SSCA Proxy Servers 2 — Questions and Answers
Question 1: What is the primary difference between a stateful SIP proxy and a stateless SIP proxy?
- A stateful proxy retains transaction state while a stateless proxy forwards each message independently (Correct answer)
- A stateful proxy only handles INVITE requests while a stateless proxy handles all methods
- A stateful proxy requires TLS while a stateless proxy uses UDP only
- A stateful proxy modifies the SDP body while a stateless proxy does not
Correct answer: A stateful proxy retains transaction state while a stateless proxy forwards each message independently
A stateful proxy maintains transaction state and can make routing decisions based on prior messages, while a stateless proxy treats each message independently.
Question 2: When a SIP proxy adds a Record-Route header to an INVITE request, what is its primary purpose?
- To record the call duration for billing purposes
- To ensure subsequent in-dialog requests route through the proxy (Correct answer)
- To indicate the maximum number of proxies allowed in the path
- To specify the codec preferences of the proxy
Correct answer: To ensure subsequent in-dialog requests route through the proxy
A proxy inserts a Record-Route header so that it remains in the signaling path for all subsequent in-dialog requests (e.g., re-INVITE, BYE).
Question 3: Which SIP header is decremented by each proxy that forwards a request, and causes the request to be rejected with a 483 response when it reaches zero?
- Expires
- Max-Forwards (Correct answer)
- Retry-After
- CSeq
Correct answer: Max-Forwards
Each proxy decrements the Max-Forwards header by 1, and when it reaches 0 the proxy returns a 483 Too Many Hops response.
Question 4: In SIP loose routing, what distinguishes a loose-route URI from a strict-route URI?
- Loose-route URIs contain the 'lr' parameter in the Route header (Correct answer)
- Loose-route URIs use the SIPs scheme instead of SIP
- Loose-route URIs always include a port number
- Loose-route URIs are placed in the Contact header instead of Route
Correct answer: Loose-route URIs contain the 'lr' parameter in the Route header
A URI with the 'lr' parameter in the Route header indicates loose routing, where the proxy forwards based on the Request-URI without requiring it to be the proxy's own address.
Question 5: What type of SIP proxy forking sends an INVITE simultaneously to multiple potential destinations?
- Sequential forking
- Parallel forking (Correct answer)
- Recursive forking
- Blind forking
Correct answer: Parallel forking
Parallel forking sends the INVITE to multiple destinations at the same time, while sequential forking tries each destination one at a time.
Question 6: When a SIP proxy challenges a UAC for credentials, which response code does it use?
- 401 Unauthorized
- 403 Forbidden
- 407 Proxy Authentication Required (Correct answer)
- 491 Request Pending
Correct answer: 407 Proxy Authentication Required
A proxy uses 407 Proxy Authentication Required to challenge the client, while 401 Unauthorized is used by UAS/registrars.
Question 7: Which header does a UAC include in a re-submitted request after receiving a 407 challenge from a proxy?
- Authorization
- Proxy-Authorization (Correct answer)
- WWW-Authenticate
- Proxy-Authenticate
Correct answer: Proxy-Authorization
After receiving a 407, the UAC re-sends the request with a Proxy-Authorization header containing the credentials.
SSCA Proxy Servers 2
What is the primary difference between a stateful SIP proxy and a stateless SIP proxy?