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SPRING Working Group                                         Dave Allan
Internet Draft                                                 Ericsson
Intended status: Standards Track                          Jeff Tantsura
Expires: October 2017
                                                             April 2017

     A Framework for Computed Multicast applied to MPLS based Segment


   This document describes a multicast solution for Segment Routing with
   MPLS data plane. It is consistent with the Segment Routing
   architecture in that an IGP is augmented to distribute information in
   addition to the link state. In this solution it is multicast group
   membership information sufficient to synchronize state in a given
   network domain. Computation is employed to determine the topology of
   any loosely specified multicast distribution tree.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance
   with the provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet
   Engineering Task Force (IETF), its areas, and its working
   groups.  Note that other groups may also distribute working
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   Internet-Drafts are draft documents valid for a maximum of six
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   in progress".

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire in October 2017.

Copyright and License Notice

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   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
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   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...................................................3
   1.1. Authors......................................................3
   1.2. Requirements Language........................................3
   2. Conventions used in this document..............................3
   2.1. Terminology..................................................3
   3. Solution Overview..............................................4
   3.1. Mapping source specific trees onto the segment routing
   3.2. Role of the Routing System...................................6
   3.3. MDT Construction Requirements................................6
   3.4. Pruning - theory of operation................................6
   4. Elements of Procedure..........................................7
   4.1. Triggers for Computation.....................................7
   4.2. FIB Determination............................................7
   4.2.1. Information in the IGP.....................................7
   4.2.2. Computation of individual segments.........................8
   4.3. FIB Generation..............................................11
   4.4. FIB installation............................................11
   5. Related work..................................................12
   5.1. IGP Extensions..............................................12
   5.2. BGP Extensions..............................................12
   6. Observations..................................................12
   7. Acknowledgements..............................................13
   8. Security Considerations.......................................13
   9. IANA Considerations...........................................13
   10. References...................................................13
   10.1. Normative References.......................................13
   10.2. Informative References.....................................13
   11. Authors' Addresses...........................................14

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1. Introduction

   This memo describes a solution for multicast for Segment Routing with
   MPLS data plane in which source specific multicast distribution trees
   (MDTs) are computed from information distributed via an IGP.
   Computation can use information in the IGP to determine if a given
   node in the network has a role as a root, leaf or replication point
   in a given MDT. Unicast tunnels are employed to interconnect the
   nodes determined to have a role. Therefore state only need be
   installed in nodes that have one of these three roles to fully
   instantiate an MDT.
   Although this approach is computationally intensive, a significant
   amount of computation can be avoided when the computing agent
   determines that the node it is computing for has no role in a given
   MDT. This permits a computed approach to multicast convergence to be
   computationally tractable.
1.1. Authors

   David Allan, Jeff Tantsura

1.2. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119 [RFC2119].

2. Changes from the last version

      Clarifications in the pruning and simplification algorithm

3. Conventions used in this document

3.1. Terminology

   Candidate replication point (CRP) - is a node that potentially needs
   to install state to replicate multicast traffic as determined at an
   intermediate step in multicast segment computation. It will either
   resolve to having no role or a role as a replication point once
   multicast has converged.

   Candidate role - refers to any potential combination of roles on a
   given multicast segment as determined at some intermediate step in
   MDT computation. For example, a node with a candidate role may be a
   leaf and may be a candidate replication point.

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   Downstream - refers to the direction along the shortest path to one
   or more leaves for a given multicast distribution tree

   Multicast convergence - is when all computation and state
   installation to ensure the FIB reflects the multicast information in
   the IGP is complete.

   MDT - multicast distribution tree. Is a tree composed of one or more
   multicast segments.

   Multicast segment - is a portion of the multicast tree where only the
   root and the leaves have been specified, and computation based upon
   the current state of the IGP database is employed to determine and
   install the required state to implement the segment. For MPLS a
   multicast segment is implemented as a p2mp LSP. A multicast segment
   is identified by a multicast SID.

