Dynamic payload types are described in the RTP A/V Profile. Unlike static payload types, dynamic payload types are not assigned in the RTP A/V Profile or by IANA. They map an RTP payload type to an AUDIO and video encoding for the duration of a session. Different members of a session could, but typically do not, use different mappings. Dynamic payload types use the RANGE 96 to 127. They are assigned by means outside of the RTP profile or protocol specification, including

session descriptions like SDP (using the a:rtpmap parameter), used in announcements and invitations (e.g., SIP);

for example:

m=audio 12345 RTP/AVP/121

a=rtpmap:121 RT24

other signaling protocols (HOWEVER, H.245 does not appear to have a mechanism for doing this, at least not for non-ITU protocols).

Note that a number of encodings are described in the RTP A/V profile which do not have a static (permanent) payload type. The RTP A/V Profile defines names for encodings which may be used by SDP or other mechanisms to specify the mapping. Encodings may ALSO be identified by object identifiers or other names.

SINCE the space for payload types is limited, only very common encodings should be assigned static types. These are typically audio and video encodings "blessed" by international standardization bodies, such as the G. series of ITU-T audio encodings. The RTP A/V Profile defines a set of criteria for making static assignments.

Dynamic payload types are described in the RTP A/V Profile. Unlike static payload types, dynamic payload types are not assigned in the RTP A/V Profile or by IANA. They map an RTP payload type to an audio and video encoding for the duration of a session. Different members of a session could, but typically do not, use different mappings. Dynamic payload types use the range 96 to 127. They are assigned by means outside of the RTP profile or protocol specification, including

session descriptions like SDP (using the a:rtpmap parameter), used in announcements and invitations (e.g., SIP);

for example:

m=audio 12345 RTP/AVP/121

a=rtpmap:121 RT24

other signaling protocols (however, H.245 does not appear to have a mechanism for doing this, at least not for non-ITU protocols).

Note that a number of encodings are described in the RTP A/V profile which do not have a static (permanent) payload type. The RTP A/V Profile defines names for encodings which may be used by SDP or other mechanisms to specify the mapping. Encodings may also be identified by object identifiers or other names.

Since the space for payload types is limited, only very common encodings should be assigned static types. These are typically audio and video encodings "blessed" by international standardization bodies, such as the G. series of ITU-T audio encodings. The RTP A/V Profile defines a set of criteria for making static assignments.