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Non-source-routed Multicast in SR Networks

Document Type Active Internet-Draft (individual)
Authors Zhaohui (Jeffrey) Zhang , IJsbrand Wijnands , Hooman Bidgoli , Yisong Liu
Last updated 2024-03-04
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PIM                                                             Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Informational                               I. Wijnands
Expires: 5 September 2024                                         Arrcus
                                                              H. Bidgoli
                                                                  Y. Liu
                                                            China Mobile
                                                            4 March 2024

               Non-source-routed Multicast in SR Networks


   With Segment Routing (SR) architecture, a unicast flow can be source-
   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.  In the case of
   multicast, traffic can be either source-routed or non-source-routed,
   and this document discusses non-sourced-routed options for multicast
   in an SR network with either MPLS or IPv6/SRv6 data plane.

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

   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 5 September 2024.

Copyright Notice

   Copyright (c) 2024 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 (
   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Traditional Multicast Technologies  . . . . . . . . . . . . .   3
   3.  Bit Index Explicit Replication  . . . . . . . . . . . . . . .   3
   4.  Multicast Trees Set up by Other Means . . . . . . . . . . . .   4
   5.  SR Specific Solutions . . . . . . . . . . . . . . . . . . . .   5
   6.  IPv6 Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. Informative References  . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   With Segment Routing (SR) architecture [RFC8402], a unicast flow can
   be source-routed through an SR network following an explicit path
   specified in the packet, without 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 non-source-routed multicast in
   an SR network, including traditional multicast technologies, BIER,
   and various SR specific solutions.  The pros and cons of each
   solution are listed for considerations by operators and vendors.

   Source-routed multicast in an SRv6 network has also been discussed in
   IETF [
   routing-over-ipv6msr6/] and some work is ongoing.  However, that is
   outside the scope of this document.

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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
   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 (referred to as SR policies), which could be MPLS/

   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 protocol packet encoding)
   as 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.

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   Even though BIER is designed and developed independent of SR, it is a
   perfect fit for an SR network because it shares with SR the number
   one principle of "no per-flow state".  A BIER domain can be used as
   provider/underlay tunnels for MVPN/EVPN-BUM (just like 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

   *  Several major ASIC makers have BIER support on their ASIC

   *  Several Major router vendors have BIER hardware capability across
      their access/aggregation/edge/core platforms.

   *  Some major router vendors have had or will soon have interoperable
      production support of BIER on certain platforms.  Roadmaps for
      BIER on more platforms also exist.

   *  There have been very good discussions and progress in BIER WG for
      methods of brown field deployments (
      [I-D.przygienda-bier-migration-options] [I-D.ietf-bier-tether]
      [I-D.ietf-bier-multicast-as-a-service] [I-D.ietf-bier-php])

   *  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/PCEP [I-D.ietf-bess-bgp-multicast]
   [I-D.ietf-idr-sr-p2mp-policy] [I-D.ietf-pce-sr-p2mp-policy], as
   described in [I-D.ietf-pim-sr-p2mp-policy].

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5.  SR Specific Solutions

   There are two multicast solutions that are specifically tied to SR
   technologies - "SR-P2MP" [I-D.ietf-pim-sr-p2mp-policy] and "Spray".

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

   2.  Spray is Ingress Replication with 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.

   The above methods are really no different from existing technologies.
   Spray is just IR with a different encapsulation (e.g.  SRv6).  While
   SR-P2MP does not use existing tree signaling protocol, it still
   requires per-tree forwarding state on the tree nodes (roots, leaves
   and branching points), and it is identical to mLDP/RSVP P2MP tunnels
   in the MPLS data plane on those tree nodes.

   mLDP/PIM trees are typically hop-by-hop - the tree nodes are directly
   connected, though tunnels can be used between mLDP, PIM and SR-P2MP
   tree nodes to avoid tree states on the non-branching nodes between
   tree nodes.  This is important in some deployment scenarios (e.g.,
   mobile user plane transport) where the replication points are
   scattered with many non-replication points among them.

   SR-P2MP can use both MPLS and SRv6 data plane.  The next section
   discusses some SRv6 aspects of SR-P2MP.

6.  IPv6 Considerations

   Some operators run IPv6/SRv6 networks without using MPLS at all.  As
   a result, Ingress Replication or SR-P2MP with MPLS data plane is not
   an option and IPv6 based forwarding must be used.  There are two
   options in this case - traditional IPv6 multicast (signaled by either
   PIM or BGP [I-D.ietf-bess-bgp-multicast]
   [I-D.ietf-bess-bgp-multicast-controller]) and SRv6-P2MP (SR-P2MP with
   SRv6 data plane).

   SR-P2MP with MPLS data plane (SRmpls-P2MP) is identical to mLDP/RSVP-
   P2MP in the data plane of the tree nodes, in that incoming multicast
   traffic carries a tree-identifing label and is replicated with
   outgoing label stacks.  The label stack includes a label that
   identifies the tree to the downstream node, and optional labels that
   leads the traffic to the downstream node.

