When to Send an ACK Segment

During one of my experimental studies I noticed a pattern in TCP ACK’s frequency.
ACK segments are sent after receiving every 2 TCP segment. I kept asking why not 3 or 1 but 2. Then I found the answer in RFC1122 “Requirements for Internet Hosts — Communication Layers” when I was trying to find some information about delayed ACK concept. I am also quoting the related section. If you have also asked yourself, this is possibly the reason behind this pattern!

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