The cover medium: RTP Packet

Figure below, reproduced verbatim from the RTP specification[4], describes the RTP packet header. Of special interest are the payload type (PT), sequence number, and timestamp fields, all of which will become relevant when building, encrypting, and steganographically embedding the message data into the packet's payload. The remainder of the packet contains an optional number of header extensions which are irrelevant to the SteganRTP communication protocol, and finally the encoded media data, otherwise known as the RTP packet's payload, which will be utilized by SteganRTP as cover-medium.

Figure: RTP packet header, from RFC-1889.

The 7-bit payload type field indicates the audio Codec used to encode the payload. The 16-bit sequence number field is a standard incrementing sequence number. The 32-bit timestamp field describes the sampling instant of the first sample in the payload, and the remaining packet data is the audio payload as encoded by the indicated Codec.