Do the simplest thing that could possibly work, and make it more complex only as necessary:
use a single TCP connection and multiplex logical sessions over it
- if these logical entities are just asynchronous request/response pairs, for example, you may be able to dispense with explicit logical sessions entirely
if you have multiple concurrent components in each instance which really need their own queues with pushback to throttle over-eager senders:
- first consider just capping the number of outstanding requests/active sessions on the sending side, instead of requiring a specific ack
- only if you need to dynamically vary queue length (eg. because you're really trying to limit working memory, which varies by session) use an explicit logical ack for this
only if you hit some case where your logical sessions really interact badly with TCP, then consider implementing your own reliable-flow-controlled-datagram protocol