<pre>Internet Engineering Task Force (IETF)                      D. Farinacci
Request for Comments: 6831                                      D. Meyer
Category: Experimental                                        J. Zwiebel
ISSN: 2070-1721                                                S. Venaas
                                                           Cisco Systems
                                                            January 2013


  <span class="h1">The Locator/ID Separation Protocol (LISP) for Multicast Environments</span>

Abstract

   This document describes how inter-domain multicast routing will
   function in an environment where Locator/ID Separation is deployed
   using the Locator/ID Separation Protocol (LISP) architecture.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6831">http://www.rfc-editor.org/info/rfc6831</a>.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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 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.



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Table of Contents

   <a href="#section-1">1</a>.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-3">3</a>
   <a href="#section-2">2</a>.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  <a href="#page-4">4</a>
   <a href="#section-3">3</a>.  Definition of Terms  . . . . . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
   <a href="#section-4">4</a>.  Basic Overview . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-8">8</a>
   <a href="#section-5">5</a>.  Source Addresses versus Group Addresses  . . . . . . . . . . . <a href="#page-10">10</a>
   <a href="#section-6">6</a>.  Locator Reachability Implications on LISP-Multicast  . . . . . <a href="#page-11">11</a>
   <a href="#section-7">7</a>.  Multicast Protocol Changes . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
   <a href="#section-8">8</a>.  LISP-Multicast Data-Plane Architecture . . . . . . . . . . . . <a href="#page-14">14</a>
     <a href="#section-8.1">8.1</a>.  ITR Forwarding Procedure . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
       <a href="#section-8.1.1">8.1.1</a>.  Multiple RLOCs for an ITR  . . . . . . . . . . . . . . <a href="#page-15">15</a>
       <a href="#section-8.1.2">8.1.2</a>.  Multiple ITRs for a LISP Source Site . . . . . . . . . <a href="#page-15">15</a>
     <a href="#section-8.2">8.2</a>.  ETR Forwarding Procedure . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
     <a href="#section-8.3">8.3</a>.  Replication Locations  . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
   <a href="#section-9">9</a>.  LISP-Multicast Interworking  . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
     <a href="#section-9.1">9.1</a>.  LISP and Non-LISP Mixed Sites  . . . . . . . . . . . . . . <a href="#page-17">17</a>
       <a href="#section-9.1.1">9.1.1</a>.  LISP Source Site to Non-LISP Receiver Sites  . . . . . <a href="#page-18">18</a>
       <a href="#section-9.1.2">9.1.2</a>.  Non-LISP Source Site to Non-LISP Receiver Sites  . . . <a href="#page-20">20</a>
       <a href="#section-9.1.3">9.1.3</a>.  Non-LISP Source Site to Any Receiver Site  . . . . . . <a href="#page-20">20</a>
       <a href="#section-9.1.4">9.1.4</a>.  Unicast LISP Source Site to Any Receiver Sites . . . . <a href="#page-21">21</a>
       <a href="#section-9.1.5">9.1.5</a>.  LISP Source Site to Any Receiver Sites . . . . . . . . <a href="#page-21">21</a>
     <a href="#section-9.2">9.2</a>.  LISP Sites with Mixed Address Families . . . . . . . . . . <a href="#page-22">22</a>
     <a href="#section-9.3">9.3</a>.  Making a Multicast Interworking Decision . . . . . . . . . <a href="#page-24">24</a>
   10. Considerations When RP Addresses Are Embedded in Group
       Addresses  . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-24">24</a>
   <a href="#section-11">11</a>. Taking Advantage of Upgrades in the Core . . . . . . . . . . . <a href="#page-25">25</a>
   <a href="#section-12">12</a>. Mtrace Considerations  . . . . . . . . . . . . . . . . . . . . <a href="#page-25">25</a>
   <a href="#section-13">13</a>. Security Considerations  . . . . . . . . . . . . . . . . . . . <a href="#page-25">25</a>
   <a href="#section-14">14</a>. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-26">26</a>
   <a href="#section-15">15</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-26">26</a>
     <a href="#section-15.1">15.1</a>. Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-26">26</a>
     <a href="#section-15.2">15.2</a>. Informative References . . . . . . . . . . . . . . . . . . <a href="#page-27">27</a>


















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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   The Locator/ID Separation Protocol [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] architecture provides a
   mechanism to separate out Identification and Location semantics from
   the current definition of an IP address.  By creating two namespaces,
   an Endpoint ID (EID) namespace used by sites and a Routing Locator
   (RLOC) namespace used by core routing, the core routing
   infrastructure can scale by doing topological aggregation of routing
   information.

   Since LISP creates a new namespace, a mapping function must exist to
   map a site's EID-Prefixes to its associated Locators.  For unicast
   packets, both the source address and destination address must be
   mapped.  For multicast packets, only the source address needs to be
   mapped.  The destination group address doesn't need to be mapped
   because the semantics of an IPv4 or IPv6 group address are logical in
   nature and not topology dependent.  Therefore, this specification
   focuses on mapping a source EID address of a multicast flow during
   distribution tree setup and packet delivery.

   This specification will address the following scenarios:

   1.  How a multicast source host in a LISP site sends multicast
       packets to receivers inside of its site as well as to receivers
       in other sites that are LISP enabled.

   2.  How inter-domain (or between LISP sites) multicast distribution
       trees are built and how forwarding of multicast packets leaving a
       source site toward receivers sites is performed.

   3.  What protocols are affected and what changes are required to such
       multicast protocols.

   4.  How ASM-mode (Any Source Multicast), SSM-mode (Single Source
       Multicast), and Bidir-mode (Bidirectional Shared Trees) service
       models will operate.

   5.  How multicast packet flow will occur for multiple combinations of
       LISP-enabled and non-LISP-enabled source and receiver sites.  For
       example:

       A.  How multicast packets from a source host in a LISP site are
           sent to receivers in other sites when they are all non-LISP
           sites.

       B.  How multicast packets from a source host in a LISP site are
           sent to receivers in both LISP-enabled sites and non-LISP
           sites.



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       C.  How multicast packets from a source host in a non-LISP site
           are sent to receivers in other sites when they are all LISP-
           enabled sites.

       D.  How multicast packets from a source host in a non-LISP site
           are sent to receivers in both LISP-enabled sites and non-LISP
           sites.

   This specification focuses on what changes are needed to the
   multicast routing protocols to support LISP-Multicast as well as
   other protocols used for inter-domain multicast, such as
   Multiprotocol BGP (MBGP) [<a href="./rfc4760" title="&quot;Multiprotocol Extensions for BGP-4&quot;">RFC4760</a>].  The approach proposed in this
   specification requires no packet format changes to the protocols and
   no operational procedural changes to the multicast infrastructure
   inside of a site when all sources and receivers reside in that site,
   even when the site is LISP enabled.  That is, internal operation of
   multicast is unchanged, regardless of whether or not the site is LISP
   enabled or whether or not receivers exist in other sites that are
   LISP enabled.

   Therefore, we see only operational (and not protocol) changes for
   PIM-ASM [<a href="./rfc4601" title="&quot;Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)&quot;">RFC4601</a>], Multicast Source Discovery Protocol (MSDP)
   [<a href="./rfc3618" title="&quot;Multicast Source Discovery Protocol (MSDP)&quot;">RFC3618</a>], and PIM-SSM [<a href="./rfc4607" title="&quot;Source-Specific Multicast for IP&quot;">RFC4607</a>].  BIDIR-PIM [<a href="./rfc5015" title="&quot;Bidirectional Protocol Independent Multicast (BIDIR- PIM)&quot;">RFC5015</a>], which
   typically does not run in an inter-domain environment, is not
   addressed in depth in this RFC.

