draft-ietf-idr-bgp-multisession-00.txt   draft-ietf-idr-bgp-multisession-01.txt 
Network Working Group John G. Scudder Network Working Group Chandra Appanna
Internet Draft Chandra Appanna Internet Draft John G. Scudder
Expiration Date: November 2004 Cisco Systems Expiration Date: March 2006 Cisco Systems
File name: draft-ietf-idr-bgp-multisession-00.txt May 2004 File name: draft-ietf-idr-bgp-multisession-01.txt October 2005
Multisession BGP Multisession BGP
draft-ietf-idr-bgp-multisession-00.txt draft-ietf-idr-bgp-multisession-01.txt
Status of this Memo Status of this Memo
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Abstract Abstract
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A common criticism of BGP is the fact that most malformed messages A common criticism of BGP is the fact that most malformed messages
cause the session to be terminated. While this behavior is necessary cause the session to be terminated. While this behavior is necessary
for protocol correctness, one may observe that the protocol machinery for protocol correctness, one may observe that the protocol machinery
of a given implementation may only be defective with respect to a of a given implementation may only be defective with respect to a
given AFI/SAFI. Thus, it would be desirable to allow the session given AFI/SAFI. Thus, it would be desirable to allow the session
related to that family to be terminated while leaving other AFI/SAFI related to that family to be terminated while leaving other AFI/SAFI
unaffected. As BGP is commonly deployed, this is not possible. unaffected. As BGP is commonly deployed, this is not possible.
In this specification, we propose a mechanism by which multiple In this specification, we propose a mechanism by which multiple
transport sessions may be established between a pair of peers. Each transport sessions may be established between a pair of peers.
transport session can be used for one or more AFI/SAFI. Each session
is distinct from a BGP protocol point of view; an error or other Each transport session can be used for one or more AFI/SAFI.
event on one session has no implications for any other session. All
protocol modifications proposed by this specification take place While AFI/SAFI based sessions is one way to group data being exchanged
between BGP peers it is also possible to concieve of sessions being
based on other characteristics of the BGP data. In this draft we also
propose a scheme for creating multiple BGP sessions between BGP peers
based on criteria other than AFI/SAFI. However, the definition of specific
criteria for grouping exchanged data into sessions is beyond the scope of
this proposal. The goal of this proposal is to provide a mechanism for
doing that if such criteria is available. For ease of understandability
and readability, AFI/SAFI will be used as an illustrative grouping
criteria through out the document.
Each session is distinct from a BGP protocol point of view; an error
or other event on one session has no implications for any other session.
All protocol modifications proposed by this specification take place
during the OPEN exchange phase of the session, there are no during the OPEN exchange phase of the session, there are no
modifications to the operation of the protocol once a session reaches modifications to the operation of the protocol once a session reaches
ESTABLISHED state. ESTABLISHED state.
Routers implementing this specification MUST also implement [MP-BGP]. Routers implementing this specification MUST also implement the base
criteria that is used to define sessions. For example if AFI/SAFI based
sessions are desired then peers implementing this specification MUST
also implement [MP-BGP].
2. Definitions 2. Definitions
"MP-BGP capability" refers to the capability [BGP-CAP] with code 1, "MP-BGP capability" refers to the capability [BGP-CAP] with code 1,
specified in [MP-BGP] section 10. specified in [MP-BGP] section 10.
A BGP speaker is said to "support" some feature or functionality (for A BGP speaker is said to "support" some feature or functionality (for
example, to support this specification, or to support a particular example, to support this specification, or to support a particular
AFI/SAFI) when the BGP implementation supports the feature AND the AFI/SAFI) when the BGP implementation supports the feature AND the
feature has not been disabled by configuration. feature has not been disabled by configuration.
A pair of AFI/SAFI groups is said to "conflict" when considering the The Session Identifier is a capability or group of capabilities that
two groups as two sets, there is an intersection between the groups will be used to differentiate individual BGP sessions between two IP
but neither group is a subset of the other. endpoints. When the AFI/SAFI is used to distinguish sessions, the MP-BGP
capability is the session identifier.
