SIP Session Management 3

Question 1

What happens if a SIP UAS receives a re-INVITE with an SDP offer it cannot support?

Accepted Answer

It responds with 488 Not Acceptable Here

Answer

488 Not Acceptable Here indicates the SDP offer parameters (e.g., codecs) in the request are not acceptable to the UAS.

Question 2

Which SIP mechanism allows a UA to detect if a session is still active when no media is flowing?

Accepted Answer

Session timers (RFC 4028)

Answer

RFC 4028 session timers use periodic re-INVITEs or UPDATE requests to verify both sides are still alive.

Question 3

In SIP session timers, what does the 'refresher' parameter in the Session-Expires header indicate?

Accepted Answer

Which UA is responsible for sending the refresh request

Answer

The refresher=uac or refresher=uas parameter specifies which endpoint must send the periodic re-INVITE or UPDATE to keep the session alive.

Question 4

What SIP response is sent by a UAS when it receives a BYE request that references a dialog it cannot find?

Accepted Answer

481 Call/Transaction Does Not Exist

Answer

481 indicates the BYE references a call-ID or dialog that the UAS has no record of, often due to a crash or restart.

Question 5

During a SIP call transfer using REFER, which header carries the transfer target's URI?

Accepted Answer

Refer-To

Answer

The Refer-To header contains the URI of the transfer target that the referee should establish a new session with.

(SIP) Session Initiation Protocol Practice Test