   Also, the current version of this specification does not describe
   multicast-based Traffic Engineering (TE) relative to the TE-ITR
   (TE-based Ingress Tunnel Router) and TE-ETR (TE-based Egress Tunnel
   Router) descriptions in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>].  Further work is also needed to
   determine the detailed behavior for multicast Proxy-ITRs (mPITRs)
   (<a href="#section-9.1.3">Section 9.1.3</a>), mtrace (<a href="#section-12">Section 12</a>), and locator reachability
   (<a href="#section-6">Section 6</a>).  Finally, further deployment and experimentation would
   be useful to understand the real-life performance of the LISP-
   Multicast solution.  For instance, the design optimizes for minimal
   state and control traffic in the core, but can in some cases cause
   extra multicast traffic to be sent <a href="#section-8.1.2">Section 8.1.2</a>.

   Issues and concerns about the deployment of LISP for Internet traffic
   are discussed in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>].  <a href="#section-12">Section 12</a> of that document provides
   additional issues and concerns raised by this document.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Requirements Notation</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].




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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Definition of Terms</span>

   The terminology in this section is consistent with the definitions in
   [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] but is extended specifically to deal with the application
   of the terminology to multicast routing.

   LISP-Multicast:   a reference to the design in this specification.
      That is, when any site that is participating in multicast
      communication has been upgraded to be a LISP site, the operation
      of control-plane and data-plane protocols is considered part of
      the LISP-Multicast architecture.

   Endpoint ID (EID):   a 32-bit (for IPv4) or 128-bit (for IPv6) value
      used in the source address field of the first (most inner) LISP
      header of a multicast packet.  The host obtains a destination
      group address the same way it obtains one today, as it would when
      it is a non-LISP site.  The source EID is obtained via existing
      mechanisms used to set a host's "local" IP address.  An EID is
      allocated to a host from an EID-Prefix block associated with the
      site in which the host is located.  An EID can be used by a host
      to refer to another host, as when it joins an SSM (S-EID,G) route
      using IGMP version 3 [<a href="./rfc4604" title="&quot;Using Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery Protocol Version 2 (MLDv2) for Source- Specific Multicast&quot;">RFC4604</a>].  LISP uses Provider-Independent
      (PI) blocks for EIDs; such EIDs MUST NOT be used as LISP RLOCs.
      Note that EID blocks may be assigned in a hierarchical manner,
      independent of the network topology, to facilitate scaling of the
      mapping database.  In addition, an EID block assigned to a site
      may have site-local structure (subnetting) for routing within the
      site; this structure is not visible to the global routing system.

   Routing Locator (RLOC):   the IPv4 or IPv6 address of an Ingress
      Tunnel Router (ITR), the router in the multicast source host's
      site that encapsulates multicast packets.  It is the output of an
      EID-to-RLOC mapping lookup.  An EID maps to one or more RLOCs.
      Typically, RLOCs are numbered from topologically aggregatable
      blocks that are assigned to a site at each point to which it
      attaches to the global Internet; where the topology is defined by
      the connectivity of provider networks, RLOCs can be thought of as
      Provider-Assigned (PA) addresses.  Multiple RLOCs can be assigned
      to the same ITR device or to multiple ITR devices at a site.

   Ingress Tunnel Router (ITR):   a router that accepts an IP multicast
      packet with a single IP header (more precisely, an IP packet that
      does not contain a LISP header).  The router treats this "inner"
      IP destination multicast address opaquely so it doesn't need to
      perform a map lookup on the group address because it is
      topologically insignificant.  The router then prepends an "outer"
      IP header with one of its globally routable RLOCs as the source
      address field.  This RLOC is known to other multicast receiver



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      sites that have used the mapping database to join a multicast tree
      for which the ITR is the root.  In general, an ITR receives IP
      packets from site end-systems on one side and sends LISP-
      encapsulated multicast IP packets out all external interfaces that
      have been joined.

      An ITR would receive a multicast packet from a source inside of
      its site when 1) it is on the path from the multicast source to
      internally joined receivers, or 2) when it is on the path from the
      multicast source to externally joined receivers.

   Egress Tunnel Router (ETR):   a router that is on the path from a
      multicast source host in another site to a multicast receiver in
      its own site.  An ETR accepts a PIM Join/Prune message from a
      site-internal PIM router destined for the source's EID in the
      multicast source site.  The ETR maps the source EID in the Join/
      Prune message to an RLOC address based on the EID-to-RLOC mapping.
      This sets up the ETR to accept multicast encapsulated packets from
      the ITR in the source multicast site.  A multicast ETR
      decapsulates multicast encapsulated packets and replicates them on
      interfaces leading to internal receivers.

   xTR:   is a reference to an ITR or ETR when direction of data flow is
      not part of the context description. xTR refers to the router that
      is the tunnel endpoint; it is used synonymously with the term
      "tunnel router".  For example, "an xTR can be located at the
      Customer Edge (CE) router" means that both ITR and ETR
      functionality can be at the CE router.

   LISP Header:   a term used in this document to refer to the outer
      IPv4 or IPv6 header, a UDP header, and a LISP header.  An ITR
      prepends headers, and an ETR strips headers.  A LISP-encapsulated
      multicast packet will have an "inner" header with the source EID
      in the source field, an "outer" header with the source RLOC in the
      source field, and the same globally unique group address in the
      destination field of both the inner and outer header.

   (S,G) State:   the formal definition is in the PIM Sparse Mode
      [<a href="./rfc4601" title="&quot;Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)&quot;">RFC4601</a>] specification.  For this specification, the term is used
      generally to refer to multicast state.  Based on its topological
      location, the (S,G) state that resides in routers can be either
      (S-EID,G) state (at a location where the (S,G) state resides) or
      (S-RLOC,G) state (in the Internet core).








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   (S-EID,G) State:   refers to multicast state in multicast source and
      receiver sites where S-EID is the IP address of the multicast
      source host (its EID).  An S-EID can appear in an IGMPv3 report,
      an MSDP SA message or a PIM Join/Prune message that travels inside
      of a site.

   (S-RLOC,G) State:   refers to multicast state in the core where S is
      a source locator (the IP address of a multicast ITR) of a site
      with a multicast source.  The (S-RLOC,G) is mapped from the
      (S-EID,G) entry by doing a mapping database lookup for the EID-
      Prefix that S-EID maps to.  An S-RLOC can appear in a PIM Join/
      Prune message when it travels from an ETR to an ITR over the
      Internet core.

   uLISP Site:   a unicast-only LISP site according to [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] that
      has not deployed the procedures of this specification and,
      therefore, for multicast purposes, follows the procedures from
      <a href="#section-9">Section 9</a>.  A uLISP site can be a traditional multicast site.

   LISP Site:   a unicast LISP site (uLISP Site) that is also multicast
      capable according to the procedures in this specification.

   mPETR:   this is a multicast proxy-ETR that is responsible for
      advertising a very coarse EID-Prefix to which non-LISP and uLISP
      sites can target their (S-EID,G) PIM Join/Prune messages. mPETRs
      are used so LISP source multicast sites can send multicast packets
      using source addresses from the EID namespace. mPETRs act as
      Proxy-ETRs for supporting multicast routing in a LISP
      infrastructure.  It is likely a uPITR [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>] and an mPETR will
      be co-located since the single device advertises a coarse EID-
      Prefix in the underlying unicast routing system.

   Mixed Locator-Sets:   this is a Locator-Set for a LISP database
      mapping entry where the RLOC addresses in the Locator-Set are in
      both IPv4 and IPv6 format.

   Unicast Encapsulated PIM Join/Prune Message:   this is a standard PIM
      Join/Prune message (LISP-encapsulated with destination UDP port
      4341) that is sent by ETRs at multicast receiver sites to an ITR
      at a multicast source site.  This message is sent periodically as
      long as there are interfaces in the OIF-list for the (S-EID,G)
      entry for which the ETR is joining.

   OIF-list:   this is notation to describe the outgoing interface list
      a multicast router stores per multicast routing table entry so it
      knows on which interfaces to replicate multicast packets.





