[OpenSER-Users] IP fragmentation but MB bit not being set

[Juan Pablo Fernandez]

Hi Bogdan,

We are using UDP transport for endpoints (xlite and Polycom IP 650) and TCP transport for our gateways (Avaya PBX with SIP support). Our proxy is record-routing and speaks UDP on the client side and TCP on the gateway side.

In the client side, INVITE requests sent via UDP to the proxy are always small enough not to need fragmentation (at least for now). When the proxy adds Record-route headers, P-Asserted-Identity and some other headers, the packets length sometimes increases enough to need fragmentation.

The problem was rather random but once we identify it we inserted a long custom header to reproduce it. 

We have been making some research and someone pointed out (thanks Tomasz Radziszewski)that in our case we are not having fragmented IP traffic but segmented TCP traffic, therefore there is no need to assert MF bit in IP flags.

Because of the UDP/TCP reencapsulation we have 2 RR headers which add to the total segment length making it more prone to needing segmentation.

Do you believe this is a problem in Avaya SIP implementation (Avaya CM/R013x.01.3.640.2)? Assuming the later, is there any possible workaround, may be a way to shorten SIP message body?

Thanks for your help

Regards

Juan Pablo



----- Original Message -----
From: "Bogdan-Andrei Iancu" <bogdan at voice-system.ro>
To: "Juan Pablo Fernandez" <juanpablo.fernandez at globant.com>
Cc: users at lists.openser.org
Sent: Tuesday, July 1, 2008 9:45:46 AM GMT -03:00 Argentina
Subject: Re: [OpenSER-Users] IP fragmentation but MB bit not being set

Hi Juan Pablo,

usually this kind of fragmentation problems were an issue for UDP, but 
never found similar problem for TCP. Actually this the reason why the 
IETF guys want to obsolete UDP for SIP and keep only TCP...

how is generating the packages ? what device? because more or less it 
will responsible for framing the content.

Regards,
Bogdan

Juan Pablo Fernandez wrote:
> Hi all,
>
> We are having some problems with long SIP IP packets that are being fragmented.
>
> We are using TCP as transport and packets bigger than ethernet MTU (1500) are being fragmented, but the "more-fragments" bit in IP flags is not being set. This is causing the remote gateway to timeout and close TCP session.
>
> Have somebody come across this issue before? Any suggestions?
>
> Thanks
>
> Juan Pablo
>
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