BGP-4+
John W. Stewart III
jstewart@metro.isi.edu
Wed, 18 Dec 1996 17:21:15 EST
yakov,
under what circumstances does a router send SNPAs? and
what does a router do on receipt of SNPAs?
also, from reading the draft, i'm assuming that you plan
to support only ASs and not RDs? did you consider typing
routing domains in addition to network addresses?
thanks,
/jws
> Sue,
>
> >
> > Hi all:
> >
> >
> > FYI - Neither Dimitry or Dave Katz have forwarded a specification
> > for the BGP-4+. Please encourage them to forward
> > a description of the protocol to the idr working group.
> > An open discussion of changes to bgp-4 is always welcome.
> > Yakov and I both expressed interest in getting a specification
> > before the working group. We did not discuss it at the
> > December meeting because there was not a specification.
> >
> > [This ends my speaking as the co-chair of IDR working group.]
>
> Attached is the proposal from Tony Bates, Ravi Chandra, Dave Katz
> and myself for backward compatible multiprotocol extensions to
> BGP-4.
>
> Yakov.
>
> P.S. I added bgp@ans.net to the cc: list.
> --------------------------------------------------------------------
>
>
>
>
>
> Network Working Group Tony Bates
> Internet Draft Cisco Systems
> Expiration Date: June 1997 Ravi Chandra
> Cisco Systems
> Dave Katz
> Cisco Systems
> Yakov Rekhter
> Cisco Systems
>
>
> Multiprotocol Extensions for BGP-4
>
> draft-bates-bgp4-multiprotocol-00.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.
>
>
>
>
>
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>
> Bates, Chandra, Katz, Rekhter [Page 1]
>
>
>
>
>
> Internet Draft draft-bates-bgp4-multiprotocol-00.txt December 1996
>
>
> 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) will have to support IPv4 and
> have an IPv4 address. 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 would just ignore the information carried
> in these attributes, and wouldn't pass it to other BGP speakers.
>
>
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>
> Bates, Chandra, Katz, Rekhter [Page 2]
>
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>
> Internet Draft draft-bates-bgp4-multiprotocol-00.txt December 1996
>
>
> 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 consists of two components, the next hop information
> and the list of reachable destinations.
>
> The attribute is encoded as shown below:
>
>
> +---------------------------------------------------+
> | Address Family (2 octets) |
> +---------------------------------------------------+
> | Length of 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-bates-bgp4-multiprotocol-00.txt December 1996
>
>
> The use and meaning of these fields are as follows:
>
> Address Family
>
> 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.
>
> Length of 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:
>
> A variable length field that lists the destinations for the
> feasible routes that are being advertised in this attribute.
> 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
>
>
>
> Bates, Chandra, Katz, Rekhter [Page 4]
>
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>
>
> Internet Draft draft-bates-bgp4-multiprotocol-00.txt December 1996
>
>
> 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.
>
> 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.
>
>
> 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 is encoded as shown below:
>
> +-----------------------------+
> | Address Family (2 octets) |
> +-----------------------------+
> | Withdrawn Routes (variable) |
> +-----------------------------+
>
>
>
>
> Bates, Chandra, Katz, Rekhter [Page 5]
>
>
>
>
>
> Internet Draft draft-bates-bgp4-multiprotocol-00.txt December 1996
>
>
> The use and the meaning of these fields are as follows:
>
> Address Family
>
> 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.
>
> Withdrawn Routes:
>
> This is a variable length field that contains a list of NLRIs
> for the routes that are being withdrawn from service. Each NLRI
> is encoded as specified in the "NLRI encoding" section of this
> document.
>
>
>
> 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.
>
>
>
>
>
> Bates, Chandra, Katz, Rekhter [Page 6]
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>
> Internet Draft draft-bates-bgp4-multiprotocol-00.txt December 1996
>
>
> 7. Security Considerations
>
> Security issues are not discussed in this document.
>
>
> 8. Acknowledgements
>
> To be supplied.
>
>
> 9. References
>
>
> [BGP-4]
>
> [IPv4]
>
> [IPv6]
>
> [RFC1700]
>
>
> 10. Author Information
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> Bates, Chandra, Katz, Rekhter [Page 7]
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> Internet Draft draft-bates-bgp4-multiprotocol-00.txt December 1996
>
>
>
> 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
> Cisco Systems, Inc.
> 170 West Tasman Drive
> San Jose, CA 95134
> email: dkatz@cisco.com
>
> Yakov Rekhter
> Cisco Systems, Inc.
> 170 West Tasman Drive
> San Jose, CA 95134
> email: yakov@cisco.com
>
> Internet Draft draft-bates-bgp4-multiprotocol-00.txt December 1996
>
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> Bates, Chandra, Katz, Rekhter [Page 8]
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>