Internet Engineering Task Force J. Scudder
Internet-Draft Juniper Networks
Intended status: Standards Track C. Appanna
Expires: March 16, 2013 Arista Networks
I. Varlashkin
Easynet Global Services
September 12, 2012
Multisession BGP
draft-ietf-idr-bgp-multisession-07
Abstract
This specification augments "Multiprotocol Extensions for BGP-4" (MP-
BGP) by proposing a mechanism to facilitate the use of multiple
sessions between a given pair of BGP speakers. Each session is used
to transport routes related by some session-based attribute such as
AFI/SAFI. This provides an alternative to the MP-BGP approach of
multiplexing all routes onto a single connection.
Use of this approach is expected to provide finer-grained fault
management and isolation as the BGP protocol is used to support more
and more diverse services.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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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."
This Internet-Draft will expire on March 16, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview of operations . . . . . . . . . . . . . . . . . . . . 5
4. Multisession BGP Capability Code . . . . . . . . . . . . . . . 6
5. New NOTIFICATION Subcodes . . . . . . . . . . . . . . . . . . 7
6. Modified Connection Collision Handling . . . . . . . . . . . . 7
7. Connection establishment . . . . . . . . . . . . . . . . . . . 8
8. Graceful restart . . . . . . . . . . . . . . . . . . . . . . . 10
9. Error handling . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Operational considerations . . . . . . . . . . . . . . . . . . 10
11. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 11
12. State Machine . . . . . . . . . . . . . . . . . . . . . . . . 11
13. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 11
14. Security Considerations . . . . . . . . . . . . . . . . . . . 12
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
17.1. Normative References . . . . . . . . . . . . . . . . . . . 13
17.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Multisession usage scenarios . . . . . . . . . . . . 13
A.1. Single session on both sides . . . . . . . . . . . . . . . 13
A.2. Single session on one side, multiple sessions on the
other . . . . . . . . . . . . . . . . . . . . . . . . . . 14
A.3. Multiple sessions based on AFI/SAFI . . . . . . . . . . . 15
A.4. Multiple sessions based on arbitrary BGP Capabilities . . 17
A.5. Process level separation of multiple sessions . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
Most BGP [RFC4271] implementations only permit a single ESTABLISHED
connection to exist with each peer. More precisely, they only permit
a single ESTABLISHED connection for any given pair of IP endpoints.
BGP Capabilities [RFC5492] extend BGP to allow diverse information
(encoded as "capabilities") to be associated with a session. In some
cases, a capability may relate to the operation of the protocol
machinery; an example is Route Refresh [RFC2918]. However, in other
cases a capability may relate specifically to some common
distinguishing characteristic of the routes carried over the session;
an example is Multiprotocol BGP [RFC4760].
Multiprotocol BGP [RFC4760] extends BGP to allow information for
multiple NLRI families and sub-families to be transported in BGP.
Routes for different families are distinguished by AFI and SAFI.
Routes for different families are commonly multiplexed onto a single
BGP session.
A common criticism of BGP is the fact that most malformed messages
cause the session to be terminated. While this behavior is necessary
for protocol correctness, one may observe that the protocol machinery
of a given implementation may only be defective with respect to a
given AFI/SAFI. Thus, it would be desirable to allow the session
related to that family to be terminated while leaving other AFI/SAFI
unaffected. As BGP is commonly deployed, this is not possible.
A second criticism of BGP is that it is difficult or in some cases
impossible to manage control plane resource contention when BGP is
used to support diverse services over a single session. In contrast,
if a single BGP session carries only information for a single service
(or related set of services) it may be easier to manage such
contention.
In this specification, we propose a mechanism by which multiple
transport sessions may be established between a pair of peers. Each
transport session is identified by a distinct set of BGP
capabilities, notably the MP-BGP capability.
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 modifications to the operation of the protocol once a session
reaches ESTABLISHED state.
Although AFI/SAFI is perhaps the most obvious way to group sets of
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routes being exchanged between BGP peers, sessions can also be
distinguished by other BGP capabilities. In general, any capability
used in this fashion would be expected to have semantics of
identifying some common distinguishing characteristic of a set of
routes, just as AFI/SAFI does; however, specifics are beyond the
scope of this document. Most examples in this document are focusing
on MP-BGP capability (or interchangeably, AFI/SAFI) based grouping
for simplicity reason. However actual application of multisessions
extension . Such use is illustrative and is not intended to be
limiting.
