BGP unsupported capability code

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1

If A tries to peer with B, and B sends a BGP capability 64 to A, if A does not support that capability what would proper and/or reasonable behavior for A be?

(a "published" source for it, if you could possibly do so.)

a) send

unsupported capability code 64 lengh 6
## 2006-08-17 19:17:05 : [bgp/stack]: send NOTIFICATION msg code (Open-Error)
subcode(Open Message Error: Unsupported Capability) to peer w9.x4.y7.z9 via socket 415

b) ignore it

c) admin defined

rfc3392.txt only appears to address the case where if A tries to peer with B, and B sends a BGP capability 64 to A, if A does not support that capability, B MAY terminate the connection with the above notification.

(section 3 paragraph 5)

Thanks,

Joe

2

If A tries to peer with B, and B sends a BGP capability 64 to A, if A does not support that capability what would proper and/or reasonable behavior for A be?

Speaker A MAY send a NOTIFICATION message with Open
Message Error/Error Subcode "Unsupported Capability" and
a data field listing capability 64 as the trigger for the
NOTIFICATION to speaker B (thereby terminating the session).
However, RFC 3392 does NOT require that the local BGP
speaker terminate the connection, which has particular utility
when considering the implications such a hard requirement
may otherwise impose on functions such as dynamic BGP
capabilities.

(a "published" source for it, if you could possibly do so.)

a) send

unsupported capability code 64 lengh 6
## 2006-08-17 19:17:05 : [bgp/stack]: send NOTIFICATION msg code (Open-Error)
subcode(Open Message Error: Unsupported Capability) to peer w9.x4.y7.z9 via socket 415

b) ignore it

c) admin defined

rfc3392.txt only appears to address the case where if A tries to peer with B, and B sends a BGP capability 64 to A, if A does not support that capability, B MAY terminate the connection with the above notification.

(section 3 paragraph 5)

If the peer doesn't support the capability and this is conveyed
from A to B via a NOTIFICATION message (as illustrated in the
log message above), then given the scenario you provide B
SHOULD NOT include the capability (64) in any subsequent
OPEN message sent to A when attempting to reestablish a
BGP connection -- IF B so chooses to attempt to automatically
reestablish a connection. Note that the above says SHOULD
NOT, not MUST NOT.

I'm not quite sure what your point above is, as I think the
document sufficiently outlines the required behavior.

Although BGP is perhaps (?) still of interest to the NANOG
community, I suspect such protocol intricacies are not and
would submit that queries of nature are best directed to
idr@ietf.org.

-danny

3

Danny McPherson wrote:

If A tries to peer with B, and B sends a BGP capability 64 to A, if A does not support that capability what would proper and/or reasonable behavior for A be?

Speaker A MAY send a NOTIFICATION message with Open
Message Error/Error Subcode "Unsupported Capability" and
a data field listing capability 64 as the trigger for the
NOTIFICATION to speaker B (thereby terminating the session).
However, RFC 3392 does NOT require that the local BGP
speaker terminate the connection, which has particular utility
when considering the implications such a hard requirement
may otherwise impose on functions such as dynamic BGP
capabilities.

Unless there is actual utility gained by B learning the hard way that A doesnt support this capability, rather than by not receiving the same capability in A's OPEN, I would not consider this behavior reasonable.

And rfc3392 specificaly says

    A BGP speaker determines the capabilities supported by its peer by
    examining the list of capabilities present in the Capabilities
    Optional Parameter carried by the OPEN message that the speaker
    receives from the peer.

Not by hit or miss with NOTIFICATION messages.

And as I read it, rfc3392 makes absolutely no mention of my case.

(section 3 paragraph 5)

If the peer doesn't support the capability and this is conveyed
from A to B via a NOTIFICATION message (as illustrated in the
log message above), then given the scenario you provide B
SHOULD NOT include the capability (64) in any subsequent
OPEN message sent to A when attempting to reestablish a
BGP connection -- IF B so chooses to attempt to automatically
reestablish a connection. Note that the above says SHOULD
NOT, not MUST NOT.

I'm not quite sure what your point above is, as I think the
document sufficiently outlines the required behavior.

Which document is this? Are you quoting?

In any event the above is not observed behavior either, as the 500 log messages on the peer's (the one sending capability 64) support desk attest to.

Does your paragraph also suggest that B SHOULD NOT send the capability when A automaticaly tries to reconnect?

Should A (the peer that sent the notification upon receipt of capability 64) NOT try to automatically reconnect?

Although BGP is perhaps (?) still of interest to the NANOG
community, I suspect such protocol intricacies are not and
would submit that queries of nature are best directed to
idr@ietf.org.

This isnt an intellectual excercise, its something that operationaly affects me. Perhaps it has, is, or will affect any of the operators who subscribe to this list.

Since I may have to go to bat against the vendor on this one, I thought I would obtain some operator opinion beforehand.

I hope that satsifies the on-topic police.

-danny

Thanks for your reply.

4

I can't see the on-topic police having any problems with this, unless
"network operations" have degraded to a point marginally above machine-level
automation. If it has, what is NANOG's purpose?

As one who has both created a BGP implementation from scratch and have
operated many, I think I can help you.

It's easy to confuse the announcement of a particular capability with
announcement of capabilities altogether. RFC3392 is clear enough, but
doesn't underline the point, and also doesn't mention any administrative
aspects.

The idea is that if a speaker supports capabilities negotiation, it
will include a BGP optional parameter in its OPEN message, listing the
capabilities it supports [Sec 3 Para 1]. The receiving speaker may
or may not support capability negotiation; if it doesn't it has no
choice except to send an error notification with error code "unsupported
optional parameter", and the originator should restart but not send any
capabilities parameter [Sec 3 Para 4]. (If that is actually acceptable
is partly an administrative issue, but implementations vary.)

Assuming both peers support capability announcements, they would each
inspect the list of capabilities supported by the other [Sec 3 Para 2],
and determine if this satisfies their needs [Sec 3 Para 3,5]. This is
a partly administrative issue, and implementations have, well, varied.
If one peer strictly requires a particular capability that the other
doesn't support, it will send an error notification, terminate the
session, and not restart [Sec 3 Para 5]. The idea here is that the
problem is beyond protocol level negotiation (ie, it isn't eg just an
optimization), and requires human intervention.

Otherwise, assuming no strictly required capabilities are missing
on either side, the session goes ahead on each side, with each peer
respecting the capabilities it understands the other peer supports.

The whole capability thing is very much an afterthought to BGP; proper
negotiation could well require several transactions, but that would
require obvious changes to the BGP protocol itself. Consequently,
actual support and behaviour for capabilities has seen some variation
between implementations and also between different versions and releases.
Things should have been reasonably settled by now, though, so I suspect
one of your peers is running old software.

As for the capability you mention (code 64), this is Graceful Restart, see

http://www.iana.org/assignments/capability-codes

Graceful Restart is an optimization, and nothing prevents peering
without it. Hence, you should not see repeated terminations and restarts,
the session should simply go ahead without using Graceful Restart.

Best,

  -- Per