TCP

How a web page arrives whole when the internet drops packets

IP only tries its best: packets get lost, damaged or reordered. TCP (Transmission Control Protocol) runs on both computers on top of IP and turns that into a reliable byte stream: the app gets exactly what was sent, in order.

Each byte gets a sequence number, and the receiver sends acknowledgements (ACKs) naming the next byte it expects. A three-way handshake first swaps starting numbers: SYN ("mine start at X"), SYN-ACK ("mine at Y"), ACK.

TCP

Web pages, SSH and email all ride on TCP

Loading a page over HTTP/1.1 or HTTP/2, logging into a server with SSH, and sending email with SMTP all use TCP connections. A missing byte in HTML or a keystroke would break them, so they want every byte, in order.

Video calls usually go the other way and use UDP, a bare protocol with no retransmission. Audio that arrives half a second late is useless, and TCP would hold back everything behind a lost packet while it waits for the resend.

TCP

Every byte is numbered, so missing ones get sent again

The sender keeps each segment until an ACK covers it. Say bytes 1–3000 go out as segments starting at 1, 1001 and 2001, and the middle one is lost. The receiver buffers 2001 and keeps replying ACK 1001.

The sender resends from byte 1001 after a timeout, or sooner after three duplicate ACKs (fast retransmit). Then the receiver hands bytes 1–3000 to the app in order and replies ACK 3001.

A checksum catches damaged segments, which are dropped and resent like lost ones. The receiver also advertises a receive window, the bytes it can buffer, so a fast sender can't overflow it.