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   RPF:   Reverse Path Forwarding is a procedure used by multicast
      routers.  A router will accept a multicast packet for forwarding
      if the packet was received on the path that the router would use
      to forward unicast packets to the multicast packet's source.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Basic Overview</span>

   LISP, when used for unicast routing, increases the site's ability to
   control ingress traffic flows.  Egress traffic flows are controlled
   by the IGP in the source site.  For multicast, the IGP coupled with
   PIM can decide which path multicast packets ingress.  By using the
   Traffic Engineering features of LISP [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>], a multicast source
   site can control the egress of its multicast traffic.  By controlling
   the priorities of Locators from a mapping database entry, a source
   multicast site can control which way multicast receiver sites join to
   the source site.

   At this point in time, there is no requirement for different Locator-
   Sets, priority, and weight policies for multicast than there is for
   unicast.  However, when Traffic Engineering policies are different
   for unicast versus multicast flows, it will be desirable to use
   multicast-based priority and weight values in Map-Reply messages.

   The fundamental multicast forwarding model is to encapsulate a
   multicast packet into another multicast packet.  An ITR will
   encapsulate multicast packets received from sources that it serves in
   a LISP-Multicast header.  The destination group address from the
   inner header is copied to the destination address of the outer
   header.  The inner source address is the EID of the multicast source
   host and the outer source address is the RLOC of the encapsulating
   ITR.

   The LISP-Multicast architecture will follow this high-level protocol
   and operational sequence:

   1.  Receiver hosts in multicast sites will join multicast content the
       way they do today -- they use IGMP.  When they use IGMPv3 where
       they specify source addresses, they use source EIDs; that is,
       they join (S-EID,G).  If the multicast source is external to this
       receiver site, the PIM Join/Prune message flows toward the ETRs,
       finding the shortest exit (that is, the closest exit for the
       Join/Prune message and the closest entrance for the multicast
       packet to the receiver).

   2.  The ETR does a mapping database lookup for S-EID.  If the mapping
       is cached from a previous lookup (from either a previous Join/
       Prune for the source multicast site or a unicast packet that went
       to the site), it will use the RLOC information from the mapping.



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       The ETR will use the same priority and weighting mechanism as for
       unicast.  So, the source site can decide which way multicast
       packets egress.

   3.  The ETR will build two PIM Join/Prune messages, one that contains
       an (S-EID,G) entry that is unicast to the ITR that matches the
       RLOC the ETR selects, and the other that contains an (S-RLOC,G)
       entry so the core network can create multicast state from this
       ETR to the ITR.

   4.  When the ITR gets the unicast Join/Prune message (see <a href="#section-3">Section 3</a>
       for formal definition), it will process (S-EID,G) entries in the
       message and propagate them inside of the site where it has
       explicit routing information for EIDs via the IGP.  When the ITR
       receives the (S-RLOC,G) PIM Join/Prune message, it will process
       it like any other join it would get in today's Internet.  The
       S-RLOC address is the IP address of this ITR.

   5.  At this point, there is (S-EID,G) state from the joining host in
       the receiver multicast site to the ETR of the receiver multicast
       site.  There is (S-RLOC,G) state across the core network from the
       ETR of the multicast receiver site to the ITR in the multicast
       source site and (S-EID,G) state in the source multicast site.
       Note, the (S-EID,G) state is the same S-EID in each multicast
       site.  As other ETRs join the same multicast tree, they can join
       through the same ITR (in which case the packet replication is
       done in the core) or a different ITR (in which case the packet
       replication is done at the source site).

   6.  When a packet is originated by the multicast host in the source
       site, the packet will flow to one or more ITRs that will prepend
       a LISP header.  By copying the group address to the outer
       destination address field, the ITR inserts its own locator
       address in the outer source address field.  The ITR will look at
       its (S-RLOC,G) state, where S-RLOC is its own locator address,
       and replicate the packet on each interface on which an (S-RLOC,G)
       join was received.  The core has (S-RLOC,G) so where fan-out
       occurs to multiple sites, a core router will do packet
       replication.

   7.  When either the source site or the core replicates the packet,
       the ETR will receive a LISP packet with a destination group
       address.  It will decapsulate packets because it has receivers
       for the group.  Otherwise, it would not have received the packets
       because it would not have joined.  The ETR decapsulates and does
       an (S-EID,G) lookup in its multicast Forwarding Information Base
       (FIB) to forward packets out one or more interfaces to forward
       the packet to internal receivers.



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   This architecture is consistent and scalable with the architecture
   presented in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] where multicast state in the core operates on
   Locators, and multicast state at the sites operates on EIDs.

   Alternatively, [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] also has a mechanism where (S-EID,G) state
   can reside in the core through the use of RPF Vectors [<a href="./rfc5496" title="&quot;The Reverse Path Forwarding (RPF) Vector TLV&quot;">RFC5496</a>] in
   PIM Join/Prune messages.  However, few PIM implementations support
   RPF Vectors, and LISP should avoid S-EID state in the core.  See
   <a href="#section-5">Section 5</a> for details.

   However, some observations can be made on the algorithm above.  The
   control plane can scale but at the expense of sending data to sites
   that may have not joined the distribution tree where the encapsulated
   data is being delivered.  For example, one site joins (S-EID1,G), and
   another site joins (S-EID2,G).  Both EIDs are in the same multicast
   source site.  Both multicast receiver sites join to the same ITR with
   state (S-RLOC,G) where S-RLOC is the RLOC for the ITR.  The ITR joins
   both (S-EID1,G) and (S-EID2,G) inside of the site.  The ITR receives
   (S-RLOC,G) joins and populates the OIF-list state for the (S-RLOC,G)
   entry.  Since both (S-EID1,G) and (S-EID2, G) map to the one
   (S-RLOC,G), packets will be delivered by the core to both multicast
   receiver sites even though each have joined a single source-based
   distribution tree.  This behavior is a consequence of the many-to-one
   mapping between S-EIDs and a S-RLOC.

   There is a possible solution to this problem that reduces the number
   of many-to-one occurrences of (S-EID,G) entries aggregating into a
   single (S-RLOC,G) entry.  If a physical ITR can be assigned multiple
   RLOC addresses and these addresses are advertised in mapping database
   entries, then ETRs at receiver sites have more RLOC address options
   and therefore can join different (RLOC,G) entries for each (S-EID,G)
   entry joined at the receiver site.  It would not scale to have a one-
   to-one relationship between the number of S-EID sources at a source
   site and the number of RLOCs assigned to all ITRs at the site, but
   "n" can reduce to a smaller number in the "n-to-1" relationship.  And
   in turn, this reduces the opportunity for data packets to be
   delivered to sites for groups not joined.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Source Addresses versus Group Addresses</span>

   Multicast group addresses don't have to be associated with either the
   EID or RLOC namespace.  They actually are a namespace of their own
   that can be treated as logical with relatively opaque allocation.
   So, by their nature, they don't detract from an incremental
   deployment of LISP-Multicast.






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   As for source addresses, as in the unicast LISP scenario, there is a
   decoupling of identification from location.  In a LISP site, packets
   are originated from hosts using their allocated EIDs.  EID addresses
   are used to identify the host as well as where in the site's topology
   the host resides but not how and where it is attached to the
   Internet.

   Therefore, when multicast distribution tree state is created anywhere
   in the network on the path from any multicast receiver to a multicast
   source, EID state is maintained at the source and receiver multicast
   sites, and RLOC state is maintained in the core.  That is, a
   multicast distribution tree will be represented as a 3-tuple of
   {(S-EID,G) (S-RLOC,G) (S-EID,G)}, where the first element of the
   3-tuple is the state stored in routers from the source to one or more
   ITRs in the source multicast site; the second element of the 3-tuple
   is the state stored in routers downstream of the ITR, in the core, to
   all LISP receiver multicast sites; and the third element in the
   3-tuple is the state stored in the routers downstream of each ETR, in
   each receiver multicast site, reaching each receiver.  Note that
   (S-EID,G) is the same in both the source and receiver multicast
   sites.

