SSCA Session Management 5 — Questions and Answers
Question 1: What is a 'back-to-back user agent' (B2BUA) in SIP session management?
- A UA that registers on behalf of two users simultaneously
- A device that terminates one SIP dialog and originates a new one, controlling both legs independently (Correct answer)
- A proxy that records both legs of a call for billing
- A UA that supports both IPv4 and IPv6 simultaneously
Correct answer: A device that terminates one SIP dialog and originates a new one, controlling both legs independently
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?
- The session continues indefinitely until a BYE is received
- The refresher terminates the session by sending a BYE (Correct answer)
- The proxy automatically extends the session timer
- A new INVITE is sent to re-establish the session from scratch
Correct answer: The refresher terminates the session by sending a BYE
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)?
- NOTIFY with Subscription-State: terminated
- REFER with a Refer-To URI and no prior session to the target (Correct answer)
- re-INVITE with a Replaces header
- INVITE with a Referred-By header only
Correct answer: REFER with a Refer-To URI and no prior session to the target
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?
- It replaces a codec in the current SDP offer
- It instructs the UAS to replace an existing dialog with the new one being established (Correct answer)
- It substitutes a failed proxy route with an alternate path
- It replaces the From address with an anonymous identity
Correct answer: It instructs the UAS to replace an existing dialog with the new one being established
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?
- SIP OPTIONS pings or STUN keepalives over the transport connection (Correct answer)
- Periodic REGISTER refresh messages
- RTP RTCP Receiver Reports sent every 5 seconds
- SIP INFO messages with a keepalive body
Correct answer: SIP OPTIONS pings or STUN keepalives over the transport connection
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.
Question 6: What is the purpose of the 'Supported' header in a SIP session establishment request?
- It mandates that the remote UA implement the listed features or reject the request
- It advertises optional SIP extensions the UA supports but does not require (Correct answer)
- It lists the codecs supported for the session media
- It indicates the SIP version supported by the UA
Correct answer: It advertises optional SIP extensions the UA supports but does not require
The Supported header lists option tags for SIP extensions the UA supports; unlike Require, the remote UA is not obligated to implement them to proceed with the request.
Question 7: During SIP session establishment, what does a 183 Session Progress response typically signal?
- The session has been fully established and media can flow
- Early media is available and the session description may be included (Correct answer)
- The request has been redirected to another server
- The UA requires authentication before proceeding
Correct answer: Early media is available and the session description may be included
A 183 Session Progress is a provisional response that may carry an SDP body enabling early media (such as ringback tones) to flow before the session is accepted.
SSCA Session Management 5
What is a 'back-to-back user agent' (B2BUA) in SIP session management?