In this document the MP-BGP capability is used as a Session Identifier
for illustrative and explanatory purpose, i.e as the capability whose
values differentiates the multiple session between two IP endpoints.
MP-BGP and AFI/SAFI can be replaced by any other capability and the
values of that capability to setup multiple sessions, without any
modifications to the procedures described.
A pair of session indentifiers are said to conflict when considering the
two groups as two sets, there is an intersection between the groups either
in the capabilities or the values in the capabilities, but neither session
group is a subset of the other. For example, a pair of AFI/SAFI is said
to "conflict" when considering the two groups as two sets, there is an
intersection between the groups but neither group is a subset of the other.
3. Use of BGP Capability Advertisement 3. Use of BGP Capability Advertisement
This specification defines the Multisession capability [BGP-CAP]: This specification defines the Multisession capability [BGP-CAP]:
Capability code (1 octet): TBD Capability code (1 octet): TBD
Capability length (1 octet): 1 Capability length (1 octet): variable
Capability value (1 octet): Flags as below Capability value (1 octet): Flags followed by the list of capabilities
that define a session.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|G| Reserved | |G| Reserved |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| One or more list of Capability codes (1 octet each) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The most significant bit is defined as the Grouping Support (G) bit. The most significant bit is defined as the Grouping Support (G) bit.
It can be used to indicate support for the ability to group multiple It can be used to indicate support for the ability to group multiple
AFI/SAFI into one session. When set (value 1) this bit indicates capabilty values into one session. When set (value 1) this bit indicates
that the BGP speaker supports grouping. that the BGP speaker supports grouping.
The remaining bits are reserved, and should be set to zero by the The remaining bits are reserved, and should be set to zero by the
sender and ignored by the receiver. sender and ignored by the receiver.
Following the reserved bits is a list of one or more Capability codes
defined in BGP. The capabilities listed here represent the session
identifier for sessions between the BGP speakers.
For example peers wishing to establish sessions based on AFI/SAFI would
exchange the Multiprotocol Extensions capability code (1) only in the
list. In this case the Multisession capability would have a length of
2 octets.
4. New NOTIFICATION Subcodes 4. New NOTIFICATION Subcodes
[BGP, BGP-DRAFT] Section 4.5 provides a number of subcodes to the [BGP, BGP-DRAFT] Section 4.5 provides a number of subcodes to the
NOTIFICATION message, and Section 6.2 elaborates on the use of those NOTIFICATION message, and Section 6.2 elaborates on the use of those
subcodes. subcodes.
This specification introduces two new subcodes: This specification introduces two new subcodes:
OPEN Message Error subcodes: OPEN Message Error subcodes:
7 - No Supported AFI/SAFI. 7 - No Supported Capability Value.
8 - Grouping Conflict 8 - Grouping Conflict
9 - Grouping Required 9 - Grouping Required
The No Supported AFI/SAFI code MAY be used when an OPEN message The No Supported Capability Value code MAY be used when an OPEN message
contains one or more capabilities, none of which list an
capability supported by the local BGP speaker. It is observed that
this subcode may be useful for BGP speakers in general, even if
they do not (otherwise) implement this specification.
The Grouping Conflict code MAY be used when an OPEN message contains
several capabilities whose values conflict with one or more
capability groups configured on the local BGP speaker. The Data field
SHOULD indicate one of the conflicting locally-configured capability
groups, encoded as theh appropriate capabilities.
The Grouping Required code MAY be used when a BGP speaker which is
configured to require grouping attempts to establish a connection
with a BGP speaker which does not support grouping. (While it is
true that it might be possible to communicate much the same
information using the Unsupported Capability NOTIFICATION message,
this more explicit method is felt to be more transparent.)
If MP-BGP capability is used as the session identifier, the notification
codes could be used as follows:
The No Supported Capability Value code MAY be used when an OPEN message
contains one or more MP-BGP capabilities, none of which list an contains one or more MP-BGP capabilities, none of which list an
AFI/SAFI supported by the local BGP speaker. It is observed that AFI/SAFI supported by the local BGP speaker. It is observed that
this subcode may be useful for MP-BGP speakers in general, even if this subcode may be useful for MP-BGP speakers in general, even if
they do not (otherwise) implement this specification. they do not (otherwise) implement this specification.
