Behavior of a Sender

As noted in Chapter 1, An Introduction to RTP, a sender is responsible for capturing audiovisual data, whether live or from a file, compressing it for transmission, and generating RTP packets. It may also participate in error correction and congestion control by adapting the transmitted media stream in response to receiver feedback. Figure 1.2 in Chapter 1 shows the process.

The sender starts by reading uncompressed media data ”audio samples or video frames ”into a buffer from which encoded frames are produced. Frames may be encoded in several ways depending on the compression algorithm used, and encoded frames may depend on both earlier and later data. The next section, Media Capture and Compression, describes this process.

Compressed frames are assigned a timestamp and a sequence number, and loaded into RTP packets ready for transmission. If a frame is too large to fit into a single packet, it may be fragmented into several packets for transmission. If a frame is small, several frames may be bundled into a single RTP packet. The section titled Generating RTP Packets later in this chapter describes these functions, both of which are possible only if supported by the payload format. Depending on the error correction scheme in use, a channel coder may be used to generate error correction packets or to reorder frames for interleaving before transmission (Chapters 8 and 9 discuss error concealment and error correction). The sender will generate periodic status reports , in the form of RTCP packets, for the media streams it is generating. It will also receive reception quality feedback from other participants , and it may use that information to adapt its transmission. RTCP was described in Chapter 5, RTP Control Protocol.