Re: Networking slowdowns

Eric Schenk (
Sun, 8 Sep 1996 01:01:33 -0400

Christoph Lameter <> writes:
>I think I found a way to demonstrate networking slowdown.
>Set the MTU/MRU of a PPP Link to 3000. Then telnet into your
>machine and cat a file of 10-20K.
>The session will stall while you still can telnet to the machine in a second
>session working just fine.
>Hope I did not shoot myself in the foot here...

Ok. I tried this here with a test rig. Both ends running 2.0.15.
I see odd "stalling" patterns in the telnet test, in that output
is bursty. This is not entirely unexpected, since a 3000 byte packet
takes a second or so to pass through the link, but much less than
a second to display on the telnet screen.
I do not see a complete "stall", that is, at no time have I been able
to make the traffic stop completely. I have, however, observed pauses
that last a few (4-5) seconds.
Looking at the tcpdump I also see that the system is very rarely
sending a full sized packet, which is quite odd. This means that telnet
is not feeding characters to the PPP link fast enough to prevent
a Nagle timeout, and occasionaly appears to have to characters ready
to send for several seconds.

As a test of this theory I ran FTP sessions across the same link.
All FTP sessions ran a full speed, no stalling, with a sequence of
full sized packets only.

This makes me suspect something in either telnet, or the pty layer.
Further evidence of this: when running a tcpdump in parallel with a telnet
I observe that packets finish crossing the link several seconds before
they get displayed by telnet.

I'd be interested to know if others see similar results with this test.
If so, then perhaps we can narrow the range of inquery into this problem.

-- eric

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