[SIPForum-discussion] Offer-answer

[caglar er]

Correct. You can send more than one 18x messages before Invıte transactıon closure. 18x messages are used to carry media SDP informations , for renegotiation of media from UAS and for PSTN terminations 18x messages are used to map ISUP CPG and ACM messages. Note: a CPG message can come after or before ACM  and the second one goes with another 18x.


B. Caglar ER

From: Hemant Kumar <uniqhemant at gmail.com>
To: mrinal kuila <mrinal.80 at gmail.com>
Cc: discussion at sipforum.org
Sent: Wednesday, June 29, 2011 9:51 PM
Subject: Re: [SIPForum-discussion] Offer-answer


Hi Mrinal,

"Once offer answer is complete Is it possible to UAS to send more 18x responses?"
Yes.
But the call flow that you have shown is not correct. Once UAS sends a final response 2xx/4xx/5xx/6xx for an INVITE it can't send any further responses for that INVITE and your flow shows 18x from UAS to UAC after 200 response.

If UAS wants to update the SDP in early dialog only(before responding with a final resposne) it can do so by sending multiple 18x reponses with SDP offer. So the call flow should be as below - 

       UAC               UAS
        |----INVITE------>| - with Supported:100rel or Require:100rel
        |                        |
        |<----18x(o)----- | - provisional responses with Require:100rel
        |                        |
        |----PRACK(a)->| - send PRACK with answer
        |                        |
        |                        |
        |                        |
        |<----18x(o) -----| - to update the SDP
        |---PRACK(a)-->|
        |                         |
        |      200OK        |
        |<------------------|
        |                        |
        |        ACK         |
        |------------------>|


Regards,
Hemant


On Wed, Jun 29, 2011 at 11:15 AM, mrinal kuila <mrinal.80 at gmail.com> wrote:

Hi  Forum,
>
>Coiuld you please answer my below question.
>Once offer answer is complete Is it possible to UAS to send more 18x responses?
>
>         UAC               UAS
>        |----INVITE------>|
>        |                 |
>         |<----18x (o)-----| - provisional responses with REQUIRE 100rel
>         |                 |
>         |-----PRACK(a)--->| - send PRACK with answer
>         |<---- 2xx--------|                 |
>          |                 |
>         |                 |
>         |<----18x --------| - >         |                 |
>        |-----PRACK------>|
>         |<----2xx---------|
>
>              200OK
>        <-----------------
>         ------------------>
>              ACK
>
>
>Regards
>Mrinal
>
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-- 
- Hemant

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