Re: Multiple TCP/IP servers on one host


Roger Hayes (rogerh@arizona.edu)
14 Sep 88 03:43:48 GMT


In article <957@helios.ee.lbl.gov> dagg@lbl.gov (Darren Griffiths) writes:
>[...] In
>many cases there are different paths to the same host, [...]
>there are two ways of making use of this path.
>
> 1) If the second path was 25% of the speed of the first path then 25% of
> the packets could be sent that way. [...]
> if the two end sides were running the Van Jacobsen/Mike Karels code
> I believe this wouldn't be to much of a problem. [...]
>
> 2) The second thing that can be done is a little nicer. [...]

The first thing (splitting load among routes) would screw up the
Jacobson/Karels improved TCP completely. They get a big win by
estimating the variance of the round trip time; using alternating
routes for different packets would drive this variance way up, causing
the timeout to be set high, causing long stoppages on lost packets.

Further, their code for calculating optimum window size, which is
pretty subtle, tests for increased available bandwidth by increasing
window size. I can imagine that this code would get pretty thoroughly
confused if the period of route-switching was near the window size, as
that would cause successive windows to have wildly varying proportions
of samples from the two routes. The slower route would periodically be
heavily overloaded (when the outstanding packets in the window
happened to mostly be sent via the slow route) while the faster route
would operate below its optimal point.

Seems like the best way to make use of alternate routes is by
implementing the type-of-service stuff.

                                Roger Hayes
                                rogerh@arizona.edu



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