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Proxy Servers Flashcards

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  1. When a SIP proxy detects a routing loop, which response code should it return?

    Answer: 482 Loop Detected

    A proxy returns 482 Loop Detected when it finds its own address in the Via header stack, indicating the request has looped back.

  2. What is the purpose of the 'branch' parameter in the Via header added by a SIP proxy?

    Answer: It identifies the transaction and must start with the magic cookie 'z9hG4bK'

    The branch parameter uniquely identifies a transaction and RFC 3261 requires compliant implementations to begin it with the magic cookie 'z9hG4bK'.

  3. How does a SIP proxy using strict routing (RFC 2543 behavior) differ in handling the Request-URI compared to loose routing?

    Answer: Strict routing requires the proxy's address to be in the Request-URI, which the proxy replaces with the next Route header

    In strict routing, the proxy must be the Request-URI target; it pops itself from the route set and places the next route entry as the new Request-URI.

  4. Which SIP method is a proxy NOT permitted to fork by default, according to RFC 3261?

    Answer: REGISTER

    REGISTER requests should not be forked by a proxy because they are intended for a specific registrar, not multiple destinations.

  5. What information does a SIP proxy use from the location service (registrar database) when routing an incoming request?

    Answer: The registered Contact URI(s) mapped to the Address of Record (AOR)

    The proxy queries the location service using the AOR from the Request-URI to retrieve the registered Contact URI(s) for the actual device(s) to forward the request to.

  6. When a proxy receives multiple 2xx responses due to parallel forking, what must it do with the additional 2xx responses after forwarding the first one?

    Answer: Send ACK and then BYE to terminate the extra dialogs

    For parallel forking, after forwarding the first 2xx, the proxy must acknowledge and tear down the remaining answered branches with ACK+BYE to avoid multiple active sessions.

  7. What does a SIP proxy do when it receives a response and needs to forward it upstream?

    Answer: It removes the topmost Via header and forwards to the address in the next Via header

    When forwarding a response upstream, a proxy removes its own (topmost) Via header and sends the response to the address specified in the next Via header.

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  1. thesipschool.com — Test delivery, fees, exam format, renewal, retake policy, score release, time limit, score validity, retake fee · verified Oct 2026