Multiprotocol Extensions for BGP-4
RFC 4760

Network Working Group                                           T. Bates
Request for Comments: 4760                                 Cisco Systems
Obsoletes: 2858                                               R. Chandra
Category: Standards Track                                  Sonoa Systems
                                                                 D. Katz
                                                              Y. Rekhter
                                                        Juniper Networks
                                                            January 2007

                   Multiprotocol Extensions for BGP-4

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.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document defines extensions to BGP-4 to enable it to carry
   routing information for multiple Network Layer protocols (e.g., IPv6,
   IPX, L3VPN, etc.).  The extensions are backward compatible - a router
   that supports the extensions can interoperate with a router that
   doesn't support the extensions.

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1.  Introduction

   The only three pieces of information carried by BGP-4 [BGP-4] that
   are IPv4 specific are (a) the NEXT_HOP attribute (expressed as an
   IPv4 address), (b) AGGREGATOR (contains an IPv4 address), and (c)
   NLRI (expressed as IPv4 address prefixes).  This document assumes
   that any BGP speaker (including the one that supports multiprotocol
   capabilities defined in this document) has to have an IPv4 address
   (which will be used, among other things, in the AGGREGATOR
   attribute).  Therefore, to enable BGP-4 to support routing for
   multiple Network Layer protocols, the only two things that have to be
   added to BGP-4 are (a) the ability to associate a particular Network
   Layer protocol with the next hop information, and (b) the ability to
   associate a particular Network Layer protocol with NLRI.  To identify
   individual Network Layer protocols associated with the next hop
   information and semantics of NLRI, this document uses a combination
   of Address Family, as defined in [IANA-AF], and Subsequent Address
   Family (as described in this document).

   One could further observe that the next hop information (the
   information provided by the NEXT_HOP attribute) is meaningful (and
   necessary) only in conjunction with the advertisements of reachable
   destinations - in conjunction with the advertisements of unreachable
   destinations (withdrawing routes from service), the next hop
   information is meaningless.  This suggests that the advertisement of
   reachable destinations should be grouped with the advertisement of
   the next hop to be used for these destinations, and that the
   advertisement of reachable destinations should be segregated from the
   advertisement of unreachable destinations.

   To provide backward compatibility, as well as to simplify
   introduction of the multiprotocol capabilities into BGP-4, this
   document uses two new attributes, Multiprotocol Reachable NLRI
   (MP_REACH_NLRI) and Multiprotocol Unreachable NLRI (MP_UNREACH_NLRI).
   The first one (MP_REACH_NLRI) is used to carry the set of reachable
   destinations together with the next hop information to be used for
   forwarding to these destinations.  The second one (MP_UNREACH_NLRI)
   is used to carry the set of unreachable destinations.  Both of these
   attributes are optional and non-transitive.  This way, a BGP speaker
   that doesn't support the multiprotocol capabilities will just ignore
   the information carried in these attributes and will not pass it to
   other BGP speakers.

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 RFC 2119 [RFC2119].

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3.  Multiprotocol Reachable NLRI - MP_REACH_NLRI (Type Code 14):

   This is an optional non-transitive attribute that can be used for the
   following purposes:

   (a) to advertise a feasible route to a peer

   (b) to permit a router to advertise the Network Layer address of the
       router that should be used as the next hop to the destinations
       listed in the Network Layer Reachability Information field of the
       MP_NLRI attribute.

   The attribute is encoded as shown below:

        +---------------------------------------------------------+
        | Address Family Identifier (2 octets)                    |
        +---------------------------------------------------------+
        | Subsequent Address Family Identifier (1 octet)          |
        +---------------------------------------------------------+
        | Length of Next Hop Network Address (1 octet)            |
        +---------------------------------------------------------+
        | Network Address of Next Hop (variable)                  |
        +---------------------------------------------------------+
        | Reserved (1 octet)                                      |
        +---------------------------------------------------------+
        | Network Layer Reachability Information (variable)       |
        +---------------------------------------------------------+

   The use and meaning of these fields are as follows:

      Address Family Identifier (AFI):

         This field in combination with the Subsequent Address Family
         Identifier field identifies the set of Network Layer protocols
         to which the address carried in the Next Hop field must belong,
         the way in which the address of the next hop is encoded, and
         the semantics of the Network Layer Reachability Information
         that follows.  If the Next Hop is allowed to be from more than
         one Network Layer protocol, the encoding of the Next Hop MUST
         provide a way to determine its Network Layer protocol.

