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100rel (SIP Provisional Message Reliability)

The use of the PRACK (Provisional Acknowledgment) method enables reliability to be offered to SIP (Session Initiation Protocol) provisional responses (1xx series). This is very important if PSTN interworking is to be supported. A user?s wish to use reliable provisional responses is invoked by the 100rel tag which is appended to the value of the required header of initial signalling messages.

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