SSCA Session Management 5

Question 1

What is a 'back-to-back user agent' (B2BUA) in SIP session management?

Accepted Answer

A device that terminates one SIP dialog and originates a new one, controlling both legs independently

Answer

A B2BUA acts as both a UAS for the incoming dialog and a UAC for the outgoing dialog, giving it full control over both call legs unlike a transparent proxy.

Question 2

When a SIP session timer refresh fails because the remote UA does not respond, what action should occur?

Accepted Answer

The refresher terminates the session by sending a BYE

Answer

Per RFC 4028, if a session refresh (re-INVITE or UPDATE) fails without a response, the refresher must send a BYE to cleanly terminate the session.

Question 3

Which SIP extension allows a UA to transfer a call to a third party without consulting the transfer target first (blind transfer)?

Accepted Answer

REFER with a Refer-To URI and no prior session to the target

Answer

A blind transfer is executed by sending REFER to the current session peer with a Refer-To URI pointing to the transfer target, without the transferor first establishing a session with that target.

Question 4

What does the 'Replaces' header in a SIP INVITE request accomplish?

Accepted Answer

It instructs the UAS to replace an existing dialog with the new one being established

Answer

The Replaces header (RFC 3891) identifies an existing dialog that should be replaced by the new INVITE dialog, enabling attended call transfer scenarios.

Question 5

In SIP, which mechanism provides session keepalive at the transport layer below the SIP session timer?

Accepted Answer

SIP OPTIONS pings or STUN keepalives over the transport connection

Answer

SIP clients often use lightweight OPTIONS pings or STUN binding requests (RFC 5626) to maintain NAT bindings and verify transport connectivity below the SIP session timer level.

(SSCA) Sip School Certified Associate Practice Test

(SSCA) Sip School Certified Associate Practice Test

SSCA Session Management 5