[SIPForum-discussion] Possible responses to 180 Ringing Messages

[Stephen James]

PRACK, CANCEL, or BYE. Only PRACK is a true response the other two would be sent if the initiator wanted to terminate the session. CANCEL is most common, but BYE can be used if an early dialog has been established. 

Sent from my iPad

On Dec 20, 2011, at 22:35, Bhanumati Behera <bbehera09 at gmail.com> wrote:

> Hi,
>      Three options are INVITE(in case of re-invite),CANCEL AND PRACK(It will  send when 100rel comes with 180 ringing)
> 
> On Sat, Dec 17, 2011 at 11:01 PM, Rudra Kant Kanth <rudra.k1 at gmail.com> wrote:
> 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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