   Multicast SID - Is the data plane identifier that is used to
   implement a multicast segment. As per a unicast MPLS segment, the
   rightmost 20 bits of a multicast SID is encoded as a label. It is
   drawn from an SRGB that is global to the SR domain.

   Pinned path - Is a unique shortest path extending from a leaf
   upstream towards the root for a given multicast segment. Therefore is
   a component of the multicast segment that it has been determined must
   be there. It will not necessarily extend from the leaf all the way to
   the root during intermediate computation steps. A pinned path can
   result from pruning operations.

   Role - refers specifically to a node that is either a root, a leaf, a
   replication node, or a pinned waypoint for a given MDT.

   Unicast convergence - is when all computation and state installation
   to ensure the FIB reflects the unicast information in the IGP is

   Upstream - refers to the direction along the shortest path to the
   root of a given MDT.

4. Solution Overview

   This memo describes a multicast architecture in which multicast state
   is only installed in those nodes that have roles as a root, leaves,
   and replication points for a given multicast segment. The a-priori
   established segment routing unicast tunnels are used as interconnect
   between the nodes that have a role in a given multicast SID.

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   A loosely specified MDT is composed of a single multicast segment and
   the routing of the MDT is delegated entirely to computation driven by
   information in the IGP database.

   Explicitly routed MDTs are expressed as a tree of concatenated
   multicast segments where both the leaves of each segment and the
   waypoints coupling a given segment to the upstream and/or downstream
   segment(s) is specified in information flooded in the IGP by the
   overall root of the MDT. The segments themselves will be computed as
   per a loosely specified MDT.

   A PE acting as an overall root for a given tree is expected to be
   configured by the operator as to where to source multicast traffic
   from, be it an attachment circuit, interworking function for client
   technology or other. Similarly a leaf for a given tree is expected to
   be configured by the operator as to the disposition of received
   multicast traffic.

   A computed segment is guaranteed to be loop free in a stable system.
   A concatenation of segments to construct an MDT will similarly be
   loop free as any collision of segments can be disambiguated in the
   data plane via the SIDs.

   This architecture significantly reduces the amount of state that
   needs to be installed in the data plane to support multicast. This
   also means that the impact of many failures in the network on
   multicast traffic distribution will be recovered by unicast local
   repair or unicast convergence with subsequent multicast convergence
   acting in the role of network re-optimization (as opposed to

4.1. Mapping source specific trees onto the segment routing architecture

   A computed source specific tree for a given multicast group
   corresponds to one or more multicast segments in the SR architecture.
   Each multicast segment is assigned a SID, typically by management
   configuration of the node that will be the overall root for the
   source specific tree. The root node then uses the IGP to advertise
   this information to all nodes in the IGP area/domain.

   A multicast group is implemented as the set of source specific trees
   from all nodes that have registered transmit interest to all nodes
   that have registered receive interest in a multicast group.

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4.2. Role of the Routing System

   The role of the IGP is to communicate topology information, multicast
   capability and associated algorithm, multicast registrations, unicast
   to SID bindings, multicast to SID bindings and waypoints in multi-
   segment MDTs. No changes to topology or unicast to SID binding
   advertisements are proposed by this memo.

   The multicast registrations/bindings will be in the form of source,
   group, transmit/receive interest and the SID to use for the source
   specific multicast tree. Registrations are originated by any node
   that has send or receive interest in a given multicast group. Nodes
   will use the combination of topology and multicast registrations to
   determine the nodes that have a role in each source specific tree and
   the SID information to then derive the required FIB state.

4.3. MDT Construction Requirements

   A multicast segment in an MDT is constructed such that between any
   pair of nodes that have a role in the segment and are connected by a
   unicast tunnel, there is not another node on the shortest path
   between the two with a role in that segment. This ensures that copies
   of a packet forwarded by an multicast segment will traverse a link
   only once in a stable system.

