[SIPForum-discussion] Possible responses to 180 Ringing Messages

Thu Jan 5 06:54:08 UTC 2012

Hi Rudra,

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.

I completely disagree with the above analysis:

How does a UA sends another INVITE,  without getting the final response
to the Initial INVITE?

In this case if the UA wants to change(codec etc) he has to Send the
UPDATE not the INVITE message.

Regards

Srinivas

From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Rudra Kant Kanth
Sent: Saturday, December 17, 2011 11:01 PM
To: AMIT ANAND
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Possible responses to 180 Ringing
Messages

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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