   The concatenation/mapping from the first element to the second
   element of the 3-tuples is done by the ITR, and from the second
   element to the third element is done at the ETRs.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Locator Reachability Implications on LISP-Multicast</span>

   Multicast state as it is stored in the core is always (S,G) state as
   it exists today or (S-RLOC,G) state as it will exist when LISP sites
   are deployed.  The core routers cannot distinguish one from the
   other.  They don't need to because it is state that uses RPF against
   the core routing tables in the RLOC namespace.  The difference is
   where the root of the distribution tree for a particular source is.
   In the traditional multicast core, the source S is the source host's
   IP address.  For LISP-Multicast, the source S is a single ITR of the
   multicast source site.

   An ITR is selected based on the LISP EID-to-RLOC mapping used when an
   ETR propagates a PIM Join/Prune message out of a receiver multicast
   site.  The selection is based on the same algorithm an ITR would use
   to select an ETR when sending a unicast packet to the site.  In the
   unicast case, the ITR can change on a per-packet basis depending on
   the reachability of the ETR.  So, an ITR can change relatively easily
   using local reachability state.  However, in the multicast case, when
   an ITR becomes unreachable, new distribution tree state must be built
   because the encapsulating root has changed.  This is more significant
   than an RPF-change event, where any router would typically locally



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   change its RPF-interface for its existing tree state.  But when an
   encapsulating LISP-Multicast ITR goes unreachable, new distribution
   state must be built and reflect the new encapsulator.  Therefore,
   when an ITR goes unreachable, all ETRs that are currently joined to
   that ITR will have to trigger a new Join/Prune message for (S-RLOC,G)
   to the new ITR as well as send a unicast encapsulated Join/Prune
   message telling the new ITR which (S-EID,G) is being joined.

   This issue can be mitigated by using anycast addressing for the ITRs,
   so the problem does reduce to an RPF change in the core, but still
   requires a unicast encapsulated Join/Prune message to tell the new
   ITR about (S-EID,G).  The problem with this approach is that the ETR
   really doesn't know when the ITR has changed, so the new anycast ITR
   will get the (S-EID,G) state only when the ETR sends it the next time
   during its periodic sending procedures.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Multicast Protocol Changes</span>

   A number of protocols are used today for inter-domain multicast
   routing:

   IGMPv1-v3, MLDv1-v2:   These protocols [<a href="./rfc4604" title="&quot;Using Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery Protocol Version 2 (MLDv2) for Source- Specific Multicast&quot;">RFC4604</a>] do not require any
      changes for LISP-Multicast for two reasons.  One is that they are
      link-local and not used over site boundaries, and the second is
      that they advertise group addresses that don't need translation.
      Where source addresses are supplied in IGMPv3 and Multicast
      Listener Discovery version 2 (MLDv2) messages, they are
      semantically regarded as EIDs and don't need to be converted to
      RLOCs until the multicast tree-building protocol, such as PIM, is
      received by the ETR at the site boundary.  Addresses used for IGMP
      and MLD come out of the source site's allocated addresses, which
      are therefore from the EID namespace.

   MBGP:   Even though the Multiprotocol Extensions for BGP-4 (MBGP)
      [<a href="./rfc4760" title="&quot;Multiprotocol Extensions for BGP-4&quot;">RFC4760</a>] are not part of a multicast routing protocol, they are
      used to find multicast sources when the unicast BGP peering
      topology and the multicast MBGP peering topology are not
      congruent.  When MBGP is used in a LISP-Multicast environment, the
      prefixes that are advertised are from the RLOC namespace.  This
      allows receiver multicast sites to find a path to the source
      multicast site's ITRs.  MBGP peering addresses will be from the
      RLOC namespace.  There are no MBGP changes required to support
      LISP-Multicast.

   MSDP:   MSDP [<a href="./rfc3618" title="&quot;Multicast Source Discovery Protocol (MSDP)&quot;">RFC3618</a>] is used to announce active multicast sources
      to other routing domains (or LISP sites).  The announcements come
      from the PIM Rendezvous Points (RPs) from sites where there are
      active multicast sources sending to various groups.  In the



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      context of LISP-Multicast, the source addresses advertised in MSDP
      will semantically be from the EID namespace since they describe
      the identity of a source multicast host.  It will be true that the
      state stored in MSDP caches from core routers will be from the EID
      namespace.  An RP address inside of the site will be from the EID
      namespace so it can be advertised and reached by an internal
      unicast routing mechanism.  However, for MSDP peer-RPF checking to
      work properly across sites, the RP addresses must be converted or
      mapped into a routable address that is advertised and maintained
      in the BGP routing tables in the core.  MSDP peering addresses can
      come out of either the EID or a routable address namespace.  Also,
      the choice can be made unilaterally because the ITR at the site
      will determine which namespace the destination peer address is out
      of by looking in the mapping database service.  There are no MSDP
      changes required to support LISP-Multicast.

   PIM-SSM:   In the simplest form of distribution tree building, when
      PIM operates in SSM mode [<a href="./rfc4607" title="&quot;Source-Specific Multicast for IP&quot;">RFC4607</a>], a source distribution tree is
      built and maintained across site boundaries.  In this case, there
      is a small modification to how PIM Join/Prune messages are sent by
      the LISP-Multicast component.  No modifications to any message
      format, but to support taking a Join/Prune message originated
      inside of a LISP site with embedded addresses from the EID
      namespace and converting them to addresses from the RLOC namespace
      when the Join/Prune message crosses a site boundary.  This is
      similar to the requirements documented in [<a href="./rfc5135" title="&quot;IP Multicast Requirements for a Network Address Translator (NAT) and a Network Address Port Translator (NAPT)&quot;">RFC5135</a>].

   BIDIR-PIM:   Bidirectional PIM [<a href="./rfc5015" title="&quot;Bidirectional Protocol Independent Multicast (BIDIR- PIM)&quot;">RFC5015</a>] is typically run inside of a
      routing domain, but if deployed in an inter-domain environment,
      one would have to decide if the RP address of the shared tree
      would be from the EID namespace or the RLOC namespace.  If the RP
      resides in a site-based router, then the RP address is from the
      EID namespace.  If the RP resides in the core where RLOC addresses
      are routed, then the RP address is from the RLOC namespace.  This
      could be easily distinguishable if the EID address were in a well-
      known address allocation block from the RLOC namespace.  Also,
      when using Embedded-RP for RP determination [<a href="./rfc3956" title="&quot;Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address&quot;">RFC3956</a>], the format
      of the group address could indicate the namespace the RP address
      is from.  However, refer to <a href="#section-10">Section 10</a> for considerations core
      routers need to make when using Embedded-RP IPv6 group addresses.
      When using BIDIR-PIM for inter-domain multicast routing, it is
      recommended to use statically configured RPs.  This allows core
      routers to associate a Bidir group's RP address with an ITR's RLOC
      address, and site routers to associate the Bidir group's RP
      address as an EID address.  With respect to Designated Forwarder
      (DF) election in BIDIR-PIM, no changes are required since all
      messaging and addressing is link-local.




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   PIM-ASM:   The ASM mode of PIM [<a href="./rfc4601" title="&quot;Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)&quot;">RFC4601</a>], the most popular form of
      PIM, is deployed in the Internet today by having shared trees
      within a site and using source trees across sites.  By the use of
      MSDP and PIM-SSM techniques described above, multicast
      connectivity can occur across LISP sites.  Having said that, that
      means there are no special actions required for processing (*,G)
      or (S,G,R) Join/Prune messages since they all operate against the
      shared tree that is site resident.  Just like with ASM, there is
      no (*,G) in the core when LISP-Multicast is in use.  This is also
      true for the RP-mapping mechanisms Auto-RP and Bootstrap Router
      (BSR) [<a href="./rfc5059" title="&quot;Bootstrap Router (BSR) Mechanism for Protocol Independent Multicast (PIM)&quot;">RFC5059</a>].

   Based on the protocol description above, the conclusion is that there
   are no protocol message format changes, just a translation function
   performed at the control plane.  This will make for an easier and
   faster transition for LISP since fewer components in the network have
   to change.

