wdiff draft-ietf-idr-bgp-multisession-05.txt draft-ietf-idr-bgp-multisession-06.txt

Internet Engineering Task Force J. Scudder
Internet-Draft Juniper Networks
Intended status: Standards Track C. Appanna
Expires: September 24, 2010 29, 2011 Cisco Systems
I. Varlashkin
Easynet Global Services
March 23, 2010 28, 2011

Multisession BGP
draft-ietf-idr-bgp-multisession-05
draft-ietf-idr-bgp-multisession-06

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

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 5
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 5
3. Use Overview of BGP Capability Advertisement operations . . . . . . . . . . . . . . . . . . . . 5
4. Multisession BGP Capability Code . . . . . . . . . . . . . . . 6
5. New NOTIFICATION Subcodes . . . . . . . . . . . . . . . . . . 6
5. Overview of Operation 7
6. Modified Connection Collision Handling . . . . . . . . . . . . 7
7. Connection establishment . . . . . . . . . . 7
5.1. Using Multisession . . . . . . . . . 8
8. Graceful restart . . . . . . . . . . . 8
5.1.1. Initiating Connections . . . . . . . . . . . . 10
9. Error handling . . . . 8
5.1.1.1. Continuing a Redirected Connection . . . . . . . . 10
5.1.2. Accepting Connections . . . . . . . . . . . . 10
10. Operational considerations . . . . 10
5.1.3. Collision Detection, Graceful Restart . . . . . . . . 11
6. . . . . . . 10
11. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 11
7.
12. State Machine . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Modifications to Connect State and Active State 11
13. Discussion . . . . . . . 12
7.2. Addition of WaitForOpen State, Deletion of OpenSent
State . . . . . . . . . . . . . . . . . . . 11
14. Security Considerations . . . . . . . . . 13
8. Discussion . . . . . . . . . . 12
15. Acknowledgements . . . . . . . . . . . . . . . . 13
9. Security . . . . . . . 12
16. IANA Considerations . . . . . . . . . . . . . . . . . . . 13
10. Acknowledgements . . 12
17. References . . . . . . . . . . . . . . . . . . . . . . 14
11. IANA Considerations . . . . 13
17.1. Normative References . . . . . . . . . . . . . . . . . 14
12. . . 13
17.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Multisession usage scenarios . . . . . . . . . . 14
12.1. Normative References . . 13
A.1. Single session on both sides . . . . . . . . . . . . . . . 13
A.2. Single session on one side, multiple sessions on the
other . . 14
12.2. Informative References . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . 15 18

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
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. For the sake of clarity, we generally use
the Most examples in this document are focusing
on MP-BGP capability (or interchangeably, AFI/SAFI) in this
document. 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 session identifiers are said BGP speakers to conflict when considering
them as two sets, there is an intersection between them either in the
capabilities or the values contained within co-
exist BGP Multisession extension modifies Connection Collision
Detection procedure of the capabilities, but
neither is a subset base BGP specification (RFC4271). Rather
than considering only IP addresses of the other. For example, a pair peers new procedure also
takes into account list of MP-BGP
capabilities is said to "conflict" when considering them certain session attributes, such as two sets
(of AFI/SAFI values), there is an intersection between the sets but
neither set is a subset AFI/
SAFI, to determine uniqueness of the other.

A BGP speaker is said sessions. When sessions are
deemed to be the "active" speaker for a given
connection if it was the party that initiated the transport open.
The active speaker's transport endpoint will typically use an
ephemeral port number.

A BGP speaker unique each of them is said then handled independently,
therefore critical conditions (such as malformed UPDATEs) in one
session won't affect others.

BGP Multisession extension introduces new BGP capability code to be the "passive" speaker for a given
connection if it was the party
indicate that received the transport open. The
passive speaker's transport endpoint typically uses the well-known a BGP port number, 179, but speaker supports protocol modification described
in this document introduces an exception
detailed in Section 5.1.1.1.

3. Use 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 Advertisement 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 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+
|G|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G| Reserved | Port number (if R is set) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| One or more Capability codes (1 octet each) |
~ Session Id ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

G - the most significant bit is defined as the Grouping Support (G) bit.
It can be used was originally intended by earlier draft
version of Multisession specification to indicate support for the ability denote capability of a BGP
speaker to group multiple capability values into one session. When set (value 1) As
this bit
indicates that information can be deduced from Session Id, the BGP speaker supports grouping. An example of
grouping is if a BGP speaker wishes to use one session for AFI/SAFI
values 1/1, 1/2 and 1/4, and another for AFI/SAFI values 2/1, 2/2 and
2/4.

