Mayank Gupta wrote:
> Hi Ritul
>
> Is there any significance of this additional message for INVITE?
>
> ACK for initial INVITE request makes sense as it may be useful for
> offer-answer when initial INVITE is without SDP and offer is received from
> called party in 200 OK. In that case ACK will send the answer.
>
> But what is the use of ACK for re-Invite message?
The main reason for ACK (IIUC) is because the INVITE can be pending for
a long time, unlike other transactions. (The lifetime of the INVITE
transaction can be extended indefinitely by sending provisional
responses.) Once UAC receives a provisional response, it ceases
retransmission of the INVITE. As a consequence, if the 200 response is
lost, there will be no retransmission of the INVITE to remind the UAS to
retransmit the 200. In lieu of that, the 200 is retransmitted until the
ACK is received.
Even a reINVITE can result in alerting, and hence may be extended a long
time. So the same logic applies there.
If you do an O/A using UPDATE, then it really is infeasible for the UAS
to alert before responding, so it must make its decision without human
input.
Thanks,
Paul
> Mayank
>
> On Fri, Nov 20, 2009 at 3:41 PM, Ritul Sonania <ri...@in.niksun.com> wrote:
>
>>> Hi All
>>>
>>> I have a query regarding new offer-answer using Re-invite and Update
>>> methods.
>>>
>>> When a new offer is sent using a Re-invite, it is responded by 200 OK and
>>> then ACK for this 200 OK is sent.
>>>
>>> --------------------> Re-Invite (for new offer)
>>> <------------------- 200 OK
>>> --------------------> ACK
>>>
>>> However, if a new offer is sent using an UPDATE method, there is no ACK
>>> sent.
>>>
>>> ---------------------> UPDATE (for new offer)
>>> <--------------------- 200 OK
>>>
>>> My query is why there is no ACK sent for the 200 OK for update message.
>> ACK is only sent for INVITE request.
>>
>> Regards,
>> Ritul Sonania
>> Niksun
>>
>>> Mayank
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>>
>>
>>
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