Network Working Group E. Chen
Internet Draft S. Sangli
Expiration Date: June 6, 2012 Cisco Systems
December 5, 2011
Dynamic Capability for BGP-4
draft-ietf-idr-dynamic-cap-14.txt
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Internet Draft draft-ietf-idr-dynamic-cap-14.txt Dec 2011
Abstract
This document defines a new BGP capability termed "Dynamic
Capability", which would allow the dynamic update of capabilities
over an established BGP session. This capability would facilitate
non-disruptive capability changes by BGP speakers.
1. Introduction
Currently BGP capabilities [RFC5492] are only advertised in the OPEN
message during the session initialization. In order to enable a new
capability or remove an existing capability (such as an Address
Family support [RFC4760]), an established session needs to be reset,
which may disrupt other services running over the session.
This document defines a new BGP capability termed "Dynamic
Capability", which would allow the dynamic update of capabilities
over an established BGP session. This capability would facilitate
non-disruptive capability changes by BGP speakers.
1.1. 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 [RFC2119].
2. Dynamic Capability
The Dynamic Capability is a new BGP capability [RFC5492]. The
Capability Code for this capability is specified in the "IANA
Considerations" section of this document. The Capability Value field
consists of a list of capability codes (one-octet for each) that
specify the capabilities that MAY be revised dynamically by the
remote speaker.
By advertising the Dynamic Capability to a peer in the OPEN, a BGP
speaker conveys to the peer that the speaker is capable of receiving
and properly handling the CAPABILITY message (as defined in the next
Section) from the peer after the BGP session has been established.
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3. Capability Message
The CAPABILITY Message is a new BGP message type with type code 6.
In addition to the fixed-size BGP header [RFC4271], the CAPABILITY
message contains one or more of the following tuples of capability
revisions:
+------------------------------+
| Init/Ack (1 bit) |
+------------------------------+
| Ack Request (1 bit) |
+------------------------------+
| Reserved (5 bits) |
+------------------------------+
| Action (1 bit) |
+------------------------------+
| Sequence Number (4 octets) |
+------------------------------+
| Capability Code (1 octet) |
+------------------------------+
| Capability Length (2 octets) |
+------------------------------+
| Capability Value (variable) |
+------------------------------+
The Init/Ack bit indicates whether a capability revision is being
initiated (when set to 0), or being acknowledged (when set to 1).
The Ack Request bit indicates whether an acknowledgment is requested
(when set to 1), or not (when set to 0) for a capability revision
being initiated.
The Reserved bits should be set to zero by the sender and ignored by
the receiver.
The Action bit is 0 for advertising a capability, and 1 for removing
a capability.
The Sequence Number field can be used by a BGP speaker to match an
acknowledgment with a capability revision that the speaker initiated
previously.
Conceptually the triple <Capability Code, Capability Length,
Capability Value> is the same as the one defined in [RFC5492], and it
specifies a capability for which the "Action" shall be applied.
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Internet Draft draft-ietf-idr-dynamic-cap-14.txt Dec 2011
4. Operation
A BGP speaker that is willing to receive the CAPABILITY message (for
one or more capability codes) from its peer SHOULD use the BGP
Capabilities Advertisement [RFC5492] to advertise the Dynamic
Capability for these capability codes.
A BGP speaker MAY send to its peer a CAPABILITY message to initiate
revisions for one or more capability codes only if these capability
codes are listed in the Dynamic Capability of the OPEN message
received from its peer.
A CAPABILITY message MAY be received only in the Established state.
Receiving a CAPABILITY message in any other state is a Finite State
Machine Error as defined in [RFC4271]. A BGP speaker SHOULD reset the
HoldTimer upon receiving a CAPABILITY message from its peer.
When a BGP speaker sends a CAPABILITY message to its peer to initiate
a capability revision, the Init/Ack bit for the capability revision
in the message MUST be set to 0. The setting of the Ack Request bit
is capability specific. The assignment of the Sequence Number is a
local matter, but MUST allow the BGP speaker to unambiguously
identify a capability revision it initiated previously based on the
Sequence Number carried in the acknowledgment from the peer.
If the Init/Ack bit is set to 1 for a capability revision in a
CAPABILITY message received by a BGP speaker, then the BGP speaker
SHALL treat the capability revision as an acknowledgment of the
receipt of a capability revision initiated by the BGP speaker. The
BGP speaker MUST ignore the Ack Request bit, and SHALL use the
Sequence Number carried in the capability revision to match with the
capability revision previously initiated. The BGP speaker SHALL
ignore an acknowledgment for a capability revision in which an
acknowledgment was not requested by the BGP speaker. If the Sequence
Number carried in the capability revision does not match any of the
the Sequence Numbers used in the capability revisions initiated by
the BGP speaker, then the BGP speaker SHOULD send a NOTIFICATION
message as specified in the Error Handling section.
