tcp - When does a Java socket send an ack?

Question

My question is that when a socket at the receiver-side sends an ack? At the time the application read the socket data or when the underlying layers get the data and put it in the buffer?

I want this because I want both side applications know whether the other side took the packet or not.

Answer 1

It's up to the operating system TCP stack when this happens, since TCP provides a stream to the application there's no guarenteed 1:1 correlation between the application doing read/writes and the packets sent on the wire and the TCP acks.

If you need to be assured the other side have received/processed your data, you need to build that into your application protocol - e.g. send a reply stating the data was received.