We already use 7 bits for the flags of the QUERY message, and since they are encoded with a fixed size byte, we may be forced to change the structure of the message soon, and I'd like to do this in version 5 but without wasting bytes on the wire. Therefore, I propose to convert fixed flag's bytes to unsigned vints, as defined in
CASSANDRA-9499. The only exception would be the flags in the frame, which should stay as fixed size.
Up to 7 bits, vints are encoded the same as bytes are, so no immediate change would be required in the drivers, although they should plan to support vint flags if supporting version 5. Moving forward, when a new flag is required for the QUERY message, and eventually when other flags reach 8 bits in other messages too, the flag's bitmaps would be automatically encoded with a size that is big enough to accommodate all flags, but no bigger than required. We can currently support up to 8 bytes with unsigned vints.
I would also like to add the list of versions to the SUPPORTED message, in order to simplify the handshake for drivers that prefer to send an OPTION message, rather than rely on receiving an error for an unsupported version in the STARTUP message. Said error should also contain the full list of supported versions, not just the min and max, for clarity, and because the latest version is now a beta version.
Finally, we currently store versions as integer constants in Server.java, and we still have a fair bit of hard-coded numbers in the code, especially in tests. I plan to clean this up by introducing a ProtocolVersion enum.