What is Selective Acknowledgment (SACK)?

Selective Acknowledgment (SACK) is like when you're playing a game and some pieces get lost on the way, but instead of starting over, you just say which ones are missing.

Imagine you're sending a long message in the mail. Each letter is a packet, little pieces of your message that travel one by one. If all packets arrive safely, great! But sometimes some get stuck or lost, like when a letter gets lost in the post.

Without SACK, if even one packet is missing, you have to start over and send the whole message again, like if you had to resend your entire game just because one piece didn’t make it.

But with Selective Acknowledgment, you can say: "Hey, I got most of your message!" You tell which parts arrived and which ones are still missing. That way, you only need to send the missing part again, like if you just had to resend that one lost game piece instead of the whole game.

It makes things faster and smoother, like having a helper who points out exactly what went wrong so you don’t have to redo everything!