Re: Weird Modem issue

Greg Stark ( )
23 Aug 1998 15:47:00 -0400

"Matthew Andersen" <> writes:

> Where do I configure the MTU on NT4 SP3 with RRAS? And what should I be
> looking at changing it to?

This is a tuning question and it depends a lot on the types of traffic
you actually see. You really have to do some experiments while
watching tcpdump traces to see the effects of various values to judge
what the best value is.

In general lower MTUs should result in lower total bandwidth but lower
latency while the line is under load as well. The default 1500 byte
mss is very high for a 14.4 or 28.8 kbs link. I typically saw latency
on the order of 10s on my 14.4kbs modem while an ftp transfer was

I experimented with values like 296 for mtu and mru which reduced the
latency to about 6-7s but found that with the services I used they
caused more fragments, which meant more retransmits and more
delays. If you use mostly TCP traffic this may not be an issue for
you. I typically use a few UDP services which send payloads slightly
larger than 256 bytes.

I currently have them set at 400 bytes, which still causes some
fragments, but mostly avoids them. Most places will suggest values
like 576, presumably to be big enough for a 512 byte packet.

Note that you have to set both mru and mtu, mru is the value requested
for the peer to set as its mtu. I have no idea where to set these on
NT, I assume they would be in the Control Panel, Network settings
somewhere, or else the DUN settings somewhere.