SSCA Call Flows 5

Question 1

In a SIP outbound proxy call flow, why does a UA send all requests to the outbound proxy even for destinations it could reach directly?

Accepted Answer

To allow the proxy to apply policy, authentication, and routing on behalf of the UA

Answer

An outbound proxy centralizes policy enforcement, NAT traversal, authentication, and routing, simplifying UA configuration.

Question 2

What happens to the RTP stream when a SIP call is put on hold using a=inactive in the SDP?

Accepted Answer

Both sending and receiving of RTP packets are stopped

Answer

a=inactive means the UA will neither send nor receive media, effectively stopping RTP in both directions for that media stream.

Question 3

In a SIP forking scenario, a proxy receives a 180 Ringing from one branch and a 200 OK from another branch simultaneously. What should the proxy forward to the UAC?

Accepted Answer

The 200 OK to establish the call and cancel the branch that sent 180

Answer

The proxy forwards the 200 OK to the UAC to confirm the session and sends CANCEL to the branch that is still ringing (180 Ringing branch).

Question 4

Which SIP call flow element is responsible for generating the Call-ID for a new dialog?

Accepted Answer

The UAC that originates the INVITE request

Answer

The UAC generates the Call-ID, which must be globally unique and remain constant for all messages within the same dialog.

Question 5

In a SIP call flow involving NAT traversal, what is the purpose of the Via header's rport parameter?

Accepted Answer

To request the server to record the actual source port from which the request was received and include it in the response

Answer

The rport parameter asks the server to use the source port of the UDP packet (the NAT-mapped port) when sending the response, solving symmetric NAT issues.

(SSCA) Sip School Certified Associate Practice Test

(SSCA) Sip School Certified Associate Practice Test

SSCA Call Flows 5