[SIPForum-discussion] SIP uses UDP

[M. Ranganathan]

On Thu, May 29, 2008 at 5:38 PM,  <cperry15 at gmail.com> wrote:
> 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
> Sent via BlackBerry from T-Mobile
>
> -----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
> (http://tools.ietf.org/html/draft-gurbani-sip-large-udp-response-00)
> etc.


It was thought ( perhaps incorrectly ) that scalability would be
better for udp as it is connectionless. Fail over and handoff is also
easier to implement for connectionless protocols.


Ranga



>
>> 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
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-- 
M. Ranganathan