Who has AS 1712?

Joe Abley jabley at hopcount.ca
Wed Nov 25 06:34:30 UTC 2009

On 2009-11-24, at 20:58, Randy Bush wrote:

>> Right. You can't advertise an ASN
>> you can only advertise a route and include an AS_PATH attribute on it
>> which makes mention of a particular AS number.
> that bit of biff-like pedantry quickly leads to you can't advertise a
> prefix.

Apologies if the pedantry seems unnecessary. I think the parallel between the announcement of a route (which has inherent reachability information contained within it) and use of an ASN in an AS_PATH attribute (which doesn't always) are different with respect to identifying use of a resource.

Overloading "advertise" for both suggests you can identify use of a resource elsewhere using the same measurement technique, which I think is broken logic.

> a bgp announcement has, in the case of ip unicast, an nlri and,
> among other things, an as-path.  see rfc 1771 4.3 on Path Attributes.

I used the word "route" in the sense that it's defined in 4271.

> as to what is being announced and what is merely loitering waiting for a
> hot pick-up, you can work that out with your mullah, priest, rabbi,
> spouse, ...

As a divorced atheist I guess I'll just read RFCs :-)

> for unusual utility of intentionally announcing a particular asn, see
> as-path poisoning, e.g. lorenzo's thesis [0], the talk which was banned
> at nanog [1], or the full paper [2].

Josh and I also talked about it at NANOG 24. I remember using it to poison routes advertised through certain edges of AS 1221 a decade ago after the idea was suggested to me by Geoff Huston, and I'm sure it was probably old news then.

>> My point is that in the absence of any mechanism for announcing an
>> ASN, a plan to gate assignment of numbers based on an announcement
>> doesn't make any sense.
> seeing if an asn is in a currently-announced as-path is useful, as has
> been pointed out in this discussion.

I don't think it's as simple as people have suggested.

The fact that nobody has ever seen a particular number present in an AS_PATH attribute might mean that the ASN has never been configured on a router, or it might mean that nobody has ever taken a measurement from a router who has seen such a route.

The fact that someone has seen a particular number present in an AS_PATH attribute might mean that that number has been used for a particular autonomous system, or it might mean that someone is doing something (intentional or otherwise) with AS_PATHs for their own personal reasons.

The topic of this thread is really concerned with database hygiene in a distributed system which, as you have pointed out repeatedly, lacks procedural or mathematical rigour. Checking whether or not a particular AS_PATH regex matches anything in one or more RIBs might tell you something, or it might give you clues as to who to call to find out more, but it can never tell you anything definitively. Definitive knowledge sure seems like it's what you want if your job is to guarantee uniqueness.

It seems to me that at some point we need to stop trying to put dresses on the pig.


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