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   SRv6-P2MP is very similar to SRmpls-P2MP in the data plane.
   Multicast traffic has an IPv6 header and the destination address has
   the LOC:FUNCT:ARGS encoding format for SRv6 [RFC8986].  The FUNCT
   bits correspond to the tree-identifying label in SRmpls-P2MP, and the
   LOC bits get the traffic from an upstream node to a downstream node
   (which could be directly or indirectly connected).

   SRv6-P2MP allows native tunneling of multicast traffic between
   indirectly connected upstream and downstream nodes, but at the cost
   of changing destination address at each tree node, which requires
   either programmable ASIC or new generation of non-programmable ASIC.
   It also requires a new control plane that involves controller-based
   calculation and signaling of the tree state.

   On the other hand, traditional IPv6 multicast is mature in both
   control plane and data plane.  PIM has been widely deployed for many
   years without platform restrictions, and the PIM-SSM protocol is very
   straightforward without the complexity of PIM-ASM.

   Even if an operator wants to use controller based calculation and
   signaling, or does not want to run PIM protocol, traditional
   multicast can still be used with BGP/controller-based signaling as
   specified in [I-D.ietf-bess-bgp-multicast] and

7.  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 when source-
   routing is not used:

   *  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 does).

   *  If efficient multicast replication is required, mLDP/RSVP-TE/PIM
      can still be used for multicast purpose if that is acceptable.

   *  If the replication points are scattered with many non-replication
      points between them, or if there is a strong desire to create
      multicast forwarding state without relying on PIM/mLDP or RSVP-TE,
      one can use static configuration (e.g., netconf based
      provisioning) or controller-based tree calculation/signaling (SR-
      P2MP, BGP-signaled IP multicast or mLDP).

   *  Use Ingress Replication with SR unicast tunnels.

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

8.  Security Considerations

   This informational document does not introduce new security risks.

9.  Acknowledgements

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

10.  Informative References

              Zhang, Z. J., Giuliano, L., Patel, K., Wijnands, I.,
              Mishra, M. P., and A. Gulko, "BGP Based Multicast", Work
              in Progress, Internet-Draft, draft-ietf-bess-bgp-
              multicast-07, 2 December 2023,

              Zhang, Z. J., Raszuk, R., Pacella, D., and A. Gulko,
              "Controller Based BGP Multicast Signaling", Work in
              Progress, Internet-Draft, draft-ietf-bess-bgp-multicast-
              controller-12, 30 December 2023,

              Zhang, Z. J., Rosen, E. C., Awduche, D. O., Shepherd, G.,
              Zhang, Z., and G. S. Mishra, "Multicast/BIER As A
              Service", Work in Progress, Internet-Draft, draft-ietf-
              bier-multicast-as-a-service-03, 6 February 2024,

              Zhang, Z. J., "BIER Penultimate Hop Popping", Work in
              Progress, Internet-Draft, draft-ietf-bier-php-11, 6
              February 2024, <

              Bidgoli, H., Xu, F., Kotalwar, J., Wijnands, I., Mishra,
              M. P., and Z. J. Zhang, "PIM Signaling Through BIER Core",
              Work in Progress, Internet-Draft, draft-ietf-bier-pim-

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              signaling-12, 25 July 2021,

              Zhang, Z. J., Warnke, N., Wijnands, I., and D. O. Awduche,
              "Tethering A BIER Router To A BIER incapable Router", Work
              in Progress, Internet-Draft, draft-ietf-bier-tether-05, 16
              February 2024, <

              Bidgoli, H., Voyer, D., Stone, A., Parekh, R., Krier, S.,
              and S. Agrawal, "Advertising p2mp policies in BGP", Work
              in Progress, Internet-Draft, draft-ietf-idr-sr-p2mp-
              policy-00, 27 May 2022,

              Bidgoli, H., Voyer, D., Rajarathinam, S., Budhiraja, A.,
              Parekh, R., and S. Sivabalan, "PCEP extensions for p2mp sr
              policy", Work in Progress, Internet-Draft, draft-ietf-pce-
              sr-p2mp-policy-05, 3 March 2024,

              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
              J. Zhang, "Segment Routing Point-to-Multipoint Policy",
              Work in Progress, Internet-Draft, draft-ietf-pim-sr-p2mp-
              policy-07, 11 October 2023,

              Przygienda, T. and Z. J. Zhang, "BIER Migration
              Frameworks", Work in Progress, Internet-Draft, draft-
              przygienda-bier-migration-options-00, 22 June 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,

   [RFC7473]  Raza, K. and S. Boutros, "Controlling State Advertisements
              of Non-negotiated LDP Applications", RFC 7473,
              DOI 10.17487/RFC7473, March 2015,

   [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, <>.

   [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, <>.

   [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,

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks

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   IJsbrand Wijnands

   Hooman Bidgoli

   Yisong Liu
   China Mobile

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