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Versions: (draft-rosen-mpls-multicast-encaps) 00 01 02 03 04 05 06 07 08 09 10 RFC 5332

Network Working Group                                    Toerless Eckert
Internet Draft                                    Eric C. Rosen (editor)
Expiration Date: March 2007                          Cisco Systems, Inc.
Updates RFCs 3032 and 4023
                                                          Rahul Aggarwal
                                                           Yakov Rekhter
                                                  Juniper Networks, Inc.

                                                          September 2006


                     MPLS Multicast Encapsulations


                draft-ietf-mpls-multicast-encaps-02.txt

Status of this Memo

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Abstract

   RFC 3032 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.  This specification updates
   RFC3032 by redefining the meaning of these two codepoints.  The
   former "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".  The former "unicast codepoint" is to



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   be used in all other cases.  Whether the data link layer payload is a
   unicast MPLS packet or a multicast MPLS packet is now to be
   determined by looking up the top label, rather than by the codepoint.

   RFC3032 does not specify the destination address to be placed in the
   "MAC DA" field of an ethernet frame which carries an MPLS multicast
   packet.  This document provides that specification.

   This document updates RFC 3032 and RFC 4023.




Contents

    1        Introduction  .........................................   2
    2        Upstream-Assigned vs. Downstream-Assigned  ............   3
    3        Ethernet Codepoints  ..................................   5
    4        PPP Protocol Field  ...................................   6
    5        GRE Protocol Type  ....................................   6
    6        IP Protocol Number  ...................................   6
    7        Ethernet MAC DA for Multicast MPLS  ...................   7
    8        IANA Considerations  ..................................   7
    9        Security Considerations  ..............................   7
   10        Normative References  .................................   7
   11        Informative References  ...............................   8
   12        Authors' Addresses  ...................................   8
   13        Full Copyright Statement  .............................   8
   14        Intellectual Property  ................................   9





1. Introduction

   RFC 3031 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



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   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 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 RFC3032.

   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
   the label is a multicast label or not.

   This document updates RFC 3032 and RFC 4023 by re-specifying the use
   of the codepoints.

   While RFC 3032 allows an MPLS packet to be carried in an ethernet
   multicast frame, it fails to specify how the Medium Access Layer
   Destination Address (MAC DA) field is to be set in that case.  This
   document provides that specification.


2. Upstream-Assigned vs. Downstream-Assigned

   According to RFC 3031, if two MPLS Label Switching Routers (LSRs) are
   adjacent in a label switched path (LSP), with respect to that LSP,
   one of them may be called the "upstream" LSR and the other the
   "downstream" LSR.  Call these Ru and Rd respectively.  Before Ru can
   send an MPLS packet to Rd with label L at the top of the label stack,
   Ru and Rd must agree on the Forwarding Equivalence Class (FEC) which
   is bound to L.  A particular binding of L to FEC F is called a
   "downstream-assigned" binding if the binding is first made by Rd and
   then advertised to Ru.  If the binding is first made by Ru and then
   advertised to Rd, it is called an "upstream-assigned" binding.

   If Ru and RD are LSP adjacencies, then they transmit a MPLS packet to
   each other through one of the following mechanisms:






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      1. by putting the MPLS packet in a data link layer frame and
         transmitting the frame

      2. by transmitting the MPLS packet through an MPLS tunnel, i.e.,
         by pushing an additional label (or labels) onto the label
         stack, and then invoking mechanism 1,

      3. by transmitting the MPLS packet through an IP-based tunnel
         (e.g., via RFC 4023), and then invoking mechanisms 1 and/or 2.

   In short, an MPLS packet is transmitted either through a data link or
   through an MPLS tunnel or through an IP tunnel.  In any of those
   cases, when the packet emerges through the tunnel, the downstream LSR
   must know whether the label that now appears at the top of the label
   stack has an upstream-assigned label binding or a downstream-assigned
   label binding.  For convenience, we will speak of a label with an
   upstream-assigned label binding as an "upstream-assigned label".

   Unicast labels MUST be downstream-assigned.

   Under certain conditions, specified below, multicast labels MAY be
   upstream-assigned.  The ability to use upstream-assigned labels is an
   OPTIONAL feature.  Upstream-assigned labels MUST NOT be used unless
   it is known that the downstream LSR supports them.  How this is known
   is outside the scope of this document.

   We discuss three different types of data link or tunnel:

     - Point-to-Point.  A point-to-point data link or tunnel associates
       two systems, such that transmissions on that link or tunnel made
       by the one are received by the other, and only by the other.

       When an MPLS packet is transmitted on a point-to-point data link
       or tunnel, its top label (before applying the data link or tunnel
       encapsulation) MUST be a downstream-assigned label.

     - Point-to-Multipoint.  A point-to-multipoint link or tunnel
       associates n systems, such that only one of them can transmit
       onto the link or tunnel, and the transmissions may be received by
       the other n-1 systems.

       The top labels (before applying the data link or tunnel
       encapsulation) of all MPLS packets which are transmitted on a
       particular point-to-multipoint data link or tunnel MUST be of the
       same type; either all upstream-assigned or all downstream-
       assigned.  This means that all the receivers on the MPLS or IP
       tunnel must know a priori whether upstream-assigned or
       downstream-assigned labels are being used in the tunnel.  How



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       this is known is outside the scope of this document.

     - Multipoint-to-Multipoint. A multipoint-to-multipoint link or
       tunnel associates n systems, such that any of them can transmit
       on the link or tunnel, and the transmissions may be received by
       the other n-1 systems.

