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

Thu Mar 3 23:09:44 UTC 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