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SubjectRe: PPP slow between Linux and Solaris - why is it so?

Dave Cole <> writes:
>I have been monitoring this list for quite a while now and I remember a
>discussion of the problem with TCP over PPP being slow between Linux and
>I remember someone mentioned that Solaris was retransmitting packets too
>quickly or something causing >50% of bandwidth to be used up by duplicate
>packets. I got the impression that Solaris was to blame. Is this
>correct? In any case, I do not remember anyone ever mentioning a fix /
>trick / work around to get the throughput back up to what it should be.
>This subject appears from time to time on comp.os.linux.networking, but I
>am still in the dark, and out of pocket with my ISP :-(

This is indeed a rather well known Solaris problem. This caused me no
end of trouble when I was trying to figure out what was wrong with the
Linux TCP stack. :-) The only thing you can do on your end is to reduce
your MTU. I find that an MTU of 296 is necessary with a 14.4k modem line,
but 576 can be used with a 28.8k modem without much trouble. With these
settings I get close to 90% utilization over short (100k) uncompressible
transmissions from a Solaris host, and slightly better over larger
transmissions. If your modem is slower than 14.4k there's not much
you can do without choosing really tiny MTU values.

On the Solaris end of things you can encourage sites you
access that use Solaris to upgrade their networking. Sun has
announced a set of patches that fix the problem. I'm not entirely
sure that the patches are actually available yet but I've been
lead to believe that they should be. I have a copy of a posting
from Sun announcing these fixes if anyone wants specific information
on what Sun patch numbers are needed.

-- eric

Eric Schenk www:
Department of Computer Science email:
University of Toronto

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