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Re: Heads up: Long AS-sets announced in the next few days

  • From: Lorenzo Colitti
  • Date: Thu Mar 03 18:10:14 2005

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




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