IP Fragments - Worth doing, testing strategies?

Sean Conner spc at conman.org
Thu Sep 30 15:33:58 CDT 2010


It was thus said that the Great Michael B. Brutman once stated:
>
> Correct, but actually in the case of large UDP packets the source 
> machine sends the fragments.  TCP is too dang smart and tries all of the 
>  path MTU discovery tricks, but UDP doesn't have that luxury.  Which is 
> great because it does give me an easy way to test - but only with UDP.
> 
> I put the gateway in the middle of the machines to try to force TCP 
> fragments.  I may have to get more perverse and use a SLIP connection to 
> the DOS machine, which has a much smaller MTU.  But I suspect that the 
> TCP path length discovery will get in the way again. (I'm really going 
> to have to try to turn all of that off.)

  While I'm familiar wih IP, I haven't tried implementing it, but, what if
you were to set an MTU size of around 50 bytes?  The IP header (sans
options) is 20 bytes, and the TCP header is another 20.  I would think
setting the MTU to just above 40 might cause some fragmentation (I thought
of suggesting an MTU of just above 20, but then you're testing for IP
fragmentation, and not TCP fragmentation.

  I tend to do stupid stuff like that, but then again, I *was* hired to
write testing code at my current job ... 

  -spc (In thoery, IP should work with an MTU of 30 bytes, right?)




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