[SIPForum-discussion] Dose PRACK comes under Future Request

[Paul Kyzivat]

On 6/5/14 4:44 AM, Peeyush Sharma wrote:
> Dear All,
>
> In one of my setup I can see that the Prack response is being sent to
> the contact IP address of 183 w/d SDP message. I have quickly gone
> through RFC 3262 but unable to locate where it is mentioned that PRACK
> should go to the contact IP of 18x message.
>
> Please help me to locate where this behaviour has mentioned in RFC?

3262 says:

    Assuming the response is to be transmitted reliably, the UAC MUST
    create a new request with method PRACK.  This request is sent within
    the dialog associated with the provisional response (indeed, the
    provisional response may have created the dialog).  PRACK requests
    MAY contain bodies, which are interpreted according to their type and
    disposition.

Sending within a dialog means, among other things, sending using the 
remote contact established for that dialog. 3261 says:

    Dialogs are created through the generation of non-failure responses
    to requests with specific methods.  Within this specification, only
    2xx and 101-199 responses with a To tag, where the request was
    INVITE, will establish a dialog.

...

    When a UAS responds to a request with a response that establishes a
    dialog ...  The UAS MUST add a Contact header field to
    the response.  The Contact header field contains an address where the
    UAS would like to be contacted for subsequent requests in the dialog

(Note that the response doesn't even have to be reliable to establish 
the dialog. It must have the to-tag set.)

So, in your case you send an INVITE, and get reliable 183 reliable 183 
response with to-tag and contact. That establishes the dialog. Because 
it is reliable, you must send PRACK within that dialog - using the 
contact uri for the dialog, which came from the 183.

	Good luck,
	Paul



> Regards,
> Peeyush
>
>
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