The next of G bit is defined as the Redirect (R) bit. When set, it
indicates that the sender wishes
deprecated - implementations conforming to continue the current BGP session
using a different transport endpoint. This entails the active
speaker dropping the current session and starting a fresh one using
the proposed endpoint; this is detailed in Section 5.1.1.1 below.
When set, the transport endpoint information is encoded in the port
number field final version of
Multisession specification SHOULD NOT rely on value of the capability as detailed below.

The remaining bits are reserved, and should G bit.

Reserved - MUST be set to zero by the
sender and sender, MUST be ignored by the receiver.

If the R bit is set, following the reserved bits is the two-octet TCP
port number to which the passive speaker wishes to redirect the
session.

Following the reserved bits and the transport endpoint information if
present is a receiver

Session Id(entifier) - list of one zero or more Capability capability codes (1 octet
each) defined in BGP. 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 if the R bit is not
set, or minus three if the R bit is set. The capabilities listed
specify which capabilities in the OPEN message comprise the session
identifier. one.
The Multisession capability code itself MUST NOT be listed; if listed
it MUST be ignored upon receipt.

For example, peers wishing to establish sessions based on AFI/SAFI
would exchange the

Empty Session Id list and Session Id containing 1 (one, Multiprotocol Extensions capability code (1)
Extensions) as the only value are considered equal and indicate that
AFI/SAFI list in the list. In this case OPEN message is used to distinguish the Multisession capability would have a
length 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 octets, 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 four octets if redirect none sessions. This is being requested.

4. 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.
subcodes specific to OPEN message.

This specification introduces five three new subcodes: subcodes for OPEN Message
Error subcodes: 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

9 - Grouping Required

10 - Redirecting Now

11 - Redirect Required

The Capability Value Mismatch code MAY be used when an OPEN message
received contains one or more capabilities whose values are
inconsistent with the corresponding capabilities of the local BGP
speaker. The Data field MUST list the offending capability code(s).

The Grouping Conflict code MAY be codes used when an OPEN message contains
one or more capabilities whose values conflict with the values of one
or more capability groups configured on the local BGP speaker. The
Data field MUST indicate one in
Session Id of the conflicting locally-configured
capability group, encoded as the appropriate capabilities.

The received message cannot be unambiguously
mapped to a locally configured group

9 - Grouping Required code MAY (from earlier drafts, perhaps should be used when a
removed if not used)

BGP speaker that is
configured to require grouping attempts implementations conforming to establish a connection
with a 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 that does not support grouping. (While it is true
that it might be possible conforming to communicate much the same information and actively using the Unsupported Capability NOTIFICATION message, this more
explicit method is felt specification MUST
use modified connection collision handling procedure as described in
this section.

Two sessions are said to be more transparent.)

If collide if and only if both of following
conditions are true:

1: the MP-BGP IP addresses on of peers are the same on both sessions

2: values of capability is codes used as the in session identifier, identifier are either
the
notifications could be used as follows:

Capability Value Mismatch MAY be used when an OPEN message contains
one same or more MP-BGP capabilities, none of which lists an AFI/SAFI
supported by overlapping (regardless fully or partially) within
given capability code

Otherwise two sessions are considered unique and both MAY transition
to the local ESTABLISHED state (subject to rest of BGP speaker. It specification).

Before attempting to create new session local system SHOULD evaluate
existing sessions with the same peer. If there is observed that this subcode
may be useful for MP-BGP speakers already a session
with the same peer in general, even if they do not
(otherwise) implement this specification.

The Grouping Conflict code MAY be used when an OPEN message contains
several MP-BGP capabilities whose AFI/SAFI conflict ESTABLISHED state and new session would collide
with one or more
AFI/SAFI groups configured on 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. The Data field speaker MUST indicate one evaluate existing
sessions with the same peer. If there is already a session in
ESTABLISHED state and multisession distinguisher values of the conflicting locally-configured AFI/SAFI
groups, encoded as MP-BGP capabilities. (One might think of this as
indicating "I'm not willing to combine AFI/SAFI foo old
and bar as you've
tried to do.")