   It should also be stated just like it is in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] that no host
   changes, whatsoever, are required to have a multicast source host
   send multicast packets and for a multicast receiver host to receive
   multicast packets.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  LISP-Multicast Data-Plane Architecture</span>

   The LISP-Multicast data-plane operation conforms to the operation and
   packet formats specified in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>].  However, encapsulating a
   multicast packet from an ITR is a much simpler process.  The process
   is simply to copy the inner group address to the outer destination
   address.  And to have the ITR use its own IP address (its RLOC) as
   the source address.  The process is simpler for multicast because
   there is no EID-to-RLOC mapping lookup performed during packet
   forwarding.

   In the decapsulation case, the ETR simply removes the outer header
   and performs a multicast routing table lookup on the inner header
   (S-EID,G) addresses.  Then, the OIF-list for the (S-EID,G) entry is
   used to replicate the packet on site-facing interfaces leading to
   multicast receiver hosts.

   There is no Data-Probe logic for ETRs as there can be in the unicast
   forwarding case.









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<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  ITR Forwarding Procedure</span>

   The following procedure is used by an ITR, when it receives a
   multicast packet from a source inside of its site:

   1.  A multicast data packet sent by a host in a LISP site will have
       the source address equal to the host's EID and the destination
       address equal to the address of the multicast group.  It is
       assumed the group information is obtained by current methods.
       The same is true for a multicast receiver to obtain the source
       and group address of a multicast flow.

   2.  When the ITR receives a multicast packet, it will have both S-EID
       state and S-RLOC state stored.  Since the packet was received on
       a site-facing interface, the RPF lookup is based on the S-EID
       state.  If the RPF check succeeds, then the OIF-list contains
       interfaces that are site facing and external facing.  For the
       site-facing interfaces, no LISP header is prepended.  For the
       external-facing interfaces a LISP header is prepended.  When the
       ITR prepends a LISP header, it uses its own RLOC address as the
       source address and copies the group address supplied by the IP
       header that the host built as the outer destination address.

<span class="h4"><a class="selflink" id="section-8.1.1" href="#section-8.1.1">8.1.1</a>.  Multiple RLOCs for an ITR</span>

   Typically, an ITR will have a single RLOC address, but in some cases
   there could be multiple RLOC addresses assigned from either the same
   or different service providers.  In this case, when (S-RLOC,G) Join/
   Prune messages are received for each RLOC, there is a OIF-list
   merging action that must take place.  Therefore, when a packet is
   received from a site-facing interface that matches on an (S-EID,G)
   entry, the interfaces of the OIF-list from all (RLOC,G) entries
   joined to the ITR as well as the site-facing OIF-list joined for
   (S-EID,G) must be included in packet replication.  In addition to
   replicating for all types of OIF-lists, each OIF-list entry must be
   tagged with the RLOC address, so encapsulation uses the outer source
   address for the RLOC joined.

<span class="h4"><a class="selflink" id="section-8.1.2" href="#section-8.1.2">8.1.2</a>.  Multiple ITRs for a LISP Source Site</span>

   Note that when ETRs from different multicast receiver sites receive
   (S-EID,G) joins, they may select a different S-RLOC for a multicast
   source site due to policy (the multicast ITR can return different
   multicast priority and weight values per ETR Map-Request).  In this
   case, the same (S-EID,G) is being realized by different (S-RLOC,G)
   state in the core.  This will not result in duplicate packets because





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   each ITR in the multicast source site will choose their own RLOC for
   the source address for encapsulated multicast traffic.  The RLOC
   addresses are the ones joined by remote multicast ETRs.

   When different (S-EID,G) traffic is combined into a single (RLOC,G)
   core distribution tree, this may cause traffic to go to a receiver
   multicast site when it does not need to.  This happens when one
   receiver multicast site joins (S1-EID,Gi) through a core distribution
   tree of (RLOC1,Gi) and another multicast receiver site joins
   (S2-EID,Gi) through the same core distribution tree of (RLOC1,Gi).
   When ETRs decapsulate such traffic, they should know from their local
   (S-EID,G) state if the packet should be forwarded.  If there is no
   (S-EID,G) state that matches the inner packet header, the packet is
   discarded.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  ETR Forwarding Procedure</span>

   The following procedure is used by an ETR, when it receives a
   multicast packet from a source outside of its site:

   1.  When a multicast data packet is received by an ETR on an
       external-facing interface, it will do an RPF lookup on the S-RLOC
       state it has stored.  If the RPF check succeeds, the interfaces
       from the OIF-list are used for replication to interfaces that are
       site facing as well as interfaces that are external facing (this
       ETR can also be a transit multicast router for receivers outside
       of its site).  When the packet is to be replicated for an
       external-facing interface, the LISP encapsulation header is not
       stripped.  When the packet is replicated for a site-facing
       interface, the encapsulation header is stripped.

   2.  The packet without a LISP header is now forwarded down the
       (S-EID,G) distribution tree in the receiver multicast site.

<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>.  Replication Locations</span>

   Multicast packet replication can happen in the following topological
   locations:

   o  In an IGP multicast router inside a site that operates on S-EIDs.

   o  In a transit multicast router inside of the core that operates on
      S-RLOCs.

   o  At one or more ETR routers depending on the path a Join/Prune
      message exits a receiver multicast site.





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   o  At one or more ITR routers in a source multicast site depending on
      what priorities are returned in a Map-Reply to receiver multicast
      sites.

   In the last case, the source multicast site can do replication rather
   than having a single exit from the site.  But this can occur only
   when the priorities in the Map-Reply are modified for different
   receiver multicast sites so that the PIM Join/Prune messages arrive
   at different ITRs.

   This policy technique, also used in [<a href="./rfc6836" title="&quot;Locator/ID Separation Protocol Alternative Logical Topology (LISP+ALT)&quot;">RFC6836</a>] for unicast, is useful
   for multicast to mitigate the problems of changing distribution tree
   state as discussed in <a href="#section-6">Section 6</a>.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  LISP-Multicast Interworking</span>

   This section describes the multicast corollary to [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>] regarding
   the interworking of multicast routing among LISP and non-LISP sites.

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  LISP and Non-LISP Mixed Sites</span>

   Since multicast communication can involve more than two entities to
   communicate together, the combinations of interworking scenarios are
   more involved.  However, the state maintained for distribution trees
   at the sites is the same, regardless of whether or not the site is
   LISP enabled.  So, most of the implications are in the core with
   respect to storing routable EID-Prefixes from either PA or PI blocks.

   Before enumerating the multicast interworking scenarios, let's define
   three deployment states of a site:

   o  A non-LISP site that will run PIM-SSM or PIM-ASM with MSDP as it
      does today.  The addresses for the site are globally routable.

   o  A site that deploys LISP for unicast routing.  The addresses for
      the site are not globally routable.  Let's define the name for
      this type of site as a uLISP site.

   o  A site that deploys LISP for both unicast and multicast routing.
      The addresses for the site are not globally routable.  Let's
      define the name for this type of site as a LISP-Multicast site.

   A LISP site enabled for multicast purposes only will not be
   considered in this document, but a uLISP site as documented in
   [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>] will be considered.  In this section there is no discussion
   of how a LISP site sends multicast packets when all receiver sites
   are LISP-Multicast enabled; that has been discussed in previous
   sections.



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<span class="grey"><a href="./rfc6831">RFC 6831</a>             LISP for Multicast Environments        January 2013</span>


   The following scenarios exist to make LISP-Multicast sites interwork
   with non-LISP-Multicast sites:

   1.  A LISP site must be able to send multicast packets to receiver
       sites that are a mix of non-LISP sites and uLISP sites.

   2.  A non-LISP site must be able to send multicast packets to
       receiver sites that are a mix of non-LISP sites and uLISP sites.

   3.  A non-LISP site must be able to send multicast packets to
       receiver sites that are a mix of LISP sites, uLISP sites, and
       non-LISP sites.

   4.  A uLISP site must be able to send multicast packets to receiver
       sites that are a mix of LISP sites, uLISP sites, and non-LISP
       sites.

