📝 Session Initiation Protocol (SIP) in VOIP (65 MCQs)

📖 From Data Communication and Networks • 28. Multimedia • 65 questions available

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What is Session Initiation Protocol (SIP) in VOIP?

Session Initiation Protocol (SIP) in VOIP is a signaling protocol used to establish, modify, and terminate multimedia communication sessions such as voice and video calls over IP networks by exchanging messages that negotiate capabilities, locate endpoints, and manage call state independently of the actual media transport, enabling interoperability across diverse devices and service providers.

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📝 All Session Initiation Protocol (SIP) in VOIP MCQs

Q1. What is the primary purpose of the SIP INVITE method?

A.To terminate an existing session

B.To modify an ongoing session ✅

C.To initiate a new session

D.To query user capabilities

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The INVITE method is used to establish a new session between user agents. It carries the session description and triggers the call setup process. Options A and D refer to other functions, while C describes a different method (OPTIONS). Therefore, the correct choice is B.

Q2. Which SIP header field contains the display name and URI of the caller?

A.Contact

B.From ✅

C.To

D.Via

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The From header field identifies the originator of the request, including a display name and a SIP URI. The Contact header provides a reachable address for future requests, To indicates the recipient, and Via tracks the request path. Hence, option B correctly describes the From header.

Q3. In SIP, what does a 200 OK response to an INVITE indicate?

A.Call has been rejected

B.Call has been accepted and can be established

C.Request was malformed ✅

D.SERVER IS BUSY

💡 Difficulty: easy | ✅ Correct: C

Q4. Which SIP method is used solely for querying the capabilities of a user agent without establishing a session?

A.REGISTER

B.OPTIONS ✅

C.ACK

D.BYE

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The OPTIONS method asks a user agent to report its capabilities (e.g., supported methods, codecs) without creating a dialog. REGISTER binds an address to a user, ACK acknowledges responses, and BYE terminates a session. Therefore, OPTIONS is the correct choice.

Q5. What is the function of the SIP Via header?

A.It indicates the destination address

B.It records the path taken by the request ✅

C.It carries the session description

D.It authenticates the user

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The Via header records each hop a SIP request traverses, allowing responses to be routed back correctly. It does not specify the ultimate destination (that's the Request-URI), nor does it carry SDP or authentication data. Hence, option B is correct.

Q6. If a SIP user agent receives a 486 Busy Here response to its INVITE, what is the most appropriate next action?

A.Immediately resend the INVITE

B.Terminate the call attempt

C.Retry after a short delay

D.SEND A CANCEL REQUEST ✅

💡 Difficulty: medium | ✅ Correct: D

Q7. A SIP proxy receives an INVITE with a Contact header that includes a private IP address. Which logical inference is most likely correct?

A.The proxy must rewrite the address

B.The request is malformed ✅

C.The user agent is behind NAT

D.The proxy should reject the request

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: When a Contact header contains a private IP, it usually means the user agent resides behind a Network Address Translation (NAT) device. The proxy does not automatically rewrite the address nor reject the request solely for this reason. Hence, the inference that the user agent is behind NAT (option B) is most plausible.

Q8. Consider two SIP messages: one with a Record-Route header and another without. What can be inferred about the routing behavior?

A.Both will follow the same path

B.The one with Record-Route will enforce a strict route set ✅

C.The one without Record-Route will be blocked

D.The one with Record-Route will be rejected

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: A Record-Route header forces subsequent requests in the dialog to pass through the same proxies, establishing a strict route set. The message lacking this header does not impose such constraints, so the inference about enforced routing (option B) is correct.

Q9. If a SIP UA sends an INVITE with an SDP body offering a codec that the callee does not support, what is the logical consequence?

A.The call will be established using the unsupported codec

B.The callee will send a 488 Not Acceptable Here response

C.The INVITE will be ignored

D.THE CALL WILL FALL BACK TO THE DEFAULT CODEC ✅

💡 Difficulty: medium | ✅ Correct: D

Q10. A SIP server logs a sequence of messages: INVITE → 100 Trying → 180 Ringing → 200 OK → ACK. What does this sequence indicate about the call state?

