rfc3353.txt

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Network Working Group                                            D. Ooms
Request for Comments: 3353                                       Alcatel
Category: Informational                                         B. Sales
                                                                 Alcatel
                                                               W. Livens
                                                            Colt Telecom
                                                              A. Acharya
                                                                     IBM
                                                             F. Griffoul
                                                                 Ulticom
                                                               F. Ansari
                                                               Bell Labs
                                                             August 2002


                     Overview of IP Multicast in a
           Multi-Protocol Label Switching (MPLS) Environment

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document offers a framework for IP multicast deployment in an
   MPLS environment.  Issues arising when MPLS techniques are applied to
   IP multicast are overviewed.  The pros and cons of existing IP
   multicast routing protocols in the context of MPLS are described and
   the relation to the different trigger methods and label distribution
   modes are discussed.  The consequences of various layer 2 (L2)
   technologies are listed.  Both point-to-point and multi-access
   networks are considered.













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Table of Contents

   1.     Introduction .............................................  3
   2.     Layer 2 Characteristics ..................................  4
   3.     Taxonomy of IP Multicast Routing Protocols
          in the Context of MPLS ...................................  5
   3.1.   Aggregation ..............................................  5
   3.2.   Flood & Prune ............................................  5
   3.3.   Source/Shared Trees ......................................  6
   3.4.   Co-existence of Source and Shared Trees ..................  7
   3.5.   Uni/Bi-directional Shared Trees .......................... 10
   3.6.   Encapsulated Multicast Data .............................. 11
   3.7.   Loop-free-ness ........................................... 11
   3.8.   Mapping of Characteristics on Existing Protocols ......... 11
   4.     Mixed L2/L3 Forwarding in a Single Node .................. 12
   5.     Taxonomy of IP Multicast LSP Triggers .................... 14
   5.1.   Request Driven ........................................... 14
   5.1.1. General .................................................. 14
   5.1.2. Multicast Routing Messages ............................... 15
   5.1.3. Resource Reservation Messages ............................ 15
   5.2.   Topology Driven .......................................... 16
   5.3.   Traffic Driven ........................................... 16
   5.3.1. General .................................................. 16
   5.3.2. An Implementation Example ................................ 17
   5.4.   Combinations of Triggers and Label Distribution Modes .... 18
   6.     Piggy-backing ............................................ 18
   7.     Explicit Routing ......................................... 20
   8.     QoS/CoS .................................................. 20
   8.1.   DiffServ ................................................. 20
   8.2.   IntServ and RSVP ......................................... 21
   9.     Multi-access Networks .................................... 21
   10.    More Issues .............................................. 22
   10.1.  TTL Field ................................................ 22
   10.2.  Independent vs. Ordered Label Distribution Control ....... 23
   10.3.  Conservative vs. Liberal Label Retention Mode ............ 24
   10.4.  Downstream vs. Upstream Label Allocation ................. 25
   10.5.  Explicit vs. Implicit Label Distribution ................. 25
   11.    Security Considerations .................................. 26
   12.    Acknowledgements ......................................... 26
   Informative References........................................... 27
   Authors' Addresses .............................................. 28
   Full Copyright Statement ........................................ 30









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RFC 3353          IP Multicast in an MPLS Environment        August 2002


Table of Abbreviations

   ATM     Asynchronous Transfer Node
   CBT     Core Based Tree
   CoS     Class of Service
   DLCI    Data Link Connection Identifier
   DRrecv  Designated Router of the receiver
   DRsend  Designated Router of the sender
   DVMRP   Distant Vector Multicast Routing Protocol
   FR      Frame Relay
   IGMP    Internet Group Management Protocol
   IP      Internet Protocol
   L2      layer 2 (e.g. ATM, Frame Relay)
   L3      layer 3 (e.g. IP)
   LSP     Label Switched Path
   LSR     Label Switching Router
   LSRd    Downstream LSR
   LSRu    Upstream LSR
   MOSPF   Multicast OSPF
   mp2mp   multipoint-to-multipoint
   MRT     Multicast Routing Table
   p2mp    point-to-multipoint
   PIM-DM  Protocol Independent Multicast-Dense Mode
   PIM-SM  Protocol Independent Multicast-Sparse Mode
   QoS     Quality of Service
   RP      Rendezvous Point
   RPT-bit RP Tree bit [DEER]
   RSVP    Resource reSerVation Protocol
   SPT-bit Shortest Path Tree [DEER]
   SSM     Source Specific Multicast
   TCP     Transmission Control Protocol
   UDP     User Datagram Protocol
   VC      Virtual Circuit
   VCI     Virtual Circuit Identifier
   VP      Virtual Path
   VPI     Virtual Path Identifier

1. Introduction

   In an MPLS cloud the routes are determined by a L3 routing protocol.
   These routes can then be mapped onto L2 paths to enhance network
   performance.  Besides this, MPLS offers a vehicle for enhanced
   network services such as QoS/CoS, traffic engineering, etc.

