Proxy Servers Flashcards

What is the significance of a SIP proxy being 'transaction stateful' versus 'dialog stateful'?

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.

When a SIP proxy performs load balancing across multiple servers, which approach distributes requests based on measured server performance?

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.

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?

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.

A SIP proxy receives an INVITE with no Route headers and the Request-URI is a SIP AOR. What should the proxy do next?

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.

What is the purpose of the 'Proxy-Require' header in a SIP request?

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.

In a SIP network, what is the function of an 'edge proxy' or 'Session Border Controller (SBC)' compared to a core proxy?

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.

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?

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.