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 routers implementing this
specification MUST also implement MP-BGP [RFC4760].
1.1. Requirements Language
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].
2. Definitions
"MP-BGP capability" refers to the capability [RFC5492] with code 1,
specified in MP-BGP [RFC4760] section 8.
A BGP speaker is said to "support" some feature or functionality (for
example, to support this specification, or to support a particular
AFI/SAFI) when the BGP implementation supports the feature AND the
feature has not been disabled by configuration.
The Session Identifier is a capability or group of capabilities that
will be used to differentiate individual BGP sessions between two IP
endpoints. When the AFI/SAFI is used to distinguish sessions, the
MP-BGP capability is the session identifier.
3. Overview of operations
To allow multiple sessions between same pair of BGP speakers to co-
exist BGP Multisession extension modifies Connection Collision
Detection procedure of the base BGP specification (RFC4271). Rather
than considering only IP addresses of the peers new procedure also
takes into account list of certain session attributes, such as AFI/
SAFI, to determine uniqueness of the sessions. When sessions are
deemed to be unique each of them is then handled independently,
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therefore critical conditions (such as malformed UPDATEs) in one
session won't affect others.
BGP Multisession extension introduces new BGP capability code to
indicate that a BGP speaker supports protocol modification described
in this document and new error message sub-codes that facilitate
handling of incompatible configurations between two speakers.
Following sections provide formal description of the protocol
enhancement. Additionally, Appendix contains non-normative examples
of desired behaviour for Multisession-enabled BGP speakers, which is
intended only for illustrative purpose.
4. Multisession BGP Capability Code
This specification defines the Multisession capability [RFC5492]:
Capability code (1 octet): 68
Capability length (1 octet): variable
Capability value (1 octet): Flags followed by the list of
capabilities that define a session.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G| Reserved | Session Id ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
G - the most significant bit was originally intended by earlier draft
version of Multisession specification to denote capability of a BGP
speaker to group multiple capability values into one session. As
this information can be deduced from Session Id, the use of G bit is
deprecated - implementations conforming to final version of
Multisession specification SHOULD NOT rely on value of the G bit.
Reserved - MUST be set to zero by sender, MUST be ignored by receiver
Session Id(entifier) - list of zero or more capability codes (1 octet
each) defined in BGP, whose values will be used to distinguish one
group from another. The size of the list is inferred from the length
of the overall capability; it is the capability length minus one.
The Multisession capability code itself MUST NOT be listed; if listed
it MUST be ignored upon receipt.
Empty Session Id list and Session Id containing 1 (one, Multiprotocol
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Extensions) as the only value are considered equal and indicate that
AFI/SAFI list in the OPEN message is used to distinguish the groups.
However, if BGP speaker wishes to use compound Session Id that
includes AFI/SAFI list as one of the components, then Capability Code
1 MUST be explicitly included in the Session Id. For example, if BGP
speaker Session Id to 'X' (denoting Capability Foo) then only Foo
will be used as Session Id, i.e. session where Foo is 1 and AFI/SAFI
is 1/1 and session where Foo is 1 and AFI/SAFI is 1/2 will be
considered as conflicting. On the other hand Session Id set to '1 X'
or 'X 1' indicates that groups are identified by combination of Foo
and AFI/SAFI, i.e. above two sessions as well as session where Foo is
2 and AFI/SAFI is 2/4 will be considered unique.
For given pair of BGP peers Multisession capability MUST be used
either on all or none sessions. This is required due to different
connection collision handling procedure used by multisession.
5. New NOTIFICATION Subcodes
BGP [RFC4271] Section 4.5 provides a number of subcodes to the
NOTIFICATION message, and Section 6.2 elaborates on the use of those
subcodes specific to OPEN message.
This specification introduces three new subcodes for OPEN Message
Error code:
7 - Capability Value Mismatch - Session Id mismatch, i.e.
remote speaker whishes to use different capability codes in
Session Id compare to local speaker
8 - Grouping Conflict - values of capability codes used in
Session Id of the received message cannot be unambiguously
mapped to a locally configured group
9 - Grouping Required (from earlier drafts, perhaps should be
removed if not used)
BGP implementations conforming to this specification SHOULD use new
sub-codes as described further down in section "Connection
establishment" of this document.
