The TCP Acknowledgements you see in the diagram's green arrows, have ACK numbers separated by 3000(two 1500 bytes).

I have a question, why the Ack number is separated by two packets? Or is it just convenient for the drawing? Should every TCP packet have a Ack reply?

1 Answer 1


This is a standard TCP feature (see RFC 1122) called Delayed ACK. The spec says you can wait before ACKing a packet but :

  • You shouldn't wait more than 500ms
  • You should ack every other packet if you are receiving full size packets.

It cuts down on the number of ACKs that you need to send.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.