Meaning and use of prack in SIP messages

Prack English translation (the provisional Response acknowledgement), you can call IT security information! This compares the image.

The final response in the SIP is understood to be reliably transmitted, such as a 200OK response to the invite, and UAC will give an ACK telling UAS that it has received 200OK. The reliability between 200 and ACK is end-to-end. Prack is a mechanism for guaranteeing the reliable transmission of temporary messages (101-199) in SIP messages. Prack is the reliability of the 200OK response, in addition to 100 of the 1xx temporary response (100 is hop-to-hop), for reliable transmission.Prack is generally a confirmation of receiving 183 call in progress/180 ringing, Prack's implementation of UAC and UAS negotiations on whether to support the extension is through an option tag-100rel.
To achieve this, UAC has two options to add Require:100rel or Supported:100rel to the invite message.  In accepting the above message, UAS also has a choice problem. When invite contains supported:100rel,uas in sending a temporary message, the parameters in the temporary message can be sent based on whether or not the UAS supports Prack decision.  If support is added to the Require:100rel and Rseq fields in the temporary message, UAC sends Prack to the message, otherwise without the above parameters, UAC does not send prack. When the invite contains Require:100rel. If UAS does not support prack, it needs to use 420 (incorrect extension) to reject the call.  Otherwise, the Require:100rel and Rseq fields are added to the temporary message, and UAC is sent Prack to the message. For example, the UAC-initiated invite contains supported:100 rel, and UAS supports the extension and has require:100rel in the 183 response, stating that the next session will have a prack response to all 1xx responses outside of 100. Prack is defined in the RFC3262.

Meaning and use of prack in SIP messages