BIER Working Group                                          IJ. Wijnands
Internet-Draft                                                P. Pfister
Intended status: Standards Track                           Cisco Systems
Expires: April 21, 2016                                 October 19, 2015

               Generic Multicast Router Election on LAN's


   When a host is connected to multiple multicast capable routers, each
   of these routers is a candidate to process the multicast flow for
   that LAN, but only one router should be elected to process it.  This
   document proposes a generic multicast router election mechanism using
   Internet Group Management Protocol (IGMP) and Multicast Listener
   Discovery (MLD) that can be used by any Multicast Overlay Signalling
   Protocol (MOSP).  Having such generic election mechanism removes a
   dependency on Protocol Independent Multicast (PIM).

Status of This Memo

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   This Internet-Draft will expire on April 21, 2016.

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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Specification of Requirements . . . . . . . . . . . . . . . .   4
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Receiver side . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Sender side . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Proposal  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Hosts connected to Local Area Networks (LAN) use Internet Group
   Management Protocol (IGMP) [RFC4605] or Multicast Listener Discovery
   (MLD) [RFC3810] to report their interest in a particular multicast
   flow.  A multicast flow is identified by a Group or a combination of
   Group and Source address.  Routers connected to a LAN listen to these
   membership reports and signal that information to the Multicast
   Overlay Signalling Protocol (MOSP).  When a host is connected to
   multiple routers, each of these routers is a candidate to forward the
   multicast flow onto that LAN, but only one of them should forward the
   packets for a given flow to avoid duplication of Multicast packets.
   A similar requirement exists for hosts that are sending multicast
   traffic and are connected to multiple routers on a LAN.  If multiple
   routers accept the multicast packets from the LAN, duplication may
   occur and/or routing loops may be created.

   Protocol Independent Multicast (PIM) [RFC4601] is a MOSP and has a
   built-in mechanism to elect a Designated Router (DR) on the receiver
   LAN and a Designated Forwarder (DF) on the senders LAN.  The DR/DF
   election avoids duplication and looping of multicast packets.  Other
   existing or candidate MOSPs, like Border Gateway Protocol (BGP)
   [RFC6514], Multi-point Label Distribution Protocols (mLDP) [RFC6826],
   Locator ID Seperation Protocol (LISP) [RFC6830] and IGMP/MLD
   [I-D.pfister-bier-mld] have no embedded LAN DR/DF election mechanism.
   These MOSPs still rely on PIM to perform DR/DF election on LANs.

   With the introduction of mLDP and Bit Indexed Explicit Replication
   (BIER) [I-D.ietf-bier-architecture], there is no dependency on PIM to

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   transport multicast packets through the network.  Having a dependency
   on PIM just for DR/DF election is undesirable if PIM is not selected
   as the MOSP.  This document proposes a generic DR/DF election which
   can be used by any MOSP without having a dependency on PIM.  It
   potentially allows for different MOSPs to coexistence on single LANs.

2.  Terminology and Definitions

   Readers of this document are assumed to be familiar with the
   terminology and concepts of the documents listed as Normative
   References.  For convenience, some of the more frequently used terms
   appear below.

      Local Area Network.

      Internet Group Management Protocol.

      Multicast Listener Discovery.

      Multipoint LDP.

      Protocol Independent Multicast.

      Any Source Multicast.

      The PIM Rendezvous Point.

      Locator ID Seperation Protocol.

      Bit Indexed Explicit Replication.

      Multicast Overlay Signalling Protocol.  This is a protocol that is
      (potentially) capable of announcing multicast flow membership
      across the network between multicast routers.  For example PIM,
      mLDP, BGP, IGMP, MLD and LISP.

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3.  Specification of Requirements

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

4.  Problem Statement

   In the following sections we describe the requirements for DR/DF
   election in more detail for hosts that are multicast senders and
   receivers connected to multiple routers on a single LAN.

4.1.  Receiver side

   Consider the network below in Topology1.

                       +---- MOSP ----+
                                    ( R3 ) -|
                  LAN1             /        |
              S H1-|-( R1 )--( R2 )         |- H2  (joined G)
                                   \        |
                                    ( R4 ) -|

                                 Figure 1

   Suppose that H2 on LAN2 is joining a multicast Group G.  The MOSP
   runs between R1, R3 and R4.  Both R3 and R4 will receive the IGMP/MLD
   report, but only one of these should become the DR.  One might
   consider that this problem can be detected and resolved by the MOSP.
   The MOSP could be enhanced to allow R1 to detect that both R2 and R4
   are connected to the same LAN, and select only to forward the
   multicast flow to R3.  That would solve the problem in the above
   topology, but would fail in the topology below:

                       +---- MOSP ----+
                                    ( R3 ) -|
                  LAN1             /        |
              S H1-|-( R1 )--( R2 )         |- H2  (joined G)
                                   \        |
                                    ( R4 ) -|
                                       - LAN3
                                       H3 (joined G)

                                 Figure 2

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   Consider that H3 on LAN3 joined the same multicast Group G.  Since H3
   is singly connected to R4, router R1 needs to forward the multicast
   flow to R4 in order for H3 to receive the packets.  R4 does not have
   enough information to determine whether or not to forward on LAN2 for
   H2 when it receives the multicast packets due to H3.  In other words,
   R4 needs DR state to avoid sending packets to H2 on LAN2.