6. Modified Connection Collision Handling
BGP speaker conforming to and actively using this specification MUST
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use modified connection collision handling procedure as described in
this section.
Two sessions are said to collide if and only if both of following
conditions are true:
1: the IP addresses on of peers are the same on both sessions
2: values of capability codes used in session identifier are either
the same or overlapping (regardless fully or partially) within
given capability code
Otherwise two sessions are considered unique and both MAY transition
to the ESTABLISHED state (subject to rest of BGP specification).
Before attempting to create new session local system SHOULD evaluate
existing sessions with the same peer. If there is already a session
with the same peer in ESTABLISHED state and new session would collide
with it, BGP speaker SHOULD NOT attempt creating new session; it's a
good idea to notify operator of the local system about such potential
collision.
Upon receipt of an OPEN messages BGP speaker MUST evaluate existing
sessions with the same peer. If there is already a session in
ESTABLISHED state and multisession distinguisher values of the old
and the new OPEN messages fully match, the old session remains and
the new MUST be closed.
If there is a session in OpenConfirm or OpenSent state and two
sessions do not collide according to this document, then both
sessions proceed as normally and section 6.8 of RFC4271 MUST NOT be
applied. If on the other hand two sessions collide according to
definition of this document, then original procedure from section 6.8
of RFC4271 MUST be applied, except for the NOTIFICATION type.
Whereas original specification prescribes to use 'Cease' error code,
multisession enabled BGP speaker SHOULD send NOTIFICATION message as
described in this document.
7. Connection establishment
When BGP Multisession is enabled by configuration for given peer and
configuration dictates that multiple sessions can potentially be
established with given peer, BGP speaker MUST advertise Multisession
Capability code in the OPEN message on every session with given peer.
In all other cases Multisession capability SHOULD NOT be advertised.
The value of Session Id MUST be the same on every session.
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When Multisession-enabled BGP speaker receives an OPEN message
without BGP Multisession Capability code it MUST assume that peer is
not capable of multiple sessions and MUST use original Connection
Collision Detection procedure as described in section 6.8 of RFC4271.
When Multisession-enabled BGP speaker receives an OPEN message
containing BGP Multisession Capability Code but with Session Id not
matching its own Session Id, local BGP speaker MUST send NOTIFICATION
message with Error Code set to 2 ("OPEN Message Error") and Error
Sub-code set to 8 ("Grouping Conflict") and drop the session. If
received Session Id matches locally configured Session Id then BGP
speaker MUST verify whether this session would collide with any of
the existing as described in section "Modified Connection Collision
Handling".
If session is allowed to continue by connection collision detection
procedure, the next step for local speaker is to find matching group
as follow:
1. If BGP capability code values used in Session Id of the received
message match exactly (i.e. for every value in the received OPEN
message there is corresponding value in a locally configured
group) then local BGP speaker proceeds with this session
2. If values in the received message do not match any of the locally
configured groups exactly, but there is one and only one locally
configured group such that for every capability code the
intersection between received and local values is non-empty set,
then this group is selected for continuing the session. Note,
such partial match results in behaviour similar to non-
multisession BGP when capability codes overlap partially.
Rationale behind allowing only one group for partial matching is
that it simplifies specification and implementation; from
operational perspective multiple partially matching groups
suggest significant descrepancy in configuration between peers
and therefore unlikely to be required in real-life networks.
3. In all other cases local BGP speaker MUST send NOTIFICATION
message with Error Code set to 2 (OPEN Message Error) and Error
Sub-code set to 8 (Grouping conflict).
Once local BGP speaker has identified which locally configured group
corresponds to received OPEN message it proceeds with the session
like it would have been regular non-multisession one, particularly -
the original Finite State Machine applies. BGP speaker is free to
handle such session either in the same process/thread as the one that
received OPEN message, or it can hand over connection to another
process/thread. If uses, the connection handover is local-matter of
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BGP implementation and not part of this specification. Appendix
contains an example how such handover could be done.
8. Graceful restart
With respect to Section 4.2 of BGP Graceful Restart [RFC4724], when
determining whether a new connection BGP speaker evaluate values of
all capability codes used in Session Identifier.
9. Error handling
If multisession-enabled BGP speaker detects an error condition that
warrants session reset, it SHOULD reset only session that was
affected by the error. Resetting other sessions with the same peer
would significantly diminish value of multisession extensions.
