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Network Working Group                                          T. EckertRequest for Comments: 5332                                 E. Rosen, Ed.Category: Standards Track                            Cisco Systems, Inc.Updates:3032,4023                                          R. Aggarwal                                                              Y. Rekhter                                                  Juniper Networks, Inc.                                                             August 2008MPLS Multicast EncapsulationsStatus 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.AbstractRFC 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 updatesRFC 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 updatesRFC 3032 andRFC 4023.Eckert, et al.              Standards Track                     [Page 1]

RFC 5332             MPLS Multicast Encapsulations           August 2008Table of Contents1. Introduction ....................................................22. Specification of Requirements ...................................33. Upstream-Assigned vs. Downstream-Assigned .......................34. Ethernet Codepoints .............................................65. PPP Protocol Field ..............................................66. GRE Protocol Type ...............................................67. IP Protocol Number ..............................................78. Ethernet MAC DA for Multicast MPLS ..............................79. IANA Considerations .............................................810. Security Considerations ........................................811. Normative References ...........................................91.  IntroductionRFC 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 inRFC 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 byRFC 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 updatesRFC 3032 andRFC 4023 by re-specifying the use   of the codepoints.  The old use of the "multicast codepoint", as   specified in those two RFCs, is hereby deprecated.   Note that an implementation that does MPLS multicast according toRFC3032 and/or 4023 will be unable to interoperate with implementations   that do MPLS multicast according to this document.  There may be some   deployed platforms that support the deprecated use of the codepoints,   but those platforms do not support the control plane mechanisms to   support MPLS multicast.  The absence of the control plane will   prevent a system that implements the deprecated use of codepoints   from attempting to interoperate with a system that uses the   codepoints as specified herein.  (If an MPLS multicast control plane   were to be implemented on a platform that only supports the   deprecated codepoint, interoperability problems such as black holes   and/or misrouting would arise.  This does not seem like a potential   problem in practice.)   WhileRFC 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.  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].3.  Upstream-Assigned vs. Downstream-Assigned   Suppose a labeled packet P is sent from Label Switching Router (LSR)   R1 to LSR R2, where R1 puts label L on the packet's label stack, and   R2 has to look up label L in order to determine the corresponding   Forwarding Equivalence Class (FEC), call it F.   If the binding between L and F was made by R2 and advertised to R1,   then the label binding is known as "downstream-assigned".RFC 3031   only discusses downstream-assigned label bindings.   If the binding between L and F was made by R1 and advertised to R2,   then the label binding is known as "upstream-assigned".Eckert, et al.              Standards Track                     [Page 3]

RFC 5332             MPLS Multicast Encapsulations           August 2008   If the binding between L and F was made by a third party, say R3, and   then advertised to both R1 and R2, we also refer to the label binding   as "upstream-assigned".   Upstream-assigned labels are not required to come from the same   "label space" as downstream-assigned labels.  SeeSection 3.14 of   [RFC3031] and especially [RFC5331] for a discussion of the notion of   "label space".  The procedures for properly interpreting an upstream-   assigned label are given in [RFC5331].   If Ru and Rd are LSP adjacencies, then they transmit an 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, or      3. by transmitting the MPLS packet through an IP-based tunnel         (e.g., viaRFC 4023 [RFC4023]), and then invoking mechanisms 1         and/or 2.   In short, an MPLS packet is transmitted through a data link, 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".   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.   This document makes no changes to the procedures regarding unicast   labels.   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 one are received by the other, and only by the other.Eckert, et al.              Standards Track                     [Page 4]

RFC 5332             MPLS Multicast Encapsulations           August 2008        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 that 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.         3. Some MPLS packets on the link may have upstream-assigned top            labels while some may have downstream-assigned top labels.Eckert, et al.              Standards Track                     [Page 5]

RFC 5332             MPLS Multicast Encapsulations           August 2008      If (and only if) the third scenario applies, the data link or      tunnel encapsulation MUST provide a codepoint that 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 TypeRFC 4023 is modified as described below.   If the IP destination address of the Generic Routing Encapsulation   (GRE) 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.Eckert, et al.              Standards Track                     [Page 6]

RFC 5332             MPLS Multicast Encapsulations           August 2008   Through procedures that 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 NumberRFC 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 MAC Destination Address to the value   01-00-5e-8v-wx-yz, where vwxyz is a 20-bit (five-nibble) value set as   follows:      1. vwxyz MAY be set to 0,      2. vwxyz MAY be set to the value of one of the MPLS labels on the         packet's label stack.   Which of these procedures is the default procedure in any particular   LSR is implementation-dependent.  However, LSRs using the two   different procedures MUST interoperate.  That is, an LSR MUST NOT   filter packets for which vwxyz has been set to zero, and it MUST NOT   indiscriminately filter all packets for which vwxyz has not been set   to zero.   If an LSR follows the procedure of setting vwxyz to the value of one   of the MPLS labels on the packet's label stack, and if that label   stack contains two or more labels, then by default, vwxyz MUST be set   to the 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 label other than the secondEckert, et al.              Standards Track                     [Page 7]

RFC 5332             MPLS Multicast Encapsulations           August 2008   to be used for this purpose.  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 [RFC5331],   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 label, more of the LSP-specific information about the packet   appears in the MAC DA field.  This can be used to filter multicast   packets with "unexpected" non-zero values of vwxyz.  Further   discussion of such filtering or its uses is outside the scope of this   document.   The use of ethernet and/or IP broadcast addresses (as distinguished   from multicast addresses) does not fall within the scope of this   specification.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 already reserved for use   when an ethernet multicast frame carries an IP multicast packet.   IANA has reserved 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.  Addresses in this range are valid when   used with ethertype 8847 or 8848.   As this document modifies the usage of ethertypes 8847 and 8848, IANA   has changed the description of these ethertypes as follows.   Ethertype 8847 is defined as "MPLS", as defined inRFC 3032 and in   this document.  Ethertype 8848 is defined as "MPLS with upstream-   assigned label", as defined in this document.10.  Security Considerations   The security considerations ofRFC 3032 andRFC 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.Eckert, et al.              Standards Track                     [Page 8]

RFC 5332             MPLS Multicast Encapsulations           August 200811.  Normative References   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol             Label Switching Architecture",RFC 3031, January 2001.   [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,             Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack             Encoding",RFC 3032, January 2001.   [RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed., "Encapsulating             MPLS in IP or Generic Routing Encapsulation (GRE)",RFC4023, March 2005.   [RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream             Label Assignment and Context-Specific Label Space",RFC5331, August 2008.Eckert, et al.              Standards Track                     [Page 9]

RFC 5332             MPLS Multicast Encapsulations           August 2008Authors' 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.netEckert, et al.              Standards Track                    [Page 10]

RFC 5332             MPLS Multicast Encapsulations           August 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 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, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Eckert, et al.              Standards Track                    [Page 11]