      D. Voyer, Ed.
Internet-Draft                                                 C. Hassen
Intended status: Standards Track                               K. Gillis
Expires: September 12, 2019                                  Bell Canada
                                                             C. Filsfils
                                                               R. Parekh
                                                     Cisco Systems, Inc.
                                                              H. Bidgoli
                                                                   Nokia
                                                          March 11, 2019


            SR Replication Policy for P2MP Service Delivery
                  draft-voyer-spring-sr-p2mp-policy-02

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 September 12, 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.  Steering  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Spray P2MP segment  . . . . . . . . . . . . . . . . . . . . .   4
   5.  TreeSID P2MP segment  . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Using Controller to build a P2MP Segment  . . . . . . . .   4
       5.1.1.  SR Replication Policy Creation  . . . . . . . . . . .   5
       5.1.2.  TreeSID P2MP Segment Computation  . . . . . . . . . .   6
       5.1.3.  Instantiating TreeSID P2MP segment nodes  . . . . . .   7
       5.1.4.  Protection  . . . . . . . . . . . . . . . . . . . . .   7
   6.  Illustration  . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   This document defines a variant of the SR Policy [I-D.  ietf-spring-
   segment-routing-policy] for constructing a P2MP segment to support
   Point-to-Multipoint service delivery.  We call it an SR Replication
   Policy.

   A Point-to-Multipoint (P2MP) segment connects a Root node to a set of
   Leaf nodes in a Segment Routing Domain.  We define two types of a
   P2MP segment: Spray and TreeSID.

   Spray P2MP segment enables a Root node to directly replicate a packet
   using a SR path to each Leaf node.

   For a TreeSID P2MP segment, a controller computes a tree from a Root
   node to a set of Leaf nodes via a set of Replication nodes.  A packet
   is replicated at the Root node and on Replication nodes towards each
   Leaf node.



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2.  SR Replication Policy

   The SR Replication policy is a variant of an SR policy
   [I-D.ietf-spring-segment-routing-policy].  This section is similar to
   section 2 of SR Policy draft
   [I-D.ietf-spring-segment-routing-policy], and applies equally to the
   Spray and TreeSID P2MP segments unless explicitly specified.  A SR
   replication policy can be provisioned either locally or setup via
   controller.

   A SR replication 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 constraints
      and optimization objectives to be satisfied by the P2MP segment.

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

   Like any SR policy, a SR Replication Policy has a BSID
   [I-D.ietf-spring-segment-routing-policy] instantiated into the
   forwarding plane.  For P2MP segments, the BSID is applicable only at
   the Root node.

   For a TreeSID P2MP segment, the SR Replication policy also has an
   associated identifier, a TreeSID.  The TreeSID is instantiated into
   the forwarding plane at Replication nodes and Leaf nodes of a P2MP
   segment.  A packet is steered towards the set of Leaf nodes when the
   active SID of the packet is a TreeSID.

   A SR Replication may comprise of multiple candidate paths.  A
   candidate path is valid when all its SID-Lists are valid.  The active
   candidate path is selected based on the tie breaking rules amongst
   the valid candidate-paths.

   In the context of a SR Replication Policy, the selected path MAY have
   more than one SID-List.  The weights of the SID-Lists is not
   applicable for a SR Replication Policy.  They MUST be set to 1.

   Any traffic steered into a SR Replication Policy is replicated along
   the SID-Lists of its selected path.  Each SID-List takes a packet to
   either a Replication node or a Leaf node of a P2MP segment.






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3.  Steering

   Traffic is steered into a SR Replication 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
      Replication Policy at the Root node.

4.  Spray P2MP segment

   In a Spray P2MP segment, packet replication occurs only at the Root
   node.  A SR Replication policy for a Spray P2MP segment is
   instantiated only at the Root node.  There are no Replication nodes
   in these segments.

   A packet, using this approach, is replicated directly to each Leaf
   node via a SR path from the Root to a given Leaf node.

5.  TreeSID P2MP segment

   In a TreeSID P2MP segment, packet replication occurs at the Root node
   and on Replication nodes towards the Leaf node.

   A SR Replication policy instantiated on the Root node takes a packet
   from the Root node to Replication nodes towards the Leaf node.  A
   Replication node MAY also be a Leaf node.  The SR Replication policy
   instantiated at the Replication nodes take the packet down further to
   other Replication nodes or Leaf nodes.

5.1.  Using Controller to build a P2MP Segment




















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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

5.1.1.  SR Replication Policy Creation

   A SR Replication policy can be instantiated and maintained in a
   centralized fashion using a Path Computation Element (PCE).  This
   section outlines a high-level architecture for such an approach.

5.1.1.1.  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








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5.1.1.2.  Invoking API

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

5.1.2.  TreeSID 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, and if successful 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).

5.1.2.1.  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.

5.1.2.2.  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 Replication policies, as well a PCE to
   dynamically identify TreeSID capable nodes.




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5.1.3.  Instantiating TreeSID 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
   protocols to program the forwarding entries, and these protocols are
   described below.

5.1.3.1.  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.

5.1.3.2.  BGP

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

5.1.3.3.  NetConf

   TBD

5.1.4.  Protection

5.1.4.1.  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.

5.1.4.2.  Path Protection

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








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6.  Illustration

   TBD

7.  IANA Considerations

   This document makes no request of IANA.

8.  Security Considerations

   There are no additional security risks introduced by this design.

9.  Acknowledgements

   The authors would like to acknowledge Siva Sivabalan.

10.  Contributors

   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



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   US

   Email: tanmoy.kundu@nokia.com

11.  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-02 (work in progress), October 2018.

   [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


   Clayton Hassen
   Bell Canada
   Vancouver
   CA

   Email: clayton.hassen@bell.ca


   Kurtis Gillis
   Bell Canada
   Halifax
   CA

   Email: kurtis.gillis@bell.ca










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   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





























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