       If MPLS packets are transmitted on a particular multipoint-to-
       multipoint link or tunnel, one of the following scenarios
       applies:

          1. It is known (by methods outside the scope of this document)
             that the top label of every MPLS packet on the link or
             tunnel is downstream-assigned

          2. It is known (by methods outside the scope of this document)
             that the top label of every MPLS packet on the link or
             tunnel is upstream-assigned

          3. Some MPLS packets on the link may have upstream-assigned
             top labels while some may have downstream-assigned top
             labels

       If (and only if) the third scenario applies, the data link or
       tunnel encapsulation MUST provide a codepoint which specifies
       whether the top label of the encapsulated MPLS packet is
       upstream-assigned or downstream-assigned.  If a particular type
       of data link or tunnel does not provide such a codepoint, then
       the third scenario MUST NOT be used.

   The remainder of this document specifies procedures for setting the
   data link layer codepoints and address fields.


3. Ethernet Codepoints

   Ethernet is an example of a multipoint-to-multipoint data link.

   Ethertype 0x8847 is used whenever a unicast ethernet frame carries an
   MPLS packet.

   Ethertype 0x8847 is also used whenever a multicast ethernet frame
   carries an MPLS packet, EXCEPT for the case where the top label of
   the MPLS packet has been upstream-assigned.

   Ethertype 0x8848, formerly known as the "MPLS multicast codepoint",
   is to be used only when an MPLS packet whose top label is upstream-
   assigned is carried in a multicast ethernet frame.



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4. PPP Protocol Field

   PPP is an example of a point-to-point data link.  When a PPP frame is
   carrying an MPLS packet, the PPP Protocol field is always set to
   0x0281.


5. GRE Protocol Type

   RFC 4023 is modified as described below.

   If the IP destination address of the GRE encapsulation is a unicast
   IP address, then the ethertype value 0x8847 MUST be used in all cases
   for the MPLS-in-GRE encapsulation.

   If the IP destination address of the GRE encapsulation is a multicast
   IP address, then:

     - the ethertype value 0x8847 MUST be used when the top label of the
       encapsulated MPLS packet is downstream-assigned,

     - the ethertype value 0x8848 MUST be used when the top label of the
       encapsulated MPLS packet is upstream-assigned.

   Through procedures which are outside the scope of this specification,
   it may be known that if the destination address of a GRE packet is a
   multicast IP address, then the top label of the GRE payload is
   upstream-assigned.  In such a case, the occurrence of the 8847
   codepoint in a GRE packet with a multicast destination IP address
   MUST be considered an error, and the packet MUST be discarded.


6. IP Protocol Number

   RFC 4023 is modified as follows: the IPv4 Protocol Number field or
   the IPv6 Next Header field is always set to 137, whether or not the
   encapsulated MPLS packet is an MPLS multicast packet.

   If the IP destination address of the IP encapsulation is an IP
   multicast address, the IP tunnel may be considered to be a point-to-
   multipoint tunnel or a multipoint-to-multipoint tunnel.  In either
   case, either all encapsulated MPLS packets in the particular tunnel
   have a downstream-assigned label at the top of the stack, or all
   encapsulated MPLS packets in that tunnel have an upstream-assigned
   label at the top of the stack.  The means by which this is determined
   for a particular tunnel is outside the scope of this specification.





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7. Ethernet MAC DA for Multicast MPLS

   When a multicast MPLS packet is carried in a multicast ethernet
   frame, the Destination MAC Address shall be set to the value 01-00-
   5e-8a-bc-de, where abcde is the twenty-bit (5-nibble) value of the
   topmost MPLS label of the MPLS packet.


8. IANA Considerations

   IANA already owns the set of ethernet multicast addresses in the
   range 01-00-5e-00-00-00 to 01-00-5e-ff-ff-ff.  Addresses in the range
   01-00-5e-00-00-00 to 01-00-5e-7f-ff-ff are reserved for use when an
   ethernet multicast frame carries an IP multicast packet.  IANA shall
   reserve ethernet addresses in the range 01-00-5e-80-00-00 to 01-00-
   5e-8f-ff-ff for use when an ethernet multicast frame carries an MPLS
   multicast packet.


9. Security Considerations

   The security considerations of RFC 3032 and RFC 4023 apply.

   Malicious changing of the codepoint may result in loss or misrouting
   of packets. However, altering the codepoint without also altering the
   label does not result in a predictable effect.

   Malicious alteration of the MAC DA on an ethernet can result in
   packets being received by a third party, rather than by the intended
   recipient.



10. Normative References

   [RFC3031] "Multiprotocol Label Switching Architecture", Rosen,
   Viswanathan, Callon, January 2001

   [RFC3032] "MPLS Label Stack Encoding", Rosen, et. al., January 2001

   [RFC4023] "Encapsulating MPLS in IP or GRE", Worster, Rekhter, Rosen,
   March 2005









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11. Informative References

12. Authors' Addresses


      Toerless Eckert
      Cisco Systems, Inc.
      170 Tasman Drive
      San Jose, CA, 95134
      Email: eckert@cisco.com



      Eric C. Rosen
      Cisco Systems, Inc.
      1414 Massachusetts Avenue
      Boxborough, MA 01719
      Email: erosen@cisco.com



      Rahul Aggarwal
      Juniper Networks
      1194 North Mathilda Ave.
      Sunnyvale, CA 94089
      Email: rahul@juniper.net



      Yakov Rekhter
      Juniper Networks
      1194 North Mathilda Ave.
      Sunnyvale, CA 94089
      Email: yakov@juniper.net



13. Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET



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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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   The IETF invites any interested party to bring to its attention any
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   ipr@ietf.org.






















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