3. Registering Additional Encodings

3. Registering Additional Encodings

This profile lists a set of encodings, each of which is comprised of a particular media data compression or representation plus a payload format for encapsulation within RTP. Some of those payload formats are specified here, while others are specified in separate RFCs. It is expected that additional encodings beyond the set listed here will be created in the future and specified in additional payload format RFCs.

This profile also assigns to each encoding a short name which MAY be used by higher-level control protocols, such as the Session Description Protocol (SDP), RFC 2327 [6], to identify encodings selected for a particular RTP session.

In some contexts it may be useful to refer to these encodings in the form of a MIME content-type. To facilitate this, RFC 3555 [7] provides registrations for all of the encodings names listed here as MIME subtype names under the "audio" and "video" MIME types through the MIME registration procedure as specified in RFC 2048 [8].

Any additional encodings specified for use under this profile (or others) may also be assigned names registered as MIME subtypes with the Internet Assigned Numbers Authority (IANA). This registry provides a means to insure that the names assigned to the additional encodings are kept unique. RFC 3555 specifies the information that is required for the registration of RTP encodings.

In addition to assigning names to encodings, this profile also assigns static RTP payload type numbers to some of them. However, the payload type number space is relatively small and cannot accommodate assignments for all existing and future encodings. During the early stages of RTP development, it was necessary to use statically assigned payload types because no other mechanism had been specified to bind encodings to payload types. It was anticipated that non-RTP means beyond the scope of this memo (such as directory services or invitation protocols) would be specified to establish a dynamic mapping between a payload type and an encoding. Now, mechanisms for defining dynamic payload type bindings have been specified in the Session Description Protocol (SDP) and in other protocols such as ITU-T Recommendation H.323/H.245. These mechanisms associate the registered name of the encoding/payload format, along with any additional required parameters, such as the RTP timestamp clock rate and number of channels, with a payload type number. This association is effective only for the duration of the RTP session in which the dynamic payload type binding is made. This association applies only to the RTP session for which it is made, thus the numbers can be re-used for different encodings in different sessions so the number space limitation is avoided.

This profile reserves payload type numbers in the range 96-127 exclusively for dynamic assignment. Applications SHOULD first use values in this range for dynamic payload types. Those applications which need to define more than 32 dynamic payload types MAY bind codes below 96, in which case it is RECOMMENDED that unassigned payload type numbers be used first. However, the statically assigned payload types are default bindings and MAY be dynamically bound to new encodings if needed. Redefining payload types below 96 may cause incorrect operation if an attempt is made to join a session without obtaining session description information that defines the dynamic payload types.

Dynamic payload types SHOULD NOT be used without a well-defined mechanism to indicate the mapping. Systems that expect to interoperate with others operating under this profile SHOULD NOT make their own assignments of proprietary encodings to particular, fixed payload types.

This specification establishes the policy that no additional static payload types will be assigned beyond the ones defined in this document. Establishing this policy avoids the problem of trying to create a set of criteria for accepting static assignments and encourages the implementation and deployment of the dynamic payload type mechanisms.

The final set of static payload type assignments is provided in Tables 4 and 5.