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Linux: How to find the maximun Segment Size (MSS) and how is it related to MTU

To see the MSS and MTU values, get an utility called iperf. In Fedora, type 'yum install iperf' (logged on as root). After installing it, you can run it to check the network performance. This is an example output:

[root@LinuxSrv psybnc]# iperf -m -c 192.168.0.100
connect failed: Connection refused
write1 failed: Broken pipe
write2 failed: Broken pipe
------------------------------------------------------------
Client connecting to 192.168.0.100, TCP port 5001
TCP window size: 16.0 KByte (default)
------------------------------------------------------------
[ 3] local 0.0.0.0 port 56715 connected with 192.168.0.100 port 5001
[ 3] 0.0- 0.0 sec 0.00 Bytes 0.00 bits/sec
WARNING: Path MTU Discovery may not be enabled.
[ 3] MSS size 524 bytes (MTU 576 bytes, minimum)


The MTU and MSS are related depending on the packet state.

In worst case when the headers are full (both IP and TCP maximum sized): MSS = MTU - 60 - 60 = MTU - 120 

Best case is when MSS = MTU - 40

Read more on this in RFC 879 - TCP maximum segment size and related topics: http://www.faqs.org/rfcs/rfc879

 




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