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Generic Multicast Router Election on LAN's

Document Type Active Internet-Draft (individual)
Authors IJsbrand Wijnands , Stig Venaas , Zhaohui (Jeffrey) Zhang , Zheng Zhang
Last updated 2024-04-21
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Internet Engineering Task Force                             IJ. Wijnands
Internet-Draft                                                    Arrcus
Intended status: Informational                                 S. Venaas
Expires: 23 October 2024                             Cisco Systems, Inc.
                                                                J. Zhang
                                                        Juniper Networks
                                                                Z. Zhang
                                                         ZTE Corporation
                                                           21 April 2024

               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

   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
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   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 23 October 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Receiver side . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Sender side . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Proposal  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  DF Election Mechanism Requirements  . . . . . . . . . . . . .   8
   7.  The DF Election mechanism . . . . . . . . . . . . . . . . . .   8
     7.1.  Highest Random Weight . . . . . . . . . . . . . . . . . .   8
     7.2.  The DF Hello Message  . . . . . . . . . . . . . . . . . .   9
     7.3.  The Designated Announcer  . . . . . . . . . . . . . . . .   9
       7.3.1.  DAL Hello Option  . . . . . . . . . . . . . . . . . .   9
       7.3.2.  A new Candidate DF  . . . . . . . . . . . . . . . . .  10  The Hello List is empty . . . . . . . . . . . . .  10  The New DF is not the DA  . . . . . . . . . . . .  10  The New DF is the DA  . . . . . . . . . . . . . .  10
       7.3.3.  A candidate DF goes down  . . . . . . . . . . . . . .  11  The DF was not the DA . . . . . . . . . . . . . .  11  The DF was the DA . . . . . . . . . . . . . . . .  11
     7.4.  DA Inconsistency  . . . . . . . . . . . . . . . . . . . .  11
   8.  The Hello Message Packet Format . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   12. Contributing authors  . . . . . . . . . . . . . . . . . . . .  12
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     13.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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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.ietf-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) [RFC8279], there is no dependency on PIM to 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.

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           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.

           A Designated Forwarder is responsible for accepting a
           multicast packet from a LAN.

           A Designated Router is responsible for forwarding a multicast
           packet onto a LAN.

           A Designated Announcer is a router that is responsible for
           announcing a list of candidate Designated Forwarders.

           A Designated Announcer List is generated by the DA and holds
           the candidate Designated Forwarders.

3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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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 sam LAN.  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 network for a given (S,G) flow.

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5.  Proposal

   As explain in section 4, it is desirable to have a generic DR/DF
   election mechanism that can be used by 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
   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 for
   Group membership discovery, 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 for 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 election mechanism for the DR and the DF is very similar.  In
   fact, when a DF is elected, it MUST always be used as the DR as well
   to avoid multicast packet looping.  The procedures in this document
   always elect a DF on the LAN, and for that reason will always be the
   DR.  In the sections that follow, we don't refer to the DR anymore.
   Everywhere where we reference DF, we implicitly mean it applies to
   both the DR and DF.

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6.  DF Election Mechanism Requirements

   When electing a DF on the LAN, it is important to have a single DF
   for a given Multicast flow at all times.  If during the election
   process (or changes to it), there is no DF, it will cause traffic
   loss to the end user.  If there are two (or more) DFs at the same
   time, it may cause traffic duplication or even loops.  Since the
   election is done among different routers, it is not so trivial to
   guarantee that there will never be inconsistency in the DF election.
   There is also a tradeoff between the complexity introduced and the
   incremental benefit it brings.  The procedures in this document are
   designed to detect inconsistency and recover from it as fast as
   possible.  During inconsistency, we prefer traffic loss over possible
   duplication or looping of multicast packets.

   When there are multiple candidate DF routers on the LAN, it is
   beneficial to load-balance the traffic over the different candidate
   DFs.  This helps to distribute the bandwidth usage among the routers,
   reduce the impact of a router failure and shorten the failover time
   when changing the DF for effected flows.  For that reason the DF
   procedures MUST support DF election per multicast group address.

7.  The DF Election mechanism

7.1.  Highest Random Weight

   The method proposed to select a DF is based on the Highest Random
   Weight (HRM) as described in [RFC2291].  The paragraph below is
   mostly taken (and modified) from [RFC2291].

