0 8 a=rtpmap 0 PCMU/8000 a=rtpmap 8 PCMA/8000 SIP: Understanding the Session Initiation Protocol Table 7.2 SDP Attribute Values Attribute a=rtpmap: RTP/AVP list a=cat: Category of the session a=keywds: Keywords of session a=tool: Name of tool used to create SDP a=ptime: Length of time in milliseconds for each packet a=recvonly Receive only mode a=sendrecv Send and receive mode a=sendonly Send only mode a=orient: Orientation for whiteboard sessions a=type: Type of conference a=charset: Character set used for subject and information fields a=sdplang: Language for the session description a=lang: Default language for the session a=framerate: Maximum video frame rate in frames per second a=quality: Suggests quality of encoding a=fmtp: Format transport a=mid: Media identification grouping a=direction: Direction for symmetric media a=rtcp: Explicit RTCP port (and address) a=inactive Inactive mode m=video 49172 RTP/AVP 32 a=rtpmap 32 MPV/90000 The codecs are referenced by the RTP/AVP profile numbers 0, 8, and 32. The called party Marconi answers the call, chooses the second codec for the first media field and declines the second media field, only wanting a PCM A-Law audio session. o=Marconi 2890844526 2890844526 IN IP4 tower.radio.org C=IN IP4 200.201.202.203 Related Protocols t=0 0 m=audio 60000 RTP/AV 8 a=rtpmap 8 PCMA/8000 m=video 0 RTP/AVP 32 If this audio-only call is not acceptable, then Tesla would send an ACK then a BYE to cancel the call. Otherwise, the audio session would be estab- lished and RTP packets exchanged. As this example illustrates, unless the number and order of media fields is maintained, the calling party would not know for certain which media sessions were being accepted and declined by the called party. One party in a call can temporarily place the other on hold (i.e., suspend- ing the media packet sending). This is done by sending an INVITE with identical SDP to that of the original INVITE but with a=sendonly attrib- ute present. The call is made active again by sending another INVITE with the 