[78417] in North American Network Operators' Group
Re: Heads up: Long AS-sets announced in the next few days
daemon@ATHENA.MIT.EDU (Lorenzo Colitti)
Thu Mar 3 18:09:40 2005
Date: Fri, 04 Mar 2005 00:09:44 +0100
From: Lorenzo Colitti <lorenzo@ripe.net>
To: davids@webmaster.com
Cc: jabley@isc.org, nanog@merit.edu, compunet@dia.uniroma3.it
In-Reply-To: <MDEHLPKNGKAHNMBLJOLKMEBLCDAB.davids@webmaster.com>
Errors-To: owner-nanog@merit.edu
David Schwartz wrote:
>>Prepending announcements with remote AS numbers has been a well-known
>>technique for preventing prefixes from propagating to particular ASes
>>for a long time.
>
> And therefore such use would not be considered experimental. We are talking
> about experimenting with routes that falsely claim to have passed through
> another autonymous system.
They are experimental in that yes, we are experimenting with a new
technique for topology discovery which to our knowledge has not been
proposed before.
As regards "falsely claim to have passed through an autonomous system",
that is not accurate:
1. RFC 1771, paragraph 5.1.6 says that in the presence of an
ATOMIC_AGGREGATE attribute, "the actual path to destinations, [...] may
traverse ASs that are not listed in the AS_PATH attribute." So an
AS-path does not claim to contain all the ASes that the announcement has
passed through.
2. Given an AS-set such as {1,2}, if you concluded that the announcement
had passed through both AS1 and AS2, you would be wrong (most of the
time, at least). So an AS-path does not claim that all the ASes in the
path are ASes that the announcement has passed through.
So, given these considerations, is everyone announcing an AS-set
announcing "routes that falsely claim to have passed through another
autonymous system"?
> Every piece of BGP documentation I have ever seen says that this attribute
> documents the ASes that the route has actually passed through.
I think the above paragraph of RFC 1771 disagrees with you.
Regards,
Lorenzo
