QuickTime Streaming

The Base Reassembler

Your packet reassembler relies on a base reassembler component to do most of the routine reassembly work. Your reassembler can modify some of the base reassembler's default behaviors simply by setting flags.With a few exceptions, your reassembler implements reassembler functions only when it needs to override the normal behavior of the base reassembler.

The base reassembler's main function is to assemble incoming packets into "chunks", then pass the chunks to a stream handler. A chunk is the amount of data useful to a particular stream handler, such as a video frame or twenty milliseconds of audio.

Your reassembler must specify the type of stream handler needed, but your reassembler doesn't work with stream handlers directly. The base reassembler does this for you.

The default behavior of the base reassembler is as follows:

The base reassembler fills out an RTPRssmPacket struct for each packet it receives.
The base reassembler puts the packets into a packet list, which corresponds to a data chunk.
By default, the packet list contains all the packets with the same RTP transmission time, which is also the sample time. The base reassembler will start a new packet list when it receives a packet with a different RTP transmission time or with the RTP marker bit set.

Your reassembler can change this default by telling the base reassembler that every packet is a chunk.

Once a packet list is complete, the base reassembler creates a chunk from it.
By default, the chunk is made by concatenating the payload of each packet in the packet list.
By default, the payload is assumed to be the entire contents of the packet following the RTP header and the payload header.

The base reassembler assumes a zero-length payload header (no payload header) unless your reassembler sets a nonzero payload header length as the default.

By default, the base reassembler discards chunks with missing packets.

The base reassembler can be set to inform your reassembler that a chunk has missing packets.

By implementing certain functions, you can cause the base reassembler to call your reassembler at one of several points to override or modify the default behavior:

Implement this function	Your reassembler will be called...
`RTPRssmAdjustPacketParams`	When each packet is received, after the base reassembler fills out the `RTPRssmPacket` struct
`RTPRssmSendPacketList`	When the packet list is complete
`RTPRssmComputeChunkSize`	When the chunk size is calculated
`RTPRssmCopyDataToChunk`	When the data is copied into the chunk record

This is discussed further in the sections "Handling Packets Yourself" and "Handling Chunks Yourself".