[Kamailio-Users] switch to TCP when UDP message is bigger than MTU

[Stanisław Pitucha]

2009/4/25 Andreas Heise <aheise at gmx.de>:
> Klaus Darilion schrieb:
>> Practically this paragraph from RFC 3261 is non-sense as it brakes
>> communication with SIP clients.
>>
>> Residential customers SIP clients are mostly behind NAT and often do not
>> even support TCP. Thus, automatic switching to TCP will cause problems
>> as you can not reach the clients anymore.
>>
> you are right, but RFC 3261 says also...
>
> 18.2.1 Receiving Requests
>
> "For any port and interface that a server listens on for UDP, it MUST
> listen on that same port and interface for TCP.
> This is because a message may need to be sent using TCP, rather than
> UDP, if it is too large."
>
> ... so the SIP clients you mentioned are not conform with RFC 3261!

Even if they do, the NATting node will simply reject the connection.
You will have 50/50 chances that it's the client starting the
connectino (works), and server starting the connection (fails on nat).
It's not a nat-friendly standard, so it's safer to just ignore it.
Also if your udp packet is bigger than MTU, it will simply get
fragmented and transmitted anyways. If you have packets bigger than
max udp size, then that's your main problem ;) If your equipment can't
handle fragmentation, you will get a lot of strange problems one way
or another.

Stan