SSCA Call Flows 3

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?

Accepted Answer

503 Service Unavailable

Answer

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?

Accepted Answer

It indicates the next-hop has received the INVITE and is processing it to prevent retransmissions

Answer

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?

Accepted Answer

INVITE

Answer

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?

Accepted Answer

A provisional response in the 101-199 range

Answer

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?

Accepted Answer

It ensures mid-dialog requests route through the proxy that inserted it

Answer

Proxies insert Record-Route to remain in the signaling path for subsequent in-dialog requests such as re-INVITE and BYE.

(SSCA) Sip School Certified Associate Practice Test