The Grouping Conflict code MAY be used when an OPEN message contains The Grouping Conflict code MAY be used when an OPEN message contains
several MP-BGP capabilities whose AFI/SAFI conflict with one or more several MP-BGP capabilities whose AFI/SAFI conflict with one or more
AFI/SAFI groups configured on the local BGP speaker. The Data field AFI/SAFI groups configured on the local BGP speaker. The Data field
SHOULD indicate one of the conflicting locally-configured AFI/SAFI SHOULD indicate one of the conflicting locally-configured AFI/SAFI
groups, encoded as MP-BGP capabilities. groups, encoded as MP-BGP capabilities.
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configured to require grouping attempts to establish a connection configured to require grouping attempts to establish a connection
with a BGP speaker which does not support grouping. (While it is with a BGP speaker which does not support grouping. (While it is
true that it might be possible to communicate much the same true that it might be possible to communicate much the same
information using the Unsupported Capability NOTIFICATION message, information using the Unsupported Capability NOTIFICATION message,
this more explicit method is felt to be more transparent.) this more explicit method is felt to be more transparent.)
The use of these subcodes is further elaborated below. The use of these subcodes is further elaborated below.
5. Overview of Operation 5. Overview of Operation
Until a BGP speaker has initiated or accepted one connection from a The operation section is divided into two main subsections.
given peer, it is unknown whether the peer supports this
specification or not. Two strategies can be considered for making
this initial determination -- either the BGP speaker can initially
assume that the peer does not support this specification, and switch
modes if it is discovered that it does, or vice-versa. Either
approach is acceptable.
The "Using Multisession" sections below discuss the BGP speaker's The "Using Multisession" sections below discuss the BGP speaker's
behavior when the peer does support this specification or is assumed behavior when the peer does support this specification or is assumed
to. The "Backward Compatibility" section discusses the BGP speaker's to. The "Backward Compatibility" section discusses the BGP speaker's
behavior when the peer does not support this specification, or is behavior when the peer does not support this specification, or is
assumed not to. Both sections discuss how to switch to the other assumed not to. Both sections also discuss how to switch to the other
mode. mode.
A BGP speaker which supports this specification SHOULD always A BGP speaker which supports this specification SHOULD always
advertise the Multisession capability, regardless of its peer's known advertise the Multisession capability, regardless of its peer's known
or presumed capability set. or presumed capability set.
5.1. Using Multisession: In all cases until a BGP speaker has initiated or accepted one connection
from a given peer, it is unknown whether the peer supports this
specification or not. Two strategies can be considered for making
this initial determination -- either the BGP speaker can initially
assume that the peer does not support this specification, and switch
modes if it is discovered that it does, or vice-versa. Either
approach is acceptable.
The following subsections discuss a BGP speaker's behavior towards a Again for illustration, section 5.0 describes the operation from the
peer which is known or assumed to support this specification. point of view of the MP-BGP capability and the associated AFI/SAFI
values as the session identifier. It can be replaced with any other
capability or groups of capabilties without any changes to the behaviour
described below.
Note that if a BGP speaker only wishes to support a single AFI/SAFI Note that if a BGP speaker only wishes to support a single AFI/SAFI
in its communications with a given peer only one session is needed in in its communications with a given peer only one session is needed in
any case, and so the "multisession" feature is moot. In such a case any case, and so the "multisession" feature is moot. In such a case
the behavior required would be indistinguishable from that given in the behavior required would be indistinguishable from that given in
the "backward compatibility" section below. In the following the "backward compatibility" section below. In the illustrative examples
sections, it is generally assumed that a BGP speaker does wish to in the following sections, it is generally assumed that a BGP speaker
support multiple AFI/SAFI in its communications with a given peer. does wish to support multiple AFI/SAFI in its communications with a
given peer.
5.1. Using Multisession:
The following subsections discuss a BGP speaker's behavior towards a
peer which is known or assumed to support this specification.
