The attribute lines themselves provide additional information about the session.
Some are self-explanatory. They are provided here simply for reference. Their usage
is explained in another text, Session Initiation Protocol (SIP): Controlling Convergent
Networks (McGraw-Hill, forthcoming).
Media Attribute Lines (a=)
?– cat = category
?– keywds = keywords
?– tool = name and version of tool
?– ptime = packet time
?– maxptime = maximum packet time
?– recvonly = receive-only mode
?– sendrecv = send and receive mode
?– orient = whiteboard orientation
?– type = conference type
?– charset = character set
?– sdplang = language tag
?– lang = language tag
?– framerate = frame rate
?– quality = quality
?– fmtp = format-specific parameters
?– rtpmap = rtpmap attribute
?– curr = current status attribute
?– des = desired-status attribute
?– conf = confirm-status attribute
?– mid = media stream identification attribute
?– group = group attribute
The SDP, then, is used to describe the parameters for a session, whether it is a voice
call or a Webinar. If a conference call provides Webinar-type service, there are many
86 Chapter 3
attributes that must be defined, but keep in mind that each part of a Webinar is a
separate session.
In other words, the voice portion of the Webinar is one session, while the display of a
whiteboard application is a different session.
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