A.The call failed before establishment

B.The call was successfully established ✅

C.The call was rejected

D.The call was paused

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The progression from INVITE through provisional responses (100, 180) to a final 200 OK, followed by an ACK, represents a successful call setup. No failure or rejection codes appear, and there is no pause indication. Hence, the call was successfully established (option B).

Q11. Which of the following best explains why SIP can operate over both TCP and UDP?

A.Because it requires reliable transport only

B.Because it needs both connection‑oriented and connectionless transport options ✅

C.Because TCP is mandatory for all SIP messages

D.Because UDP provides encryption

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: SIP is designed to be transport‑agnostic, allowing it to run over UDP (connectionless, low latency) and TCP (reliable, ordered delivery). This flexibility enables deployment in diverse network environments. TCP is not mandatory, and UDP itself does not provide encryption. Thus, option B correctly captures the rationale.

Q12. A SIP proxy receives an INVITE with a Max‑Forwards value of 0. What should the proxy logically do?

A.Forward the INVITE to the destination

B.Decrease Max‑Forwards and forward ✅

C.Return a 483 Too Many Hops response

D.Ignore the message

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: When Max‑Forwards reaches 0, the request must not be forwarded further. The proxy should generate a 483 Too Many Hops response, indicating that the request has traversed too many hops. Therefore, option B is the correct logical action.

Q13. If a SIP user agent receives a 401 Unauthorized response without a WWW‑Authenticate header, what is the most appropriate inference?

A.The server supports authentication

B.The response is malformed ✅

C.The request was successful

D.THE SERVER IS OFFLINE

💡 Difficulty: medium | ✅ Correct: B

Q14. When comparing SIP and H.323, which analytical statement is accurate?

A.SIP uses binary encoding while H.323 uses text

B.Both protocols rely on identical signaling models

C.SIP is more flexible due to its text‑based nature ✅

D.H.323 PROVIDES BETTER NAT TRAVERSAL

💡 Difficulty: medium | ✅ Correct: C

Q15. In a SIP dialog, what is the effect of adding a Route header to subsequent requests?

A.It forces the request to bypass the proxy

B.It ensures the request follows a strict path ✅

C.It changes the destination URI

D.IT REMOVES THE NEED FOR A VIA HEADER

💡 Difficulty: medium | ✅ Correct: B

Q16. A SIP registrar stores a binding of a user’s public address to a private address. Which conceptual advantage does this provide?

A.It enables end‑to‑end encryption

B.It allows users to register multiple devices ✅

C.It simplifies NAT traversal

D.It eliminates the need for DNS

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Registrars map a SIP URI to one or more contact addresses, allowing a single user to be reachable on multiple devices. While NAT traversal benefits can arise, the primary conceptual advantage is multi‑device registration. Hence, option B captures the core benefit.

Q17. Consider a SIP network where a proxy adds a Record‑Route header to an INVITE. How does this affect later BYE messages?

A.The BYE will ignore the Record‑Route

B.The BYE must follow the same route set ✅

C.The BYE will be rejected

D.The BYE will use a new route

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: When a Record‑Route header is present in the initial INVITE, the dialog establishes a strict route set. All subsequent in‑dialog requests, including BYE, must traverse the same proxies in the order recorded. Therefore, the BYE must follow the same route set, making option B correct.

Q18. A SIP implementation uses a hash‑based algorithm to generate a nonce value $N = \text{hash}(timestamp \| secret)$ . If the timestamp changes every second, what security property does this provide?

A.Confidentiality

B.Integrity ✅

C.Replay protection

D.AUTHENTICATION

💡 Difficulty: hard | ✅ Correct: B

Q19. Which analytical comparison correctly distinguishes SIP’s handling of media from its signaling?