   The router computes the weight for EACH candidate DF by performing a
   hash over the Group address that identifies the flow, as well as over
   the address of the candidate DF.  The router then chooses the
   candidate DF with the highest resulting weight value.  This has the
   advantage of minimizing the number of flows affected by a candidate
   DF addition or deletion (only 1/N of them), but is approximately N
   times as expensive as a modulo-N hash.

   In order to get a good distribution of the Group addresses over the
   candidate DFs, it is important we choose a good Pseudorandom function
   to calculate the Weight.  The Weight is calculated using the Group
   (G) IP address and the Candidate DF (CDF) IP address.

   Weight(G, CDF) =
           (1103515245((1103515245.G+12345)XOR CDF)+12345)(mod 2^31)

   If multiple Candidate DFs end up with the same highest weight, the DF
   with the lowest IP address MUST be selected.

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   If every candidate DF on the LAN uses the same HRW algorithm to
   select the DF for a particular Group out of the same list of
   candidate DFs, they all will reach the same conclusion and there will
   be no inconsistency.  It is very important every router on the LAN
   has the same list of candidate DFs.  The mechanism proposed in this
   draft to generate a consistent list is based on the new Hello

7.2.  The DF Hello Message

   In order to discover the candidate DFs we need a mechanism to learn
   them.  We introduce a new (IGMP/MLD) message type called the DF
   Hello.  Routers on a LAN that are candidate DFs periodically send DF
   Hellos.  The message format is specified later in a later revision
   document.  Based on the DF Hellos it is possible to generate a list
   of candidate DFs.  However, it is challenging to keep the candidate
   DF list synchronized between the routers when DFs are added or
   removed from the list as each router will do that based on its own
   scheduling.  Especially when candidate DFs timeout, it is very likely
   this happens at different times and opens up the opportunity for
   inconsistency.  Also, when a new candidate DF is added to the network
   and one of the routers did not get the initial DF Hello message, its
   candidate DF list will be out of sync until the next DF Hello is
   received, leading to a inconsistent candidate DF list for a
   relatively long period.  In order to help synchronize the candidate
   DF List we elect a Designated Announcer (DA).

7.3.  The Designated Announcer

   The router that will act as the Designated Announcer is determined by
   the Priority value as included in the Hello message, using the IP
   address as tiebreaker.  The router with the highest priority is
   preferred, if there are multiple routers with the same priority, the
   router with the highest IP address is preferred.  The DA determines
   which routers from the Hello List (HL) are included in the Designated
   Announcer List (DAL).  By default all the routers in the HL are
   considered to be included in the DAL.  It is however possible to
   filter certain candidates and not include these in the list based on
   some sort of preference.

7.3.1.  DAL Hello Option

   The DAL is sent out by the DA as an Option included in its Hello
   message.  In order to reliably transmit the Hello Message with the
   DAL option, a DAL sequence number is included in the packet along
   with an acknowledgement flag for each router in the DAL.  Every
   router in the DAL MUST respond by triggering a Hello message
   including this sequence number.  If the DA has not received a

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   response within a given timeout from certain routers in the DAL it
   will re-transmit the Hello message with the Acknowledgement flag not
   set for the routers that have not responded.  The routers on the LAN
   that see their IP address in the DAL without the acknowledgement flag
   set will re-transmit their Hello.  This process continues until the
   DA has received a response from all the routers in the DAL.  Using
   this mechanism we minimize the time an inconsistency can occur when a
   router has missed a Hello message that includes that DAL.

7.3.2.  A new Candidate DF

   When a new candidate DF becomes active on the LAN, it first has to
   learn if there are other candidate DFs on the LAN.  Learning about
   other candidate DFs is accelerated by setting the Learn Flag in the
   Hello message.  Routers on the LAN that receive a Hello with the
   Learn Flag set will trigger a Hello message in response.  After the
   learning delay the new DF assumes all candidate DFs on the LAN have
   responded and the Hello List is complete.  There are three different
   scenarios the new DF has to consider.  The Hello List is empty

   When the HL is empty, the new DF will become the DA with only its own
   address in the DAL.  The DF will start to act as DF for all the
   groups.  The New DF is not the DA

   When there are other candidate DFs on the LAN, the Hello List is
   populated.  If the new DF is not the DA, it will have to wait for the
   DA to include its address in the DAL.  As soon as it sees its own
   address in the ADA with the acknowledgement flag not set, it will
   trigger a Hello message with the DAL sequence number and start to act
   as DF.  Note, it is likely that new DFs IP address is already
   included in the first Hello message it receives from the DA.  The New DF is the DA

