Delayed ACK is basically the tcp stack being smart.
Pasted:
Delayed ACK means TCP doesn't immediately ack every single received TCP segment. (When reading the following, think about an interactive ssh session, not bulk transfer.) If you receive a single lone TCP segment, you wait 100-200ms, on the assumption that the receiving application will probably generate a response of some kind. (E.g., every time sshd receives a keystroke, it typically generates a character echo in response.) You don't want the TCP stack to send an empty ACK followed by a TCP data packet 1ms later, every time, so you delay a little, so you can combine the ACK and data packet into one. So far so good. But what if the application doesn't generate any response data? Well, in that case, what difference can a little delay make? If there's no response data, then the client can't be waiting for anything, can it? Well, the application-layer client can't be waiting for anything, but the TCP stack at the end can be waiting: This is where Nagle's Algorithm enters the story:
Delayed ACK is basically the tcp stack being smart. Pasted:
Delayed ACK means TCP doesn't immediately ack every single received TCP segment. (When reading the following, think about an interactive ssh session, not bulk transfer.) If you receive a single lone TCP segment, you wait 100-200ms, on the assumption that the receiving application will probably generate a response of some kind. (E.g., every time sshd receives a keystroke, it typically generates a character echo in response.) You don't want the TCP stack to send an empty ACK followed by a TCP data packet 1ms later, every time, so you delay a little, so you can combine the ACK and data packet into one. So far so good. But what if the application doesn't generate any response data? Well, in that case, what difference can a little delay make? If there's no response data, then the client can't be waiting for anything, can it? Well, the application-layer client can't be waiting for anything, but the TCP stack at the end can be waiting: This is where Nagle's Algorithm enters the story:
Read more at TCP Performance problems caused by interaction between Nagle's Algorithm and Delayed ACK