SIP Protocol Fundamentals 1 — Questions and Answers
Question 1: Which SIP method is used to request information about the capabilities of a SIP endpoint?
- REGISTER
- OPTIONS (Correct answer)
- BYE
- INVITE
Correct answer: OPTIONS
The SIP OPTIONS method is specifically used to query a SIP endpoint or proxy server about its capabilities and supported features without establishing a call. This allows a client to determine what methods, content types, and extensions the server supports. It's crucial for interoperability, enabling endpoints to understand what types of sessions can be initiated and ensuring compatible communication.
Question 2: What does a SIP response code of 180 Ringing indicate?
- The call has been successfully established
- The request has been received and is being processed
- The destination endpoint is currently ringing (Correct answer)
- The request cannot be fulfilled due to an error
Correct answer: The destination endpoint is currently ringing
A SIP response code of 180 Ringing is a provisional response indicating that the called party's user agent has received the INVITE and is currently alerting the user. It informs the calling party that the call is progressing and the destination is being notified, often accompanied by a ringback tone. This response does not establish the call but signals that the recipient is being alerted.
Question 3: Which SIP header identifies the unique identifier for a particular SIP transaction or call?
- To
- From
- Call-ID (Correct answer)
- CSeq
Correct answer: Call-ID
The Call-ID header field in SIP identifies a unique identifier for a particular SIP transaction or call. It remains constant for all messages within a single call, allowing all related requests and responses to be grouped together. This is crucial for tracking and managing the state of a call across multiple SIP entities and for debugging purposes.
Question 4: What is the correct order of the main components in a SIP message?
- Request Line, Headers, Body (Correct answer)
- Headers, Body, Request Line
- Body, Request Line, Headers
- Request Line, Body, Headers
Correct answer: Request Line, Headers, Body
A SIP message, similar to HTTP, is structured into three main components in a specific order. It begins with the Request Line (for requests) or Status Line (for responses), followed by various Headers that provide message details and routing information. Finally, an optional Body contains session description or other payload data, ensuring consistent message parsing and processing across SIP networks.
Question 5: Which component of a SIP transaction is responsible for maintaining state between a client request and the server response?
- SIP Proxy
- SIP Registrar
- SIP Dialog (Correct answer)
- SIP Gateway
Correct answer: SIP Dialog
A SIP Dialog is a peer-to-peer relationship between two SIP user agents that persists for the duration of a call or session. It maintains state information, such as sequence numbers and route sets, between a client request and subsequent server responses. This stateful connection ensures that related messages are correctly associated and processed within the context of an ongoing session, providing continuity.
SIP Protocol Fundamentals 1
Which SIP method is used to request information about the capabilities of a SIP endpoint?