[e2e] a new IRTF group on Transport Models

Michael B Greenwald mbgreen at dsl.cis.upenn.edu
Fri Jun 17 06:29:28 PDT 2005

To answer one specific question Doug raises below: as of three
years ago an appreciable number of flows traveled along paths
where packets were queued at two or more hops.

We studied this issue in a paper (``On the Sensitivity of Network
Simulation to Topology'', K. G.  Anagnostakis, M. Greenwald,
R. Ryger. Proceedings of MASCOTS 2002., Postscript:
ftp://ftp.cis.upenn.edu/pub/mbgreen/papers/mascots02.ps.gz, or
PDF: http://www.cis.upenn.edu/~anagnost/papers/mbottle.pdf) a
couple of years ago. (We did coarse probing of 38k paths, but the
paper focused on ~2k paths).  We were very conservative in
deciding that a path experienced multiple congestion points, so
the number was an underestimate at that time.  Our main question
was whether such multi-congestion-point paths occured in
significant numbers --- the answer was "yes", even our
conservative lower bound represented a measurable fraction of
congested paths.

(Because things have changed in 3 years, and the number was an
underestimate even then, our exact numbers are not germane at
this point [although the curious should feel free to look at the

   Wed, 8 Jun 2005 18:06:59 +0100
   "Douglas Leith" <d.leith at eee.strath.ac.uk>

   Following up on Frank's question, one area where I suspect more data
   would help is in defining  topologies to test TCP performance over. 
   Most work to date has focussed on a dumbell topology.  While this seems
   like a useful starting point, it would be good to have a better 
   understanding of the range of end-to-end topologies experienced by TCP
   flows in practice.  For example, it would be good to know what 
   proportion of flows travel along paths where packets are queued at two
   or more hops (due to cross-traffic etc) and to better understand the 
   character of such paths assuming they exist in appreciable numbers.  
   This seems to require additional measurement information from that which 
   is currently available - probing from the edge alone can probably only 
   yield limited/ambiguous information on what's happening inside the 
   network and so router information might help out a lot.
   -----Original Message-----
   From: end2end-interest-bounces at postel.org on behalf of =
   frank at kastenholz.org
   Sent: Wed 6/8/2005 1:49 PM
   To: end2end-interest at postel.org
   Subject: Re: [e2e] a new IRTF group on Transport Models

   While all this chatter about certain actions TCP can
   or can not take and perfect nets with theorem
   provers in all the routers is as interesting and
   amusing as brain surgery, my original question stands:
        Is there any thought to identifying information
        that routers and end systems might provide that
        either can be fed back into the models to refine
        them or used in parallel to (in)validate them?
   Frank Kastenholz


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