QuickTime Streaming

Writing Media Packetizers and Reassemblers

Media packetizers are components that understand how to break QuickTime media into packets for RTP transmission. Packet reassemblers are components that understand how to put those packets back together to reconstitute the media. Packetizers and reassemblers are written in pairs; one to packetize for transmission, the other to reassemble during reception. Media packetizers are used during live transmissions and during the creation of hint tracks. Packet reassemblers are used when receiving streaming content.

RTP transmission is a best-effort delivery system. Over the Internet, it is inherently lossy. Packets may be dropped and not retransmitted; they may arrive out of order or after substantial delay. For streaming to work under these conditions, the sender and receiver must deal intelligently with unpredictable loss. In QuickTime, this intelligence resides primarily in the packetizers and reassemblers.

For example, a packetizer may break a video frame into a grid of independently coherant rectangles. If a packet is lost, only one rectangle needs to be discarded; the rest of the video frame can be reconstructed. The reassembler in this case might substitute the old rectangle for that part of the grid when a packet is lost, or slowly fade that rectangle to black, or use some other recovery scheme.

QuickTime includes a generic packetizer/reassembler pair that works with most QuickTime content. It also includes specialized packetizer/reassembler pairs optimized for specific media types and compression formats, such as Sorenson video, Qualcomm Purevoice audio, and QDesign music.

You can write your own packetizer/reassembler pairs and add them to QuickTime. You might want to do this if you have written your own codec, for example, or if you have a particularly clever scheme for packing media or recovering from packet loss.

Writing Media Packetizers and Reassemblers

Passing Non-Media Data

Writing a Media Packetizer

Writing a Packet Reassembler