[SIPForum-discussion] Possible responses to 180 Ringing Messages

Sun Dec 25 05:10:07 UTC 2011

Hi Keyur,

Yes, you are correct in saying that a BYE cannot follow a 180 Ringing
(since BYE is used to terminate an established session).

In normal cases UAC-1 sends a CANCEL to terminate his INVITE after 180
Ringing and before UAC-2 sends his 200OK. However in race situations, while
CANCEL was sent by UAC-1 but, before UAC-2 receives the CANCEL it sends
it's 200OK (assuming there is some delay b/w UAC-1 and UAC-2, OR some
proxies in between OR both UAC-1 and UAC-2 pressed Cancel and Accept-Call
almost at same instant);

IN THESE CASES:-
One transaction has
INVITE ----------------------> C_seq =1 invite
<------------------ 100 Trying C_seq =1 invite
<------------------ 180 Ringing C_seq =1 invite

Now, a CANCEL is sent and parallely a 200OK is also sent:-
CANCEL -------------> C_seq =1 Cancel                              || both
happening at same
<--------------- 200 OK C_seq =1 invite                                 ||
instants of time
<--------------- 200 OK C_seq =1 Cancel

UAS are stack-coded in a way that the moment they receive a Cancel, they
immediately responds with a 200OK to update successful receipt; also CANCEL
is a separate transaction. Any by default, to terminate original
transaction of INVITE, generally a 487 is sent (which is a Final Response).
However, in our case; UAC-1 finds that it has already received a final
response in form of 200 OK for it's INVITE transaction; so it may not get
another final response from UAC-2.

Thus, UAC-1 has to send a BYE to terminate the unwanted established
transaction, thus, ask UAC-2 to release off the unwanted ports.

I hope this makes sense.

MEANWHILE, my take for the question as "subsequent messages for a 180
Ringing" continues to be PRACK, INVITE and CANCEL.

Regards,
Rudra

On Sat, Dec 24, 2011 at 11:51 PM, Keyur Amin <fossil0681 at yahoo.com> wrote:

> Rudra,
>
> You cannot have a BYE following 180 unless it has been ACKed. Also, its
> quite curious though, why would you need a BYE after CANCEL, since the
> session has already been terminated. Seems redundant to me.
>
> In term of the original disucussion point, INVITE/ACK/CANCEL are all
> requests. There is not response needed for 180. If there is a need for
> reliable responses, you can have a PRACK transaction followed by 180, but
> again its not a response to 180. Not sure if the question is stated
> correctly.
>
> *Thanks,*
> *Keyur Amin*
> ------------------------------
>
> **
>
>   ------------------------------
> *From:* Rudra Kant Kanth <rudra.k1 at gmail.com>
> *To:* Stephen James <sjames_1958 at yahoo.com>
> *Cc:* "discussion at sipforum.org" <discussion at sipforum.org>
> *Sent:* Thursday, December 22, 2011 12:42 AM
> *Subject:* Re: [SIPForum-discussion] Possible responses to 180 Ringing
> Messages
>
> Hi Stephan,
>
> So, are there instances possible where we can have a BYE following after a
> 180 Ringing (apart from a CANCEL and PRACK)?
>
> I have not seen such an instance myself. The only thing which I have seen
> is that BYE request follows a CANCEL (that too only in race-conditions). I
> think it's an unique Finding and I would be interested to see such a trace
> where it depicts that even before the final response is received, a BYE is
> sent ahead of CANCEL.
>
> Regards,
> Rudra
>
> On Thu, Dec 22, 2011 at 10:20 AM, Stephen James <sjames_1958 at yahoo.com>wrote:
>
> 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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