The page that never arrived (TCP reliable delivery)

The postcard and the phone call. This lesson stays on the call after the handshake and asks what happens when a page read down the line goes missing. The lab at the end of the story lets you set how often that happens.

Before you start. You know from TCP vs UDP, and the 3-way handshake that the network only carries postcards: chunks of data, called packets, that arrive or silently don’t. TCP builds a phone call on top of them, opened by a three-message handshake in which each side says a random starting number and hears it read back. This lesson is what happens after the handshake: how every page arrives, in order, when postcards keep getting lost.