Which message acknowledges a reliable 183 response?

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Q. Which message acknowledges a reliable 183 response?

a) INFO
b) ACK
c) UPDATE
d) PRACK

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:arrow_right: Correct answer: Letter d.

In SIP (including VoLTE IMS), if a provisional response like 183 Session Progress is sent reliably (using the 100rel mechanism), it must be acknowledged by the UE with a PRACK (Provisional Response ACKnowledgement) message.

A normal ACK is only used for final responses (e.g., 200 OK), while PRACK is specifically designed to acknowledge reliable provisional responses like 183, ensuring early media or session progress information is confirmed.

Q. Which message acknowledges a reliable 183 response?

a) :x: INFO
→ Incorrect. INFO is used to carry mid-session information, not for acknowledgements.

b) :x: ACK
→ Incorrect. ACK acknowledges final responses (like 200 OK), not reliable provisional responses.

c) :x: UPDATE
→ Incorrect. UPDATE is used to modify the session parameters before completion, not to acknowledge a provisional response.

d) :white_check_mark: PRACK
→ Correct. PRACK is the mechanism for acknowledging reliable provisional responses, like a 183 with 100rel.