MPLS Multicast Encapsulations
RFC 5332

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Network Working Group                                          T. Eckert
Request for Comments: 5332                                 E. Rosen, Ed.
Category: Standards Track                            Cisco Systems, Inc.
Updates: 3032, 4023                                          R. Aggarwal
                                                              Y. Rekhter
                                                  Juniper Networks, Inc.
                                                             August 2008

                     MPLS Multicast Encapsulations

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   RFC 3032 established two data link layer codepoints for MPLS, used to
   distinguish whether the data link layer frame is carrying an MPLS
   unicast or an MPLS multicast packet.  However, this usage was never
   deployed.  This specification updates RFC 3032 by redefining the
   meaning of these two codepoints.  Both codepoints can now be used to
   carry multicast packets.  The second codepoint (formerly the
   "multicast codepoint") is now to be used only on multiaccess media,
   and it is to mean "the top label of the following label stack is an
   upstream-assigned label".

   RFC 3032 does not specify the destination address to be placed in the
   "MAC DA" (Medium Access Layer Destination Address) field of an
   ethernet frame that carries an MPLS multicast packet.  This document
   provides that specification.

   This document updates RFC 3032 and RFC 4023.

Eckert, et al.              Standards Track                     [Page 1]
RFC 5332             MPLS Multicast Encapsulations           August 2008

Table of Contents

   1. Introduction ....................................................2
   2. Specification of Requirements ...................................3
   3. Upstream-Assigned vs. Downstream-Assigned .......................3
   4. Ethernet Codepoints .............................................6
   5. PPP Protocol Field ..............................................6
   6. GRE Protocol Type ...............................................6
   7. IP Protocol Number ..............................................7
   8. Ethernet MAC DA for Multicast MPLS ..............................7
   9. IANA Considerations .............................................8
   10. Security Considerations ........................................8
   11. Normative References ...........................................9

1.  Introduction

   RFC 3031 [RFC3031] defines the "Next Hop Label Forwarding Entry"
   (NHLFE).  The NHLFE for a particular label maps the label into a next
   hop (among other things).  When an MPLS packet is received, its top
   label is mapped to an NHLFE, and the packet is sent to the next hop
   specified by the NHLFE.

   We define a particular MPLS label to be a "multicast label" in a
   particular context if the NHLFE to which it is mapped, in that
   context, specifies a set of next hops, with the semantics that the
   packet is to be replicated and a copy of the packet sent to each of
   the specified next hops.  Note that this definition accommodates the
   case where the set of next hops contains a single member.  What makes
   a label a multicast label in a particular context is the semantics
   attached to the set, i.e., the intention to replicate the packet and
   transmit to all members of the set if the set has more than one
   member.

   RFC 3032 [RFC3032] established two data link layer codepoints for
   MPLS: one to indicate that the data link layer frame is carrying an
   MPLS unicast packet, and the other to indicate that the data link
   layer frame is carrying an MPLS multicast packet.  The term
   "multicast packet" is not precisely defined in RFC 3032, though one
   may presume that the "multicast" codepoint is intended to identify
   the packet's top label as a multicast label.  However, the multicast
   codepoint has never been deployed, and further development of the
   procedures for MPLS multicast have shown that, while there is a need
   for two codepoints, the use of the two codepoints is not properly
   captured by RFC 3032.

Eckert, et al.              Standards Track                     [Page 2]
RFC 5332             MPLS Multicast Encapsulations           August 2008

   In particular, there is no need for the codepoint to indicate whether
   the top MPLS label is a multicast label.  When the receiver of an
   MPLS packet looks up the top label, the NHLFE will specify whether or
   not the label is a multicast label.

   This document updates RFC 3032 and RFC 4023 by re-specifying the use
   of the codepoints.  The old use of the "multicast codepoint", as