SSCA Call Flows 3 — Questions and Answers
Question 1: What SIP response is sent by a proxy when it is temporarily unable to process a request and the client should retry later?
- 486 Busy Here
- 503 Service Unavailable (Correct answer)
- 500 Server Internal Error
- 408 Request Timeout
Correct answer: 503 Service Unavailable
503 Service Unavailable indicates the server is temporarily unable to handle the request, often including a Retry-After header.
Question 2: In a SIP call flow, the UAC sends an INVITE and receives a 100 Trying. What is the purpose of 100 Trying?
- It confirms the call is connected
- It indicates the next-hop has received the INVITE and is processing it to prevent retransmissions (Correct answer)
- It means the remote UA is ringing
- It signals that authentication is required
Correct answer: It indicates the next-hop has received the INVITE and is processing it to prevent retransmissions
100 Trying is a provisional response that stops the client's retransmission timer (T1) and indicates the request is being forwarded.
Question 3: Which SIP dialog-creating method establishes the dialog parameters using the To-tag in the response?
- REGISTER
- OPTIONS
- INVITE (Correct answer)
- MESSAGE
Correct answer: INVITE
The INVITE method creates a dialog, and the To-tag included in the 1xx or 2xx response from the UAS establishes the full dialog ID.
Question 4: In a SIP early dialog, which response code initiates it?
- 200 OK
- 100 Trying
- A provisional response in the 101-199 range (Correct answer)
- 302 Moved Temporarily
Correct answer: A provisional response in the 101-199 range
An early dialog is created by a provisional response (1xx other than 100) that contains a To-tag, such as 180 Ringing.
Question 5: What is the role of the Record-Route header in a SIP call flow?
- It stores the history of visited proxies for billing
- It ensures mid-dialog requests route through the proxy that inserted it (Correct answer)
- It prevents loops by recording each hop
- It specifies the codec priority list
Correct answer: It ensures mid-dialog requests route through the proxy that inserted it
Proxies insert Record-Route to remain in the signaling path for subsequent in-dialog requests such as re-INVITE and BYE.
Question 6: A SIP UA receives a 491 Request Pending response to a re-INVITE. What should it do?
- Immediately retransmit the re-INVITE
- Send a BYE to terminate the session
- Wait for a random interval before retrying the re-INVITE (Correct answer)
- Send a CANCEL to abort the request
Correct answer: Wait for a random interval before retrying the re-INVITE
491 Request Pending means the remote UA already has a pending transaction; the sender should wait a random time (typically 1-2 seconds for the initiator) before retrying.
Question 7: During a SIP call teardown, which party sends the BYE request in a normal call termination initiated by the caller?
- The proxy always sends BYE
- Either UA can send BYE; in this case the caller (UAC) sends it (Correct answer)
- Only the UAS can send BYE
- The registrar sends BYE on behalf of the UA
Correct answer: Either UA can send BYE; in this case the caller (UAC) sends it
Either party in an established dialog can send BYE to terminate the session; if the caller ends the call, the UAC sends BYE.
SSCA Call Flows 3
What SIP response is sent by a proxy when it is temporarily unable to process a request and the client should retry later?