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

Lorenzo Colitti lorenzo at ripe.net
Fri Mar 4 00:50:07 UTC 2005

David Schwartz wrote:
>>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.
> 	So you do not know what affect your announcements will have.

We don't know the effectiveness of the technique. That depends on the 
topology of the Internet, where you run the announcements from, etc. 
etc. We do know the effect that the announcements will have on BGP 
routers, i.e., cause them to ignore the path if they are in the AS-set, 
and accept them otherwise (modulo policy, max aspath length, etc. etc., 
of course).

>>So, given these considerations, is everyone announcing an AS-set
>>announcing "routes that falsely claim to have passed through another
>>autonymous system"?
> 	Yes. From RFC1771:

Ok, so if everyone announcing an AS-set is announcing "routes that 
falsely claim to have passed through another autonymous system", and you 
are saying this shouldn't be done, then why aren't you complaining with 
everyone who is announcing an AS-set?

> [Quote of section 5.1.2 almost in its entirety]
> 	So you are violating RFC1771, plain and simple. To then go and cite one
> small section of RFC1771 in your defense is hypocritical.

You quote Section 5.1.2, but you don't mention that if you follow 
Section 5.1.2 to the letter there is no way that an AS-path may contain 
an AS-set. To summarize the whole of section 5.1.2, the various cases are:

Propagating a route learned from an UPDATE message:

  a) To another router in same AS: don't modify AS-path
  b) To a neighboring AS:
     1. Path starts with AS_SEQUENCE: prepend own ASn
     2. Path starts with AS_SET: prepend new AS_SEQUENCE with own AS in it

Originating a route:

   a) To neighboring AS: announce own ASn as only element in path
   b) To another router in same AS: announce empty AS-path

If you follow this to the letter, you must rule out both prepending "(In 
this case, the AS number of the originating speaker's autonomous system 
will be the only entry in the AS_PATH attribute)" and any form of 
AS-set, since there is no way, following these rules, that an AS-set may 
enter the AS-path in the first place.

If we are violating this section, then everyone else announcing an 
AS-set, and - at least the way I read it - anyone doing prepending, is 
doing so too. But nobody is suggesting that these things shouldn't be done.


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