Use of the Redirecting Now new OPEN messages fully match, the old session remains and Redirect Required codes
the new MUST be closed.

If there is detailed a session in
Section 5.1.1.1.

The use of these subcodes is further elaborated below.

5. Overview of Operation

The operation section is divided into OpenConfirm or OpenSent state and two main subsections.

The "Using Multisession" sections below discuss the BGP speaker's
behavior when
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 peer does support other hand two sessions collide according to
definition of this specification or is assumed
to. The "Backward Compatibility" document, then original procedure from section discusses 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's
behavior when the peer does not support speaker SHOULD send NOTIFICATION message as
described in this specification, or document.

7. Connection establishment

When BGP Multisession is
assumed not to. Both sections also discuss how to switch to the
other mode.

A enabled by configuration for given peer and
configuration dictates that multiple sessions can potentially be
established with given peer, BGP speaker that supports this specification MUST always advertise
the Multisession capability, regardless of its peer's known or
presumed capability set.
Capability code in the OPEN message on every session with given peer.
In all other 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 Multisession capability SHOULD NOT be advertised.
The value of Session Id MUST be considered for
making this initial determination -- either the same on every session.

When Multisession-enabled BGP speaker can
initially receives an OPEN message
without BGP Multisession Capability code it MUST assume that the peer does is
not support this specification, capable of multiple sessions and switch modes if it is discovered that it does, or vice-versa.
Either approach is acceptable.

As discussed previously, this MUST use original Connection
Collision Detection procedure as described in section describes the operation from
the point of view 6.8 of the MP-BGP capability 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 associated AFI/
SAFI values as the session. If
received Session Id matches locally configured Session Id then BGP
speaker MUST verify whether this session identifier. It can be replaced would collide with any
other capability or groups of capabilities without any changes to
the
behavior existing as described below.

Note that if a BGP speaker only wishes to support a single AFI/SAFI in its communications with a given peer only one section "Modified Connection Collision
Handling".

If session is needed in
any case, and so allowed to continue by connection collision detection
procedure, the "multisession" feature next step for local speaker is moot. In such a case
the behavior required would be indistinguishable from that given to find matching group
as follow:

1. If BGP capability code values used in Session Id of the "backward compatibility" section below. In the illustrative
examples received
message match exactly (i.e. for every value in the following sections, it received OPEN
message there is generally assumed that a
BGP speaker does wish to support multiple AFI/SAFI corresponding value in its
communications with a given peer.

5.1. Using Multisession

The following subsections discuss a BGP speaker's behavior towards a
peer that is known or assumed to support this specification.

5.1.1. Initiating Connections

When a locally configured
group) then local BGP speaker (the "active" speaker) attempts BGP communication proceeds with its peer (the "passive" speaker), it initiates this session

2. If values in the received message do not match any of the locally
configured groups exactly, but there is one connection
per and only one locally
configured group of AFI/SAFI it wishes to support. (This implies such that a new
local TCP port will be allocated for each new connection.) The OPEN
sent on each connection MUST include the Multisession every capability and
one or more MP-BGP capabilities indicating code the AFI/SAFI to be
supported on that session. If a non-trivial group of AFI/SAFI (i.e.,
a
intersection between received and local values is non-empty set,
then this group of two or more) is proposed, selected for continuing the session. Note,
such partial match results in behaviour similar to non-
multisession BGP speaker MUST also set
the G bit of the Multisession capability. Even if a trivial when capability codes overlap partially.
Rationale behind allowing only one group of
AFI/SAFI for partial matching is proposed, the G bit SHOULD
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 set if grouping is
supported. The active required in real-life networks.

3. In all other cases local BGP speaker MUST NOT send NOTIFICATION
message with Error Code set the R bit nor include an
associated TCP port number.

Note that any "group of AFI/SAFI" may be a singleton group, i.e. the
speaker may wish to use a separate BGP connection for each AFI/SAFI.

If the peer also supports this specification 2 (OPEN Message Error) and also wishes Error
Sub-code set to
support the AFI/SAFI in question, 8 (Grouping conflict).