   5.  A LISP site must be able to send multicast packets to receiver
       sites which are a mix of LISP sites, uLISP sites, and non-LISP
       sites.

<span class="h4"><a class="selflink" id="section-9.1.1" href="#section-9.1.1">9.1.1</a>.  LISP Source Site to Non-LISP Receiver Sites</span>

   In the first scenario, a site is LISP enabled for both unicast and
   multicast traffic and as such operates on EIDs.  Therefore, there is
   a possibility that the EID-Prefix block is not routable in the core.
   For LISP receiver multicast sites, this isn't a problem, but for non-
   LISP or uLISP receiver multicast sites, when a PIM Join/Prune message
   is received by the edge router, it has no route to propagate the
   Join/Prune message out of the site.  This is no different than the
   unicast case that LISP Network Address Translation (LISP-NAT) in
   [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>] solves.

   LISP-NAT allows a unicast packet that exits a LISP site to get its
   source address mapped to a globally routable address before the ITR
   realizes that it should not encapsulate the packet destined to a non-
   LISP site.  For a multicast packet to leave a LISP site, distribution
   tree state needs to be built so the ITR can know where to send the
   packet.  So, the receiver multicast sites need to know about the
   multicast source host by its routable address and not its EID
   address.  When this is the case, the routable address is the
   (S-RLOC,G) state that is stored and maintained in the core routers.
   It is important to note that the routable address for the host cannot
   be the same as an RLOC for the site because it is desirable for ITRs
   to process a PIM Join/Prune message that is received from an
   external-facing interface.  If the message will be propagated inside
   of the site, the site-part of the distribution tree is built.




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<span class="grey"><a href="./rfc6831">RFC 6831</a>             LISP for Multicast Environments        January 2013</span>


   Using a globally routable source address allows non-LISP and uLISP
   multicast receivers to join, create, and maintain a multicast
   distribution tree.  However, the LISP-Multicast receiver site will
   want to perform an EID-to-RLOC mapping table lookup when a PIM Join/
   Prune message is received on a site-facing interface.  It does this
   because it wants to find an (S-RLOC,G) entry to Join in the core.
   So, there is a conflict of behavior between the two types of sites.

   The solution to this problem is the same as when an ITR wants to send
   a unicast packet to a destination site but needs to determine if the
   site is LISP enabled or not.  When it is not LISP enabled, the ITR
   does not encapsulate the packet.  So, for the multicast case, when
   the ETR receives a PIM Join/Prune message for (S-EID,G) state, it
   will do a mapping table lookup on S-EID.  In this case, S-EID is not
   in the mapping database because the source multicast site is using a
   routable address and not an EID-Prefix address.  So, the ETR knows to
   simply propagate the PIM Join/Prune message to an external-facing
   interface without converting the (S-EID,G) because it is an (S,G),
   where S is routable and reachable via core routing tables.

   Now that the multicast distribution tree is built and maintained from
   any non-LISP or uLISP receiver multicast site, the way the packet
   forwarding model is used can be explained.

   Since the ITR in the source multicast site has never received a
   unicast encapsulated PIM Join/Prune message from any ETR in a
   receiver multicast site, it knows there are no LISP-Multicast
   receiver sites.  Therefore, there is no need for the ITR to
   encapsulate data.  Since it will know a priori (via configuration)
   that its site's EIDs are not routable (and not registered to the
   mapping database system), it assumes that the multicast packets from
   the source host are sent by a routable address.  That is, it is the
   responsibility of the multicast source host's system administrator to
   ensure that the source host sends multicast traffic using a routable
   source address.  When this happens, the ITR acts simply as a router
   and forwards the multicast packet like an ordinary multicast router.

   There is an alternative to using a LISP-NAT scheme just as there is
   an alternative to using unicast [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>] forwarding by employing
   Proxy Tunnel Routers (PxTRs).  This can work the same way for
   multicast routing as well, but the difference is that non-LISP and
   uLISP sites will send PIM Join/Prune messages for (S-EID,G) that make
   their way in the core to multicast PxTRs.  Let's call this use of a
   PxTR as a "Multicast Proxy-ETR" (or mPETR).  Since the mPETRs
   advertise very coarse EID-Prefixes, they draw the PIM Join/Prune
   control traffic making them the target of the distribution tree.  To
   get multicast packets from the LISP source multicast sites, the tree




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<span class="grey"><a href="./rfc6831">RFC 6831</a>             LISP for Multicast Environments        January 2013</span>


   needs to be built on the path from the mPETR to the LISP source
   multicast site.  To make this happen, the mPETR acts as a "Proxy-ETR"
   (where in unicast it acts as a "Proxy-ITR", or an uPITR [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>]).

   The existence of mPETRs in the core allows source multicast site ITRs
   to encapsulate multicast packets according to (S-RLOC,G) state.  The
   (S-RLOC,G) state is built from the mPETRs to the multicast ITRs.  The
   encapsulated multicast packets are decapsulated by mPETRs and then
   forwarded according to (S-EID,G) state.  The (S-EID,G) state is built
   from the non-LISP and uLISP receiver multicast sites to the mPETRs.

<span class="h4"><a class="selflink" id="section-9.1.2" href="#section-9.1.2">9.1.2</a>.  Non-LISP Source Site to Non-LISP Receiver Sites</span>

   Clearly non-LISP-Multicast sites can send multicast packets to non-
   LISP receiver multicast sites.  That is what they do today.  However,
   discussion is required to show how non-LISP-Multicast sites send
   multicast packets to uLISP receiver multicast sites.

   Since uLISP receiver multicast sites are not targets of any (S,G)
   state, they simply send (S,G) PIM Join/Prune messages toward the non-
   LISP source multicast site.  Since the source multicast site in this
   case has not been upgraded to LISP, all multicast source host
   addresses are routable.  So, this case is simplified to where a uLISP
   receiver multicast site appears to the source multicast site to be a
   non-LISP receiver multicast site.

<span class="h4"><a class="selflink" id="section-9.1.3" href="#section-9.1.3">9.1.3</a>.  Non-LISP Source Site to Any Receiver Site</span>

   When a non-LISP source multicast site has receivers in either a non-
   LISP/uLISP site or a LISP site, one needs to decide how the LISP
   receiver multicast site will attach to the distribution tree.  It is
   known from <a href="#section-9.1.2">Section 9.1.2</a> that non-LISP and uLISP receiver multicast
   sites can join the distribution tree, but a LISP receiver multicast
   site ETR will need to know if the source address of the multicast
   source host is routable or not.  It has been shown in <a href="#section-9.1.1">Section 9.1.1</a>
   that an ETR, before it sends a PIM Join/Prune message on an external-
   facing interface, does an EID-to-RLOC mapping lookup to determine if
   it should convert the (S,G) state from a PIM Join/Prune message
   received on a site-facing interface to an (S-RLOC,G).  If the lookup
   fails, the ETR can conclude the source multicast site is a non-LISP
   site, so it simply forwards the Join/Prune message.  (It also doesn't
   need to send a unicast encapsulated Join/Prune message because there
   is no ITR in a non-LISP site and there is namespace continuity
   between the ETR and source.)

   For a non-LISP source multicast site, (S-EID,G) state could be
   limited to the edges of the network with the use of multicast proxy-
   ITRs (mPITRs).  The mPITRs can take native, unencapsulated multicast



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<span class="grey"><a href="./rfc6831">RFC 6831</a>             LISP for Multicast Environments        January 2013</span>


   packets from non-LISP source multicast and uLISP sites and
   encapsulate them to ETRs in receiver multicast sites or to mPETRs
   that can decapsulate for non-LISP receiver multicast or uLISP sites.
   The mPITRs are responsible for sending (S-EID,G) joins to the non-
   LISP source multicast site.  To connect the distribution trees
   together, multicast ETRs will need to be configured with the mPITR's
   RLOC addresses so they can send both (S-RLOC,G) joins to build a
   distribution tree to the mPITR as well as configured for sending
   unicast joins to mPITRs so they can propagate (S-EID,G) joins into
   source multicast sites.  The use of mPITRs is undergoing more study
   and is a work in progress.

