A session description is composed of general information that applies to the whole session (such as the timing) followed by sections that are specific to each medium. Thus in the above description, the ``a=orient:portrait'' line applies to the ``application/wb'' medium, whereas the ``a=recvonly'' applies to the whole session.
For each medium SDP includes the type of media (video, audio, etc), the transport protocol (RTP/UDP/IP, H.320, etc), the format of the media (H.261 video, MPEG video, etc), and media or codec specific attributes.
For an IP multicast session, the multicast address for that medium and the transport port are also conveyed. This address and port are the destination address and destination port of the multicast stream, whether being sent, received, or both.
For an IP unicast session, the remote address for the media stream and the transport port for the contact address are given. This makes sense in the context of an invitation such as SIP, where the person making the call gives information about where the person being called should send his audio and video streams. The response to the invitation would then give similar information about where the caller should send her audio and video streams.