SSCA Security, QoS & Regulatory Compliance 2 — Questions and Answers
Question 1: Which SIP security mechanism uses a challenge-response scheme based on MD5 to authenticate users?
- TLS mutual authentication
- Digest Authentication (Correct answer)
- S/MIME signing
- SRTP key exchange
Correct answer: Digest Authentication
SIP Digest Authentication (RFC 3261) uses an MD5-based challenge-response mechanism where the server issues a nonce and the client responds with a hashed credential.
Question 2: A SIP proxy receives a REGISTER request from a user. Which response code does it send to initiate Digest Authentication?
- 403 Forbidden
- 407 Proxy Authentication Required
- 401 Unauthorized (Correct answer)
- 400 Bad Request
Correct answer: 401 Unauthorized
A registrar or UAS sends 401 Unauthorized with a WWW-Authenticate header to challenge a REGISTER request, prompting the client to resend with credentials.
Question 3: Which DSCP value is the IETF-recommended marking for interactive voice traffic to achieve Expedited Forwarding (EF) per-hop behavior?
- DSCP 46 (EF) (Correct answer)
- DSCP 0 (BE)
- DSCP 26 (AF31)
- DSCP 34 (AF41)
Correct answer: DSCP 46 (EF)
DSCP 46 maps to the Expedited Forwarding (EF) PHB, which provides low latency and low jitter treatment required for real-time voice.
Question 4: CALEA requires US telecommunications carriers to do which of the following?
- Encrypt all voice calls with AES-256
- Provide lawful intercept capability to authorized agencies (Correct answer)
- Block international SIP traffic without a license
- Store all call recordings for 7 years
Correct answer: Provide lawful intercept capability to authorized agencies
The Communications Assistance for Law Enforcement Act (CALEA) mandates that carriers build lawful intercept interfaces so authorized law enforcement can access communications.
Question 5: Which attack involves an adversary flooding a SIP server with spoofed REGISTER messages to exhaust memory or processing resources?
- SIP registration hijacking
- INVITE flood (DoS)
- REGISTER flood (Correct answer)
- RTP injection
Correct answer: REGISTER flood
A REGISTER flood attack sends large volumes of REGISTER requests—often with spoofed source IPs—to exhaust server resources and cause a denial of service.
Question 6: In QoS terms, what does 'jitter' refer to in the context of VoIP?
- Packet loss rate measured per second
- Variation in packet arrival delay (Correct answer)
- Total one-way end-to-end delay
- The difference between send and receive bitrates
Correct answer: Variation in packet arrival delay
Jitter is the statistical variance (variation) in the inter-packet arrival times, which causes audio quality problems if not compensated by a jitter buffer.
Question 7: Which SIP header is used to carry the SRTP master key parameters negotiated via SDP in a secure call setup?
- Encryption: header
- a=crypto: SDP attribute (Correct answer)
- SDES-SRTP in P-header
- Via branch parameter
Correct answer: a=crypto: SDP attribute
The 'a=crypto:' SDP attribute (SDP Security Descriptions, RFC 4568) carries the SRTP master key, salt, and cipher suite during offer/answer negotiation.
SSCA Security, QoS & Regulatory Compliance 2
Which SIP security mechanism uses a challenge-response scheme based on MD5 to authenticate users?