SSCA SDP Protocol 5 — Questions and Answers
Question 1: What is the significance of the session version number in the SDP 'o=' line?
- It increments with each updated SDP offer/answer to signal a change (Correct answer)
- It specifies which version of RTP to use
- It identifies the SIP dialog version
- It is a fixed value set by the SDP library
Correct answer: It increments with each updated SDP offer/answer to signal a change
The session version in the o= line must be incremented each time the SDP is modified so that the remote party can detect that the session description has changed.
Question 2: In SDP, what does 'a=recvonly' signify when included in an offer?
- The offerer wants to receive media but not send it (Correct answer)
- The offerer wants to send media but not receive it
- The offerer wants full bidirectional media
- The offerer wants to pause the media stream
Correct answer: The offerer wants to receive media but not send it
An 'a=recvonly' offerer indicates it will only receive media from the remote endpoint, not transmit any.
Question 3: How does SDP represent multicast sessions in the connection (c=) line?
- By specifying a multicast address with an optional TTL value appended (Correct answer)
- By adding 'multicast=true' as a session attribute
- By using a separate 'm=multicast' line
- Multicast is handled outside SDP via IGMP only
Correct answer: By specifying a multicast address with an optional TTL value appended
SDP represents multicast by including a multicast IP address in the c= field, optionally followed by a slash and TTL value (e.g., 'IN IP4 224.2.1.1/127').
Question 4: What is the purpose of the 'a=maxptime' attribute in SDP?
- Specifies the maximum packet duration in milliseconds the receiver can accept (Correct answer)
- Sets the maximum number of RTP packets per second
- Limits the maximum codec bitrate
- Defines the largest allowed RTP payload size in bytes
Correct answer: Specifies the maximum packet duration in milliseconds the receiver can accept
The a=maxptime attribute tells the sender the maximum length of time (in ms) of audio that can be encapsulated in a single RTP packet the receiver will accept.
Question 5: In the SDP offer/answer model, what RFC defines the core rules for offer and answer negotiation?
- RFC 3264 (Correct answer)
- RFC 4566
- RFC 3261
- RFC 2327
Correct answer: RFC 3264
RFC 3264 (An Offer/Answer Model with Session Description Protocol) defines the rules for how SDP is used in offer/answer exchanges within SIP.
Question 6: Which SDP attribute is used to identify the mid (media identification) tag for BUNDLE grouping?
- a=mid (Correct answer)
- a=id
- a=tag
- a=label
Correct answer: a=mid
The a=mid attribute assigns a unique identifier to each media section, allowing the a=group:BUNDLE attribute to reference which streams are bundled together.
Question 7: What does a port value of '9' in the SDP m= line conventionally indicate in WebRTC?
- ICE is being used and the actual port will be determined by candidate negotiation (Correct answer)
- The media stream is disabled
- The media uses port 9 as a fallback
- Port 9 is reserved for RTCP in WebRTC
Correct answer: ICE is being used and the actual port will be determined by candidate negotiation
In WebRTC SDP, a port of 9 in the m= line is a placeholder indicating ICE is in use; the actual usable address and port come from the a=candidate lines.
SSCA SDP Protocol 5
What is the significance of the session version number in the SDP 'o=' line?