A.SIP embeds media directly in messages

B.Media negotiation occurs via SDP within SIP ✅

C.Media is handled by a separate protocol

D.Media is negotiated using RTP

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: SIP itself carries only signaling; media description is carried in the SDP payload, and the actual media streams use RTP. Therefore, the accurate analytical distinction is that media negotiation occurs via SDP inside SIP messages, not that media is embedded or directly handled by SIP. Option B captures this.

Q20. A SIP user agent receives a 403 Forbidden response to a REGISTER request. What is the most logical next step?

A.Retry the registration immediately

B.Modify the request credentials and resend

C.Terminate the registration process ✅

D.Send a BYE request

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: A 403 response indicates that the server understood the request but refuses to fulfill it, often due to authentication or policy issues. The correct reaction is to cease registration attempts until the underlying issue is resolved, not to retry instantly or send unrelated messages. Hence, terminating the registration (option C) is the logical next step.

Q21. What conceptual relationship exists between SIP dialogs and call legs?

A.A dialog represents a single call leg

B.Multiple dialogs can exist within a single call leg ✅

C.Dialogs are unrelated to call legs

D.Each dialog maps to a media stream

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A call leg is the media path between two endpoints, while a SIP dialog is the signaling context that may control one or more call legs. Thus, multiple dialogs can be associated with a single call leg (e.g., for re‑INVITE renegotiations). Option B correctly describes the conceptual relationship.

Q22. If a SIP server implements the SIP‑outbound extension, which conceptual benefit does it provide?

A.End‑to‑end encryption

B.Improved NAT traversal ✅

C.Reduced latency

D.AUTOMATIC REGISTRATION

💡 Difficulty: hard | ✅ Correct: B

Q23. A SIP UA sends an INVITE with a Session‑Expires header of 1800 seconds. If the UA does not receive a re‑INVITE before 1800 seconds, what is the logical outcome?

A.The session continues indefinitely

B.The session is terminated by the UA ✅

C.The session is refreshed automatically

D.THE SERVER SENDS A 200 OK

💡 Difficulty: hard | ✅ Correct: B

Q24. Which analytical statement best describes the impact of using TLS over TCP for SIP signaling?

A.It reduces the need for retransmissions

B.It guarantees message ordering

C.It provides confidentiality and integrity ✅

D.It eliminates the need for SIP authentication

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: TLS over TCP encrypts the SIP messages, offering confidentiality and integrity, while TCP already ensures ordering. Retransmissions are still handled at the application layer, and authentication may still be required. Hence, the primary impact is confidentiality and integrity, making option C correct.

Q25. A SIP proxy receives an INVITE with a Contact header containing a malformed URI. What is the most appropriate logical action?

A.Forward the INVITE regardless

B.Return a 400 Bad Request ✅

C.Strip the Contact header and forward

D.Ignore the request

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A malformed Contact URI violates SIP syntax rules, and the proxy must reject the request with a 400 Bad Request response. Forwarding or stripping the header would be non‑compliant, and ignoring the request provides no feedback. Thus, returning 400 is the correct logical response.

Q26. In SIP, what is the conceptual purpose of the Allow header?

A.To list supported media codecs

B.To specify permitted SIP methods ✅

C.To indicate the maximum call duration

D.TO PROVIDE AUTHENTICATION CREDENTIALS

💡 Difficulty: hard | ✅ Correct: B

Q27. A SIP endpoint uses the P‑Asserted‑Identity header. Which conceptual advantage does this provide over the From header?

A.It hides the user's identity

B.It ensures identity integrity across trusted networks ✅

C.It encrypts the SIP message

D.It reduces message size

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: P‑Asserted‑Identity is used within trusted networks to convey the verified identity of the user, offering stronger assurance than the untrusted From header. It does not hide identity, encrypt messages, or affect size. Hence, option B accurately reflects its conceptual advantage.

Q28. Which logical inference can be made if a SIP call fails with a 404 Not Found response to an INVITE?

