[SIPForum-discussion] SIP uses UDP

cperry15 at gmail.com cperry15 at gmail.com
Thu May 29 21:38:18 UTC 2008

The longer setup time will increase the post dial delay (PDD). Which the time elapsed from when the subscriber finishes dialing digits and the subscriber hears ringback
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-----Original Message-----
From: "Raj Jain" <rj2807 at gmail.com>

Date: Thu, 29 May 2008 12:24:54 
To:"Ashis Jena" <jenaashis at gmail.com>
Cc:discussion at sipforum.org
Subject: Re: [SIPForum-discussion] SIP uses UDP

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

> 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

> 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
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