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There are two types of responses defined by SIP. They are provisional and final. Final responses (2xx-6xx) convey the result of the request processing and are sent reliably. SIP Provisional responses do not have an acknowledgement system so they are not reliable. There are certain scenarios in which the provisional SIP responses (1xx) must be delivered reliably.  For example in a SIP/ PSTN inter-working scenario it is crucial that the 180 and 183 messages are not missed. To solve this problem the SIP PRACK (Provisional Response Acknowledgement) method described in RFC 3262 guarantees a reliable and ordered delivery of provisional responses in SIP. PRACK Improves network reliability by adding an acknowledgement system to the provisional Responses (1xx).

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SIP PRACK is configured in the SIP SGP object as displayed below. For information on configuring the SIP Signaling and associated functionality, refer to the Configure SIP (Single SIP IP) topic.

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PRACK Support

There is a drop down menu with the following selections. PRACK is disabled by Default.

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The PRACK Timer is used to request an extension of the transaction at proxies in case the INVITE transaction takes some time to generate a final response (RFC 3262 page 3). When the PRACK Refresh Timer expires the IMG 2020 will send out a 1xx reliable response.

60 (seconds) - Wait 60 seconds before sending the 1xx message.

150 (seconds) (default) - Wait 150 seconds before sending the 1xx message.

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• An invalid SDP message will be ignored by the IMG 2020.

• The IMG 2020 will accept a valid PRACK message which has invalid SDP information embedded. It will discard only the SDP portion of the PRACK message. (Media parameters will not change.)

• The IMG 2020 does not support sending out a PRACK message with the SDP information embedded. It will only send out a PRACK message without the SDP information embedded.

Call Flows

Refer to the SIP PRACK Call Flows topic for call flow information

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