Multiprotocol Extensions for BGP-4
draft-ietf-idr-bgp4-multiprotocol-01

Network Working Group Tony Bates
Internet Draft Cisco Systems
Expiration Date: March 1998 Ravi Chandra
 Cisco Systems
 Dave Katz
 Juniper Networks
 Yakov Rekhter
 Cisco Systems
 Multiprotocol Extensions for BGP-4
 draft-ietf-idr-bgp4-multiprotocol-01.txt
1. Status of this Memo
 This document is an Internet-Draft. Internet-Drafts are working
 documents of the Internet Engineering Task Force (IETF), its areas,
 and its working groups. Note that other groups may also distribute
 working documents as Internet-Drafts.
 Internet-Drafts are draft documents valid for a maximum of six months
 and may be updated, replaced, or obsoleted by other documents at any
 time. It is inappropriate to use Internet-Drafts as reference
 material or to cite them other than as ``work in progress.''
 To learn the current status of any Internet-Draft, please check the
 ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
 Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
 munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
 ftp.isi.edu (US West Coast).
2. Abstract
 Currently BGP-4 [BGP-4] is capable of carrying routing information
 only for IPv4 [IPv4]. This document defines extensions to BGP-4 to
 enable it to carry routing information for multiple Network Layer
 protocols (e.g., IPv6, IPX, etc...). The extensions are backward
 compatible - a router that supports the extensions can interoperate
 with a router that doesn't support the extensions.
Bates, Chandra, Katz, Rekhter 
[Page 1]
Internet Draft draft-ietf-idr-bgp4-multiprotocol-01.txt September 1997
3. Overview
 The only three pieces of information carried by 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
 associated a particular Network Layer protocol with NLRI. To identify
 individual Network Layer protocols this document uses Address Family,
 as defined in [RFC1700].
 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.
Bates, Chandra, Katz, Rekhter 
[Page 2]
Internet Draft draft-ietf-idr-bgp4-multiprotocol-01.txt September 1997
4. 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.
 (c) to allow a given router to report some or all of the
 Subnetwork Points of Attachment (SNPAs) that exist within the
 local system
 The attribute contains one or more triples <Address Family
 Information, Next Hop Information, Network Layer Reachability
 Information>, where each triple 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) |
 +---------------------------------------------------------+
 | Number of SNPAs (1 octet) |
 +---------------------------------------------------------+
 | Length of first SNPA(1 octet) |
 +---------------------------------------------------------+
 | First SNPA (variable) |
 +---------------------------------------------------------+
 | Length of second SNPA (1 octet) |
 +---------------------------------------------------------+
 | Second SNPA (variable) |
 +---------------------------------------------------------+
 | ... |
 +---------------------------------------------------------+
 | Length of Last SNPA (1 octet) |
 +---------------------------------------------------------+
 | Last SNPA (variable) |
 +---------------------------------------------------------+
 | Network Layer Reachability Information (variable) |
 +---------------------------------------------------------+
Bates, Chandra, Katz, Rekhter 
[Page 3]
Internet Draft draft-ietf-idr-bgp4-multiprotocol-01.txt September 1997
 The use and meaning of these fields are as follows:
 Address Family Identifier:
 This field carries the identity of the Network Layer protocol
 associated with the Network Address that follows. Presently
 defined values for this field are specified in RFC1700 (see the
 Address Family Numbers section).
 Subsequent Address Family Identifier:
 This field provides additional information about the type of
 the Network Layer Reachability Information carried in the
 attribute.
 Length of Next Hop Network Address:
 A 1 octet field whose value expresses the length of the
 "Network Address of Next Hop" field as 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
 Number of SNPAs:
 A 1 octet field which contains the number of distinct SNPAs to
 be listed in the following fields. The value 0 may be used to
 indicate that no SNPAs are listed in this attribute.
 Length of Nth SNPA:
 A 1 octet field whose value expresses the length of the "Nth
 SNPA of Next Hop" field as measured in semi-octets
 Nth SNPA of Next Hop:
 A variable length field that contains an SNPA of the router
 whose Network Address is contained in the "Network Address of
 Next Hop" field. The field length is an integral number of
 octets in length, namely the rounded-up integer value of one
 half the SNPA length expressed in semi-octets; if the SNPA
 contains an odd number of semi-octets, a value in this field
 will be padded with a trailing all-zero semi-octet.