Once local BGP speaker has identified which locally configured group
corresponds to received OPEN message it will respond proceeds with an OPEN that
includes the Multisession capability and session
like it would have been regular non-multisession one, particularly -
the AFI/SAFI included original Finite State Machine applies. BGP speaker is free to
handle such session either in the
active speaker's OPEN. If same process/thread as the active speaker's OPEN included a non-
trivial group of AFI/SAFI one that the peer supports, then the peer's
Multisession capability will have the G bit set.
received OPEN message, or it can hand over connection to another
process/thread. If uses, the peer also supports this specification and wishes to support
some but not all connection handover is local-matter of the AFI/SAFI in question, it will respond with an
OPEN that includes the Multisession capability
BGP implementation and a subset of AFI/
SAFI included in the active speaker's OPEN. The reason for listing
only a subset may be because some of the AFI/SAFI are simply not
supported, or because the peer does not wish to support the AFI/SAFI
as a group (i.e. it may part of this specification. Appendix
contains an example how such handover could be configured done.

8. Graceful restart

With respect to use Section 4.2 of BGP Graceful Restart [RFC4724], when
determining whether a smaller group). In
this case, the new connection BGP speaker MAY consider the set evaluate values of AFI/SAFI that were
not included
all capability codes used in the peer's OPEN to form a new group, and MAY try to
initiate a new Session Identifier.

9. Error handling

If multisession-enabled BGP speaker detects an error condition that
warrants session reset, it SHOULD reset only session using that group.

If was
affected by the error. Resetting other sessions with the same peer also
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 this specification but does not support
grouping, and a non-trivial group of AFI/SAFI has been proposed, 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 respond as given be carried in the previous paragraph but with the
additional proviso that the G bit one session, and
which address families will be clear. In this case, the
BGP speaker MAY accept the connection as given carried in the previous
paragraph, or it MAY reply with a another session.

BGP implementation supporting multisession extension SHOULD allow
operator to view state of each individual group and at least last
NOTIFICATION message with ERROR
Code OPEN Message Error and Error Subcode Grouping Required, and the that caused connection will be closed.

If the peer wishes to continue reset.

For the sake of interoperability between BGP connection 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 a different
transport endpoint, in addition to responding as detailed above, it
will set the R bit and will include the TCP port number that should
be used to continue the connection. See Section 5.1.1.1 for details
regarding how
attribute. Let's consider this is handled. on an example. If the peer does not wish implementation A
requires AFI/SAFI 1/1 and 1/4 to support the be always carried within same
session, while implementation B requires AFI/SAFI in question, it
will reply with a NOTIFICATION message 1/4 to be always
carried only with Error Code OPEN Message
Error, and Error Subcode Capability Value Mismatch, 1/128 and the
connection will be closed.

A not with any other, then it's not
possible to establish session between such BGP speaker MUST NOT attempt speakers. However if
implementations A and B both allow each AFI/SAFI to initiate connections be carried each
in its own group, then we can establish three sessions - one for any AFI/
SAFI 1/1, another one for which AFI/SAFI 1/4 and third one for AFI/SAFI
1/128.

11. Backward Compatibility

This subsection discusses a BGP speaker's behavior towards a connection already exists.

If the peer does
that is known or assumed not to support this specification, it will respond with
an OPEN that does not include the Multisession capability. specification. In this
case the connection SHOULD be terminated, and future connections to
short, the BGP speaker's behavior towards such a peer should be attempted in as
otherwise defined for the "backward compatibility" mode
discussed in Section 6.

5.1.1.1. Continuing a Redirected Connection

When BGP protocol, according to [RFC4271] and
any other extension supported by the active speaker BGP speaker.

If a BGP speaker receives an OPEN from message that doesn't include
Multisession Capability and local BGP speaker is required to use
multisession (e.g. through configuration by operator), the passive local BGP
speaker
that includes transport redirect information, it MUST reply with an
Open Message Error drop the session and send appropriate NOTIFICATION
message as described in Section 5. If multisession is not required,
local BGP speaker proceeds with its subcode set to Redirecting
Now and close the session. Subsequently, multisession extension disabled, so
it MUST attempt appears as regular implementation to initiate
a new session using the transport endpoint that peer.

As previously mentioned, the passive BGP speaker
has proposed SHOULD always advertise the
Multisession capability in lieu 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 original one (which typically would have
been the well-known scope of this document.

12. State Machine

This specification does not modify BGP port, 179). The new session should proceed
exactly FSM as the such, but all
references to execution of collision handling procedure of original one did; that is, the active speaker SHOULD
send an OPEN
BGP specification are replaced with the same content, and can expect call to receive from
the passive speaker an OPEN with the same content collision handling
procedure described in this document.