A.The destination user does not exist ✅

B.The network is congested

C.The caller lacks authentication

D.The proxy is misconfigured

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: A 404 response indicates that the requested user or resource cannot be found at the given address. It does not imply congestion, authentication issues, or misconfiguration of the proxy. Therefore, the logical inference is that the destination user does not exist (option A).

Q29. A SIP UA receives a 503 Service Unavailable response with a Retry‑After header of 30 seconds. What is the appropriate logical next step?

A.Immediately resend the request

B.Wait 30 seconds before retrying ✅

C.Terminate the session

D.Send a CANCEL request

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The Retry‑After header explicitly instructs the client to wait the indicated duration before sending another request. Ignoring it or terminating the session would be non‑compliant. Hence, waiting 30 seconds before retrying (option B) is the correct logical action.

Q30. Which analytical comparison correctly contrasts SIP’s stateless proxy behavior with a stateful proxy?

A.Stateless proxies store dialog state

B.Stateful proxies do not alter headers

C.Stateless proxies forward without storing ✅

D.Stateful proxies drop all requests

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: A stateless proxy forwards SIP messages without maintaining any dialog state, merely relaying them and possibly rewriting headers. In contrast, a stateful proxy retains transaction and dialog information, allowing more sophisticated processing. Therefore, the accurate analytical contrast is that stateless proxies forward without storing (option C).

Q31. What conceptual relationship exists between SIP and RTP?

A.SIP transports media, RTP carries signaling

B.SIP negotiates media parameters used by RTP ✅

C.They are unrelated protocols

D.BOTH ARE TRANSPORT LAYERS

💡 Difficulty: hard | ✅ Correct: B

Q32. A SIP client implements the PRACK method. What conceptual benefit does PRACK provide?

A.It replaces the ACK method

B.It allows early media to be reliably acknowledged ✅

C.It speeds up call setup

D.It encrypts the call

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: PRACK (Provisional Response ACK) enables reliable acknowledgment of provisional responses (e.g., 180 Ringing) that carry early media. It does not replace ACK, nor does it directly affect speed or encryption. Thus, the benefit is reliable acknowledgment of early media, option B.

Q33. If a SIP request contains multiple Via headers, what logical effect does this have on the response path?

A.The response will follow only the first Via

B.The response will follow the last Via ✅

C.The response will be sent to all Via addresses

D.THE RESPONSE WILL BE DROPPED

💡 Difficulty: hard | ✅ Correct: B

Q34. Which analytical statement best captures the trade‑off when choosing UDP over TCP for SIP transport?

A.UDP guarantees delivery

B.TCP reduces latency

C.UDP reduces overhead but may lose packets ✅

D.TCP provides built‑in encryption

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: UDP offers lower overhead and latency but lacks reliability, meaning packets can be lost, whereas TCP provides reliable delivery at the cost of additional overhead. Therefore, the trade‑off is accurately described by option C.

Q35. A SIP endpoint receives a 407 Proxy Authentication Required response without a Proxy‑Authenticate header. What logical conclusion should the endpoint draw?

A.The proxy supports authentication

B.The response is malformed ✅

C.The request succeeded

D.The server is offline

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Similar to a 401 response, a 407 must include a Proxy‑Authenticate header detailing the challenge. Its absence signals a protocol violation, making the response malformed. Hence, the logical conclusion is that the response is malformed (option B).

Q36. In SIP, what is the conceptual purpose of the Diversion header?

A.To indicate call forwarding ✅

B.To specify media codecs

C.To encrypt the message

D.TO TERMINATE A SESSION

💡 Difficulty: hard | ✅ Correct: A

Q37. A SIP registrar receives a REGISTER request with an Expires header of 0. What is the logical effect?

A.It refreshes the registration

B.It creates a new registration

C.It removes the existing registration ✅

D.It updates the contact address

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: An Expires value of 0 in a REGISTER request signals that the client wishes to deregister, causing the server to remove the existing binding. Therefore, the logical effect is the removal of the registration (option C).

Q38. Which analytical comparison correctly contrasts the use of the Replaces header with a normal re‑INVITE?