10. Operational considerations
Multisession feature SHOULD be disabled by default. BGP
implementation SHOULD provide configuration-time option to enable
multisession extension on per-peer basis. If BGP implementation
supports non-trivial groups, then it SHOULD provide configuration-
time option for operator to control how sessions are grouped. An
example of such option would be possibility for an operator to
specify which address families will be carried in one session, and
which address families will be carried in another session.
BGP implementation supporting multisession extension SHOULD allow
operator to view state of each individual group and at least last
NOTIFICATION message that caused connection reset.
For the sake of interoperability between BGP speakers supporting
multisession, an implementation SHOULD NOT impose hard-coded
restrictions on groups based on particular Session Id are put
together. If such restrictions are unavoidable, then BGP
implementation MUST support at least trivial groups based on that
attribute. Let's consider this on an example. If implementation A
requires AFI/SAFI 1/1 and 1/4 to be always carried within same
session, while implementation B requires AFI/SAFI 1/4 to be always
carried only with 1/128 and not with any other, then it's not
possible to establish session between such BGP speakers. However if
implementations A and B both allow each AFI/SAFI to be carried each
in its own group, then we can establish three sessions - one for AFI/
SAFI 1/1, another one for AFI/SAFI 1/4 and third one for AFI/SAFI
1/128.
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11. Backward Compatibility
This subsection discusses a BGP speaker's behavior towards a peer
that is known or assumed not to support this specification. In
short, the BGP speaker's behavior towards such a peer should be as
otherwise defined for the BGP protocol, according to [RFC4271] and
any other extension supported by the BGP speaker.
If a BGP speaker receives OPEN message that doesn't include
Multisession Capability and local BGP speaker is required to use
multisession (e.g. through configuration by operator), the local BGP
speaker MUST drop the session and send appropriate NOTIFICATION
message as described in Section 5. If multisession is not required,
local BGP speaker proceeds with multisession extension disabled, so
it appears as regular implementation to the peer.
As previously mentioned, the BGP speaker SHOULD always advertise the
Multisession capability in its OPEN message, even towards "backward
compatibility" peers.
Use of techniques such as dynamic capabilities
[I-D.ietf-idr-dynamic-cap] for on-the-fly switching of session modes
is beyond the scope of this document.
12. State Machine
This specification does not modify BGP FSM as such, but all
references to execution of collision handling procedure of original
BGP specification are replaced with call to collision handling
procedure described in this document.
The specific state machine modifications to [RFC4271] Section 8.2.2
are as follows.
13. Discussion
Note that many BGP implementations already permit multiple sessions
to be used between a given pair of routers, typically by configuring
multiple IP addresses on each router and configuring each session to
be bound to a different IP address. The principal contribution of
this specification is to allow multiple sessions to be created
automatically, without additional configuration overhead or address
consumption.
The specification supports the simple case of one capability being
used as the session identifier and one connection per session
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identifier value. It also permits connections be established based
on multiple capabilities as a session identifier with multiple values
per capability grouped together per connection.
In the context of MP-BGP based connections, which we believe may be
the most prevalent use of this specification, it permits supporting
one AFI/SAFI per connection, and also permits arbitrary grouping of
AFI/SAFI onto BGP connections. For such grouping to function
pleasingly, both peers participating in a connection need to agree on
what AFI/SAFI 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.
14. Security Considerations
This document does not change the BGP security model.
15. Acknowledgements
The authors would like to thank Martin Djernaes, Pedro Marques, Keyur
Patel, Robert Raszuk, Yakov Rekhter, David Ward and Anton Elita for
their valuable comments.
16. IANA Considerations
IANA has allocated BGP Capability Code 68 as the Multisession BGP
Capability.
This document requests IANA to allocate three new OPEN Message Error
subcodes:
7 - Capability Value Mismatch
8 - Grouping Conflict
9 - Grouping Required
17. References
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17.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
January 2007.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, February 2009.
17.2. Informative References
[I-D.ietf-idr-dynamic-cap]
Ramachandra, S. and E. Chen, "Dynamic Capability for
BGP-4", draft-ietf-idr-dynamic-cap-14 (work in progress),
December 2011.
[RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC 2918,
September 2000.
Appendix A. Multisession usage scenarios
This section demonstrates usage of Multisession Extension in several
common scenarios. All examples presented here for illustrative
purpose only, they're not part of Multisession specification.