         Presently defined values for the Address Family Identifier
         field are specified in the IANA's Address Family Numbers
         registry [IANA-AF].

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      Subsequent Address Family Identifier (SAFI):

         This field in combination with the Address Family Identifier
         field identifies the set of Network Layer protocols to which
         the address carried in the Next Hop must belong, the way in
         which the address of the next hop is encoded, and the semantics
         of the Network Layer Reachability Information that follows.  If
         the Next Hop is allowed to be from more than one Network Layer
         protocol, the encoding of the Next Hop MUST provide a way to
         determine its Network Layer protocol.

      Length of Next Hop Network Address:

         A 1-octet field whose value expresses the length of the
         "Network Address of Next Hop" field, measured in octets.

      Network Address of Next Hop:

         A variable-length field that contains the Network Address of
         the next router on the path to the destination system.  The
         Network Layer protocol associated with the Network Address of
         the Next Hop is identified by a combination of <AFI, SAFI>
         carried in the attribute.

      Reserved:

         A 1 octet field that MUST be set to 0, and SHOULD be ignored
         upon receipt.

      Network Layer Reachability Information (NLRI):

         A variable length field that lists NLRI for the feasible routes
         that are being advertised in this attribute.  The semantics of
         NLRI is identified by a combination of <AFI, SAFI> carried in
         the attribute.

         When the Subsequent Address Family Identifier field is set to
         one of the values defined in this document, each NLRI is
         encoded as specified in the "NLRI encoding" section of this
         document.

   The next hop information carried in the MP_REACH_NLRI path attribute
   defines the Network Layer address of the router that SHOULD be used
   as the next hop to the destinations listed in the MP_NLRI attribute
   in the UPDATE message.

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   The rules for the next hop information are the same as the rules for
   the information carried in the NEXT_HOP BGP attribute (see Section
   5.1.3 of [BGP-4]).

   An UPDATE message that carries the MP_REACH_NLRI MUST also carry the
   ORIGIN and the AS_PATH attributes (both in EBGP and in IBGP
   exchanges).  Moreover, in IBGP exchanges such a message MUST also
   carry the LOCAL_PREF attribute.

   An UPDATE message that carries no NLRI, other than the one encoded in
   the MP_REACH_NLRI attribute, SHOULD NOT carry the NEXT_HOP attribute.
   If such a message contains the NEXT_HOP attribute, the BGP speaker
   that receives the message SHOULD ignore this attribute.

   An UPDATE message SHOULD NOT include the same address prefix (of the
   same <AFI, SAFI>) in more than one of the following fields: WITHDRAWN
   ROUTES field, Network Reachability Information fields, MP_REACH_NLRI
   field, and MP_UNREACH_NLRI field.  The processing of an UPDATE
   message in this form is undefined.

4.  Multiprotocol Unreachable NLRI - MP_UNREACH_NLRI (Type Code 15):

   This is an optional non-transitive attribute that can be used for the
   purpose of withdrawing multiple unfeasible routes from service.

   The attribute is encoded as shown below:

        +---------------------------------------------------------+
        | Address Family Identifier (2 octets)                    |
        +---------------------------------------------------------+
        | Subsequent Address Family Identifier (1 octet)          |
        +---------------------------------------------------------+
        | Withdrawn Routes (variable)                             |
        +---------------------------------------------------------+

   The use and the meaning of these fields are as follows:

      Address Family Identifier (AFI):

         This field in combination with the Subsequent Address Family
         Identifier field identifies the set of Network Layer protocols
         to which the address carried in the Next Hop field must belong,
         the way in which the address of the next hop is encoded, and
         the semantics of the Network Layer Reachability Information
         that follows.  If the Next Hop is allowed to be from more than
         one Network Layer protocol, the encoding of the Next Hop MUST
         provide a way to determine its Network Layer protocol.

