[SIPForum-discussion] PRACK

[Abhisek Acharya]

Hi Moh,

First of all good morning and I hope everything going well in PCCW HONGKONG
and USA.I will answer your question in a detailed way with few points.


1.100 rel stands for the support of PRACK method is SIP protocol.rel stands
for reliability and 100 stands for provisional response. 100rel is called
as option tags.No option tag is mandatory but the use of the same can be
very very tricky.

2.100 rel option tag has nothing to do with 200 OK response but it has
everything to do with the 18x response.If 100 rel is missing from the 200
OK response then it should not have any impact on the call theoretically.

3.Supported and Require headers have a bigger role to play in the use of
100 rel.If initial INVITE contains a Supported:100 rel then if the UAS
wants the 18x responses to be acknowledged then it MUST add a Require:100
rel option-tag with a Rseq header which will act as a Reliable sequence
number and UAC will acknowledge that with a PRACK message with a Rack
header which will replicate the Rseq number.

4.As long as the above process is working fine then there is no reason of
call failure.


5.Now coming back to your question regarding the Require:timer header in
200 OK response then my answer is yes.200 OK response can contain
Require:timer header as this signifies the kicking off the
Session-Refresher mechanism and this is a completely different mechanism to
PRACK.


Please send me a call trace with all the necessary information and IP
details so that I can take a look and put my points.Also please let me know
if something is not clear on the above explanation.Please remember that 100
rel and timer are two different option tags and used for two different
purposes in SIP.

Regards
Abhisek Acharya




On Thu, Nov 8, 2012 at 12:50 PM, Moh, Yee-Kuang <ykmoh at pccwglobal.com>wrote:

> Hi SIPForum,****
>
> ** **
>
> Greeting! ****
>
> ** **
>
> **1.      **I would like to know isn’t that “100rel” has to presence or
> mandatory in header of the 2nd 200ok (note: it is used for responding to
> INVITE)? ****
>
> **2.      **What happen if “100rel” was missing in this 200ok header?    *
> ***
>
> **3.      **Does 2nd 200ok has to follow the same header with
> “Request:100rel” when “183 Session Progress” is having “Request: 100rel”? I
> experience some GW respond “200ok” contain “Request: timer\r\n” the call
> was disconnected after picking up the phone. ****
>
> ** **
>
> ** **
>
> Thank you very much.****
>
> ** **
>
> Regards****
>
> moh****
>
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