A.1. Single session on both sides
BGP Speaker A and BGP Speaker B are both configured to exchange IPv4
unicast (AFI=1, SAFI=1) and IPv4 L3VPN (AFI=1, SAFI=128) prefixes
over single session. If Multisession extension is disabled by
configuration on both sides, then the session is, from every
perspective, indistinguishable from ordinary (non-multisession) BGP
peering. If only one of the speakers is enabled (through
configuration) for multisession and yet only with one session to
multiplex both AFI/SAFI, then again only single session is
established and it looks like normal session. Although multisession-
enabled BGP speaker is capable of processing new NOTIFICATION sub-
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codes, the other side (non-multisession) won't take advantage of it.
On the other hand use of new NOTIFICATION sub-codes isn't necessary
in this situation because both sides keep all AFI/SAFI within same
session. Finally, if both speakers are multisession-enabled, they
still setup single session, but now they can use new NOTIFICATION
sub-codes for more sophisticated error handling.
Note that if both speakers configured to use only single session and
their respective AFI/SAFI lists overlap but do not match exactly,
then like with ordinary (non-multisession) BGP speakers the session
will transition to ESTABLISHED state. It's possible that one of the
speakers (or both) require exact match of AFI/SAFI lists in order to
establish session (either by implementation or through
configuration). In this case such speaker will send NOTIFICATION
message with Error Code 2 (OPEN Message Error) and Sub-code 8
(Grouping conflict) and subsequently close the session.
A.2. Single session on one side, multiple sessions on the other
In this setup Speaker A is configured to carry IPv4 unicast (AFI=1,
SAFI=1) and IPv4 L3VPN (AFI=1, SAFI=128) prefixes within single
session, while Speaker B is configured with two sessions - one for
IPv4 unicast and second for IPv4 L3VPN. Several scenarios are
possible depending on which speaker sends OPEN message first and
whether Speaker A is multisession-enabled or not.
Assuming Speaker A is not multisession-enabled, it sends OPEN message
first and there is no existing session between these two peers.
Speaker B determines that OPEN message lists both AFI/SAFI and it
knows that it wants to split them into different sessions, therefore
it's obvious that setup cannot function as intended. Since
separation of two address families into two groups is performed by
operator (as per Multisession Extension specification), the most
appropriate action is to prevent any communication between Speaker A
and B until operator intervenes and resolves the conflict in
configuration. To do this BGP Speaker B sends NOTIFICATION message
with Error Code 6 (because peer is not expected to understand new
notification sub-codes). Would Speaker A be multisession enabled,
then Speaker B would send NOTIFICATION message with Error Code 1 and
Error Subcode 9 (Grouping Required).
Now let's consider reverse situation - the Speaker B sends an OPEN
message for either AFI/SAFI first. Assuming Speaker A is not
multisession-enabled, it will accept OPEN message containing either
AFI/SAFI and will reply with OPEN message containing both AFI/SAFI.
Although session might transitions for a brief period to ESTABLISHED
state, the Speaker B upon receipt of the OPEN message will detect
misconfiguration and send NOTIFICATION message with Error Code 6 as
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in previous paragraph. Would Speaker A be multisession-enabled, it
could detect misconfiguration on its own and send NOTIFICATION
message with Error Code 1 and Error Subcode 8 (Grouping conflict).
There is possibility that Speaker A opens one TCP connection and
sends its OPEN message, and simultaneously Speaker B opens one or two
TCP connection(s) and sends OPEN message on each of them. Since
Speaker A is not multisession-enabled, it will invoke original
collision detection procedure and will drop one of the sessions.
Speaker B seeing NOTIFICATION message with Cease error code concludes
that Speaker A is not multisession-capable and that setup prescribed
by Speaker B's configuration cannot be achieved. Depending on
implementation of Speaker B a session for one of the AFI/SAFI may
progress to ESTABLISHED state, but Speaker B will inform operator
about incompatible configuration.
