Can't you just use an atomic counter for requests/responses per connection, and increment/decrement it when you receive/send a packet, checking that it is posit

Can't you just use an atomic counter for requests/responses per connection, and increment/decrement it when you receive/send a packet, checking that it is positive before you try to send, and being fine if it briefly dips into the negative region because there shouldn't be too many fighting over being able to queue request? And even if, you should be able to find instructions or so that do a bounded-decrement that does not go below zero, and still ensures that no count are lost.