[SIPForum-discussion] SIP uses UDP

[Raj Jain]

On Thu, May 29, 2008 at 10:00 AM, Ashis Jena <jenaashis at gmail.com> wrote:
> why does SIP uses UDP eve though UDP is non relaible , connectionless
> protcol?

I'm not exactly sure of the historical reasons (simplicity,
scalability?), but many have since realized that transporting SIP over
UDP has many issues and is not optimal. There are issues such as lack
of NAT traversal when packet fragmentation occurs, large UDP responses
(http://tools.ietf.org/html/draft-gurbani-sip-large-udp-response-00)
etc.

> what happens if the message is lost in the way?

SIP provides its own reliability (through retransmissions) when it is
transported over UDP. Look at the transaction state machines in RFC
3261.

> what happens to the call set up time if the message is lost and sending port
> goes on retransmitting the message?

Well, call set up time increases in that case. But this can happen in
TCP as well.

--
Raj Jain