If the Init/Ack bit is set to 0 for a capability revision in a
CAPABILITY message received by a BGP speaker, then the BGP speaker
SHOULD first validate the capability code in the message. If the
capability code is not listed in the Dynamic Capability advertised by
the speaker to the peer, the BGP speaker SHOULD send a NOTIFICATION
message as specified in the Error Handling section. For a valid
capability code, if the Ack Request bit is set to 1, the BGP speaker
MUST first send a CAPABILITY message to acknowledge the receipt of
the capability revision. The Init/Ack bit in the acknowledgment MUST
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Internet Draft draft-ietf-idr-dynamic-cap-14.txt Dec 2011
be set to 1, and all the other fields in the capability revision MUST
be kept unchanged.
After receiving a capability revision initiated by a peer, the BGP
speaker SHALL update the capability previously received from that
peer based on the Action bit in the message, and then function in
accordance with the revised capability for the peer. The BGP speaker
SHALL ignore such a capbility revision that either results in no
change to an existing capability, or removes a capability that was
not advertised previously. The procedures specified in the "Error
Handling" section SHOULD be followed when an error is detected in
processing the CAPABILITY message.
In order to avoid ambiguities in sending and processing UPDATE
messages, certain capability revisions may require close coordination
between the BGP speaker (the Initiator) that initiates the capability
revisions and another BGP speaker (the Receiver) that receives the
capability revisions. The mechanism of acknowledgment defined in
this document SHALL be used for the revision of such a capability.
For the Initiator, the capability revision SHALL take effect (for the
purpose of sending updates) immediately after the capability revision
is sent, and the capability revision SHALL take effect (for the
purpose of receiving updates) immediately after an acknowledgment is
received from the Receiver. For the Receiver, the capability
revision SHALL take effect (for the purpose of receiving updates)
immediately after the capability revision is received from the
Initiator, and the capability revision SHALL take effect (for the
purpose of sending updates) immediately after an acknowledgment is
sent.
5. Error Handling
This document defines a new NOTIFICATION error code:
Error Code Symbolic Name
7 CAPABILITY Message Error
The following error subcodes are defined as well:
Subcode Symbolic Name
1 Unknown Sequence Number
2 Invalid Capability Length
3 Malformed Capability Value
4 Unsupported Capability Code
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Internet Draft draft-ietf-idr-dynamic-cap-14.txt Dec 2011
If a BGP speaker detects an error while processing a CAPABILITY
message, it MUST send a NOTIFICATION message with Error Code
CAPABILITY Message Error. If any of the defined error subcode is
applicable, the Data field of the NOTIFICATION message MUST contain
the tuple for the capability revision that causes the speaker to send
the message.
If the Sequence Number carried in a capability revision marked as
acknowledgment does not match any of the the Sequence Numbers used in
the capability revisions initiated by the BGP speaker, then the error
subcode is set to Unknown Sequence Number.
If the Capability Length field in the CAPABILITY message is incorrect
for a Capability Code, then the error subcode is set to Invalid
Capability Length.
If the Capability Value field in the CAPABILITY message is malformed
(the definition of "malformed" depends on the Capability Code), then
the error subcode is set to Malformed Capability Value.
If the Capability Code in the CAPABILITY message is not any of the
capability codes advertised in the Dynamic Capability by the speaker,
then the error subcode is set to Unsupported Capability Code.
6. IANA Considerations
This document defines the CAPABILITY message type for BGP with type
code 6, and a NOTIFICATION error code and subcodes for the errors in
a CAPABILITY message.
This document uses a BGP capability code to indicate that a BGP
speaker supports the Dynamic Capability. The capability code 67 has
been assigned by IANA.
7. Security Considerations
This extension to BGP does not change the underlying security issues.
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8. Acknowledgments
The authors would like to thank Yakov Rekhter, Ravi Chandra, Dino
Farinacci, Pedro Marques, Chandrashekhar Appanna, Derek Yeung, Bruno
Rijsman, John Scudder and Jeffrey Haas for their review and comments.
9. Normative References
[RFC4271] Rekhter, Y., T. Li, and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)," RFC 4271, January 2006.
[RFC4760] Bates, T., Chandra, R., Rekhter, Y., and D. Katz,
"Multiprotocol Extensions for BGP-4", RFC 4760, January 2007.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, February 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10. Authors' Addresses
Enke Chen
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
Email: enkechen@cisco.com
Srihari R. Sangli
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
Email: rsrihari@cisco.com
Chen & Sangli [Page 7]