   Current unicast routing protocols generate a same (optimal) shortest
   path in steady state for a certain (source, destination) pair.
   Remark that unicast protocols can behave slightly different with
   regard to equal cost paths.



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RFC 3353          IP Multicast in an MPLS Environment        August 2002


   For multicast, the optimal solution (minimum cost to interconnect N
   nodes) would impose a Steiner tree computation.  Unfortunately, no
   multicast routing protocol today is able to maintain such an optimal
   tree.  Different multicast protocols will therefore, in general,
   generate different trees.

   The discussion is focused on intra-domain multicast routing
   protocols.  Aspects of inter-domain routing are beyond the scope of
   this document.

2. Layer 2 Characteristics

   Although MPLS is multiprotocol both at L3 and at L2, in practice IP
   is the only considered L3 protocol.  MPLS can run on top of several
   L2 technologies (PPP/Sonet, Ethernet, ATM, FR, ...).

   When label switching is mapped on L2 switching capabilities (e.g.
   VPI/VCI is used as label), attention is mainly focused on the mapping
   to ATM [DAVI].  ATM offers high switching capacities and QoS
   awareness, but in the context of MPLS it poses several limitations
   which are described in [DAVI].  Similar considerations are made for
   Frame Relay on L2 in [CONT].  The limitations can be summarized as:

   - Limited Label Space:  either the standardized or the implemented
     number of bits available for a label can be small (e.g. VPI/VCI
     space, DLCI space), limiting the number of LSPs that can be
     established.

   - Merging:  some L2 technologies or implementations of these
     technologies do not support multipoint-to-point and/or
     multipoint-to-multipoint 'connections', obstructing the merging of
     LSPs.

   - TTL:  L2 technologies do not support a 'TTL-decrement' function.

   All three limitations can impact the implementation of multicast in
   MPLS as will be described in this document.

   When native MPLS is deployed the above limitations vanish.  Moreover
   on PPP and Ethernet links the same label can be used at the same time
   for a unicast and a multicast LSP because different EtherTypes for
   MPLS unicast and multicast are defined [ROSE].









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RFC 3353          IP Multicast in an MPLS Environment        August 2002


3. Taxonomy of IP Multicast Routing Protocols in the Context of MPLS

   At the moment, an abundance of IP multicast routing protocols is
   being proposed and developed.  All these protocols have different
   characteristics (scalability, computational complexity, latency,
   control message overhead, tree type, etc...).  It is not the purpose
   of this document to give a complete taxonomy of IP multicast routing
   protocols, only their characteristics relevant to the MPLS technology
   will be addressed.

   The following characteristics are considered:

   - Aggregation
   - Flood & Prune
   - Source/Shared trees
   - Co-existence of Source and Shared Trees
   - Uni/Bi-directional shared trees
   - Encapsulated multicast data
   - Loop-free-ness

   The discussion of these characteristics will not lead to the
   selection of one superior multicast routing protocol.  It is not
   impossible that different IP multicast routing protocols will be
   deployed in the Internet.

3.1. Aggregation

   In unicast different destination addresses are aggregated to one
   entry in the routing table, yielding one FEC and one LSP.

   The granularity of multicast streams is (*, G) for a shared tree and
   (S, G) for a source tree, S being the source address and G the
   multicast group address.  Aggregation of multicast trees with
   different multicast 'destination' addresses on one LSP is a subject
   for further study.

3.2. Flood & Prune

   To establish a multicast tree some IP multicast routing protocols
   (e.g. DVMRP, PIM-DM) flood the network with multicast data.  The
   branches can then be pruned by nodes which do not want to receive the
   data of the specific multicast group.  This process is repeated
   periodically.

   Flood & Prune multicast routing protocols have some characteristics
   which significantly differ from unicast routing protocols:





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   a) Volatile.  Due to the Flood & Prune nature of the protocol, very
      volatile tree structures are generated.  Solutions to map a
      dynamic L3 p2mp tree to a L2 p2mp LSP need to be efficient in
      terms of signaling overhead and LSP setup time.  The volatile L2
      LSP will consume a lot of labels throughout the network, which is
      a disadvantage when label space is limited.

   b) Traffic-driven.  The router only creates state for a certain group
      when data arrives for that group.  Routers also independently
      decide to remove state when an inactivity timer expires.

      - Thus LSPs can not be pre-established as is usually done in
        unicast.  To minimize the time between traffic arrival and LSP
        establishment a fast LSP setup method is favorable.

      - Since creation and deletion of a L3 route at each node is
        triggered by traffic, this suggests that the LSP associated with
        the route be setup and torn down in a traffic-driven manner as
        well.

      - If an LSR does not support L3 forwarding this traffic-driven
        nature even requires that the upstream LSR takes the initiative
        to create an LSP (Upstream Unsolicited or Downstream on Demand
        label advertisement).

3.3. Source/Shared Trees

   IP multicast routing protocols create either source trees (S, G),
   i.e. a tree per source (S) and per multicast group (G), or shared
   trees (*, G), i.e. one tree per multicast group (Figure 1).


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