   Note that this can be satisfied by a minimum cost shortest path tree,
   but is not an absolute requirement. The pruning rules specified in
   this memo will meet this requirement without necessarily producing
   absolutely minimum cost multicast segment (or incurring the
   associated computational cost).

4.4. Pruning - theory of operation

   The role of nodes in a given multicast segment is determined by first
   producing an inclusive shortest path tree with all possible paths
   between the root and leaves, and then applying a set of pruning rules
   repeatedly until an acyclic tree is produced or no further prunes are

   For the majority of multicast segments these rules will
   authoritatively produce a minimum cost tree. For those segments that
   have not yet been authoritatively resolved, there is a set of pruning
   operations applied that are not guaranteed to produce a tree that
   meets the requirements of 3.3, therefore these trees require auditing
   and potential correction according to a further set of agreed rules.
   This avoids the necessity of an exhaustive search of the solution

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   A node during computation of a segment may conclude that it will
   absolutely not have a role at any of numerous points in the
   computation process and abandon computation of that segment.

5. Elements of Procedure

5.1. Triggers for Computation

   MDT computation is triggered by changes to the IGP database. These
   are in the form of either changes in registered multicast group
   interest, addition or removal of a multi-segment MDT descriptor, or
   topology changes.

   A change in registered interest for a group will require re-
   computation of all MDTs that implement the multicast group.

   A topology change will require the computation of some number of
   multicast segments, the actual number will depend on the
   implementation of tree computation but at a minimum will be all trees
   for which there is not an optimal shortest path solution as a result
   of the topology change.

5.2. FIB Determination

5.2.1. Information in the IGP

   Group membership information for a multicast segment is obtained from
   the IGP. This is true for single segment MDTs as well as multi-
   segment MDTs. Included in the multi-segment MDT specification is the
   waypoint nodes in MDT and the upstream and downstream SIDs. The
   specified node is expected to cross connect the SIDs to join the
   segments together acting in the role of leaf for the upstream segment
   and root for the downstream segment.

   When a waypoint in an MDT descriptor does not exist in the IGP, the
   assumption is that the node identified by the waypoint SID has
   failed. The response of the other nodes in the system in FIB
   determination is to add the leaves of the downstream segment to the
   upstream segment.

   An example of this would be consider a node "x", and another node
   "y". At some point in time, "x" advertises a tree that identifies "y"
   as a waypoint that cross connects upstream SID "a" to downstream SID
   "b". At some later point node "y" fails. The other nodes in the
   network will compute segment "a" as if it included all leaves and
   waypoints in segment "b". All apriori state installed for segment "b"

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   would be removed as the failure of "y" has required "b" to be
   subsumed by "a".

5.2.2. Computation of individual segments

   FIB generation for a multicast segment is the result of computation,
   ultimately as applied to all source specific trees in the network.
   All computing nodes implement a common algorithm for tree generation,
   as all MUST agree on the solution.

   One algorithm is as follows:

   All possible shortest paths to the set of leaves for the MDT is
   determined. Then pruning rules are repeatedly applied until no
   further prunes are possible.

   The philosophy of the application of these rules could be expressed
   as "simplify as much as possible, and prune that which cannot be".
   The rules are:

   1) Eliminate any links and nodes not on a potential shortest path
     from the root to the leaves for the MDT under consideration.

   2) Simplify via the replacement of any nodes that do not have a
     potential role in the MDT with links.

     This will be nodes that are not a leaf, a root or a candidate
     replication point. For example:


     B is a leaf. A is not but is in a potential shortest path from root
     to B. However A will have no role in the MDT that serves B as it
     provides simple transit therefore is replaced with a direct
     connection between the root and B.


     Note that such pruning also needs to avoid the creation of
     duplicate parallel links. For example:


         Root                       B


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     Where A and C have no role and the cost root-A-B = cost root-C-B,
     they can be replaced with a single link from Root to B.

