TCP fragmentation issue

Ian Caddy ianc at
Tue Jun 1 03:28:25 UTC 2004

Hi All,

Some of our project people have reported an issue with our product 
fragmenting TCP packets destined to another machine when the length of 
the packet is between 100 and 202 bytes in length.

TCP packets of larger sizes are sent through unfragmented.

We are using RTEMS 4.5.0 for our product with 100MBit Ethernet.  The max 
MTU is not set in our stack (set to 0 so defaults to 1500) and it is not 
set to anything other than default on the Windows machine it is talking 
to.  They have tried various different machines to with the same results.

This in itself is not a problem, but a commercial package on the windows 
machine, has a problem if the packet is fragmented which is causing 
issues.  We have other systems running with no problems, so they have 
asked if I can look into it.  They even checked it with another package 
that works, and the checked the raw packet log and saw that packets of 
between 100 and 202 bytes were fragmented, so this particular package is 
not effecting the behaviour.

I had a quick look in the mail archives, and nothing turned up.

Before I start a proper investigation, has anyone seen this sort of 
behaviour before in RTEMS and have an explanation about why it is happening?

I look forward to your responses.


Ian Caddy
Microsol (Aust) Lty Ltd.

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