A.Replaces terminates the existing dialog

B.Re‑INVITE updates media without changing dialog identifiers ✅

C.Replaces creates a new dialog

D.Re‑INVITE adds a new route

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A re‑INVITE modifies an existing dialog (e.g., to change codecs) while preserving the dialog identifier. The Replaces header, on the other hand, allows a new request to take over an existing dialog, effectively replacing it. Therefore, the accurate contrast is that re‑INVITE updates media without changing identifiers (option B).

Q39. A SIP UA wants to ensure that a particular request is processed by a specific proxy. Which header should it use?

A.Via

B.Route ✅

C.Record‑Route

D.Contact

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The Route header explicitly directs a SIP request to travel through a designated proxy, overriding the default routing based on the Request‑URI. Via records the path taken but does not dictate the next hop, while Record‑Route is added by proxies for future requests. Hence, the correct header is Route (option B).

Q40. If a SIP endpoint receives a 486 Busy Here and the user decides to retry after 10 seconds, which conceptual SIP mechanism should be used to attempt the call again?

A.Send a new INVITE ✅

B.Send a re‑INVITE

C.Send an UPDATE

D.SEND A BYE

💡 Difficulty: hard | ✅ Correct: A

Q41. Which factual recall question best fits the 10% definition‑based quota?

A.What does SIP stand for? ✅

B.What is the purpose of the ACK method?

C.Which port is SIP typically assigned to?

D.What is the meaning of the Contact header?

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: SIP stands for Session Initiation Protocol, a signaling protocol used to establish, modify, and terminate multimedia sessions such as voice and video calls over IP networks.

Q42. What does the acronym SIP stand for in networking?

A.Session Initiation Protocol ✅

B.Secure Internet Protocol

C.Simple Interchange Procedure

D.Server Interface Package

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The acronym SIP is defined as Session Initiation Protocol, which is the signaling protocol used to establish, modify, and terminate multimedia sessions such as voice and video calls over IP networks. The other options are unrelated protocols or concepts, making the first choice the only correct definition.

Q43. Which default port does SIP use for UDP transport?

A.5060 ✅

B.5061

C.80

D.443

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: SIP traditionally listens on UDP port 5060 for unencrypted signaling traffic. Port 5061 is reserved for SIP over TLS, while ports 80 and 443 are associated with HTTP and HTTPS respectively. Therefore, the correct default UDP port for SIP is 5060.

Q44. What header field in a SIP message indicates the sequence number of a request?

A.Via

B.Call-ID

C.CSeq ✅

D.Max-Forwards

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: The CSeq (Command Sequence) header contains a numeric value that increments with each new request within a dialog, allowing the recipient to detect out‑of‑order or duplicate messages. Via, Call-ID, and Max-Forwards serve different purposes, so CSeq is the correct answer.

Q45. In SIP, what is the purpose of the 'REGISTER' method?

A.Establish media

B.Authenticate user

C.Update location ✅

D.Terminate session

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: The REGISTER method allows a user agent to inform a registrar of its current IP address and contact URI, effectively updating its location for incoming calls. It does not directly establish media, authenticate, or terminate sessions, making 'Update location' the correct purpose.

Q46. If a SIP INVITE request is sent without a 'Contact' header, what is the most likely outcome?

A.The request succeeds

B.The request fails with 400 Bad Request ✅

C.The request is redirected

D.The request is queued

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: RFC 3261 requires the Contact header in an INVITE that creates a dialog. Its absence violates the syntax rules, causing the UAS to reject the request with a 400 Bad Request response. The other outcomes are not mandated by the specification.

Q47. A SIP proxy receives an INVITE targeting user@example.com. The proxy cannot resolve the domain's DNS SRV record. What should the proxy do according to RFC 3261?

A.Return 404 Not Found

B.Return 503 Service Unavailable ✅

C.Forward to next hop

D.Drop silently

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: When DNS resolution fails, the proxy cannot locate the next hop and must indicate a temporary failure. RFC 3261 specifies a 503 Service Unavailable response in this situation, whereas 404 would imply a permanent address error.

