The first byte of the header consists of a three bit version, a downstream flag, and a compressed flag, than a 3 bit type value. After that, a nodeID byte, session byte, frame counter, and fragment number are passed to complete the header. After that, the rest of length is a part of a message that is not related to the header.
If the compression bit is set, the rest of the message has received brotli compression and requires decompression.
One every fragment zero of a transaction, the first two bytes of the message is the byte length as an unsigned 16-bit integer. After that, a 8-bit checksum follows and the payload than follows.