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         Presently defined values for the Address Family Identifier
         field are specified in the IANA's Address Family Numbers
         registry [IANA-AF].

      Subsequent Address Family Identifier (SAFI):

         This field in combination with the Address Family Identifier
         field identifies the set of Network Layer protocols to which
         the address carried in the Next Hop must belong, the way in
         which the address of the next hop is encoded, and the semantics
         of the Network Layer Reachability Information that follows.  If
         the Next Hop is allowed to be from more than one Network Layer
         protocol, the encoding of the Next Hop MUST provide a way to
         determine its Network Layer protocol.

      Withdrawn Routes Network Layer Reachability Information:

         A variable-length field that lists NLRI for the routes that are
         being withdrawn from service.  The semantics of NLRI is
         identified by a combination of <AFI, SAFI> carried in the
         attribute.

         When the Subsequent Address Family Identifier field is set to
         one of the values defined in this document, each NLRI is
         encoded as specified in the "NLRI encoding" section of this
         document.

   An UPDATE message that contains the MP_UNREACH_NLRI is not required
   to carry any other path attributes.

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5.  NLRI Encoding

   The Network Layer Reachability information is encoded as one or more
   2-tuples of the form <length, prefix>, whose fields are described
   below:

                       +---------------------------+
                       |   Length (1 octet)        |
                       +---------------------------+
                       |   Prefix (variable)       |
                       +---------------------------+

   The use and the meaning of these fields are as follows:

   a) Length:

      The Length field indicates the length, in bits, of the address
      prefix.  A length of zero indicates a prefix that matches all (as
      specified by the address family) addresses (with prefix, itself,
      of zero octets).

   b) Prefix:

      The Prefix field contains an address prefix followed by enough
      trailing bits to make the end of the field fall on an octet
      boundary.  Note that the value of trailing bits is irrelevant.

6.  Subsequent Address Family Identifier

   This document defines the following values for the Subsequent Address
   Family Identifier field carried in the MP_REACH_NLRI and
   MP_UNREACH_NLRI attributes:

      1 - Network Layer Reachability Information used for unicast
          forwarding

      2 - Network Layer Reachability Information used for multicast
          forwarding

   An implementation MAY support all, some, or none of the Subsequent
   Address Family Identifier values defined in this document.

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7.  Error Handling

   If a BGP speaker receives from a neighbor an UPDATE message that
   contains the MP_REACH_NLRI or MP_UNREACH_NLRI attribute, and if the
   speaker determines that the attribute is incorrect, the speaker MUST
   delete all the BGP routes received from that neighbor whose AFI/SAFI
   is the same as the one carried in the incorrect MP_REACH_NLRI or
   MP_UNREACH_NLRI attribute.  For the duration of the BGP session over
   which the UPDATE message was received, the speaker then SHOULD ignore
   all the subsequent routes with that AFI/SAFI received over that
   session.

   In addition, the speaker MAY terminate the BGP session over which the
   UPDATE message was received.  The session SHOULD be terminated with
   the Notification message code/subcode indicating "UPDATE Message
   Error"/"Optional Attribute Error".

8.  Use of BGP Capability Advertisement

   A BGP speaker that uses Multiprotocol Extensions SHOULD use the
   Capability Advertisement procedures [BGP-CAP] to determine whether
   the speaker could use Multiprotocol Extensions with a particular
   peer.

   The fields in the Capabilities Optional Parameter are set as follows.
   The Capability Code field is set to 1 (which indicates Multiprotocol
   Extensions capabilities).  The Capability Length field is set to 4.
   The Capability Value field is defined as:

                     0       7      15      23      31
                     +-------+-------+-------+-------+
                     |      AFI      | Res.  | SAFI  |
                     +-------+-------+-------+-------+

   The use and meaning of this field is as follow:

      AFI  - Address Family Identifier (16 bit), encoded the same way as
          in the Multiprotocol Extensions

      Res. - Reserved (8 bit) field.  SHOULD be set to 0 by the sender
          and ignored by the receiver.

          Note that not setting the field value to 0 may create issues
          for a receiver not ignoring the field.  In addition, this
          definition is problematic if it is ever attempted to redefine
          the field.

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      SAFI - Subsequent Address Family Identifier (8 bit), encoded the
          same way as in the Multiprotocol Extensions.