Q48. During a SIP session, the UAC sends a re‑INVITE with SDP indicating a new media port. The UAS replies with 200 OK but retains the original port. What inference can be drawn about the UAS's media handling?

A.The UAS accepts the change

B.The UAS rejects the change ✅

C.The UAS needs renegotiation

D.The UAS ignores the SDP

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A 200 OK that does not reflect the offered SDP indicates the UAS is not willing to change its media parameters. It therefore effectively rejects the proposed change, requiring the UAC to continue using the original media settings.

Q49. If a SIP response code is 486 Busy Here, what does this imply about the callee's state, and what is the appropriate next step for the caller?

A.Callee is unavailable; retry later

B.Callee is busy; try alternative ✅

C.Callee rejected; abort call

D.Callee accepted; start media

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: A 486 response signals that the called party is currently engaged in another call. The caller should either try an alternate destination or wait and retry, rather than aborting immediately or assuming media can start.

Q50. When a SIP message includes both 'Via' and 'Record-Route' headers, what logical relationship exists between them?

A.Via precedes Record-Route

B.Record-Route precedes Via ✅

C.They are independent

D.Record-Route replaces Via

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Proxies insert a Record-Route header before they add their own Via header. Consequently, the Record-Route entries appear earlier in the header list, establishing the logical order that Record-Route precedes Via.

Q51. A SIP server logs a series of 100 Trying responses followed by a 180 Ringing, then a 200 OK. What can be deduced about the call progression?

A.The call failed

B.The call succeeded after ringing ✅

C.The call is pending

D.The call was redirected

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The sequence Trying → Ringing → 200 OK reflects the normal call setup flow: the request is being processed, the callee is alerted (ringing), and finally the callee accepts, completing the call establishment.

Q52. If a SIP dialog's CSeq number is received out of order (lower than previous), what should the recipient do according to protocol?

A.Accept anyway

B.Discard and send 500 Server Internal Error

C.Send 400 Bad Request ✅

D.Increment and accept

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: RFC 3261 requires the recipient to reject out‑of‑order CSeq numbers with a 400 Bad Request response, preventing replay attacks and ensuring proper sequencing. Accepting or silently discarding would violate protocol robustness.

Q53. Suppose a SIP user agent sends a CANCEL for a transaction that has already received a 200 OK. What is the expected behavior?

A.CANCEL is ignored ✅

B.Transaction is terminated

C.A new transaction is created

D.200 OK is retransmitted

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Once a 200 OK has been sent, the transaction is considered completed; a subsequent CANCEL has no effect and is ignored by the UAS, which continues to process the already‑established dialog.

Q54. When a SIP message is sent over TLS, which of the following guarantees confidentiality?

A.Via header

B.Contact header

C.Encryption of payload ✅

D.Use of UDP

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: TLS provides transport‑layer encryption, protecting the entire SIP message—including headers and body—from eavesdropping. The other options are unrelated to confidentiality, making encrypted payload the correct guarantee.

Q55. Compare the behavior of a SIP proxy operating in 'stateful' mode versus 'stateless' mode when handling REGISTER requests.

A.Stateful stores registration info, stateless forwards only ✅

B.Stateful forwards only, stateless stores

C.Both store registration info

D.Neither store registration info

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: A stateful proxy retains registration bindings to answer future requests, while a stateless proxy simply forwards the REGISTER without retaining any state. This distinction directly influences how subsequent INVITEs are routed.

Q56. Evaluate the impact of using SIP over TCP instead of UDP on call setup latency and reliability.

A.Lower latency, higher reliability

B.Higher latency, lower reliability

C.Lower latency, lower reliability

D.Higher latency, higher reliability ✅

💡 Difficulty: hard | ✅ Correct: D

📖 Explanation: TCP introduces a three‑way handshake and retransmission mechanisms, which increase the time before the first request reaches the peer (higher latency) but guarantee delivery (higher reliability) compared with UDP's best‑effort delivery.

