[SIPForum-discussion] Offer/Answer Model

[Vivek Batra]

Hi Nora,
This is interesting one. From you call flow, it is not specified whether 180
Ringing is reliable response or unreliable response. So, there are two cases
for this type of flow that 180 Ringing can reliable or non-reliable one.

1. If 180 Ringing is reliable one.
In this case, as per RFC 3262, if initial INVITE has no SDP offer and first
SDP offer is made in reliable provisional response, UAC must send the SDP
answer in PRACK.

2. If 180 Ringing is non-reliable response
In this case, as per RFC 3261, if initial INVITE has no SDP offer then SDP
offer can be made either in 200 OK (of INVITE) or in reliable provisional
response (This means explicitly that unreliable provisonal response can not
have SDP offer)

In result, as per your call flow where you have mentioned the first SDP
offer in 180 Ringing, this provisinal response must be reliable one as per
RFC 3262. And, if this is reliable one, SDP answer must be send in PRACK by
UAC (which is not mentioned in yours call flow).

I would much appreciate you feedbacks on the same!!

--Vivek Batra


2008/8/24 雨 陈 <chen.yu26 at yahoo.com.cn>

> Hi Vivek,
> What I had mentioned 180/ACK modle is a flow with SDP negociation like
> this:
>
>          User A         User B
>           |                       |
>           |   INVITE F1     |
>           |--------------------> |
>           | 180 [SDP] F2  |   Callee B offer the session discription
> first
>           |<-------------------- |
>           | 200 [SDP] F3  |   The same SDP Msg from User B .
>           |<-------------------- |
>           | ACK [SDP] F4 |   Caller A answer with his SDP Msg .
>           |--------------------->|
>           |                       |
>           |  RTP Media     |
>           |<======== =>|
> Well, it can be found in the process to create or modify a session. BTW,
> the User A and User B represent different brand of softswitch.
>
> Any further dissucion?
>
> Regards
>
> Nora
>
> *Vivek Batra <vivek7683 at gmail.com>* 写道：
>
> Hi Nora,
> As the possibilities of SDP negotiation stated by you, how can SDP be
> negotiated between 180 and ACK? Can you please explore??
>
> --Vivek
>
>
> On Thu, Aug 21, 2008 at 6:12 AM, 雨 陈 <chen.yu26 at yahoo.com.cn> wrote:
>
>> Hi,
>> According to the practical call flow, it seem the offer/answer model of
>> SDP exchange can be found in INVITE/180, INVITE/200, 180/ACK or 200/ACK,
>> even though the 180 response is provisional and sometimes the followed
>> 200 OK send the same SDP Msg to confirm it.
>>
>> Regards
>>
>> Nora
>>
>>
>> *vijay kant gupta <vijaykant.it2002 at gmail.com>* 写道：
>>
>> See in Offer Answer Model. if u send invite without SDP at that time ACK
>> has to contain SDP for A party.
>> But if INVITE conatin SDP then 200 ok is Answer for Invite
>> ACK is final ACKNOWLEDGE.
>> On Wed, Aug 20, 2008 at 3:54 PM, zabi <mohamed.zabiulla at gmail.com> wrote:
>>
>>>    Dear All,
>>>
>>>     As per my knoweldge, Offer/Answer can occur in INVITE/2XX/ACK.
>>>
>>> Given a Sip message containing SDP how do we differentiate whether it is
>>> an OFFER/ANSWER??
>>>
>>>
>>> Regards,
>>> Zabi
>>>
>>>
>>>
>>>
>>>
>>>
>>> --
>>> why run? you'll only die tired!!
>>> Zabi.
>>>
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