   After the Learning delay the new router may find it self having the
   highest Priority and will be the new ADA.  Note, we prefer the DA to
   be deterministic so the new DF will take over the role of the DA.
   The DF which is currently the DA will have seen the Hello message
   from the new DF and will realize this is the new DA.  The current DA
   MUST respond by sending a Hello message without the DAL in it.  All
   the routers on the LAN will now know that the current DA is going
   away.  The candidate DFs MAY continue to use the old DAL until the
   new DAL list is received from the new DA.  The new DF will create the
   DAL list based on its Hello List and send out a Hello message,

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   following the procedures as described above.  If during a transition
   of the DA a router detects inconsistency between the received DAL and
   the perceived DA, the router stops using the current DAL and waits
   until the inconsistency is resolved.  This inconsistency may have
   occurred due to missing a DF Hello message (also see section DA

7.3.3.  A candidate DF goes down

   When a DF goes down there are 2 different scenarios to consider.  The DF was not the DA

   When a DF goes down, due to a failure or an operator removing it from
   the LAN, the routers on the LAN will eventually detect this because
   the Holdtime for that DF will expire.  This does not have an
   immediate effect on the DF procedures because the DF is chosen from
   the DAL, originated by the DA.  A candidate DF MUST NOT take any
   action based on a candidate DF going down, but MUST wait for the DA
   to sent out a new DAL list.  This will ensure that all candidate DFs
   on the LAN will start to use the new DAL at the same time and avoid
   any discrepancies due to routers expiring the timer associated with
   the DF that went down.  The DF was the DA

   If the DF that goes down is the DA, a new DA has to be elected.
   Note, every candidate DF on the LAN is a potential candidate to
   become the new DA.  The new DA is chosen based on the Hello List
   using the Designated Announcer election procedures.  It is possible a
   candidate DF receives the DAL from the new DA before it detected the
   current DA is down.  This may be due to a race condition where timers
   on the candidate DF expire at different times.  We use the procedures
   as described in section (DA inconsistency).

7.4.  DA Inconsistency

   A candidate DF that receives a DAL from a router that it does not
   consider to be the active DA MUST immediately stop acting as a DF.
   The candidate DF MUST wait for the DA inconsistency to be resolved
   before it is allowed to resume its role as candidate DF.  This will
   cause traffic to be blocked for the multicast groups this DF is
   responsible for, but it will not cause traffic duplication and/or
   loops due to other DFs using a different DAL list.  The inconsistency
   can be resolved due to the following events.

   *  The active DA expires.

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   *  A Hello is received from the active DA without a DAL.

   When the candidate DF detects that there is only one candidate DF
   that has announced the DAL and it is considered to be the DA, the
   inconsistency is resolved and the DF can resume its role as DF for
   the Groups it is responsible for.

8.  The Hello Message Packet Format

   The format of the Hello Message is included on the next revision of
   this document.

9.  Security Considerations


10.  IANA Considerations


11.  Acknowledgments

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

12.  Contributing authors

   Pierre Pfister

   Cisco Systems


13.  References

13.1.  Normative References

              Pfister, P., Wijnands, I., Venaas, S., Wang, C., Zhang,
              Z., and M. Stenberg, "BIER Ingress Multicast Flow Overlay
              using Multicast Listener Discovery Protocols", Work in
              Progress, Internet-Draft, draft-ietf-bier-mld-08, 2 July
              2023, <

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

   [RFC2291]  Slein, J., Vitali, F., Whitehead, E., and D. Durand,
              "Requirements for a Distributed Authoring and Versioning
              Protocol for the World Wide Web", RFC 2291,
              DOI 10.17487/RFC2291, February 1998,

   [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,

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

Wijnands, et al.         Expires 23 October 2024               [Page 13]
Internet-Draft              Abbreviated Title                 April 2024

   [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,

13.2.  Informative References

Authors' Addresses

   IJsbrand Wijnands

   Stig Venaas
   Cisco Systems, Inc.
   Tasman Drive
   San Jose, CA 95134
   United States of America

   Jeffrey Zhang
   Juniper Networks
   United States of America

   Zheng(Sandy) Zhang
   ZTE Corporation

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