Network Working Group Ravi Chandra
Internet Draft Cisco Systems
Expiration Date: February 1998 John G. Scudder
Internet Engineering Group, LLC
Capabilities Negotiation with BGP-4
draft-ietf-idr-bgp4-cap-neg-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,
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ftp.isi.edu (US West Coast).
2. Abstract
Currently BGP-4 [BGP-4] requires that when a BGP speaker receives an
OPEN message with one or more unrecognized Optional Parameters, the
speaker must terminate BGP peering. This complicates introduction of
new capabilities in BGP.
This document defines new Optional Parameter, called Capabilities,
that is expected to facilitate introduction of new capabilities in
BGP by providing graceful capability negotiation.
The proposed parameter is backward compatible - a router that
supports the parameter can maintain BGP peering with a router that
doesn't support the parameter.
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3. Overview of Operations
When a BGP speaker that supports capabilities negotiation sends an
OPEN message to its BGP peer, the message includes an Optional
Parameter, called Capabilities. The parameter lists the capabilities
supported by the speaker. The speaker can mark a listed capability as
"Required", which means that if the peer doesn't recognize/support
the capability, the BGP peering shall be terminated.
When the peer receives the OPEN message, if the message contains the
Capabilities Optional Parameter, the peer checks whether it supports
all of the listed capabilities marked as R, and if not, sends a
NOTIFICATION message, and terminates peering. The Error Subcode in
the message is set to Unsupported Capability. The message should
contain all the capabilities marked as R that are not supported by
the peer. If the peer doesn't support a capability that is not
marked as R, the peer should not use this as a reason to terminate
peering.
A BGP speaker may use a particular capability when peering with
another speaker if both speakers support that capability. A BGP
speaker determines the capabilities supported by its peer by
examining the list of capabilities present in the Capabilities
Optional Parameter carried by the OPEN message that the peer sends to
the speaker.
A BGP speaker determines that its peer doesn't support capabilities
negotiation, if in response to an OPEN message that carries the
Capabilities Optional Parameter, the speaker receives a NOTIFICATION
message with the Error Subcode set to Unsupported Optional Parameter.
4. Capabilities Optional Parameter (Parameter Type 2):
This is an Optional Parameter that is used by a BGP speaker to convey
to its BGP peer the list of capabilities supported by the speaker.
The parameter contains one or more triples <Capability Code,
Capability Length, Capability Value>, where each triple is encoded as
shown below:
+------------------------------+
| Capability Code (1 octet) |
+------------------------------+
| Capability Length (1 octet) |
+------------------------------+
| Capability Value (variable) |
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Internet Draft draft-ietf-idr-bgp4-cap-neg-00.txt August 1997
+------------------------------+
The use and meaning of these fields are as follows:
Capability Code:
Capability Code is a one octet field that unambiguously
identifies individual capabilities.
The high-order bit of this field is used to mark the capability
as "Required" (if the bit is set to 1).
Capability Length:
Capability Length is a one octet field that contains the length
of the Capability Value field in octets.
Capability Value:
Capability Value is a variable length field that is interpreted
according to the value of the Capability Code field.
5. Extensions to Error Handling
This document defines new Error Subcode - Unsupported Capability.
The value of this Subcode is 7. The Data field in the NOTIFICATION
message lists the set of capabilities that are marked as Required,
but are either unsupported or unrecognized by the BGP speaker that
sends the message. Each such capability is encoded the same way as it
was encoded in the received OPEN message.
6. Security Considerations
Security issues are not discussed in this document.
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7. Acknowledgements
To be supplied.
8. References
[BGP-4]
9. Author Information
Ravi Chandra
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
e-mail: rchandra@cisco.com
John G. Scudder
Internet Engineering Group, LLC
122 S. Main, Suite 280
Ann Arbor, MI 48104
e-mail: jgs@ieng.com
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