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Active BGP Probing and large AS-sets

  • From: Lorenzo Colitti
  • Date: Thu Jun 09 09:55:00 2005


our announcement on nanog a few months ago of experiments involving BGP updates containing large AS-sets [1] caused a few flames. Now we have an in-depth document with results on the subject and would like to explain what we intended to do.

We have presented our techniques at RIPE 50, and there is a Web page on the project, with links to the full report and the slides, up at:

What follows is a brief summary of what we do, why we believe it is safe, and why it shouldn't cause problems to current operational practices. For more details, see the presentation and the report.

We will be happy to answer any further questions you might have.

Lorenzo Colitti


Active BGP Probing

What we do

Existing topology discovery techniques are good at discovering topology but bad at discovering policy. However, to predict the effect of network faults, perform effective traffic engineering, develop peering strategies, and evaluate the quality of upstreams, it would be useful for ISPs to to know how their announcements can be propagated and how the policies of other ASes in the Internet affect their prefixes. No operational tools exist to do this.

The principle is the following: an AS using our probing techniques can announce one of its prefixes with AS-paths including the numbers of other ASes. Due to loop detection, these "prohibited" ASes will not use or propagate the announcement. To avoid influencing AS-path length, the prohibited ASes are placed in an AS-set at the end of the path.

Thus, to stop its announcement from being propagated by ASes 1, 2, and 3, an AS (say AS12654) might announce one of its prefixes with an AS-path of 12654 {1,2,3}. This allows AS 12654 to discover who propagates its announcements, find backup paths, and deduce the policies of other ASes with respect to its prefixes.

To collect data it is possible to the RIS or ORV route collectors. However, since our methods operate in steady state, the results are visible from any looking glass on the Internet.

Why it is safe

We are confident that such announcements are safe, provided that the length of the AS-set announced is limited. We say this based on:

1. Equipment tests. Juniper routers seem to have no problems with long AS-paths, while Cisco routers reset the BGP session when the AS-path more than 512 bytes long (about 254 ASes). Cisco is aware of this issue and is in the process of fixing it; in any case, our techniques limit the length of the AS-sets announced to a suitable number to account for propagation (obviously, much smaller than 254).

2. IPv6 tests. We started testing our techniques on the IPv6 backbone in November. No problems were reported. The first report (of only two) we got of someone even noticing the unusually long paths arrived at the end of February.

3. Observation. Longer AS-sets (123 [2] and 124 [3] elements) and longer AS-paths have been observed in the wild with no ill effects that we know of -- at least, we know of no operators who reported problems.

Why it doesn't impact routers

Route flap dampening limits our probing to the order of one update per hour. This is negligible compared to the over 15,000 updates/hour a typical Tier-1 router might receive. As regards impact on memory, the amount of RAM to store a 100-element AS-set for one prefix is of the order of hundreds of bytes, which is irrelevant for core routers which are already using tens of megabytes of memory for BGP.

Why it doesn't hamper debugging

Prepending other AS numbers is to a certain extent already done today. Our techniques are similar, but foreign ASes are only in the AS-set at the end of the path, so it's immediately obvious which path the announcement has taken. Due to the size of the AS-set, we doubt that anyone seeing such an announcement would believe it was due to one of the ASes in the set, but would probably look at the first AS before the set. Furthermore, the prefix immediately identifies the source of the announcements. Finally, the routes can also be tagged with communities to help identify them.


RIPE NCC Roma Tre Computer Networks research group
[email protected] [email protected]