Background: I’ve spent a while working with a variety of device interfaces and have seen a lot of protocols, many serial and UDP in which data integrity is handled at the application protocol level. I’ve been seeking to improve my receive routine handling of protocols in general, and considering the “ideal” design of a protocol.
My question is: is there any protocol framing scheme out there that can definitively identify corrupt data in all cases? For example, consider the standard framing scheme of many protocols:
Field: Length in bytes
<SOH>: 1
<other framing information>: arbitrary, but fixed for a given protocol
<length>: 1 or 2
<data payload etc.>: based on length field (above)
<checksum/CRC>: 1 or 2
<ETX>: 1
For the vast majority of cases, this works fine. When you receive some data, you search for the SOH (or whatever your start byte sequence is), move forward a fixed number of bytes to your length field, and then move that number of bytes (plus or minus some fixed offset) to the end of the packet to your CRC, and if that checks out you know you have a valid packet. If you don’t have enough bytes in your input buffer to find an SOH or to have a CRC based on the length field, then you wait until you receive enough to check the CRC. Disregarding CRC collisions (not much we can do about that), this guarantees that your packet is well formed and uncorrupted.
However, if the length field itself is corrupt and has a high value (which I’m running into), then you can’t check the (corrupt) packet’s CRC until you fill up your input buffer with enough bytes to meet the corrupt length field’s requirement.
So is there a deterministic way to get around this, either in the receive handler or in the protocol design itself? I can set a maximum packet length or a timeout to flush my receive buffer in the receive handler, which should solve the problem on a practical level, but I’m still wondering if there’s a “pure” theoretical solution that works for the general case and doesn’t require setting implementation-specific maximum lengths or timeouts.
Thanks!
The reason why all protocols I know of, including those handling “streaming” data, chop up the datastream in smaller transmission units each with their own checks on board is exactly to avoid the problems you describe. Probably the fundamental flaw in your protocol design is that the blocks are too big.
The accepted answer of this SO question contains a good explanation and a link to a very interesting (but rather heavy on math) paper about this subject.
So in short, you should stick to smaller transmission units not only because of practical programming related arguments but also because of the message length’s role in determining the security offered by your crc.