The specific state machine modifications to [RFC4271] Section 8.2.2
are as previously with
the exception that the R bit should be clear and no associated port
number should be present. If the R bit is not clear it (and the
accompanying port number) SHOULD be disregarded. follows.

13. Discussion

Note that although the OPEN messages exchanged many BGP implementations already permit multiple sessions
to be used between a given pair of routers, typically by configuring
multiple IP addresses on the reinitiated each router and configuring each session can be expected to
be the same as or similar bound to those from
the previous session as discussed above, an implementation MUST NOT
rely on or enforce this assumption when handling the received OPEN. a different IP address. The new session MUST be handled as any other new session would be in principal contribution of
this respect.

As discussed above, when the passive speaker requests a redirect, the
active speaker specification is expected to drop the current session and initiate a
new one. If it does not do so, the passive speaker MAY elect allow multiple sessions to
continue the session, be created
automatically, without additional configuration overhead or it MAY elect to terminate address
consumption.

The specification supports the simple case of one capability being
used as the session by
sending a Redirect Required NOTIFICATION.

5.1.2. Accepting Connections

When processing a identifier and one connection attempt, the BGP speaker MUST wait until
the peer's OPEN message has been received before proceeding. This is
at variance per session
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 behavior specified in the finite state machine
(FSM) context of [RFC4271], but is interoperable with that FSM. The FSM
changes are specified in Section 7.

Once MP-BGP based connections, which we believe may be
the peer's OPEN message has been received, if most prevalent use of this specification, it includes the
Multisession capability and permits supporting
one or more MP-BGP capabilities
indicating a group AFI/SAFI per connection, and also permits arbitrary grouping of
AFI/SAFI that the onto BGP speaker wishes connections. For such grouping to
support, then the BGP speaker responds with an OPEN message that
includes 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 Multisession capability and one connections may not establish, or more MP-BGP
capabilities indicating connections
may be established than were expected (in the same AFI/SAFI. 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
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]
Chen, E. and S. Ramachandra, "Dynamic Capability for
BGP-4", draft-ietf-idr-dynamic-cap-12 (work in progress),
October 2010.

[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-
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 includes the Multisession capability and one or more MP-
BGP capabilities indicating a group of message lists both AFI/SAFI and it
knows that conflicts with
an AFI/SAFI grouping it wants to split them into different sessions, therefore
it's obvious that has been configured on setup cannot function as intended. Since
separation of two address families into two groups is performed by
operator (as per Multisession Extension specification), the BGP speaker then 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 MAY reply 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 listing a set of
message for either AFI/SAFI that
intersect with those proposed by the peer (in effect overriding the
locally configured set) or first. Assuming Speaker A is not
multisession-enabled, it MAY close the connection 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 OPEN Message 6 as
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 Grouping Conflict. The former behavior 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 suggested 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 the
default if grouping described in previous subsection. Now suppose
Speaker B is supported.

If the BGP speaker configured with additional session to carry IPv4 L3VPN
prefixes. Since Speaker A does not support AFI/SAFI grouping have multiple sessions
configured, it MAY reply
with an won't send another OPEN listing one 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 the 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 out

This is most common use of those proposed multisession extension is to separate
prefixes based on AFI/SAFI. Note that use of AFI/SAFI based groups
is denoted by the
peer. It MUST also set the G bit empty Optional Data field in the Multisession capability to
zero.

If the passive speaker wishes to continue the session for this
particular grouping on a different port number, it sets Capability,
which is the R bit same as in
its OPEN and includes previous two sections. Grouping
configuration is devised from the TCP port number on which it list of actually advertised AFI/
SAFI lists (MP-BGP Capability). This will continue
the session. The passive speaker MUST be prepared to accept a
connection on the given port immediately demonstrated in
following transmission of 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 OPEN.

If the received own session. We start with no existing sessions between
these speakers. Speaker A (though roles can reverse) sends OPEN
message does not include any MP-BGP capability
indicating an AFI/SAFI the BGP speaker wishes to support, in which among other capabilities it SHOULD
close the connection announces MP-BGP
Capability for AFI=1 SAFI=1 and Multisession Capability with a NOTIFICATION empty
optional data field. Speaker B upon receipt of such message finds
that it expects to exchange IPv4 unicast with Error Code OPEN
Message Error Speaker B in a
dedicated session. It accepts connection and Error Subcode Capability Value Mismatch.

