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PRACK Confirms that the client has received a final response to an INVITE.
PRACK is similar to ACK, but specific to the reliability of provisional responses. The PRACK functionality is defined in RFC 3262 (Reliability
of Provisional Response in the Session Initiation Protocol (SIP)).

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Posted by sagitraz

PRACK- Provisional Acknowledge. Used when it is necessary to acknowledge the intermediate call progress messages reliably. (draft-ietf-sip-refer-05)

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Posted by george99