Attributes
SDP uses attributes to extend the core protocol. Attributes can appear within the Session or Media sections and are scoped accordingly as session-level or media-level. New attributes are added to the standard occasionally through registration with IANA.
Attributes take two forms:
- A property form:
a=flagconveys a simple boolean property of the media or session. - A value form:
a=attribute:valueprovides a named parameter.
Two of these attributes are specially defined:
a=charset:encodinga=sdplang:code
The first one is used in the Sesssion or Media sections to specify another character encoding (as registered in the IANA registry) than the default one highly recommanded (UTF-8) where it is used in standard protocol keys whose values are containing a text intended to be displayed to a user. The second one is used to specify in which language it is written (alternate texts in multiple languages may be carried in the protocol, and selected automatically by the user agent according to user preferences. In both cases, each textual field in the protocol which are not interpreted symbolically by the protocol itself, will be interpreted as opaque strings, but rendered to the user or application with the values indicated in the last occurence of the charset and sdplang in the current Media section, or otherwise their last value in the Session section).
Note that the first 3 mandatory parameters (v=, s= and o=), even though they seem to contain displayable text, are not intended to be displayed to users and translated. The fields present in their values are considered in the protocol as opaque strings, they are used as identifiers, just like paths in an URL or filenames in a file system: the SDP standard indicates that they must all non empty and should be UTF-8 encoded.
A few other attributes (described as part the standard SDP specifications in the same RFC) are also shown in the example above, either as a session-level attribute (such as the attribute in property form a=recvonly) which also applies to the described medias unless they override their value, or as a media-level attribute (such as the attribute in value form a=rtpmap:99 h263-1998/90000 for the video media in the example).
Read more about this topic: Session Description Protocol
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