So, I'm getting this data. From the network socket, or out of a file. I'm cobbling together code that will interpret the data. Read some bytes, check some flags, and some bytes indicate how much data follows. Read in that much data, rinse, repeat.
This task reminds me much to parsing source code. I'm comfy with lex/yacc and antlr, but they're not up to this task. You can't specify bits and raw bytes as tokens (well, maybe you could, but I wouldn't know how), and you can't coax them into "read two bytes, make them into an unsigned 16bit integer, call it n, and then read n bytes.".
Then again, when the spec of the protocol/data format is defined in a systematic manner (not all of them are), there should be a systematic way to read in data that is formatted according to the protocol. Right?
There's gotta be a tool that does that.