A Baichuan message: a header and the bytes after it, split at
header.payloadOffset.
In a reply the split separates the XML body from binary data such as a snapshot. In a request it separates the channel extension from the command's XML. Each part is encrypted on its own.
The decoded header. bodyLength covers body and payload together.
Bytes before payloadOffset, as sent on the wire, still encrypted.
Bytes after payloadOffset, empty for most messages.