   3) Simplify via the elimination of fewer hop paths

     When for a given set of leaves, a node has multiple downstream
     links that converge on a common downstream point, and that set of
     leaves is only a subset of the leaves reachable on one or more of
     the links, any link that only serves that subset of leaves can be

     For example:


          \                         /




     Link AB is cost 2, link AC and CB are cost 1 (cost of link CD does
     not affect the example).

     B and D are leaves of a root upstream of A. From A, link AB can
     reach leaf B. Path AC can reach leaf B and D. In this case path A-B
     can be pruned from consideration. The set of leaves reachable via
     link A-B is a subset of that reachable by A-C, and the paths from A
     that serves that subset converges at B.

   4) Prune upstream links.

     The normal procedure is to determine the closest upstream leaf or
     pinned path and then compare all upstream adjacencies with that

        a.  If the upstream adjacency extends closer to the root than
          the closest leaf or pinned path, then that adjacency can be

        b.  If the upstream adjacency extends the same distance towards
          the root then

            i. If it is to a non-leaf or pinned path candidate
               replication point, it can be pruned

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           ii. If it is to a pinned path, where there are equal upstream
               adjacencies that terminate on leaves, it can be pruned
               (considered inferior).

          iii. If there is more than one "equal" upstream adjacency,
               that is all terminate on nodes that are on pinned paths,
               or all terminate on nodes that are leaves, than one is
               selected. This is via the lowest node ID.

        c.  If the upstream adjacency is a candidate replication point
          closer than the closest leaf, and upstream from it is a node
          that is a leaf or pinned path equidistant with the closest
          leaf, then all adjacencies that extend to leaves ranked lower
          than the leaf or pinned path behind the CRP may be pruned.
          Note that an upstream adjacency that has a CRP closer than
          the closest leaf or pinned path cannot be pruned.

        d.  When for a given node all possible upstream adjacencies that
          can be pruned have been identified, each is removed, and any
          simplifications that can be performed as a result of the
          prune are performed. This is the equivalent of a localized
          check for 2 and 3 above and is then performed iteratively in
          response to changes to the graph as a result of pruning.

   The procedure is to implement 1, 2 and 3 above, then loop on 4 until
   such time as the MDT is fully resolved, or no further prunes are
   possible. Step 4 is performed in a specific order. The nodes are
   processed according to a ranking from closest to the root to the
   farthest, and from lowest node ID to the highest within a given
   distance from the root.

   If, at the end of pruning and simplification, all leaves in a
   multicast segment have a unique shortest path to the root, the tree
   is considered resolved, and the computation can progress directly to
   the FIB generation step.

   If not all leaves have a unique shortest path, additional pruning
   steps are applied. These steps are NOT guaranteed to produce a lowest
   cost tree, and therefore require an additional audit and possible
   modification to ensure when forwarding a maximum of one copy of a
   packet will traverse an interface.

   For segments not authoritatively resolved by the above rules, a prune
   that will not authoritatively result in a minimum cost tree is
   applied. For the purpose of interoperability, the following rule is
   proposed: A computing node will select the closest node to the root
   with a candidate role that does not have a unique shortest path to

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   the root. Where more than one such node exists, the one with the
   lowest unicast SID is selected. For that node, the best upstream link
   is selected and all other upstream links pruned. The best upstream
   link is defined as the link with the closest node with a candidate
   role that potentially serves the highest number of leaves. Where
   there is a tie, once again the node with the lowest SID is selected.

   Once the links have been pruned, rules 2 through 4 are repeatedly
   applied until either the tree is fully resolved, or again no further
   prunes are possible, in which case the next closest remaining
   unresolved node has the same prune applied.

   For all segments not resolved by the initial prune rules, they are
   audited to ensure all nodes that have a role in the tree do not have
   a node with a role between them and their upstream node on the tree.
   If they do, the old upstream adjacency is removed, and the superior
   one added.

