[OpenSIPS-Users] UDP fragmentation in reply routes

[Maxim Sobolev]

Smells like a OS/kernel bug to me. There is little application can do in
that regard, UDP fragmentation/reassembly happens at much lower layers of
the OSI stack.

However, as a workaround as long as SIP goes you can try to reduce your SIP
signalling packet size by using compact version of SIP headers, as well as
dropping headers that are not used. That would save you 100-150 bytes per
SIP message perhaps. I don't know if OpenSIP can do that in the proxy mode
out of the box though, so you might want to add b2b into the flow.

-Max

On Mon., May 18, 2020, 12:34 p.m. Olle Frimanson, <olle at zaark.com> wrote:

> Hi,
>
>
>
> We have an issue on our home proxy (opensips 2.4.6), when it receives  200
> OK (over UDP)  from our Freeswitch and the package size is higher than the
> MTU size , we sometimes get fragmentation  of the UDP packets, but only the
> first part of the fragmented package is sent to our edge proxy. Is this a
> known issue or is it a OS bug?
>
>
>
> We have not yet spotted any pattern on this and in most cases bigger
> packets with MTU around 1600 bytes gets through without an issue.
>
>
>
> I can add that in the other direction in the normal request routes we
> don’t have any issue at all can have packets > 2000 bytes without any
> issues.
>
> Does Opensips use IP_MTU_DISCOVER or how is fragmentation controlled and is it expected to have different behavior in reply routes vs other routes?
>
> We use Centos 7 in a virtual server environment.
>
>
>
> I was hoping someone can share some light on this strange issue.
>
>
>
> BR/Olle
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