5.1.1. Initiating Connections: 5.1.1. Initiating Connections:
When a BGP speaker attempts BGP communication with its peer, it When a BGP speaker attempts BGP communication with its peer, it
initiates one connection per group of AFI/SAFI it wishes to support. initiates one connection per group of AFI/SAFI it wishes to support.
(This implies that a new local TCP port will be allocated for each (This implies that a new local TCP port will be allocated for each
new connection.) The OPEN sent on each connection MUST include the new connection.) The OPEN sent on each connection MUST include the
Multisession capability and one or more MP-BGP capabilities Multisession capability and one or more MP-BGP capabilities
indicating the AFI/SAFI to be supported on that session. If a non- indicating the AFI/SAFI to be supported on that session. If a non-
trivial group of AFI/SAFI (i.e., a group of two or more) is proposed, trivial group of AFI/SAFI (i.e., a group of two or more) is proposed,
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grouping, and a non-trivial group of AFI/SAFI has been proposed, then grouping, and a non-trivial group of AFI/SAFI has been proposed, then
it will respond as given in the previous paragraph but with the it will respond as given in the previous paragraph but with the
additional proviso that the G bit will be clear. In this case, the additional proviso that the G bit will be clear. In this case, the
BGP speaker MAY accept the connection as given in the previous BGP speaker MAY accept the connection as given in the previous
paragraph, or it MAY reply with a NOTIFICATION message with ERROR paragraph, or it MAY reply with a NOTIFICATION message with ERROR
Code OPEN Message Error and Error Subcode Grouping Required, and the Code OPEN Message Error and Error Subcode Grouping Required, and the
connection will be closed. connection will be closed.
If the peer does not wish to support the AFI/SAFI in question, it If the peer does not wish to support the AFI/SAFI in question, it
will reply with a NOTIFICATION message with Error Code OPEN Message will reply with a NOTIFICATION message with Error Code OPEN Message
Error, and Error Subcode No Supported AFI/SAFI, and the connection Error, and Error Subcode No Supported Capability value, and the connection
will be closed. will be closed.
A BGP speaker SHOULD NOT attempt to initiate connections for any A BGP speaker SHOULD NOT attempt to initiate connections for any
AFI/SAFI for which a connection already exists. AFI/SAFI for which a connection already exists.
If the peer does not support this specification, it will respond with If the peer does not support this specification, it will respond with
an OPEN which does not include the Multisession capability. In this an OPEN which does not include the Multisession capability. In this
case the connection SHOULD be terminated, and future connections to case the connection SHOULD be terminated, and future connections to
the peer should be attempted in the "backward compatibility" mode the peer should be attempted in the "backward compatibility" mode
discussed below. discussed below.
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default if grouping is supported. default if grouping is supported.
If the BGP speaker does not support AFI/SAFI grouping it MAY reply If the BGP speaker does not support AFI/SAFI grouping it MAY reply
with an OPEN listing one of the AFI/SAFI out of those proposed by the with an OPEN listing one of the AFI/SAFI out of those proposed by the
peer. It SHOULD also set the G bit in the Multisession capability to peer. It SHOULD also set the G bit in the Multisession capability to
zero. zero.
If the received OPEN message does not include any MP-BGP capability If the received OPEN message does not include any MP-BGP capability
indicating an AFI/SAFI the BGP speaker wishes to support, it should indicating an AFI/SAFI the BGP speaker wishes to support, it should
close the connection with a NOTIFICATION message with Error Code OPEN close the connection with a NOTIFICATION message with Error Code OPEN
Message Error and Error Subcode No Supported AFI/SAFI. Message Error and Error Subcode No Supported Capability Value.
If the received OPEN message does not include the Multisession If the received OPEN message does not include the Multisession
capability, then the peer does not support this specification. The capability, then the peer does not support this specification. The
connection MAY be continued in the "backward compatibility" mode connection MAY be continued in the "backward compatibility" mode
discussed below, or it MAY be terminated and future connections to discussed below, or it MAY be terminated and future connections to
the peer attempted in the "backward compatibility" mode. the peer attempted in the "backward compatibility" mode.
