(PT) of the RTP header. Each profile is accompanied by several payload format specifications, each of which describes the transport of particular encoded data. Examples of audio payload formats are G.711, G.723, G.726, G.729, GSM, QCELP, MP3, and DTMF, and examples of video payloads are H.261, H.263, H.264, H.265 and MPEG-1/MPEG-2. The mapping of MPEG-4 audio/video streams to RTP packets is specified in , and H.263 video payloads are described in . Examples of RTP profiles include: The RTP profile for Audio and video conferences with minimal control () defines a set of static payload type assignments, and a dynamic mechanism for mapping between a payload format, and a PT value using Session Description Protocol (SDP). The Secure Real-time Transport Protocol (SRTP) () defines an RTP profile that provides cryptographic services for the transfer of payload data. The experimental Control Data Profile for RTP (RTP/CDP) for machine-to-machine communications. Packet header RTP packets are created at the application layer and handed to the transport layer for delivery. Each unit of RTP media data created by an application begins with the RTP packet header. The RTP header has a minimum size of 12 bytes. After the header, optional header extensions may be present. This is followed by the RTP payload, the format of which is determined by the particular class of application. The fields in the header are as follows: Version: (2 bits) Indicates the version of the protocol. Current version is 2. P (Padding): (1 bit) Used to indicate if there are extra padding bytes at the end of the RTP packet. Padding may be used to fill up a block of certain size, for example as required by an encryption algorithm. The last byte of the padding contains the number of padding bytes that were added (including itself). X (Extension): (1 bit) Indicates presence of an extension header between the header and payload data. The extension header is application or profile specific. CC (CSRC count): (4 bits) Contains the number of CSRC identifiers (defined below) that follow the SSRC (also defined below). M (Marker): (1 bit) Signaling used at the application level in a profile-specific manner. If it is set, it means that the current data has some special relevance for the application. PT (Payload type): (7 bits) Indicates the format of the payload and thus determines its interpretation by the application. Values are profile specific and may be dynamically