Extract from SDP definition documen

	For media whose transport protocol is RTP, SDP can be used to
     provide a dynamic binding of media encoding to RTP payload type.
     The encoding names in the RTP AV Profile do not specify unique
     audio encodings (in terms of clock rate and number of audio
     channels), and so they are not used directly in SDP format fields.
     Instead, the payload type number should be used to specify the
     format for static payload types and the payload type number along
     with additional encoding information should be used for dynamically
     allocated payload types.

     An example of a static payload type is u-law PCM coded single
     channel audio sampled at 8KHz.  This is completely defined in the
     RTP Audio/Video profile as payload type 0, so the media field for
     such a stream sent to UDP port 49232 is:

                           m=video 49232 RTP/AVP 0

     An example of a dynamic payload type is 16 bit linear encoded
     stereo audio sampled at 16KHz.  If we wish to use dynamic RTP/AVP
     payload type 98 for such a stream, additional information is
     required to decode it:

                          m=video 49232 RTP/AVP 98
                           a=rtpmap:98 L16/16000/2

     The general form of an rtpmap attribute is:

     a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
     parameters>]

     For audio streams, <encoding parameters> may specify the number of
     audio channels.  This parameter may be omitted if the number of
     channels is one provided no additional parameters are needed.  For
     video streams, no encoding parameters are currently specified.

     Additional parameters may be defined in the future, but
     codecspecific parameters should not be added.  Parameters added to
     an rtpmap attribute should only be those required for a session
     directory to make the choice of appropriate media too to
     participate in a session.  Codec-specific parameters should be
     added in other attributes.

     Up to one rtpmap attribute can be defined for each media format
     specified. Thus we might have:

                       m=audio 49230 RTP/AVP 96 97 98
                             a=rtpmap:96 L8/8000
                            a=rtpmap:97 L16/8000
                           a=rtpmap:98 L16/11025/2

     RTP profiles that specify the use of dynamic payload types must
     define the set of valid encoding names and/or a means to register
     encoding names if that profile is to be used with SDP.

     Experimental encoding formats can also be specified using rtpmap.
     RTP formats that are not registered as standard format names must
     be preceded by "X-".  Thus a new experimental redundant audio
     stream called GSMLPC using dynamic payload type 99 could be
     specified as:

                          m=video 49232 RTP/AVP 99
                          a=rtpmap:99 X-GSMLPC/8000

     Such an experimental encoding requires that any site wishing to
     receive the media stream has relevant configured state in its
     session directory to know which tools are appropriate.

     Note that RTP audio formats typically do not include information
     about the number of samples per packet.  If a non-default (as
     defined in the RTP Audio/Video Profile) packetisation is required,
     the "ptime" attribute is used as given below.