The one concern I'd have with this format is a length field getting corrupted in transit and causing an out-of-bounds memory access. The network protocols' checksums won't save you 100% of the time, especially if there's bad hardware in the loop. If every field is fixed length this is less of a concern, of course; you might get bad data but you won't get e.g. a string with length 64M.

In our system, if the message didn't unpack properly, the application would send a retransmit request with that message's sequence number. But in practice, this scenario never occurred because TCP already did this for us.

I do not remember it ever happening but being semi-paranoid I had length in 2 places - beginning and the end of the message.