Re: 2.0.21: TCP hangs solved by disabling Path MTU Discovery

Eric Schenk (
Wed, 25 Sep 1996 21:26:46 -0400

"Derrick J. Brashear" <> writes:
>I put back a piece of code that had been if 0'd out around 2.0.16 to get this
>to work with the path mtu discovery, but when I posted the patch as a possible
>fix for the PPP problems I was having, someone else posted that the patch was
>in there because it was making something break on his machine. Who knows?
>Note to sparclinux people: Perhaps path MTU should default to disabled? (I
>recall that it doesn't, but my SparcLinux box is, um, not here at the moment)

That was the code to keep the don't fragment bit on for ACKs during
path MTU discovery. Without this VJ compression doesn't work as well
as it should, hence the SparcLinux complaints about PPP speed?

In any case, when we put in code to do that bit, we got some reports
that networking broke badly for some people. Looking through my
archives now, however, I can only find one such report, and I have
a later confirmation that turning off VJ compression fixed the problem.
This would point to a bug in someones VJ compression code, perhaps
in our own.

So, I _think_ that we may have been a bit to hasty in backing out
that change. Can anyone who did have problems due to this change
please get in touch with me so I can track down exactly under which
conditions it caused trouble? We really should put that change back,
but if we do I need to fix any other problems uncovered by that change.


-- eric

P.S. I'm not on the sparclinux list, so could someone either point
me at an archive for that list, or give me some idea of what kind
of discussion has occured about this problem on that list?
Also, please be sure to cc: replies to either both lists, or at
least to me so I can keep track of what we know about this problem.

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