How to set the maximum TCP mss (Maximum Segment Size) on Linux?

Question

A simple question:

In Linux, how do you set the maximum segment size that is allowed on a TCP connection? I need to set this for an application I did not write (so I cannot use setsockopt to do it). I need to set this ABOVE the mtu in the network stack.

Here's why:

I have two streams sharing the same network connection. One sends small packets periodically, which need absolute minimum latency. The other sends tons of data--I am using SCP to simulate that link.

I have setup traffic control (tc) to give the minimum latency traffic high priority. The problem I am running into, though, is that the TCP packets that are coming down from SCP end up with sizes up to 64K bytes. Yes, these are broken into smaller packets based on mtu, but this unfortunately occurs AFTER tc prioritizes the packets. Thus, my low latency packet gets stuck behind up to 64K bytes of SCP traffic.

This article indicates that on Windows you can set this value: http://compnetworking.about.com/od/networkprotocols/g/mtu-maximum.htm

Is there something on Linux I can set? I've tried ip route and iptables, but these are applied too low in the network stack. I need to limit the TCP packet size before tc, so it can prioritize the high priority packets appropriately.

Thanks for any help!

Eric

Answer 1

The upper bound of the advertised TCP MSS is the MTU of the first hop route. If you're seeing 64k segments, that tends to indicate that the first hop route MTU is excessively large - are you using loopback or something for testing?