5.3. FIB Generation

   The topology components that remain at the end of the pruning
   operation will reflect all nodes that have a role in a given
   multicast segment plus the necessary tunnels (as all intervening
   multi-path scenarios will have been simplified away). From this the
   FIB can be generated:

   All nodes that have a role in a given multicast segment and have
   nodes upstream in the segment will need to accept the SID for the MDT
   from at minimum, all upstream interfaces.

   All nodes that have a role in a given segment and have nodes
   immediately downstream in the segment will need to replicate packets
   simply labelled with the multicast SID onto those interfaces.

   All nodes that have a role in a given segment and have nodes
   reachable via a tunnel downstream set the FIB to push the tunnel
   unicast SID for the downstream node onto any replicated copies of a
   received packet, and identify the set of interfaces on the shortest
   path for the tunnel SID.

5.4. FIB installation

   FIB installation needs to acknowledge two aspects of the hybrid
   tunnel and role model of multicast tree construction. The first is
   that because of the sparse state model simple tree adds, moves, and
   changes may require the installation of state where it did not
   previously exist, and such changes may impact existing services. The

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   second is that it is possible to retain the knowledge to prioritize
   computation of those trees impacted the failure of a node with a

   To address this, there are three stages of state installation for
   multicast convergence:

   1) Immediate:

        a.  Installation of state for multicast segments impacted by the
          failure of a node in the network, and installation of state
          for segments in nodes that have not previously had a role in
          the given segment.

        b.  Installation of state for waypoints in multi-segment MDTs.

   2) After T1: Update state for nodes that both had and have a role in
     a given multicast segment.

   3) After T2: Removal of state for nodes that transition from having a
     role to not having a role for a given multicast segment.

   T1 and T2 are network wide configurable values.

6. Related work

6.1. IGP Extensions

   The required IGP changes are documented in [MCAST-ISIS] and [MCAST-

6.2. BGP Extensions

   This memo will require the specification of a new PMSI Tunnel
   Attribute (SPRING P2MP tunnel, tentatively 0x09) to order to
   integrate into the multicast framework documented in RFC 6514

7. Observations

   This technique is not confined to segment routing, and with the
   provision of a global label space (to be employed as per a multicast
   SID), an MPLS-LDP network would also provide the requisite mesh of
   unicast tunnels and be capable of implementing this approach to

   This memo focuses on an implementation based upon nodes that are IGP
   speakers and converge independently so is written in a form that

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   assumes a node, computing node and IGP speaker are one in the same.
   It should be observed that the relative frugality of data plane state
   would suggest that separation of computation from nodes in the data
   plane combined with management or "software defined networking" based
   population of the multicast FIB entries may also be useful modes of
   network operation.

8. Acknowledgements

   Thanks to Uma Chunduri for his detailed review and suggestions.

9. Security Considerations

   For a future version of this document.

10. IANA Considerations

   This document requires the allocation of a PMSI tunnel type to
   identify a SPRING P2MP tunnel type from the P-Multicast Service
   Interface Tunnel (PMSI Tunnel) Tunnel Types registry.

11. References

11.1. Normative References

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

11.2. Informative References

[MCAST-ISIS] Allan et.al., "IS-IS extensions for Computed Multicast
        applied to MPLS based Segment Routing", IETF work in progress,
        draft-allan-isis-spring-multicast-00, July 2016

[MCAST-OSPF] Allan et.al., "OSPF extensions for Computed Multicast
        applied to MPLS based Segment Routing", IETF work in progress,
        draft-allan-ospf-spring-multicast-00, July 2016

[RFC6514] Aggarwal et.al., "BGP Encodings and Procedures for Multicast
         in MPLS/BGP IP VPNs", IETF RFC 6514, February 2012

[RFC7385] Andersson & Swallow "IANA Registry for P-Multicast Service
         Interface (PMSI) Tunnel Type Code Points", IETF RFC 7385,
         October 2014

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12. Authors' Addresses

   Dave Allan (editor)
   300 Holger Way
   San Jose, CA  95134
   Email: david.i.allan@ericsson.com

   Jeff Tantsura
   Email: jefftant.ietf@gmail.com

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