What is UPDATE is SIP?

[techToddler]

Hello Friends,

I would like understand the SIP call flow. When I was reading SRND and other forum article about SIP I was notified that UPDATE message is to send if something is update in the call flow like codec or payload etc.

I looked into some UPDATE messages but can't understand what I exactly says.

Example a call flow between a CUCM and SME.

SME send a update and CUCM send a 200 OK.

What exactly that we can see in UPDATE to understand what's really going on.

Your help in this highly appreciated. Thanks.

Regards

Sanjay

[Vivek Batra]

Hi Sanjay,

UPDATE message as per RFC can be used to modify media related parameters in early dialog since RE-INVITE can be used only once dialog has been confirmed. So if there is any need to initiate second offer-answer during early dialog, UPDATE can be used. Please note that you can use UPDATE in confirm dialog as well.

You can refer RFC 3311 for more detailed call flow.

- Vivek 

[techToddler]

Thank you Vivek,

But do we have some place that we can see  the change in media that happened in Update?

Regards

Sanjay

[Vivek Batra]

Hi Sanjay,

There could be numerous cases but implementation specific. One scenario I could think as of now is delayed offer with PRACK.

UAC -> INVITE (w/o SDP)

UAC <- 183 Session Progress (with SDP offer)

UAC -> PRACK (with SDP answer, omitted 200 OK of PRACK here)

UAC here doesn't like the offer SDP offer received, UAC again sends it own preference of codecs;

UAC -> UPDATE (with SDP offer)

UAC <- 200 OK (with SDP answer)

- Vivek

[techToddler]

Thanks Vivek.

Sanjay