I've always found the bitfields in the TCP header to be either too loosely defined or the standard insufficiently lax, since there are many combinations that ma

I've always found the bitfields in the TCP header to be either too loosely defined or the standard insufficiently lax, since there are many combinations that make theoretical sense but don't work well or at all in practice. E.g. SYN+ACK+FIN+DATA would be perfect for a very short-lived connection that transfers a single segment. IMHO they should've either just used a "state" field that enumerates all possible states a packet is intended to have, or usefully defined all the possible combinations of the flags.