[Serdev] SIP UDP / dont fragment issue

[Valentin Nechayev]

 Wed, Jun 01, 2005 at 14:35:14, martin.koenig wrote about "Re: [Serdev] SIP UDP / dont fragment issue": 

> It works very well for SIP and UDP with Ser. It works with all ATAs and 
> Phones that we use in our platform.

It works
1) with SIP due to its retransmissions, but not for arbitrary UDP,
2) when kernel provides fragmentation on initial transmission based on
cached MTU data in kernel.

But your statement:
> >>It is very correct to set the dont fragment bit here, because the 
> >>message is already fragmented and an endpoint would not be able to 
> >>aggregate these further fragmented messages.

isn't correct, because re-fragmenting is stable; it's only matter
of taste to set DF or not; this gives a problem of efficiency vs.
stable reachability. E.g., BSD systems don't ever set DF on UDP.
OTOH, Linux does. SER works on both if kernel support is correct
and transparent (SER sends message of any size and it's correctly
fragmented and assembled).

> Valentin, I'm not a programmer, nor do I plan to become one. It simply 
> works, please talk to the responsible Ser developer about details how 
> they did do this job.

Yes, it works. It works both with DF and without DF.

P.S. Your colleague from network department probably will say that
I described a too marginal case in previous letter. Yes, it was.

-- 
Valentin Nechayev
Porta Software Ltd., Software Engineer
mailto:netch at portaone.com