[SIPForum-discussion] sip prack

[bhanu gowda]

Hi Karthik,

Below are my comments

.
>
>
> 1.What is prack?
>
> PRACK stands for Provisonal  Response Acknowledgement  .

The PRACK method is used to acknowledge receipt of reliably transported
provisional responses (1xx). The reliability of 2xx, 3xx, 4xx, 5xx, and
6xx responses to INVITEs is achieved using the ACK method. However, in
cases where a provisional response, such as 180 Ringing, is critical in
determining
the call state, it may be necessary for the receipt of a provisional
response
to be confirmed. The PRACK method applies to all provisional responses
except
the 100 Trying response, which is never reliably transported.

2.why is prack used?
>

As per RFC 3262

 Provisional responses provide information on the
   progress of the request processing, but are not sent reliably in RFC
   3261.

   It was later observed that reliability was important in several
   cases, including interoperability scenarios with the PSTN.
   Therefore, an optional capability was needed to support reliable
   transmission of provisional responses.  That capability is provided
n this specification.i.e With PRACK


> 3.What are the reasons for using prack?
>

Same as :Answer 2

>
> 4.What is the difference between prack , ack and 200 OK?
>

PRACK:Prack, is received that indicates reception of
the 1xx by the UAC .

ACK:ACK, is received that indicates reception of
the 2xx by the UAC .

The ACK method is used to acknowledge final responses to INVITE requests.
Final responses
are defined as 2xx, 3xx, 4xx, 5xx, or 6xx class responses. The CSeq
number is never incremented for an ACK, but the CSeq method is
changed to ACK.

ACK will be used for Sending SDP if the initial INVITE is send without SDP
offer.

200 Ok: Has two properties

When used to accept a session invitation,
it will contain a message body containing the media properties of the
UAS (called party).

When used in response to other requests, it indicates successful
completion or receipt of the request. The response stops further
retransmissions
of the request.


Folks correct me if i am wrong.

Regards
>
Bhanuprakash HP

>
>
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>
On Thu, Dec 30, 2010 at 8:58 PM, karthik <karthik8989 at rediffmail.com> wrote:

> Hi ALL,
>
> Please help me understand this....
>
> 1.What is prack?
>
> 2.why is prack used?
>
> 3.What are the reasons for using prack?
>
> 4.What is the difference between prack , ack and 200 OK?
>
> Thanks,
> Karthik
>
>
>
>
> <http://sigads.rediff.com/RealMedia/ads/click_nx.ads/www.rediffmail.com/signatureline.htm@Middle?>
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