Provisional Response Acknowledgment in SIP Agent Server - Exam 400-051: CCIE Collaboration

SIP Agent Server Header Fields for Provisional Response Acknowledgment

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Which two header fields does a SIP Agent Server use to indicate that the message needs Provisional Response Acknowledgment? (Choose two.)



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A. B. C. D. E. F.


When a Session Initiation Protocol (SIP) User Agent Server (UAS) sends a Provisional Response (1xx) to a SIP User Agent Client (UAC), it can request an acknowledgment from the UAC indicating that the Provisional Response was received correctly. This acknowledgment is called a Provisional Response Acknowledgment (PRACK).

To request a PRACK, a SIP UAS can use two header fields:

  1. Require: 100rel The Require header field is used by a SIP UAS to require certain functionality from the UAC for a successful communication. In this case, the UAS includes the 100rel option tag in the Require header field to indicate that it requires support for PRACK from the UAC.

  2. RSeq: [Numeric value that uniquely identifies the response message] The RSeq header field is used by a SIP UAS to provide a sequence number for each Provisional Response that it sends. The UAS includes the RSeq header field in the Provisional Response to indicate that the response requires a PRACK from the UAC with the same RSeq value.

Therefore, options B and E are the correct answers as they refer to the Require header and the RSeq header, respectively.