How does TCP connection terminate if one of the machine dies?

Question

If a TCP connection is established between two hosts (A & B), and lets say host A has sent 5 octets to host B, and then the host B crashes (due to unknown reason). The host A will wait for acknowledgments, but on not getting them, will resend octets and also reduce the sender window size. This will repeat couple times till the window size shrinks to zero because of packet loss. My question is, what will happen next?

Answer 1

In this case, TCP eventually times out waiting for the ack's and return an error to the application. The application have to read/recv from the TCP socket to learn about that error, a subsequent write/send call will fail as well. Up till the point that TCP determined that the connection is gone, write/send calls will not fail, they'll succeed as seen from the application or block if the socket buffer is full.

In the case your host B vanishes after it has sent its ACKs, host A will not learn about that until it sends something to B, which will eventually also time out, or result in an ICMP error. (Typically the first write/send call will not fail as TCP will not fail the connection immediately, and keep in mind that write/send calls does not wait for ACKs until they complete).

Note also that retransmission does not reduce the window size.