<span class="h4"><a class="selflink" id="section-9.1.4" href="#section-9.1.4">9.1.4</a>.  Unicast LISP Source Site to Any Receiver Sites</span>

   In the last section, it was explained how an ETR in a multicast
   receiver site can determine if a source multicast site is LISP
   enabled by looking into the mapping database.  When the source
   multicast site is a uLISP site, it is LISP enabled, but the ITR, by
   definition, is not capable of doing multicast encapsulation.  So, for
   the purposes of multicast routing, the uLISP source multicast site is
   treated as a non-LISP source multicast site.

   Non-LISP receiver multicast sites can join distribution trees to a
   uLISP source multicast site since the source site behaves, from a
   forwarding perspective, as a non-LISP source site.  This is also the
   case for a uLISP receiver multicast site since the ETR does not have
   multicast functionality built-in or enabled.

   Special considerations are required for LISP receiver multicast
   sites; since they think the source multicast site is LISP enabled,
   the ETR cannot know if the ITR is LISP-Multicast enabled.  To solve
   this problem, each mapping database entry will have a multicast
   2-tuple (Mpriority, Mweight) per RLOC [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>].  When the Mpriority
   is set to 255, the site is considered not multicast capable.  So, an
   ETR in a LISP receiver multicast site can distinguish whether a LISP
   source multicast site is a LISP-Multicast site or a uLISP site.

<span class="h4"><a class="selflink" id="section-9.1.5" href="#section-9.1.5">9.1.5</a>.  LISP Source Site to Any Receiver Sites</span>

   When a LISP source multicast site has receivers in LISP, non-LISP,
   and uLISP receiver multicast sites, it has a conflict about how it
   sends multicast packets.  The ITR can either encapsulate or natively
   forward multicast packets.  Since the receiver multicast sites are
   heterogeneous in their behavior, one packet-forwarding mechanism
   cannot satisfy both.  However, if a LISP receiver multicast site acts
   like a uLISP site, then it could receive packets like a non-LISP
   receiver multicast site, thereby making all receiver multicast sites
   have homogeneous behavior.  However, this poses the following issues:



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   o  LISP-NAT techniques with routable addresses would be required in
      all cases.

   o  Or, alternatively, mPETR deployment would be required, thus
      forcing coarse EID-Prefix advertisement in the core.

   o  But, what is most disturbing is that when all sites that
      participate are LISP-Multicast sites but a non-LISP or uLISP site
      joins the distribution tree, then the existing joined LISP
      receiver multicast sites would have to change their behavior.
      This would create too much dynamic tree-building churn to be a
      viable alternative.

   So, the solution space options are:

   1.  Make the LISP ITR in the source multicast site send two packets,
       one that is encapsulated with (S-RLOC,G) to reach LISP receiver
       multicast sites and another that is not encapsulated with
       (S-EID,G) to reach non-LISP and uLISP receiver multicast sites.

   2.  Make the LISP ITR always encapsulate packets with (S-RLOC,G) to
       reach LISP-Multicast sites and to reach mPETRs that can
       decapsulate and forward (S-EID,G) packets to non-LISP and uLISP
       receiver multicast sites.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  LISP Sites with Mixed Address Families</span>

   A LISP database mapping entry that describes the Locator-Set,
   Mpriority, and Mweight per locator address (RLOC), for an EID-Prefix
   associated with a site could have RLOC addresses in either IPv4 or
   IPv6 format.  When a mapping entry has a mix of RLOC-formatted
   addresses, it is an implicit advertisement by the site that it is a
   dual-stack site.  That is, the site can receive IPv4 or IPv6 unicast
   packets.

   To distinguish if the site can receive dual-stack unicast packets as
   well as dual-stack multicast packets, the Mpriority value setting
   will be relative to an IPv4 or IPv6 RLOC See [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] for packet
   format details.

   If one considers the combinations of LISP, non-LISP, and uLISP sites
   sharing the same distribution tree and considering the capabilities
   of supporting IPv4, IPv6, or dual-stack, the number of total
   combinations grows beyond comprehension.

   Using some combinatorial math, the following profiles of a site and
   the combinations that can occur:




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<span class="grey"><a href="./rfc6831">RFC 6831</a>             LISP for Multicast Environments        January 2013</span>


   1.  LISP-Multicast IPv4 Site

   2.  LISP-Multicast IPv6 Site

   3.  LISP-Multicast Dual-Stack Site

   4.  uLISP IPv4 Site

   5.  uLISP IPv6 Site

   6.  uLISP Dual-Stack Site

   7.  non-LISP IPv4 Site

   8.  non-LISP IPv6 Site

   9.  non-LISP Dual-Stack Site

   Let's define (m n) = m!/(n!*(m-n)!), pronounced "m choose n" to
   illustrate some combinatorial math below.

   When 1 site talks to another site, the combinatorial is (9 2), when 1
   site talks to another 2 sites, the combinatorial is (9 3).  If we sum
   this up to (9 9), then:

   (9 2) + (9 3) + (9 4) + (9 5) + (9 6) + (9 7) + (9 8) + (9 9) =

     36  +   84  +  126  +  126  +   84  +   36  +   9   +   1

   which results in 502 as the total number of cases to be considered.

   This combinatorial gets even worse when one considers a site using
   one address family inside of the site and the xTRs using the other
   address family (as in using IPv4 EIDs with IPv6 RLOCs or IPv6 EIDs
   with IPv4 RLOCs).

   To rationalize this combinatorial nightmare, there are some
   guidelines that need to be put in place:

   o  Each distribution tree shared between sites will either be an IPv4
      distribution tree or an IPv6 distribution tree.  Therefore, head-
      end replication can be avoided by building and sending packets on
      each address-family-based distribution tree.  Even though there
      might be an urge to do multicast packet translation from one
      address family format to the other, it is a non-viable over-
      complicated urge.  Multicast ITRs will only encapsulate packets
      where the inner and outer headers are from the same address
      family.



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   o  All LISP sites on a multicast distribution tree must share a
      common address family that is determined by the source site's
      Locator-Set in its LISP database mapping entry.  All receiver
      multicast sites will use the best RLOC priority controlled by the
      source multicast site.  This is true when the source site is
      either LISP-Multicast or uLISP enabled.  This means that priority-
      based policy modification is prohibited.  When a receiver
      multicast site ETR receives an (S-EID,G) join, it must select a
      S-RLOC for the same address family as S-EID.

   o  When a multicast Locator-Set has more than one locator, only
      locators from the same address family MUST be set to the same best
      priority value.  A mixed Locator-Set can exist (for unicast use),
      but the multicast priorities MUST be the set for the same address
      family locators.

   o  When the source site is not LISP enabled, determining the address
      family for the flow is up to how receivers find the source and
      group information for a multicast flow.

<span class="h3"><a class="selflink" id="section-9.3" href="#section-9.3">9.3</a>.  Making a Multicast Interworking Decision</span>

   Thus far, <a href="#section-9">Section 9</a> has shown all combinations of multicast
   connectivity that could occur.  As already concluded, this can be
   quite complicated, and, if the design is too ambitious, the dynamics
   of the protocol could cause a lot of instability.

   The trade-off decisions are hard to make, and so the same single
   solution is desirable to work for both IPv4 and IPv6 multicast.  It
   is imperative to have an incrementally deployable solution for all of
   IPv4 unicast and multicast and IPv6 unicast and multicast while
   minimizing (or eliminating) both unicast and multicast EID namespace
   state.

   Therefore, the design decision to go with uPITRs [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>] for
   unicast routing and mPETRs for multicast routing seems to be the
   sweet spot in the solution space in order to optimize state
   requirements and avoid head-end data replication at ITRs.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Considerations When RP Addresses Are Embedded in Group Addresses</span>

   When ASM and PIM-BIDIR are used in an IPv6 inter-domain environment,
   a technique exists to embed the unicast address of an RP in an IPv6
   group address [<a href="./rfc3956" title="&quot;Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address&quot;">RFC3956</a>].  When routers in end sites process a PIM
   Join/Prune message that contains an Embedded-RP group address, they
   extract the RP address from the group address and treat it from the
   EID namespace.  However, core routers do not have state for the EID
   namespace and need to extract an RP address from the RLOC namespace.