 Network Layer Reachability Information:
Bates, Chandra, Katz, Rekhter 
[Page 4]
Internet Draft draft-ietf-idr-bgp4-multiprotocol-01.txt September 1997
 A variable length field that lists NLRI for the feasible routes
 that are being advertised in this 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 border router that should be
 used as the next hop to the destinations listed in the MP_NLRI
 attribute in the UPDATE message. When advertising a MP_REACH_NLRI
 attribute to an external peer, a router may use one of its own
 interface addresses in the next hop component of the attribute,
 provided the external peer to which the route is being advertised
 shares a common subnet with the next hop address. This is known as a
 "first party" next hop. A BGP speaker can advertise to an external
 peer an interface of any internal peer router in the next hop
 component, provided the external peer to which the route is being
 advertised shares a common subnet with the next hop address. This is
 known as a "third party" next hop information. A BGP speaker can
 advertise any external peer router in the next hop component,
 provided that the Network Layer address of this border router was
 learned from an external peer, and the external peer to which the
 route is being advertised shares a common subnet with the next hop
 address. This is a second form of "third party" next hop
 information.
 Normally the next hop information is chosen such that the shortest
 available path will be taken. A BGP speaker must be able to support
 disabling advertisement of third party next hop information to handle
 imperfectly bridged media or for reasons of policy.
 A BGP speaker must never advertise an address of a peer to that peer
 as a next hop, for a route that the speaker is originating. A BGP
 speaker must never install a route with itself as the next hop.
 When a BGP speaker advertises the route to an internal peer, the
 advertising speaker should not modify the next hop information
 associated with the route. When a BGP speaker receives the route via
 an internal link, it may forward packets to the next hop address if
 the address contained in the attribute is on a common subnet with the
 local and remote BGP speakers.
 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. If such a message is received from an
 external peer, the local system shall check whether the leftmost AS
 in the AS_PATH attribute is equal to the autonomous system number of
Bates, Chandra, Katz, Rekhter 
[Page 5]
Internet Draft draft-ietf-idr-bgp4-multiprotocol-01.txt September 1997
 the peer than sent the message. If that is not the case, the local
 system shall send the NOTIFICATION message with Error Code UPDATE
 Message Error, and the Error Subcode set to Malformed AS_PATH.
5. 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 contains one or more triples <Address Family
 Information, Unfeasible Routes Length, Withdrawn Routes>, where each
 triple 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:
 This field carries the identity of the Network Layer protocol
 associated with the NLRI that follows. Presently defined values
 for this field are specified in RFC1700 (see the Address Family
 Numbers section).
 Subsequent Address Family Identifier:
 This field provides additional information about the type of
 the Network Layer Reachability Information carried in the
 attribute.
 Withdrawn Routes:
 A variable length field that lists NLRI for the routes that are
 being withdrawn from service. 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.
Bates, Chandra, Katz, Rekhter 
[Page 6]
Internet Draft draft-ietf-idr-bgp4-multiprotocol-01.txt September 1997
 An UPDATE message that contains the MP_UNREACH_NLRI is not required
 to carry any other path attributes.
6. 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 address prefixes 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.
7. 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
Bates, Chandra, Katz, Rekhter 
[Page 7]
Internet Draft draft-ietf-idr-bgp4-multiprotocol-01.txt September 1997
 3 - Network Layer Reachability Information used for both unicast
 and multicast forwarding
 This document reserves values 128-255 for vendor-specific
 applications.
 This document reserves value 0.
8. Security Considerations
 Security issues are not discussed in this document.
9. Acknowledgements
 To be supplied.
10. References
 [BGP-4]
 [IPv4]
 [IPv6]
 [RFC1700]
11. Author Information
 Tony Bates
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134
 email: tbates@cisco.com
 Ravi Chandra
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134
 email: rchandra@cisco.com
 Dave Katz
 Juniper Networks, Inc.
Bates, Chandra, Katz, Rekhter 
[Page 8]
Internet Draft draft-ietf-idr-bgp4-multiprotocol-01.txt September 1997
 3260 Jay St.
 Santa Clara, CA 95054
 email: dkatz@jnx.com
 Yakov Rekhter
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134
 email: yakov@cisco.com
Bates, Chandra, Katz, Rekhter 
[Page 9]