SIP Interoperability and Integration 3 — Questions and Answers
Question 1: Which SIP extension allows a UA to learn the capabilities of a remote UA without establishing a call?
- OPTIONS method (Correct answer)
- SUBSCRIBE method
- INFO method
- MESSAGE method
Correct answer: OPTIONS method
The OPTIONS method allows a SIP UA to query another UA's capabilities, including supported methods, codecs, and SIP extensions.
Question 2: In SIP federation between two service providers, ENUM (RFC 3761) is primarily used to:
- Encrypt SIP messages between domains
- Map telephone numbers to SIP URIs via DNS (Correct answer)
- Compress SDP bodies
- Authenticate SIP registrations
Correct answer: Map telephone numbers to SIP URIs via DNS
ENUM uses DNS NAPTR records to resolve E.164 telephone numbers to SIP URIs, enabling cross-domain routing without a central clearinghouse.
Question 3: A SIP proxy receives a request with a Max-Forwards value of 0. What must the proxy do?
- Forward the request and reset the value to 70
- Return a 483 Too Many Hops response (Correct answer)
- Drop the request silently
- Forward to the next hop and decrement to -1
Correct answer: Return a 483 Too Many Hops response
Per RFC 3261, a proxy that receives a request with Max-Forwards: 0 must respond with 483 Too Many Hops.
Question 4: Which header enables a SIP UA behind NAT to advertise an externally reachable contact address learned from a STUN server?
- P-Asserted-Identity
- Contact (Correct answer)
- Via received/rport parameters
- P-Preferred-Identity
Correct answer: Contact
The UA places its STUN-discovered public IP and port in the Contact header so that remote parties can reach it through the NAT.
Question 5: When integrating SIP with Microsoft Teams Direct Routing, the Session Border Controller must support which audio codec mandatorily?
- G.729
- G.711 (PCMU/PCMA) (Correct answer)
- Opus
- G.722
Correct answer: G.711 (PCMU/PCMA)
Microsoft Teams Direct Routing requires the SBC to support G.711 (PCMU and PCMA) as a mandatory baseline codec for interoperability.
Question 6: In SIP interworking with XMPP (Jingle), which RFC defines the mapping between SIP and XMPP for instant messaging?
- RFC 3428
- RFC 7247 (Correct answer)
- RFC 4960
- RFC 5626
Correct answer: RFC 7247
RFC 7247 defines the architectural framework and SIP-XMPP interworking guidelines for presence and instant messaging.
Question 7: A call center deploys SIP and needs to fork an incoming call to multiple agents simultaneously. Which SIP feature supports this?
- Sequential forking via 301 redirect
- Parallel forking by a proxy (Correct answer)
- SIP REFER with Replaces
- SIP PRACK
Correct answer: Parallel forking by a proxy
A SIP proxy can perform parallel forking by sending the INVITE to multiple registered contacts simultaneously, ringing all agents at once.
SIP Interoperability and Integration 3
Which SIP extension allows a UA to learn the capabilities of a remote UA without establishing a call?