It's also possible that initially Speaker B has been configured with
only one AFI/SAFI, e.g. IPv4 unicast. The session between two peers
would come up as described in previous subsection. Now suppose
Speaker B is configured with additional session to carry IPv4 L3VPN
prefixes. Since Speaker A does not have multiple sessions
configured, it won't send another OPEN message as long as first
session is in ESTABLISHED state. Therefore it's only possible that
Speaker B will attempt establishing second connection and send new
OPEN message containing only IPv4 L3VPN AFI/SAFI. If Speaker A is
non-multisession enabled, it will drop second session sending
NOTIFICATION message. From this Speaker B can conclude that
configuration of two sides is incompatible, will stop attempting to
bring up IPv4 L3VPN session and will notify operator. Already
ESTABLISHED session may remain unaffected (subject to Speaker B
implementation), just like with non-multisession speakers.
A.3. Multiple sessions based on AFI/SAFI
This is most common use of multisession extension is to separate
prefixes based on AFI/SAFI. Note that use of AFI/SAFI based groups
is denoted by empty Optional Data field in Multisession Capability,
which is the same as in previous two sections. Grouping
configuration is devised from the list of actually advertised AFI/
SAFI lists (MP-BGP Capability). This will be demonstrated in
following examples.
Let's consider BGP Speaker A and BGP Speaker B both configured to
exchange IPv4 unicast, IPv4 labelled-unicast and IPv4 L3VPN prefixes
each in its own session. We start with no existing sessions between
these speakers. Speaker A (though roles can reverse) sends OPEN
message in which among other capabilities it announces MP-BGP
Capability for AFI=1 SAFI=1 and Multisession Capability with empty
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optional data field. Speaker B upon receipt of such message finds
that it expects to exchange IPv4 unicast with Speaker B in a
dedicated session. It accepts connection and sends similar OPEN
message to Speaker A. As there were no existing sessions, collision
handling procedure is not invoked at this time. Next Speaker A (but
again it could be Speaker B) starts new TCP connection to Speaker B
and sends OPEN message with MP-BGP Capability for AFI=1 SAFI=4 and
Multisession Capability with empty optional data field. Speaker B is
willing to exchange IPv4 labelled-unicast too, but before accepting
the proposal it executes collision detection procedure. Since AFI/
SAFI lists of the old (ESTABLISHED) and of the new sessions are
different, the sessions don't collide and, sending OPEN message with
AFI=1 SAFI=4, the Speaker B brings second session to ESTABLISHED
state. In the same way third session, for AFI=1 SAFI=128, is brought
up.
Note that similar behaviour will be also observed if two speakers
send OPEN messages simultaneously - modified collision handling
procedure, introduced by Multisession Extension specification, will
mark sessions as unique based on the difference in Session Id
(different AFI/SAFI lists). If Speaker A opens TCP connection and
sends an OPEN message for either AFI/SAFI, and simultanously Speaker
B opens TCP connection and send OPEN message for the same AFI/SAFI,
then modified collision handling procedure will resolve the conflict
just like original procedure would do in non-multisession
environment. Yet modified collision handling procedure allows
sessions with distinct Session Id's to coexist without affecting each
other. This behaviour applies also to more complex cases where
groups include more AFI/SAFI or based on different Capability Codes
all together. For this reason collision handling is not discussed in
remaining scenarios.
Now suppose Speaker A configuration is as above, but Speaker B is
configured to combine labelled-unicast and L3VPN prefixes into the
same session. IPv4 session is brought up as above. Next there are
two possible alternatives. Either Speaker A sends OPEN message for
one of the remaining sessions, to which Speaker B responds with
NOTIFICATION message Error Code 2 and Error Subcode 8. Or Speaker B
sends OPEN message for combined session including both of the
remaining address families, to which Speaker A responds either with
exactly the same NOTIFICATION message. At the end only IPv4 session
remains in ESTABLISHED state, while two other address families
require operator's intervention for configuring either Speaker A with
combined session for labelled-unicast and L3VPN, or Speaker B for one
session per AFI/SAFI. Note that if Speaker B would have used an
implementation that requires that labelled-unicast and L3VPN address-
families are combined into single session, then behaviour of each
side would be exactly as above.
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If Speaker A wouldn't have L3VPN configuration for Speaker B at all,
then whether second session would progress to ESTABLISHED or not
depends on whether configuration of either side requires exact match
between groups (by default implementations expected to mimim original
BGP behaviour which will bring overlapping AFI/SAFI up, but won't
require exact match, but some implementation may provide
configuration knob to require exact match).