5.1.3. Collision Detection, Graceful Restart: 5.1.3. Collision Detection, Graceful Restart:
[BGP, BGP-DRAFT] Section 6.8 (BGP connection collision detection) [BGP, BGP-DRAFT] Section 6.8 (BGP connection collision detection)
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6. State Machine 6. State Machine
As mentioned under "accepting connections" above, this specification As mentioned under "accepting connections" above, this specification
modifies the BGP finite state machine, albeit in a backward- modifies the BGP finite state machine, albeit in a backward-
compatible fashion. compatible fashion.
In addition, note that one state machine is considered to exist for In addition, note that one state machine is considered to exist for
each of the connections which may exist to a given peer. This each of the connections which may exist to a given peer. This
implies that, for example, any session flap dampening that may exist implies that, for example, any session flap dampening that may exist
is performed per AFI/SAFI. is performed per session indetifier.
The specific state machine modifications to [BGP-DRAFT] Section 8.2.2 The specific state machine modifications to [BGP-DRAFT] Section 8.2.2
are as follows. are as follows.
6.1. Modifications to Connect State and Active State 6.1. Modifications to Connect State and Active State
In the actions in response to the events Open Delay timer expires In the actions in response to the events Open Delay timer expires
[Event 12] and TCP connection succeeds [Event 16 or Event 17], an [Event 12] and TCP connection succeeds [Event 16 or Event 17], an
OPEN is not sent and the state changes to WaitForOpen and not to OPEN is not sent and the state changes to WaitForOpen and not to
OpenSent. OpenSent.
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7. Discussion 7. Discussion
Note that many BGP implementations already permit multiple sessions Note that many BGP implementations already permit multiple sessions
to be used between a given pair of routers, typically by configuring to be used between a given pair of routers, typically by configuring
multiple IP addresses on each router and configuring each session to multiple IP addresses on each router and configuring each session to
be bound to a different IP address. The principal contribution of be bound to a different IP address. The principal contribution of
this specification is to allow multiple sessions to be created this specification is to allow multiple sessions to be created
automatically, without additional configuration overhead or address automatically, without additional configuration overhead or address
consumption. consumption.
In addition to the simple mode of supporting one AFI/SAFI per The specification supports the simple case of one capability being used
connection, the procedures described here also permit arbitrary as the session identifier and one connection per session identifier value.
grouping of AFI/SAFI onto BGP connections. For such grouping to It also permits connections be established based on the multiple
function pleasingly, both peers participating in a connection need to capabilties as a sessio identifier with multiple values per capability
agree on what AFI/SAFI groupings will be used. If conflicting grouped together per connection.
groupings are configured, the connections may not establish, or more
connections may be established than were expected (in the degenerate In the context of MP-BGP based connections, which we believe may be the
case, one connection per AFI/SAFI could be established despite most prevalent use of this specification it permits supporting one
configured groupings). We observe that the potential for misbehavior AFI/SAFI per connection, and also permits arbitrary grouping of AFI/SAFI
in the presence of conflicting configuration is not unusual in BGP, onto BGP connections. For such grouping to function pleasingly, both
and that support for, and configuration of grouping is purely peers participating in a connection need to agree on what AFI/SAFI
optional. groupings will be used. If conflicting groupings are configured, the
connections may not establish, or more connections may be established
than were expected (in the degenerate case, one connection per AFI/SAFI
could be established despite configured groupings). We observe that
the potential for misbehavior in the presence of conflicting configuration
is not unusual in BGP, and that support for, and configuration of grouping
is purely optional.
8. Acknowledgements 8. Acknowledgements
To be supplied. To be supplied.
9. References 9. References
[BGP4] [BGP4]
Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)," RFC Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)," RFC
1771, March 1995. 1771, March 1995.
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100 S. Main Suite 200 100 S. Main Suite 200
Ann Arbor, MI 48104 Ann Arbor, MI 48104
Email: jgs@cisco.com Email: jgs@cisco.com
Chandra Appanna Chandra Appanna
Cisco Systems, Inc. Cisco Systems, Inc.
170 West Tasman Drive 170 West Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
e-mail: achandra@cisco.com e-mail: achandra@cisco.com
13. Full Copyright Statement 13. Intellectual Property Statement
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