If the received sends similar OPEN
message does not include the Multisession
capability, then the peer does to Speaker A. As there were no existing sessions, collision
handling procedure is not support invoked at this specification. The
connection MAY be continued in the "backward compatibility" mode
discussed in Section 6, or time. Next Speaker A (but
again it MAY could be terminated Speaker B) starts new TCP connection to Speaker B
and future
connections 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 peer attempted in the "backward compatibility"
mode.

5.1.3. Collision Detection, Graceful Restart

[RFC4271] Section 6.8 (BGP connection proposal it executes collision detection) considers
a pair detection procedure. Since AFI/
SAFI lists of connections to have collided if the source old (ESTABLISHED) and destination
IP addresses of both connections match. With respect 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 peers ESTABLISHED
state. In the same way third session, for AFI=1 SAFI=128, is brought
up.

Note that
support this 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 groups associated with the
connections must also intersect lists). If Speaker A opens TCP connection and
sends an OPEN message for them to be considered 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 have
collided. coexist without affecting each
other. This consideration also behaviour applies also to Section 4.2 of BGP Graceful
Restart [RFC4724], when determining whether a new connection should
be considered equivalent to a reset of a previous TCP session.

6. Backward Compatibility

This subsection discusses a BGP speaker's behavior towards a peer
that more complex cases where
groups include more AFI/SAFI or based on different Capability Codes
all together. For this reason collision handling is known or assumed not discussed in
remaining scenarios.

Now suppose Speaker A configuration is as above, but Speaker B is
configured to support this specification. In
short, combine labelled-unicast and L3VPN prefixes into the BGP speaker's behavior towards such a peer should be
same session. IPv4 session is brought up as
otherwise defined above. Next there are
two possible alternatives. Either Speaker A sends OPEN message for
one of the BGP protocol, according remaining sessions, to [RFC4271] which Speaker B responds with
NOTIFICATION message Error Code 2 and
any other extension supported by Error Subcode 8. Or Speaker B
sends OPEN message for combined session including both of the BGP speaker.

As previously mentioned,
remaining address families, to which Speaker A responds either with
exactly the BGP speaker SHOULD always advertise same NOTIFICATION message. At the
Multisession capability in its OPEN message, even towards "backward
compatibility" peers.

If, end only IPv4 session
remains in opening a BGP connection ESTABLISHED state, while two other address families
require operator's intervention for configuring either Speaker A with such a peer,
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 OPEN
implementation that
includes the Multisession capability is received from the peer, requires that labelled-unicast and L3VPN address-
families are combined into single session, then
the peer SHOULD behaviour of each
side would be changed exactly as above.

If Speaker A wouldn't have L3VPN configuration for Speaker B at all,
then whether second session would progress to "multisession" mode. How this is done ESTABLISHED or not
depends on whether the configuration of either side requires exact match
between groups (by default implementations expected to mimim original
BGP speaker has 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 sent an OPEN or not --

If the BGP speaker has not yet sent an OPEN to the peer, then the
connection MAY be continued described in the "multisession" mode discussed
above, or it MAY be terminated and future connections previous
case. Next let Speaker A to be the peer
attempted in "multisession" mode.

If the BGP speaker has sent an OPEN first to the peer, then the current setup second TCP session SHOULD be terminated
and future connections to the peer
attempted in "multisession" mode.

Use of techniques such as dynamic capabilities
[I-D.ietf-idr-dynamic-cap] send OPEN message for on-the-fly switching group 2. Applying collision handling
procedure as defined in Multisession specification Speaker B
continues processing of session modes received OPEN message. If Speaker B is beyond
configured for strict match between the scope groups, then it will detect
incompatibility of this document.

7. State Machine

As mentioned under "accepting connections" above, this specification
modifies 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 finite state machine, albeit implementation would in a backward-
compatible fashion.

In addition, note that one state machine is considered to exist for
each absence of the connections that may exist to a given peer. This implies
that, for example, any session flap dampening multisession), it will
reply with OPEN message that may exist is
performed per lists AFI=1 SAFI=4 and session identifier.

The specific state machine modifications
potentially progresses to [RFC4271] Section 8.2.2
are as follows.

