[SIPForum-discussion] Regarding Offer/Answer Model

[Vivek Batra]

Saraswati,
To the best of my knowledge, you can only identify whether SDP is present in
SIP message or not with the header 'Content-Type'.

--Vivek

On Wed, Aug 20, 2008 at 10:09 PM, saraswati sp <saraswatisp at gmail.com>wrote:

> Thanks a lot.
>
> Currently exploring on Offer/Answer. So need to make sure whether am moving
> in right direction. So plz guide me.
>
> So now the first SIP message that includes SDP is offer and the latter sip
> message containing SDP will be Answer.
>
> In order to know whether SDP is present or not, we need to check
> the presence of header field "Content-type". which is mandatory header
> field.
>
> According to RFC 3261 "The Content-Type header field MUST be present if
> the body is not empty. If the body is empty, and a Content-Type header
> field is present, it indicates that the body of the specific type has zero
> length".
>
> So is the presence of SDP known only by this header field or any other??
>
> Thanks in advance.
>
>
> On Wed, Aug 20, 2008 at 10:46 PM, Vivek Batra <vivek7683 at gmail.com> wrote:
>
>> Hi,
>> As per RFC 3261, SDP can be negotiated in INVITE and 200OK or 200 OK and
>> ACK.
>> At the same time, unreliable response viz 183 Session Progress can also
>> have SDP answer for early media. However, unreliable response can not have
>> SDP if first INVITE hadn't SDP offer.
>>
>> RFC3262 defines the SDP negotiation in reliable provisional responses
>> where SDP can be negotiated between:
>> INVITE and reliable response (180 Ringing)
>> 180 Ringing and PRACK
>> PRACK and 200 OK (of PRACK)
>>
>> At any time during early dialog once the SDP has been negotiated, caller
>> or callee can again send the offer in UPDATE.
>>
>> Now as per your question regarding how to identify the SDP is Offfer or
>> Answer, the first SIP message which includes SDP is alwyas Offer and
>> subsequent SIP message which includes SDP would be SDP Answer!!
>>
>> I hope, you would get brief overview.
>>
>> --Vivek
>>
>>
>>
>> On Wed, Aug 20, 2008 at 3:51 AM, saraswati sp <saraswatisp 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,
>>> Saraswati.
>>>
>>>
>>>
>>>
>>>
>>
>
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