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

                                                               June 2007


                     MPLS Multicast Encapsulations


                draft-ietf-mpls-multicast-encaps-06.txt

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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 RFC
   3032 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 be



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   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.

   RFC 3032 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        Specification of Requirements  ...........................   2
 2        Introduction  ............................................   3
 3        Upstream-Assigned vs. Downstream-Assigned  ...............   4
 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  ....................................   8
12        Informative References  ..................................   9
13        Authors' Addresses  ......................................   9
14        Full Copyright Statement  ................................  10
15        Intellectual Property  ...................................  10





1. Specification of Requirements

   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 [RFC2119].







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2. 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.

   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.








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3. Upstream-Assigned vs. Downstream-Assigned

   According to RFC 3031 [RFC3031], 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:

      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 [RFC4023]), 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:







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     - 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.

       For a given direction of a given point-to-point data link or
       tunnel, the following MUST be the case:  either every MPLS packet
       will carry an upstream-assigned label, or else every MPLS packet
       will carry a downstream-assigned label.  The procedures for
       determining whether upstream-assigned or downstream-assigned
       labels are being used are outside the scope of this
       specification. However, in the absence of any other information,
       the use of downstream-assigned labels MUST be presumed by
       default.

     - 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
       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







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          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.


4. 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.


5. 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.


6. 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:



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     - 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.


7. 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.


8. Ethernet MAC DA for Multicast MPLS

   When an LSR transmits a multicast MPLS packet in a multicast ethernet
   frame, it MUST set the Destination MAC Address to the value
   01-00-5e-8a-bc-de, where abcde MUST, by default, be the twenty-bit
   (five-nibble) value of the second MPLS label on the packet's label
   stack.  By "the second label", we mean the label that is in the label
   stack entry that immediately follows the topmost label stack entry.
   The LSR MAY, if configured to do so, allow a a label other than the
   second to be used for this purpose. However, if the MPLS packet has
   only one label, the value of that label will be used instead of the
   value of the (non-existent) second label.

   It is expected that the LSR will follow the procedures of [UPSTREAM],
   pushing on two labels, with the topmost label being a "context label"
   that is the same for all MPLS packets being transmitted by the LSR
   onto the ethernet, but with the second label being different for
   different LSPs.  Thus if the MAC DA value is a function of the second



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   label, more of the LSP-specific information about the packet appears
   in the MAC DA field.  However, the way in which that information is
   used, if any, is outside the scope of this document.


9. 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.


10. 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.



11. Normative References

   [RFC2119] "Key words for use in RFCs to Indicate Requirement
   Levels.", Bradner, March 1997

   [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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12. Informative References

   [UPSTREAM] "MPLS Upstream Label Assignment and Context Specific Label
   Space", Aggarwal, Rekhter, Rosen,  draft-ietf-mpls-upstream-
   label-02.txt, March 2007.


13. 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













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14. Full Copyright Statement

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