SSCA SDP Protocol 3 — Questions and Answers
Question 1: What does the 'm=' line in SDP specify?
- Media type, port, transport protocol, and payload type list (Correct answer)
- Media codec name and clock rate only
- The multicast address for a session
- The maximum number of media streams allowed
Correct answer: Media type, port, transport protocol, and payload type list
The m= line defines the media type (audio/video), port, transport protocol (e.g., RTP/AVP), and the list of payload format numbers.
Question 2: Which SDP attribute is used to carry SIP session-level bandwidth information for RTCP?
- b=RS and b=RR (Correct answer)
- a=maxptime
- a=bandwidth
- b=TIAS
Correct answer: b=RS and b=RR
b=RS (RTCP sender bandwidth) and b=RR (RTCP receiver bandwidth) specify the bandwidth allocated for RTCP streams.
Question 3: In SDP, what is the role of the 'a=fmtp' attribute?
- Provides format-specific parameters for a payload type (Correct answer)
- Defines the frame size for audio codecs
- Lists fallback payload types
- Specifies the RTCP feedback mechanism
Correct answer: Provides format-specific parameters for a payload type
The a=fmtp attribute conveys additional codec-specific parameters not covered by a=rtpmap, such as packetization-mode for H.264.
Question 4: What is a 'session-level' SDP attribute as opposed to a 'media-level' attribute?
- An attribute that applies to the entire session unless overridden at the media level (Correct answer)
- An attribute that only affects video streams
- An attribute defined inside an m= block
- An attribute that cannot be overridden
Correct answer: An attribute that applies to the entire session unless overridden at the media level
Session-level attributes appear before any m= line and apply globally, but can be overridden by attributes within individual m= sections.
Question 5: Which transport protocol value in the m= line indicates secure RTP (SRTP)?
- RTP/SAVP (Correct answer)
- RTP/AVP
- SRTP/AVP
- UDP/TLS
Correct answer: RTP/SAVP
RTP/SAVP (Secure Audio/Video Profile) in the m= line indicates that SRTP is to be used for media transport.
Question 6: In SDP, what does the 'a=ptime' attribute define?
- The length of time represented by each RTP audio packet in milliseconds (Correct answer)
- The maximum allowed packet size in bytes
- The preferred codec priority order
- The pause time between media bursts
Correct answer: The length of time represented by each RTP audio packet in milliseconds
The a=ptime attribute specifies the packetization interval — how many milliseconds of audio each RTP packet should contain.
Question 7: How does SDP handle IPv6 addresses in the connection (c=) line?
- By specifying 'IP6' as the address type instead of 'IP4' (Correct answer)
- By enclosing the address in square brackets
- IPv6 is not supported in SDP
- By using a special 'v6=' line
Correct answer: By specifying 'IP6' as the address type instead of 'IP4'
SDP supports IPv6 by using 'IN IP6' followed by the IPv6 address in the c= line, as defined in RFC 4566.
SSCA SDP Protocol 3
What does the 'm=' line in SDP specify?