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4. Requirements and Recommendations

The purpose of SDP is to convey information about media streams in multimedia sessions to allow the recipients of a session description to participate in the session. SDP is primarily intended for use with Internet protocols, although it is sufficiently general that it can describe multimedia conferences in other network environments. Media streams can be many-to-many. Sessions need not be continually active.

Thus far, multicast-based sessions on the Internet have differed from many other forms of conferencing in that anyone receiving the traffic can join the session (unless the session traffic is encrypted). In such an environment, SDP serves two primary purposes. It is a means to communicate the existence of a session, and it is a means to convey sufficient information to enable joining and participating in the session. In a unicast environment, only the latter purpose is likely to be relevant.

An SDP description includes the following:

  • Session name and purpose
  • Time(s) the session is active
  • The media comprising the session
  • Information needed to receive those media (addresses, ports, formats, etc.)

As resources necessary to participate in a session may be limited, some additional information may also be desirable:

  • Information about the bandwidth to be used by the session
  • Contact information for the person responsible for the session

In general, SDP must convey sufficient information to enable applications to join a session (with the possible exception of encryption keys) and to announce the resources to be used to any nonparticipants that may need to know. (This latter feature is primarily useful when SDP is used with a multicast session announcement protocol.)

4.1. Media and Transport Information​

An SDP description includes the following media information:

  • The type of media (video, audio, etc.)
  • The media transport protocol (RTP/UDP/IP, H.320, etc.)
  • The format of the media (H.261 video, MPEG video, etc.)

In addition to media format and transport protocol, SDP conveys address and port details. For an IP multicast session, these comprise:

  • The multicast group address for media
  • The transport port for media

This address and port are the destination address and destination port of the multicast stream, whether being sent, received, or both.

For unicast IP sessions, the following are conveyed:

  • The remote address for media
  • The remote transport port for media

The semantics of the address and port depend on context. Typically, this SHOULD be the remote address and remote port to which media is to be sent or received. Details may differ based on the network type, address type, protocol, and media specified, and whether the SDP is being distributed as an advertisement or negotiated in an offer/answer [RFC3264] exchange. (E.g., Some address types or protocols may not have a notion of port.) Deviating from typical behavior should be done cautiously since this complicates implementations (including middleboxes that must parse the addresses to open Network Address Translation (NAT) or firewall pinholes).

4.2. Timing Information​

Sessions may be either bounded or unbounded in time. Whether or not they are bounded, they may be only active at specific times. SDP can convey:

  • An arbitrary list of start and stop times bounding the session
  • For each bound, repeat times such as "every Wednesday at 10am for one hour"

This timing information is globally consistent, irrespective of local time zone or daylight saving time (see Section 5.9).

4.3. Obtaining Further Information about a Session​

A session description could convey enough information to decide whether or not to participate in a session. SDP may include additional pointers in the form of Uniform Resource Identifiers (URIs) [RFC3986] for more information about the session. (Note that use of URIs to indicate remote resources is subject to the security considerations from [RFC3986].)

4.4. Internationalization​

The SDP specification recommends the use of the ISO 10646 character set in the UTF-8 encoding [RFC3629] to allow many different languages to be represented. However, to assist in compact representations, SDP also allows other character sets such as [ISO.8859-1.1998] to be used when desired. Internationalization only applies to free-text subfields (session name and background information), and not to SDP as a whole.

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