   A speaker that supports multiple <AFI, SAFI> tuples includes them as
   multiple Capabilities in the Capabilities Optional Parameter.

   To have a bi-directional exchange of routing information for a
   particular <AFI, SAFI> between a pair of BGP speakers, each such
   speaker MUST advertise to the other (via the Capability Advertisement
   mechanism) the capability to support that particular <AFI, SAFI>
   route.

9.  IANA Considerations

   As specified in this document, the MP_REACH_NLRI and MP_UNREACH_NLRI
   attributes contain the Subsequence Address Family Identifier (SAFI)
   field.  The SAFI name space is defined in this document.  The IANA
   registered and maintains values for the SAFI namespace as follows:

      - SAFI values 1 and 2 are assigned in this document.

      - SAFI value 3 is reserved.  It was assigned by RFC 2858 for a use
        that was never fully implemented, so it is deprecated by this
        document.

      - SAFI values 5 through 63 are to be assigned by IANA using either
        the Standards Action process, defined in [RFC2434], or the Early
        IANA Allocation process, defined in [RFC4020].

      - SAFI values 67 through 127 are to be assigned by IANA, using the
        "First Come First Served" policy, defined in RFC 2434.

      - SAFI values 0 and 255 are reserved.

      - SAFI values 128 through 240 are part of the previous "private
        use" range.  At the time of approval of this document, the
        unused values were provided to IANA by the Routing Area
        Director.  These unused values, namely, 130, 131, 135 through
        139, and 141 through 240, are considered reserved in order to
        avoid conflicts.

      - SAFI values 241 through 254 are for "private use", and values in
        this range are not to be assigned by IANA.

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10.  Comparison with RFC 2858

   This document makes the use of the next hop information consistent
   with the information carried in the NEXT_HOP BGP path attribute.

   This document removes the definition of SAFI 3 and deprecates SAFI 3.

   This document changes partitioning of the SAFI space.  Specifically,
   in RFC 2858 SAFI values 128 through 240 were part of the "private
   use" range.  This document specifies that of this range, allocations
   that are currently in use are to be recognized by IANA, and that
   unused values, namely 130, 131, 135 through 139, and 141 through 240,
   should be considered reserved.

   This document renames the Number of SNPAs field to Reserved and
   removes the rest of the SNPA-related information from the
   MP_REACH_NLRI attribute.

11.  Comparison with RFC 2283

   This document restricts the MP_REACH_NLRI attribute to carry only a
   single instance of <AFI, SAFI, Next Hop Information, ...>.

   This document restricts the MP_UNREACH_NLRI attribute to carry only a
   single instance of <AFI, SAFI, ...>.

   This document clarifies handling of an UPDATE message that carries no
   NLRI, other than the one encoded in the MP_REACH_NLRI attribute.

   This document clarifies error handling in the presence of
   MP_REACH_NLRI or MP_UNREACH_NLRI attributes.

   This document specifies the use of BGP Capabilities Advertisements in
   conjunction with multi-protocol extensions.

   Finally, this document includes the "IANA Consideration" section.

12.  Security Considerations

   This extension to BGP does not change the underlying security issues
   inherent in the existing BGP.

13.  Acknowledgements

   The authors would like to thank members of the IDR Working Group for
   their review and comments.

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14.  Normative References

   [BGP-CAP]  Chandra, R. and J. Scudder, "Capabilities Advertisement
              with BGP-4", RFC 3392, November 2002.

   [BGP-4]    Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [IANA-AF]  "Address Family Numbers", Reachable from
              http://www.iana.org/numbers.html

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC4020]  Kompella, K. and A. Zinin, "Early IANA Allocation of
              Standards Track Code Points", BCP 100, RFC 4020, February
              2005.

Authors' Addresses

   Tony Bates
   Cisco Systems, Inc.
   EMail: tbates@cisco.com

   Ravi Chandra
   Sonoa Systems
   EMail: rchandra@sonoasystems.com

   Dave Katz
   Juniper Networks, Inc.
   EMail: dkatz@juniper.net

   Yakov Rekhter
   Juniper Networks, Inc.
   EMail: yakov@juniper.net

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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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