4.2.  Sender side

   Consider the network below in Topology3.

                        +---- MOSP ----+
                  |- ( R1 )
                  |        \               LAN2
            S H1 -|        ( R3 ) -- ( R4 ) -|- H2  (joined G)
                  |        /
                  |- ( R2 )

                                 Figure 3

   H1 is directly connected via a LAN1 to R1 and R2.  H2 joins a
   multicast Group G, without specifying the Source.  This is called Any
   Source Multicast (ASM).  The MOSP signals R4s interest in Group G to
   R1 and R2.  Note that there is no PIM deployed in this network and
   there is no Rendezvous Point (RP) that is a target for this receiving
   this Group membership.  R4 has no information which routers in this
   network have multicast packets to sent for this Group.  Since this is
   ASM, there may be multiple senders for this Group and H2 wants to
   receive them all.  For that reason, R4 will use the MOSP to announce
   the membership to all edge nodes in the network (R1 and R2).  This
   poses a potential problem since R1 and R2 are both directly connected
   to the Source S.  If both R1 and R2 will forward the multicast
   packets to R4, H2 will receive duplicate packets.  This is a problem
   that only occurs when a Source is dually connected to two or more
   routers connected to the BIER domain.  This problem can be resolved
   by doing a Designated Forwarder (DF) election, similar to the DR
   election.  If R1 and R2 are aware they are directly connected, an
   election will cause only one of them to forward the multicast packets
   into the BIER domain for a given (S,G) flow.

5.  Proposal

   As explain in Section 4, it is desirable to have a generic DR/DF
   election mechanism that can be used for existing and candidate MOSPs.
   Also, if a mix of MOSPs is used in the network, it is not obvious
   which MOSP is responsible for electing the DR/DF.  If a single DR/DF
   is to be elected, and each MOSP does its own election, the MOSPs have

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   to negotiate among each other who will be responsible for DR/DF on a
   LAN.  Independent of the MOSP, a single router connected to the LAN
   should be elected.  It seems inefficient and unpractical to have each
   MOSP implement its own DR/DF election mechanism.

   There is a process in the router that all the MOSPs depend on, that
   is the IGMP/MLD process.  The DR/DF election is typically based on
   the Group address or Group and Source address of the multicast flow.
   This information is available in the IGMP/MLD process.  In this
   document we propose to enhance the IGMP/MLD protocol to allow a DR/DF
   election among multicast routers connected to a LAN.  As soon as a
   router is elected as DR/DF, it can select the MOSP that will be
   responsible to deliver the multicast flow to this router, and onwards
   onto the LAN(s).

   IGMP/MLD has support for electing a Membership Querier based on the
   lowest IP address of the multicast routers sending out Membership
   Queries.  It would be possible to use the elected Membership Querier
   as the DR/DF on a LAN.  However, the authors believe that the
   Membership Querier procedures are not robust and extensible enough to
   be used DR/DF election on LANs.  For example, if a new multicast
   router becomes active on a LAN, it will immediately assume the role
   of a Membership Querier, which can lead to duplication and/or looping
   of packets if also used as DR/DF.  This duplication/looping will last
   until it learns about other Membership queriers with a lower IP
   address.  Having two Membership queriers on the LAN has limited
   impact on the IGMP/MLD protocol it self, it would only cause more
   Membership Reports to be received.

   The exact procedures to form a neighborship between IGMP/MLD routers
   will added in a later revision of this document.

6.  Security Considerations


7.  IANA Considerations


8.  Acknowledgments

   Many thanks to Neale Ranns and Greg Shepherd for their comments on
   this draft.

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9.  Normative References

              Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
              S. Aldrin, "Multicast using Bit Index Explicit
              Replication", draft-ietf-bier-architecture-02 (work in
              progress), July 2015.

              Pfister, P., Wijnands, I., and M. Stenberg, "BIER Ingress
              Multicast Flow Overlay using Multicast Listener Discovery
              Protocols", draft-pfister-bier-mld-00 (work in progress),
              July 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601,
              DOI 10.17487/RFC4601, August 2006,

   [RFC4605]  Fenner, B., He, H., Haberman, B., and H. Sandick,
              "Internet Group Management Protocol (IGMP) / Multicast
              Listener Discovery (MLD)-Based Multicast Forwarding
              ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605,
              August 2006, <>.

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,

   [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,

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   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,

Authors' Addresses

   IJsbrand Wijnands
   Cisco Systems
   De Kleetlaan 6a
   Diegem  1831


   Pierre Pfister
   Cisco Systems


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