Discovering AS Path confederation...

Steve Schaefer schaefer at
Wed Jul 25 21:43:40 UTC 2001

On Wed, 25 Jul 2001, Chris Rapier wrote:


> I'm plotting observed traffic against BGP reported AS paths to get a
> logical view of traffic flow in a smallish network (I2). However, it was
> pointed out to me (by Bradley Huffaker) that the BGP reported path does
> not always correspond to the ASes a packet actually traverses.

Propagation of a route in BGP requires that the network actually use that
route.  On the surface, this would seem to ensure that the actual path
must match the BGP path.  This assumes, however, that the packet follows
the same route from end to end.

If there is any route filtering, the packet may follow a short prefix
until it reaches a network which has a longer prefix in its routing table.
In that case, the packet will then follow the longer prefix.  Someone
looking at the BGP tables from the source would only see the route for the
short prefix and may infer the wrong AS path.


> My contention is that the vast majority of this seeming divergence is
> the result of AS confederation and not any specific failure of the
> protocol (or implementation) as a whole.


I haven't thought through this one.  Can you enlighten me offline?

> What I was thinking of doing was to collect a large number of traces and
> their corresponding traversed ASes and run statistical analysis on it to
> infer the confederations.


Contact me offline.  We may be able to help configure and run some tests.

Steve Schaefer

Dashbit - The Leader In Internet Topology

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