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PIM                                                             Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Informational                              IJ. Wijnands
Expires: February 16, 2019                                 Cisco Systems
                                                             A. Dolganow
                                                                 L. Geng
                                                            China Mobile
                                                         August 15, 2018

                        Multicast in SR Networks


   With Segment Routing (SR) architecture [rfc8402], a unicast flow can
   be routed through an SR network following an explicit path specified
   in the packet, w/o the need for per-flow state in the network.  As a
   result, the otherwise needed protocols to signal the per-flow unicast
   state can also be removed from the network.  This document discusses
   options for multicast in an SR network.

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 February 16, 2019.

Copyright Notice

   Copyright (c) 2018 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

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   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.  Traditional Multicast Technologies  . . . . . . . . . . . . .   2
   3.  Bit Index Explicit Replication  . . . . . . . . . . . . . . .   3
   4.  Multicast Trees Set up by Other Means . . . . . . . . . . . .   4
   5.  SR Specific Solutions . . . . . . . . . . . . . . . . . . . .   4
   6.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   With Segment Routing (SR) architecture [RFC8402], a unicast flow can
   be routed through an SR network following an explicit path specified
   in the packet, w/o the need for per-flow state in the network.  As a
   result, the otherwise needed protocols to signal the per-flow unicast
   state can also be removed from the network.

   This document summarizes options for multicast in an SR network,
   including traditional multicast technologies, BIER, and various SR
   specific proposals.  The pros and cons of each solution are listed
   for considerations by operators and vendors.

2.  Traditional Multicast Technologies

   Traditional multicast technologies include PIM [RFC7761], RSVP-TE
   P2MP [RFC4875], and mLDP [RFC6388].  They all require per-tree state
   on nodes on the tree, and the corresponding protocols to signal and
   maintain the state.  An incoming packet's IP header or MPLS label is
   looked up, and the packet is forwarded according to the matched

   While SR allows simplification of state, protocols and centralized
   SDN-control, the traditional methods of delivering multicast traffic
   run contrary to those SR goals.

   An alternative is Ingress Replication (IR) - an upstream node of a
   multicast packet tunnels individual copies to some downstream nodes

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   across some intermediate nodes.  In an SR network, the unicast
   tunnels used for IR could be traditional IP/MPLS tunnels or could be
   SR tunnels.

   While with IR there is no per-tree state on those intermediate nodes,
   multiple copies of the same packet may be sent over the same link.
   If efficient replication is required, PIM/mLDP/RSVP-TE P2MP can still
   be used for multicast even in an SR network.  Deploying SR in the
   network does not mandate changing the solution that is in place for

   While mLDP uses the same LDP Session (and packet encoding) as is used
   by unicast and there is code sharing between LDP and mLDP with
   regards to neighbor discovery, session setup and Label allocation
   management, the protocol logic for setting up mLDP tunnel is
   completely different.  It is understood that there is a desire to
   remove LDP from the network when SR is deployed, but there is really
   no problem to continue to run mLDP for Multicast.  RFC7473 documents
   a solution where LDP can be run in mLDP-Only mode by simply not
   exchanging any Unicast bindings.

   Similarly, RSVP-TE P2MP tunnel setup is orthogonal from P2P tunnel
   setup as well.  It can be used for multicast even in an SR network if
   bandwidth reservation and/or per-tunnel explicit path are required.

3.  Bit Index Explicit Replication

   Bit Index Explicit Replication (BIER) [RFC8279] is a new multicast
   technology that achieves efficient replication as with PIM tree or
   mLDP/RSVP-TE P2MP tunnels but without requiring per-tree/tunnel state
   in the transit nodes as with IR.

   BIER is a perfect fit for an SR network.  A BIER domain can be used
   as provider/underlay tunnels for MVPN/EVPN-BUM (just as traditional
   PIM tree or mLDP/RSVP P2MP tunnels), or can be part of end-to-end PIM
   trees [I-D.ietf-bier-pim-signaling] (similiar to mLDP inband
   signaling [RFC6826]).

   However, BIER uses a new forwarding plane that cannot be implemented
   on many deployed routers.  On the other hand, the entry point barrier
   is decreasing and BIER is becoming more realistic with the following

   o  Several major router vendors have wide deployment of platforms
      capable of BIER (with a software upgrade)

   o  Several ASIC vendors have BIER support on their latest/upcoming
      ASIC offering

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   o  There have been very good discussions in BIER WG for methods of
      brown field deployments ( [I-D.przygienda-bier-migration-options]

   o  More and more customers now have concrete plans and requirements
      for BIER deployment in their networks

4.  Multicast Trees Set up by Other Means

   Some operators may accept that they need to maintain per-tree state
   in their SR network for efficient multicast, and their applications
   do not require too much per-tree multicast state.  However, since
   unicast no longer needs LDP or RSVP-TE protocol in an SR network,
   they really want to remove PIM/LDP/RSVP-TE protocol from their
   networks, and make use of controllers.  In that case, the multicast
   trees could be set up statically from management plane (e.g.,
   netconf), or dynamically via BGP [I-D.zzhang-bess-bgp-multicast].