Q57. Contrast the use of 'Contact' header in initial INVITE versus in subsequent re‑INVITE messages.

A.Initial Contact defines endpoint, re‑INVITE may update it ✅

B.Re‑INVITE removes Contact

C.Both always remain unchanged

D.Contact is ignored in re‑INVITE

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: During the initial INVITE, the Contact header supplies the address where the callee should send future requests. In a re‑INVITE, the Contact can be modified to reflect a new endpoint or remain unchanged, allowing dynamic endpoint updates.

Q58. Assess the security implications of exposing SIP signaling on port 5060 versus using port 5061 with TLS.

A.Port 5060 is less secure, 5061 is more secure

B.Both ports provide equal security ✅

C.Port numbers are irrelevant to security

D.5060 is more secure than 5061

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Port 5060 typically carries clear‑text SIP, making it vulnerable to eavesdropping and tampering. Port 5061 is designated for SIP over TLS, providing encryption and authentication, thereby offering a higher security level.

Q59. Which of the following best differentiates SIP's 'User Agent Client' (UAC) from 'User Agent Server' (UAS)?

A.UAC initiates requests, UAS responds ✅

B.UAC responds, UAS initiates

C.Both initiate and respond equally

D.Neither initiates nor responds

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The UAC is the entity that sends SIP requests (e.g., INVITE), whereas the UAS receives those requests and generates appropriate responses. This functional split is fundamental to SIP's request‑response model.

Q60. Analyze how the 'Max-Forwards' header influences SIP request routing in a network with multiple proxies.

A.It limits the number of hops a request can traverse ✅

B.It determines the maximum bandwidth

C.It sets the maximum retransmission timer

D.It defines the maximum call duration

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Each proxy decrements the Max-Forwards value by one before forwarding the request. When the value reaches zero, the request is rejected with a 483 Too Many Hops response, preventing infinite looping.

Q61. Given two SIP implementations, one that supports early media and another that does not, how does this affect the call flow when a 180 Ringing is received?

A.Early media allows media before 200 OK ✅

B.Both implementations behave identically

C.The non‑early‑media implementation sends 200 OK immediately

D.Early media blocks the 180 Ringing

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: If early media is supported, the callee may send media (RTP) after the 180 Ringing, allowing the caller to hear audio before the call is fully answered. The implementation lacking early media must wait for a 200 OK before media can flow.

Q62. Compare the effect of using 'p-asserted-identity' header versus 'From' header for caller identification.

A.p‑asserted‑identity is more trustworthy ✅

B.From header is more trustworthy

C.Both provide identical trust levels

D.Neither header carries identity information

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: The p‑asserted‑identity header is inserted by trusted network elements (e.g., an operator) and is therefore considered more reliable than the user‑supplied From header, which can be spoofed.

Q63. In SIP, what is the primary distinction between 'proxy' and 'redirect' server functions?

A.Proxy forwards requests, redirect sends 3xx responses ✅

B.Redirect forwards, proxy sends 3xx

C.Both perform identical functions

D.Neither handles routing

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: A proxy acts as an intermediary, forwarding the request toward its destination. A redirect server does not forward the request; instead, it returns a 3xx response directing the client to a new URI.

Q64. Evaluate the consequences of misconfiguring the 'Expires' parameter in a REGISTER request.

A.The registration persists indefinitely

B.The registration expires too early ✅

C.The registration never expires

D.The registration is ignored

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: If the Expires value is set too low, the registrar will remove the binding before the user agent expects, causing calls to fail until the client re‑registers. An excessively high value would keep stale bindings, but the primary risk is premature expiration.

Q65. If a SIP network employs NAT traversal techniques like STUN, how does this modify the SDP 'c=' line handling?

A.Private IP is replaced with public IP ✅

B.The c= line is left unchanged

C.The c= line is removed entirely

D.Multiple c= lines are added

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: STUN discovers the public IP address and port assigned by the NAT. The endpoint then substitutes the private address in

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