Subject: [sipcore] SIP response is larger than MTU

From: "Goren, Ofer (Ofer)" <[email-masked]>
Date: Mon, 19 May 2014 09:01:35 +0000

Hi

I saw this question raised @ 2006, but I did not see any follow up or a final decision. There was a draft suggested that became dead  @ 2006 as well.
Anyway, what if I send an empty INVITE, for example, using UDP, and the resulted response with SDP offer, exceeds the MTU? I cannot switch to TCP for the response, as it needs to follow the request's VIAs.

Thank you,

Ofer.

---

From: Brett Tate <[email-masked]>
Date: Mon, 19 May 2014 07:18:37 -0400

> what if I send an empty INVITE, for example, using
> UDP, and the resulted response with SDP offer,
> exceeds the MTU? I cannot switch to TCP for the
> response, as it needs to follow the request's VIAs.

RFC 3261 section 18.1.1 discusses that 1) UDP fragmentation will occur for
SIP messages over (approximately) 1300 bytes, and 2) implementations MUST
be able to handle 65535 byte SIP messages sent over UDP.

The fragmented UDP response can typically be successfully
delivered/handled unless someone has disabled fragmentation support or
there was excessive fragmentation.  However as you noticed, the attempt to
deploy "preferring UDP and switch to TCP only for large messages" does
have limitations.

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