5.  SR Specific Solutions

   There are two multicast solutions that are specifically tied to SR
   technologies - "TreeSID" and "Spray"
   [I-D.voyer-spring-sr-p2mp-policy].  There is also a proposal
   [I-D.allan-pim-sr-mpls-multicast-framework] that uses calculated
   multicast trees based on flooded membership information.

   1.  TreeSID essentially refers to P2MP tunnels setup not by mLDP or
       RSVP-TE.  It could be statically configured, or instantiated from
       a controller.

   2.  Spray is Ingress Replication.  In particular, in case of SRv6,
       instead of regular IP/MPLS tunneling, an SRv6 header encodes the
       "tunnel end" and the original destination multicast address.
       When the packet arrives at the tunnel end, the original multicast
       address is restored from the SRv6 header.

   3.  In [I-D.allan-pim-sr-mpls-multicast-framework], multicast
       membership information is flooded, and each node will calculate
       if it plays a role (as a root, leaf, or a branching point) for
       each multicast tree.  If yes, corresponding forwarding state is
       installed to forward incoming multicast traffic accordingly.

   The above three methods are really no different from existing
   technologies. 1 and 3 do not use existing tree signaling protocol but
   still require per-tree forwarding state on roots, leaves and
   branching points (for comparison, existing technologies does have
   per-tree state on all nodes on a tree - including those non-branching

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   points).  3 is similar to MOSPF [RFC 1584] and requires flooded
   receiver information throughout the domain. 2 is just IR with a
   different encapsulation (e.g.  SRv6).

6.  Summary

   There is no one-for-all option when it comes to multicast in an SR
   network.  Depending on specific deployment scenarios and
   requirements, the following could be considered in order:

   o  If most of nodes can support it, BIER is the best choice, because
      it removes state from the network and simplifies the control-plane
      (like SR).

   o  If efficient multicast replication is required, then run mLDP/
      RSVP-TE/PIM for multicast purpose if that is acceptable.

   o  If there is a strong desire to create multicast forwarding state
      without relying on mLDP and/or RSVP-TE, one can use static
      configuration or controller signaling (e.g.  SR specific TreeSID,
      generic BGP based signaling or netconf based provisioning).

   o  Use Ingress Replication with SR unicast tunnels.  This includes
      Spray Multicast if SRv6 is desired.

   In case of MPLS based P2MP, a Binding SID could be optionally used to
   direct traffic to the root of the tunnel.

   Notice that there are already many protocols defined (PIM, mLDP,
   RSVP-TE, BGP, MOSPF) that can be used to create multicast forwarding
   state.  When designing a new protocol to do the same, we need to be
   sure the benefits are significant enough and out-weight the cost of
   developing it.

7.  Security Considerations

   This document does not seem to introduce new security risks.

8.  Acknowledgements

   The authors thank Eric Rosen, Tony Przygienda, and John Drake for
   their reviews, comments and suggestions.

9.  Informative References

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              Allan, D., Tantsura, J., and I. Duncan, "A Framework for
              Computed Multicast Applied to SR-MPLS", draft-allan-pim-
              sr-mpls-multicast-framework-00 (work in progress), June

              Bidgoli, H., Dolganow, A., Kotalwar, J., Xu, F., mishra,
              m., and Z. Zhang, "PIM Signaling Through BIER Core",
              draft-ietf-bier-pim-signaling-03 (work in progress), June

              Przygienda, T. and Z. Zhang, "BIER Migration Frameworks",
              draft-przygienda-bier-migration-options-00 (work in
              progress), June 2018.

              daniel.voyer@bell.ca, d., Hassen, C., Gillis, K.,
              Filsfils, C., and A. Venkateswaran, "SR Replication Policy
              for P2MP Service Delivery", draft-voyer-spring-sr-p2mp-
              policy-00 (work in progress), June 2018.

              Zhang, Z., Patel, K., Wijnands, I., and a.
              arkadiy.gulko@thomsonreuters.com, "BGP Based Multicast",
              draft-zzhang-bess-bgp-multicast-01 (work in progress),
              March 2017.

              Zhang, Z., Raszuk, R., Pacella, D., and A. Gulko,
              "Controller Based BGP Multicast Signaling", draft-zzhang-
              bess-bgp-multicast-controller-00 (work in progress),
              September 2017.

              Zhang, Z., "BIER Penultimate Hop Popping", draft-zzhang-
              bier-php-01 (work in progress), August 2018.

   [RFC4875]  Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
              Yasukawa, Ed., "Extensions to Resource Reservation
              Protocol - Traffic Engineering (RSVP-TE) for Point-to-
              Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
              DOI 10.17487/RFC4875, May 2007,

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   [RFC6388]  Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
              Thomas, "Label Distribution Protocol Extensions for Point-
              to-Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011,

   [RFC6826]  Wijnands, IJ., Ed., Eckert, T., Leymann, N., and M.
              Napierala, "Multipoint LDP In-Band Signaling for Point-to-
              Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6826, DOI 10.17487/RFC6826, January 2013,

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,

   [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>.

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks

   EMail: zzhang@juniper.net

   IJsbrand Wijnands
   Cisco Systems

   EMail: ice@cisco.com

   Andrew Dolganow

   EMail: andrew.dolganow@nokia.com

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   Liang Geng
   China Mobile

   EMail: gengliang@chinamobile.com

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