do not filter your customers

Dongting Yu dongting.yu at cl.cam.ac.uk
Sat Feb 25 16:39:15 CST 2012


Let me chime in and attempt to explain why a couple of solutions I've
seen so far in this thread won't work:

- rate-limiting/throttling updates: BGP by protocol does not repeat
updates; if an update is sent then the sender assumes that the
receiver has received it and will remember it until a change or a
withdrawal. If you rate limit announcements, either you hold things
off in a buffer, which would need a very large buffer, or you drop
updates, which would lead to inconsistent views on the two sides of
the session. What if a legitimate update was among the large burst?

- max-prefix: it is currently used to prevent large bursts of updates
but it won't stop Youtube incident, which was more targeted. Perhaps
the YT incident falls into a different category from 'route leaks' but
without a clear definition of the latter we simply cannot say. Also,
max-prefix causes problems in slowly-increasing peers or peers with
new large customers and people not bothered to adjust the max-prefix
value accordingly.

- max-prefix in the form of a percentage: some peers actually are very
stable in the number of prefixes they announce, and some are not. Both
are probably valid depending on your business model/requirements. A x%
may be too lax for one company but too little for another. Figuring
the right number (or even a ballpark) is probably a lot harder than a
simple max-prefix value. I have seen ASes that announce hundreds to
tens of thousands of prefixes on a periodic basis. Percentages also
don't work so well for ASes with single-digit or low-double-digit
number of of prefixes.


Dongting



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