SSCA Proxy Servers 5 — Questions and Answers
Question 1: What is the significance of a SIP proxy being 'transaction stateful' versus 'dialog stateful'?
- Transaction stateful proxies track individual SIP transactions while dialog stateful proxies track the full call lifecycle (Correct answer)
- Transaction stateful proxies only handle INVITE while dialog stateful proxies handle all methods
- Transaction stateful proxies use TCP only while dialog stateful proxies support UDP
- Transaction stateful proxies require TLS while dialog stateful proxies do not
Correct answer: Transaction stateful proxies track individual SIP transactions while dialog stateful proxies track the full call lifecycle
A transaction stateful proxy maintains state for a single request/response cycle, while a dialog stateful proxy tracks the entire dialog from INVITE to BYE.
Question 2: When a SIP proxy performs load balancing across multiple servers, which approach distributes requests based on measured server performance?
- Round-robin distribution
- Least-connections distribution
- Random distribution
- Weight-based distribution using q-values (Correct answer)
Correct answer: Weight-based distribution using q-values
SIP load balancing using q-values in Contact or Route headers allows weighted distribution where higher q-values indicate preferred targets.
Question 3: Which SIP header field does a proxy insert to indicate the path the request took through the proxy infrastructure, used primarily with the REGISTER method?
- Record-Route
- Path (Correct answer)
- Route
- Via
Correct answer: Path
The Path header (RFC 3327) is used during REGISTER to record proxy addresses, ensuring subsequent requests to the UA are routed through the same proxy path.
Question 4: A SIP proxy receives an INVITE with no Route headers and the Request-URI is a SIP AOR. What should the proxy do next?
- Return a 404 Not Found immediately
- Query the location service to resolve the AOR to a Contact URI and forward the request (Correct answer)
- Forward the request to a default gateway without consulting location service
- Return a 301 Moved Permanently with a Contact header
Correct answer: Query the location service to resolve the AOR to a Contact URI and forward the request
When no Route header is present and the Request-URI is an AOR, the proxy consults the location service to find the registered Contact URI(s) and forwards accordingly.
Question 5: What is the purpose of the 'Proxy-Require' header in a SIP request?
- It lists SIP extensions that every proxy in the path must support, or return a 420 Bad Extension (Correct answer)
- It specifies authentication credentials for the proxy
- It indicates the maximum number of proxies allowed to forward the request
- It requests the proxy to record the route for subsequent in-dialog messages
Correct answer: It lists SIP extensions that every proxy in the path must support, or return a 420 Bad Extension
The Proxy-Require header lists SIP extensions that all proxies must support; if a proxy does not support a listed extension, it must return 420 Bad Extension.
Question 6: In a SIP network, what is the function of an 'edge proxy' or 'Session Border Controller (SBC)' compared to a core proxy?
- Edge proxies handle topology hiding, NAT traversal, and security at the network border while core proxies handle internal routing (Correct answer)
- Edge proxies only support UDP while core proxies support all transports
- Edge proxies terminate media while core proxies handle only signaling
- Edge proxies manage user registration exclusively while core proxies handle call routing
Correct answer: Edge proxies handle topology hiding, NAT traversal, and security at the network border while core proxies handle internal routing
An edge proxy/SBC sits at the network boundary to handle NAT traversal, topology hiding, security enforcement, and protocol normalization before requests reach the core network.
Question 7: If a SIP proxy receives a request destined for a domain it is not responsible for and has no configured route, what is the correct behavior?
- Return 403 Forbidden
- Perform DNS resolution on the domain and attempt to forward the request (Correct answer)
- Return 484 Address Incomplete
- Return 501 Not Implemented
Correct answer: Perform DNS resolution on the domain and attempt to forward the request
A SIP proxy should attempt to resolve the destination domain via DNS (NAPTR/SRV/A records) and forward the request to the resolved address if it is configured to route externally.
SSCA Proxy Servers 5
What is the significance of a SIP proxy being 'transaction stateful' versus 'dialog stateful'?