5.1 Unicast Streams

5.1 Unicast Streams

If the offerer wishes to only send media on a stream to its peer, it MUST mark the stream as sendonly with the "a=sendonly" attribute. We refer to a stream as being marked with a certain direction if a direction attribute was present as either a media stream attribute or a session attribute. If the offerer wishes to only receive media from its peer, it MUST mark the stream as recvonly. If the offerer wishes to communicate, but wishes to neither send nor receive media at this time, it MUST mark the stream with an "a=inactive" attribute. The inactive direction attribute is specified in RFC 3108 [3]. Note that in the case of the Real Time Transport Protocol (RTP) [4], RTCP is still sent and received for sendonly, recvonly, and inactive streams. That is, the directionality of the media stream has no impact on the RTCP usage. If the offerer wishes to both send and receive media with its peer, it MAY include an "a=sendrecv" attribute, or it MAY omit it, since sendrecv is the default.

For recvonly and sendrecv streams, the port number and address in the offer indicate where the offerer would like to receive the media stream. For sendonly RTP streams, the address and port number indirectly indicate where the offerer wants to receive RTCP reports. Unless there is an explicit indication otherwise, reports are sent to the port number one higher than the number indicated. The IP address and port present in the offer indicate nothing about the source IP address and source port of RTP and RTCP packets that will be sent by the offerer. A port number of zero in the offer indicates that the stream is offered but MUST NOT be used. This has no useful semantics in an initial offer, but is allowed for reasons of completeness, since the answer can contain a zero port indicating a rejected stream (Section 6). Furthermore, existing streams can be terminated by setting the port to zero (Section 8). In general, a port number of zero indicates that the media stream is not wanted.

The list of media formats for each media stream conveys two pieces of information, namely the set of formats (codecs and any parameters associated with the codec, in the case of RTP) that the offerer is capable of sending and/or receiving (depending on the direction attributes), and, in the case of RTP, the RTP payload type numbers used to identify those formats. If multiple formats are listed, it means that the offerer is capable of making use of any of those formats during the session. In other words, the answerer MAY change formats in the middle of the session, making use of any of the formats listed, without sending a new offer. For a sendonly stream, the offer SHOULD indicate those formats the offerer is willing to send for this stream. For a recvonly stream, the offer SHOULD indicate those formats the offerer is willing to receive for this stream. For a sendrecv stream, the offer SHOULD indicate those codecs that the offerer is willing to send and receive with.

For recvonly RTP streams, the payload type numbers indicate the value of the payload type field in RTP packets the offerer is expecting to receive for that codec. For sendonly RTP streams, the payload type numbers indicate the value of the payload type field in RTP packets the offerer is planning to send for that codec. For sendrecv RTP streams, the payload type numbers indicate the value of the payload type field the offerer expects to receive, and would prefer to send. However, for sendonly and sendrecv streams, the answer might indicate different payload type numbers for the same codecs, in which case, the offerer MUST send with the payload type numbers from the answer.

Different payload type numbers may be needed in each direction because of interoperability concerns with H.323.

As per RFC 2327, fmtp parameters MAY be present to provide additional parameters of the media format.

In the case of RTP streams, all media descriptions SHOULD contain "a=rtpmap" mappings from RTP payload types to encodings. If there is no "a=rtpmap", the default payload type mapping, as defined by the current profile in use (for example, RFC 1890 [5]) is to be used.

This allows easier migration away from static payload types.

In all cases, the formats in the "m=" line MUST be listed in order of preference, with the first format listed being preferred. In this case, preferred means that the recipient of the offer SHOULD use the format with the highest preference that is acceptable to it.

If the ptime attribute is present for a stream, it indicates the desired packetization interval that the offerer would like to receive. The ptime attribute MUST be greater than zero.

If the bandwidth attribute is present for a stream, it indicates the desired bandwidth that the offerer would like to receive. A value of zero is allowed, but discouraged. It indicates that no media should be sent. In the case of RTP, it would also disable all RTCP.

If multiple media streams of different types are present, it means that the offerer wishes to use those streams at the same time. A typical case is an audio and a video stream as part of a videoconference.

If multiple media streams of the same type are present in an offer, it means that the offerer wishes to send (and/or receive) multiple streams of that type at the same time. When sending multiple streams of the same type, it is a matter of local policy as to how each media source of that type (for example, a video camera and VCR in the case of video) is mapped to each stream. When a user has a single source for a particular media type, only one policy makes sense: the source is sent to each stream of the same type. Each stream MAY use different encodings. When receiving multiple streams of the same type, it is a matter of local policy as to how each stream is mapped to the various media sinks for that particular type (for example, speakers or a recording device in the case of audio). There are a few constraints on the policies, however. First, when receiving multiple streams of the same type, each stream MUST be mapped to at least one sink for the purpose of presentation to the user. In other words, the intent of receiving multiple streams of the same type is that they should all be presented in parallel, rather than choosing just one. Another constraint is that when multiple streams are received and sent to the same sink, they MUST be combined in some media specific way. For example, in the case of two audio streams, the received media from each might be mapped to the speakers. In that case, the combining operation would be to mix them. In the case of multiple instant messaging streams, where the sink is the screen, the combining operation would be to present all of them to the user interface. The third constraint is that if multiple sources are mapped to the same stream, those sources MUST be combined in some media specific way before they are sent on the stream. Although policies beyond these constraints are flexible, an agent won't generally want a policy that will copy media from its sinks to its sources unless it is a conference server (i.e., don't copy received media on one stream to another stream).

A typical usage example for multiple media streams of the same type is a pre-paid calling card application, where the user can press and hold the pound ("#") key at any time during a call to hangup and make a new call on the same card. This requires media from the user to two destinations - the remote gateway, and the DTMF processing application which looks for the pound. This could be accomplished with two media streams, one sendrecv to the gateway, and the other sendonly (from the perspective of the user) to the DTMF application.

Once the offerer has sent the offer, it MUST be prepared to receive media for any recvonly streams described by that offer. It MUST be prepared to send and receive media for any sendrecv streams in the offer, and send media for any sendonly streams in the offer (of course, it cannot actually send until the peer provides an answer with the needed address and port information). In the case of RTP, even though it may receive media before the answer arrives, it will not be able to send RTCP receiver reports until the answer arrives.