7.1. Modifications ESTABLISHED state provided that Speaker A
doesn't require exact match on AFI/SAFI list. Similar applies to Connect State and Active State

In the actions in response to
session 3 for the events Open Delay timer expires
[Event 12] and TCP connection succeeds [Event 16 remaining AFI/SAFI. Note that configuration for
exact or Event 17], an
OPEN 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 not sent and the state changes to WaitForOpen and not to
OpenSent.

7.2. Addition of WaitForOpen State, Deletion most
comprehensive scenario, the behaviour of OpenSent State

The WaitForOpen state the BGP speakers is
essentially the same as in all respects to OpenSent, except
for the action in response to reception case of a valid OPEN message
[Event 19]. In that event, the local system sends an OPEN message
prior to sending a KEEPALIVE message.

The OpenSent state is deleted. All references AFI/SAFI based groups. However
arbitrary groups do add extra complexity because BGP speakers need to OpenSent are
replaced by references
consider not only values of single capability, but need to WaitForOpen.

8. Discussion

Note agree upon
Capability Codes that many BGP implementations already permit multiple sessions
to be used between a given pair constitute Session Id. Following example
demonstrates behaviour of routers, typically by configuring
multiple IP addresses multisession-enabled BGP speakers in
situation where Session Id on each router and configuring each session to
be bound to a side is based on different IP address. The principal contribution of
this specification
capabilities.

Let's suppose there is imaginery Capability Code X that denotes
Experiment Id, and two speakers would like 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 exchange IPv4 unicast
and one connection per session
identifier value. It also permits connections be established L3VPN prefixes for two experiments. Speaker A would like to
group prefixes into separate sessions based solely on multiple capabilities as a session identifier Experiment Id
(so two sessions 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 two AFI/SAFI in each), while Speaker B would
like to agree on
what have separate session per experiment per AFI/SAFI groupings (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 used. If conflicting groupings are
configured, non-empty. Multisession Capability
sent by Speaker A will contain only 'Experiment Id Capability Code'
in the connections may not establish, or more connections
may be established than were expected (in 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 degenerate case, one
connection per AFI/SAFI could peer, it will notice mismatch
between content of the Optional Data field and, since sessions cannot
be established despite configured
groupings). We observe that the potential for misbehavior in as intended, the
presence of conflicting configuration is not unusual in BGP, speaker will send NOTIFICATION
message with Error Code 2 and that
support for, 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 grouping is purely optional.

9. Security Considerations

The ability to redirect to a port other than the well-known BGP port
implies either
side changes.

Note that despite Multisession Capability does not containing a legitimate BGP session may exist for which neither
port is equal field
to 179. This may have implications denote support for firewall
filters used to protect the control processor.

In other respects, this document non-AFI/SAFI based groups, even an
implementation that does not change the BGP security
model.

10. Acknowledgements

The authors would like support groups based on arbitrary
capability codes will be able to thank Pedro Marques, Keyur Patel, Robert
Raszuk, Yakov Rekhter recognise configuration mismatch and David Ward for their valuable comments.

11. IANA Considerations

IANA has allocated BGP Capability Code 68
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
Capability.

This document requests IANA
implementations can leverege IPC facilities provided by host
operating systems to allocate five new OPEN Message Error
subcodes:

7 - Capability Value Mismatch

8 - Grouping Conflict

9 - Grouping Required

10 - Redirecting Now

11 - Redirect Required

12. References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs handover arbitrary session 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 appropriate
process. For example, many systems can pass connection from the
process that accepted TCP connection to a process dedicated for BGP-4", RFC 4760,
January 2007.

[RFC5492] Scudder, J.
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 R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, February 2009.

12.2. Informative References

[I-D.ietf-idr-dynamic-cap]
Chen, E. 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 S. Ramachandra, "Dynamic Capability for
BGP-4", draft-ietf-idr-dynamic-cap-10 (work in progress),
January 2010.

[RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC 2918,
September 2000. does not require specific aid from
BGP at protocol specification level. Therefore process level
separation is not part of multisession specification.

Authors' Addresses

John G. Scudder
Juniper Networks

Email: jgs@juniper.net

Chandra Appanna
Cisco Systems

Email: achandra@cisco.com

Ilya Varlashkin
Easynet Global Services

Email: ilya.varlashkin@easynet.net