[SIPForum-discussion] Possible responses to 180 Ringing Messages

[Rudra Kant Kanth]

Hi Amit and Albert,

I have seen situations where 180 Ringing is followed up with PRACK as
reliable acknowledgement to the 180 Ringing.

Though by default, the 180 Ringing doesn't need acknowledgement of
successful receipt by the far-end. However, when there is a "*required: 100
rel*" parameter in the 180 Ringing, then that 180 Ringing needs to be
acknowledged for successful receipt and thus a PRACK is sent by the UAS,
for which the client sends back 200 OK. However, in more generic cases, a
180 Ringing would have 100 rel in the supported format.

Thus,
PRACK is the ONLY VALID "RESPONSE" in this case. As Amit stated, all the
options were actually REQUESTS only.
But assuming the question is re-framed as what all are possible next
messages after a 180 Ringing, THEN among the options, we can have a CANCEL
coming in as well. If still I have to choose a 3rd option then, I would
choose an INVITE as my last option. The reason for choosing INVITE and not
BYE as my answer is that(based on my gut feeling):- in an event a user
decides to drop the call, he has to send a CANCEL. However, if the user
decides to change any of the entries in the INVITE headers (say contacts,
or codecs etc), it has to come up to the far-end client in form of INVITE
only.

OK, another thing is that in race-conditions, you will also find a BYE
being sent in a CANCEL request call, but, this BYE HAS TO FOLLOW THE
CANCEL. It will never come before the CANCEL.

So, PRACK, CANCEL and INVITE.

Regards,
Rudra


On Wed, Dec 14, 2011 at 11:44 AM, AMIT ANAND <amiit.anand at gmail.com> wrote:

> Hi Albert,
>
> Where did you get this question?
>
> If i were on your place i would say non of these,
>
> These all are requests and ideally there could be no response to a
> response.
>
> Except ACK any of other four can come in a call flow after 180.
>
> Cheers
> Amit
>
>
>
> On Fri, Dec 9, 2011 at 6:53 AM, Albert Etsebeth <umbrualbert at gmail.com>wrote:
>
>> Hi All
>>
>> I got this question in a exam, and its been driving me mad.
>>
>> Question:"what are valid responses to 180 ringing" (Choose 3)
>>
>> Options:PRACK / ACK / BYE /CANCEL /INVITE
>>
>> In my opinion there is only 2, PRACK and CANCEL
>>
>> Thanks
>>
>> umbru
>>
>>
>>
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>
>
> --
> Thanks
> Amit Anand
>
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