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   Therefore, it is the responsibility of ETRs in multicast receiver
   sites to map the group address into a group address where the
   Embedded-RP address is from the RLOC namespace.  The mapped RP
   address is obtained from an EID-to-RLOC mapping database lookup.  The
   ETR will also send a unicast (*,G) Join/Prune message to the ITR so
   the branch of the distribution tree from the source site resident RP
   to the ITR is created.

   This technique is no different than the techniques described in this
   specification for translating (S,G) state and propagating Join/Prune
   messages into the core.  The only difference is that the (*,G) state
   in Join/Prune messages are mapped because they contain unicast
   addresses encoded in an Embedded-RP group address.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Taking Advantage of Upgrades in the Core</span>

   If the core routers are upgraded to support [<a href="./rfc5496" title="&quot;The Reverse Path Forwarding (RPF) Vector TLV&quot;">RFC5496</a>], then the EID-
   specific data can be passed through the core without, possibly,
   having to store the state in the core.

   By doing this, one can eliminate the ETR from unicast encapsulated
   PIM Join/Prune messages to the source site's ITR.

   However, this solution is restricted to a small set of workable cases
   that would not be good for general use of LISP-Multicast.  In
   addition, due to slow convergence properties, it is not recommended
   for LISP-Multicast.

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Mtrace Considerations</span>

   Mtrace functionality MUST be consistent with unicast traceroute
   functionality where all hops from multicast receiver to multicast
   source are visible.

   The design for mtrace for use in LISP-Multicast environments is to be
   determined but should build upon mtrace version 2 specified in
   [<a href="#ref-MTRACE" title="&quot;Mtrace Version 2: Traceroute Facility for IP Multicast&quot;">MTRACE</a>].

<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  Security Considerations</span>

   The security concerns for LISP-Multicast are mainly the same as for
   the base LISP specification [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] and for multicast in general,
   including PIM-ASM [<a href="./rfc4601" title="&quot;Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)&quot;">RFC4601</a>].

   There may be a security concern with respect to unicast PIM messages.
   When multiple receiver sites are joining an (S-EID1,G) distribution
   tree that maps to a (RLOC1,G) core distribution tree, and a malicious
   receiver site joins an (S-EID2,G) distribution tree that also maps to



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   the (RLOC1,G) core distribution tree, the legitimate sites will
   receive data from S-EID2 when they did not ask for it.

   Other than as noted above, there are currently no known security
   differences between multicast with LISP and multicast without LISP.
   However, this has not been a topic that has been investigated deeply
   so far; therefore, additional issues might arise in future.

<span class="h2"><a class="selflink" id="section-14" href="#section-14">14</a>.  Acknowledgments</span>

   The authors would like to gratefully acknowledge the people who have
   contributed discussion, ideas, and commentary to the making of this
   proposal and specification.  People who provided expert review were
   Scott Brim, Greg Shepherd, and Dave Oran.  Other commentary from
   discussions at the Summer 2008 IETF in Dublin were Toerless Eckert
   and IJsbrand Wijnands.

   The authors would also like to thank the MBONED working group for
   constructive and civil verbal feedback when this document was
   presented at the Fall 2008 IETF in Minneapolis.  In particular, good
   commentary came from Tom Pusateri, Steve Casner, Marshall Eubanks,
   Dimitri Papadimitriou, Ron Bonica, Lenny Guardino, Alia Atlas, Jesus
   Arango, and Jari Arkko.

   An expert review of this specification was done by Yiqun Cai and
   Liming Wei.  The authors thank them for their detailed comments.

   This work originated in the Routing Research Group (RRG) of the IRTF.
   An individual submission was converted into a LISP working group
   document.

<span class="h2"><a class="selflink" id="section-15" href="#section-15">15</a>.  References</span>

<span class="h3"><a class="selflink" id="section-15.1" href="#section-15.1">15.1</a>.  Normative References</span>

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC3618">RFC3618</a>]  Fenner, B. and D. Meyer, "Multicast Source Discovery
              Protocol (MSDP)", <a href="./rfc3618">RFC 3618</a>, October 2003.

   [<a id="ref-RFC3956">RFC3956</a>]  Savola, P. and B. Haberman, "Embedding the Rendezvous
              Point (RP) Address in an IPv6 Multicast Address",
              <a href="./rfc3956">RFC 3956</a>, November 2004.

   [<a id="ref-RFC4601">RFC4601</a>]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", <a href="./rfc4601">RFC 4601</a>, August 2006.



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<span class="grey"><a href="./rfc6831">RFC 6831</a>             LISP for Multicast Environments        January 2013</span>


   [<a id="ref-RFC4604">RFC4604</a>]  Holbrook, H., Cain, B., and B. Haberman, "Using Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Protocol Version 2 (MLDv2) for Source-
              Specific Multicast", <a href="./rfc4604">RFC 4604</a>, August 2006.

   [<a id="ref-RFC4607">RFC4607</a>]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", <a href="./rfc4607">RFC 4607</a>, August 2006.

   [<a id="ref-RFC4760">RFC4760</a>]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", <a href="./rfc4760">RFC 4760</a>,
              January 2007.

   [<a id="ref-RFC5015">RFC5015</a>]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", <a href="./rfc5015">RFC 5015</a>, October 2007.

   [<a id="ref-RFC5135">RFC5135</a>]  Wing, D. and T. Eckert, "IP Multicast Requirements for a
              Network Address Translator (NAT) and a Network Address
              Port Translator (NAPT)", <a href="https://www.rfc-editor.org/bcp/bcp135">BCP 135</a>, <a href="./rfc5135">RFC 5135</a>, February 2008.

   [<a id="ref-RFC5496">RFC5496</a>]  Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
              Forwarding (RPF) Vector TLV", <a href="./rfc5496">RFC 5496</a>, March 2009.

   [<a id="ref-RFC6830">RFC6830</a>]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", <a href="./rfc6830">RFC 6830</a>,
              January 2013.

   [<a id="ref-RFC6832">RFC6832</a>]  Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
              "Interworking between Locator/ID Separation Protocol
              (LISP) and Non-LISP Sites", <a href="./rfc6832">RFC 6832</a>, January 2013.

<span class="h3"><a class="selflink" id="section-15.2" href="#section-15.2">15.2</a>.  Informative References</span>

   [<a id="ref-MTRACE">MTRACE</a>]   Asaeda, H. and W. Lee, Ed., "Mtrace Version 2: Traceroute
              Facility for IP Multicast", Work in Progress,
              October 2012.

   [<a id="ref-RFC5059">RFC5059</a>]  Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
              "Bootstrap Router (BSR) Mechanism for Protocol Independent
              Multicast (PIM)", <a href="./rfc5059">RFC 5059</a>, January 2008.

   [<a id="ref-RFC6836">RFC6836</a>]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol Alternative Logical
              Topology (LISP+ALT)", <a href="./rfc6836">RFC 6836</a>, January 2013.







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<span class="grey"><a href="./rfc6831">RFC 6831</a>             LISP for Multicast Environments        January 2013</span>


Authors' Addresses

   Dino Farinacci
   Cisco Systems
   Tasman Drive
   San Jose, CA
   USA

   EMail: farinacci@gmail.com


   Dave Meyer
   Cisco Systems
   Tasman Drive
   San Jose, CA
   USA

   EMail: dmm@cisco.com


   John Zwiebel
   Cisco Systems
   Tasman Drive
   San Jose, CA
   USA

   EMail: jzwiebel@cruzio.com


   Stig Venaas
   Cisco Systems
   Tasman Drive
   San Jose, CA
   USA

   EMail: stig@cisco.com















Farinacci, et al.             Experimental                     [Page 28]
</pre>