Network Working Group Tony Bates (Cisco Systems)
Internet Draft Ravi Chandra (Redback Networks)
Expiration Date: November 2004 Dave Katz (Juniper Networks)
Obsoles RFC2858 Yakov Rekhter (Juniper Networks)
Multiprotocol Extensions for BGP-4
draft-ietf-idr-rfc2858bis-06.txt
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
2. Abstract
Currently BGP-4 is capable of carrying routing information only for
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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3. 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].
4. Overview
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
associated 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 [RFC1700], 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
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in these attributes, and will not pass it to other BGP speakers.
5. 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 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) |
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+---------------------------------------------------------+
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 Network Layer protocol
associated with the Network Address of Next Hop and the
semantics of the Network Layer Reachability Information that
follows.
Presently defined values for the Address Family Identifier
field are specified in RFC1700 (see the Address Family Numbers
section).
Subsequent Address Family Identifier (SAFI):
This field in combination with the Address Family Identifier
field identifies the Network Layer protocol associated with the
Network Address of the Next Hop and the semantics of the
Network Layer Reachability Information that follows.
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. 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.
Number of SNPAs:
A 1 octet field which contains the number of distinct SNPAs to
be listed in the following fields. The value 0 SHALL 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
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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
MUST be padded with a trailing all-zero semi-octet.
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.
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 un-defined.
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6. 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 semantics associated with the
Network Layer Reachability Information (NLRI) that follows.
Presently defined values for the Address Family Identifier
field are specified in RFC1700 (see the Address Family Numbers
section).
Subsequent Address Family Identifier (SAFI):
This field in combination with the Address Family Identifier
field identifies the semantics associated with the Network
Layer Reachability Information (NLRI) that follows.
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
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to carry any other path attributes.
7. 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.
8. 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
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Address Family Identifier values defined in this document.
9. 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 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".
10. Use of BGP Capability Advertisement
A BGP speaker that uses Multiprotocol Extensions SHOULD use the
Capability Advertisment 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.
SAFI - Subsequent Address Family Identifier (8 bit), encoded
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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>
routes.
11. 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 will
maintain and register values for the SAFI namespace as follows. SAFI
value 0 is reserved. SAFI values 1 and 2 are assigned in this
document. SAFI values 4 through 63 are to be assigned by IANA using
the "IETF Consensus" policy defined in RFC2434. SAFI values 64
through 127 are to be assigned by IANA, using the "First Come First
Served" policy defined in RFC2434. SAFI values 128 through 255 are
for "private use", and values in this range are not to be assigned by
IANA.
12. Comparison with RFC2858
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.
13. Comparison with RFC2283
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.
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This document specifies the use of BGP Capabilities Advertisements in
conjunction with Multi-protocol extensions.
Finally, this document includes the "IANA Consideration" Section.
14. Security Considerations
This extension to BGP does not change the underlying security issues
inherent in the existing BGP.
15. Acknowledgements
The authors would like to thank members of the IDR Working Group for
their review and comments.
16. Normative References
[BGP-CAP] "Capabilities Advertisement with BGP-4", R. Chandra, J.
Scudder, RFC2842, May 2000
[BGP-4] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC1700] "Assigned Numbers", J. Reynolds, J. Postel, RFC1700,
October 1994 (see also http://www.iana.org/iana/assignments.html)
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
17. Author Information
Tony Bates
Cisco Systems, Inc.
email: tbates@cisco.com
Ravi Chandra
Redback Networks
e-mail: rchandra@redback.com
Dave Katz
Juniper Networks, Inc.
email: dkatz@juniper.com
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Yakov Rekhter
Juniper Networks, Inc.
email: yakov@juniper.com
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