SIP Protocol Fundamentals 2 — Questions and Answers
Question 1: Which SIP method is used to modify an existing session, such as changing the media parameters?
- UPDATE
- RE-INVITE (Correct answer)
- MODIFY
- PATCH
Correct answer: RE-INVITE
A re-INVITE (sending INVITE within an established dialog) is used to modify session parameters like codec or media direction.
Question 2: What does the SIP 'To' header field represent in an initial request?
- The IP address of the proxy server
- The logical recipient or callee of the request (Correct answer)
- The transport protocol to use
- The session expiration time
Correct answer: The logical recipient or callee of the request
The 'To' header identifies the logical recipient of the request, typically the callee's address-of-record (AOR).
Question 3: What is the purpose of the SIP 'Contact' header?
- Identifies the caller's email address
- Provides a direct URI for reaching the UA in subsequent requests (Correct answer)
- Specifies the media codec preferences
- Lists all proxy servers in the path
Correct answer: Provides a direct URI for reaching the UA in subsequent requests
The Contact header provides a direct SIP URI (typically with an IP address) where the UA can be reached, bypassing the AOR.
Question 4: Which SIP response code class indicates a successful request?
- 1xx
- 2xx (Correct answer)
- 3xx
- 4xx
Correct answer: 2xx
2xx responses indicate success; for example, 200 OK signals that an INVITE was accepted.
Question 5: What is the role of a SIP Registrar?
- Routes SIP messages between domains
- Accepts REGISTER requests and maps AORs to device locations (Correct answer)
- Terminates media streams
- Generates SDP offers
Correct answer: Accepts REGISTER requests and maps AORs to device locations
A Registrar accepts REGISTER requests and stores the binding between a user's AOR and their current contact URI in a location service.
Question 6: In SIP, what does the term 'dialog' refer to?
- A single SIP message exchange
- A peer-to-peer SIP relationship established by certain requests (Correct answer)
- The TCP connection between two UAs
- A conference call with three or more participants
Correct answer: A peer-to-peer SIP relationship established by certain requests
A dialog is a long-term relationship between two UAs, identified by a Call-ID, local tag, and remote tag, established by requests like INVITE.
Question 7: Which header uniquely identifies a SIP transaction?
- Call-ID
- Via branch parameter (Correct answer)
- CSeq number
- From tag
Correct answer: Via branch parameter
The Via branch parameter (starting with 'z9hG4bK') uniquely identifies a SIP transaction per RFC 3261.
SIP Protocol Fundamentals 2
Which SIP method is used to modify an existing session, such as changing the media parameters?