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Re: [RRG] Geoff Huston's article on BGP stability, update statistics and damping

Robin Whittle wrote:

Geoff Huston has a new article "Damping BGP":

  http://www.potaroo.net/ispcol/2007-06/dampbgp.html

He proposes methods of identifying updates which are usually
associated with path hunting.  Typically this is one or multiple
announcements of a longer path for the prefix, followed by the
withdrawal of the prefix.  I understand from this that he
supports "path length damping" as described by Tony Li:

   http://tools.ietf.org/html/draft-li-bgp-stability

How such filtering would work when all routers implement it I am
not sure, but it seems like a good idea.
The withdrawal would propagate, or the path would stabilise. One way to view it is as a more selective form of Min Route Advert Interval Timer. 



I learnt some interesting things, including that BGP routers
often use TCP to throttle the updates from peers to a rate they
can handle.  One reason this is done is to prevent local input
queues from taking up too much memory.


Thank you John Scudder for clearly explaining this to me earlier this year.



Meanwhile, the peers perform "output queue compression".  They
look into the queue of updates which are waiting to be sent and
remove any messages which have been rendered incorrect by events
which transpired since that message was put in the queue.


Thank you John Scudder for clearly explaining this to me earlier this year.



There is also a diagram depicting path hunting, and graphs
depicting the statistics of updates, based on research in April
this year.

  "10% of announced prefixes being responsible for 53% of all
   routing updates, and the busiest 1% of prefixes responsible
   for 24% of the routing updates for the month."

  "It can’t be the case that more than a million routing updates
   actually reflect true underlying changes in topology of the
   network, given that these one million updates only refer to
   2,000 prefixes."

I think the time analysis of the updates would be of interest to
anyone concerned about BGP stability.
I would hope so. A significant proportion of the BGP traffic appears to be concerned with a relatively small number of "pathologies" where BGP itself is the amplifier. 

   Geoff




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