Finally we look at the case where AFI/SAFI lists of different
configured sessions overlap. Suppose Speaker A is configured with
following groups: group 1 AFI=1 SAFI=1, group 2 AFI=1 SAFI=4 and
SAFI=128, group 3 AFI=2 SAFI=4; and Speaker B is configured as: group
1 AFI=1 SAFI=1, group 2 AFI=1 SAFI=4, group 3 AFI=1 SAFI=128 and
AFI=2 SAFI=4. For simplicity sake we assume that group 1 is brought
up first. Both speakers behave as already described in previous
case. Next let Speaker A to be the first to setup second TCP session
and send OPEN message for group 2. Applying collision handling
procedure as defined in Multisession specification Speaker B
continues processing of received OPEN message. If Speaker B is
configured for strict match between the groups, then it will detect
incompatibility of AFI/SAFI list between the received message and its
own configuration, therefore it will send NOTIFICATION message with
Error Code 2 and Error Subcode 8. If on the other hand Speaker B
allows partial overlapping of received and its own AFI lists (as
regular BGP implementation would in absence of multisession), it will
reply with OPEN message that lists AFI=1 SAFI=4 and session
potentially progresses to ESTABLISHED state provided that Speaker A
doesn't require exact match on AFI/SAFI list. Similar applies to the
session 3 for the remaining AFI/SAFI. Note that configuration for
exact or partial match between AFI/SAFI lists is the same for all
sessions between given peers.
A.4. Multiple sessions based on arbitrary BGP Capabilities
Although grouping based on arbitrary attributes is the most
comprehensive scenario, the behaviour of the BGP speakers is
essentially the same as in case of AFI/SAFI based groups. However
arbitrary groups do add extra complexity because BGP speakers need to
consider not only values of single capability, but need to agree upon
Capability Codes that constitute Session Id. Following example
demonstrates behaviour of multisession-enabled BGP speakers in
situation where Session Id on each side is based on different
capabilities.
Let's suppose there is imaginery Capability Code X that denotes
Experiment Id, and two speakers would like to exchange IPv4 unicast
and L3VPN prefixes for two experiments. Speaker A would like to
group prefixes into separate sessions based solely on Experiment Id
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(so two sessions with two AFI/SAFI in each), while Speaker B would
like to have separate session per experiment per AFI/SAFI (so four
sessions with one AFI/SAFI in each). Since Session Id involves
attribute other than AFI/SAFI, the Optional Data field in
Multisession Capability will be non-empty. Multisession Capability
sent by Speaker A will contain only 'Experiment Id Capability Code'
in the Optional Data, whereas Speaker B will put there both
"Experiment Id Capability Code" and "MP-BGP (AFI/SAFI)". When either
speaker receives OPEN message from the peer, it will notice mismatch
between content of the Optional Data field and, since sessions cannot
be established as intended, the speaker will send NOTIFICATION
message with Error Code 2 and Subcode 7 after which session will be
dropped. Both speakers will notify operator and will suppress
further attempt to bring session up until configuration of either
side changes.
Note that despite Multisession Capability does not containing a field
to denote support for non-AFI/SAFI based groups, even an
implementation that does not support groups based on arbitrary
capability codes will be able to recognise configuration mismatch and
provide sufficient information to the peer as described above.
A.5. Process level separation of multiple sessions
As fault isolation is the key motivation for the Multisession
Extension it's natural to consider process-level separation between
the sessions. Although Multisession specification itself does not
prescribe any particular way of handling each session, BGP
implementations can leverege IPC facilities provided by host
operating systems to handover arbitrary session to appropriate
process. For example, many systems can pass connection from the
process that accepted TCP connection to a process dedicated for
particular group using specially crafted message on Unix socket.
This is somewhat acking to inetd, but based on content of the OPEN
message (e.g. AFI/SAFI list) rather than on transport protocol
properties (e.g. TCP/UDP port numbers). At one extrimity the
process that initially accepts TCP connection may be very primitive
and can leave even connection collision handling to a specializing
process, on the other hand process could handle collision detection
itself or even handle particular group on its own while passing only
specific group to another process. This process level separation is
local implementation business and does not require specific aid from
BGP at protocol specification level. Therefore process level
separation is not part of multisession specification.
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Authors' Addresses
John G. Scudder
Juniper Networks
Email: jgs@juniper.net
Chandra Appanna
Arista Networks
Email: achandra@aristanetworks.com
Ilya Varlashkin
Easynet Global Services
Email: ilya.varlashkin@easynet.net
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