Transmission Control Protocol (TCP)

The Transmission Control Protocol (TCP) is a Layer 4 transport protocol that provides reliable, ordered, and connection-oriented delivery of data between two hosts over an IP network. It is one half of the TCP/IP suite that underpins most internet traffic, used by HTTP/HTTPS, email (SMTP), and file transfer (FTP).

TCP adds reliability on top of IP’s best-effort delivery. IP can drop packets, deliver them out of order, or duplicate them; TCP guarantees that the application layer sees a continuous, correctly-ordered byte stream.

Key features

  • Reliable delivery: Every segment is acknowledged by the receiver. If an acknowledgement is not received within a timeout, the sender retransmits the segment.

  • Ordered delivery: Each byte is assigned a sequence number. The receiver buffers out-of-order segments and delivers data to the application in the correct order.

  • Flow control: The receiver advertises a receive window — the amount of data it can buffer — and the sender limits how much unacknowledged data it keeps in flight. This prevents a fast sender from overwhelming a slow receiver.

  • Congestion control: TCP monitors the network for signs of congestion (dropped packets, increased RTT) and reduces its send rate accordingly. Algorithms such as slow start, congestion avoidance, fast retransmit, and fast recovery (CUBIC, BBR in modern kernels) manage this automatically.

  • Error detection: Each TCP segment carries a checksum over the header and data. Corrupted segments are discarded and retransmitted.

  • Full-duplex: Data can flow in both directions simultaneously over a single connection.

Connection lifecycle

Three-way handshake (connection establishment)

Before data can be exchanged, a connection is established:

  1. SYN — client sends a segment with the SYN flag set and its initial sequence number (ISN).

  2. SYN-ACK — server acknowledges the client’s ISN and sends its own ISN.

  3. ACK — client acknowledges the server’s ISN. The connection is established.

This costs one full round-trip before any application data can be sent.

Data transfer

Once connected, both sides exchange segments. Each segment carries a sequence number, an acknowledgement number (confirming bytes received), and the payload. The sender keeps a congestion window and a receive window, sending data up to the minimum of both.

Four-way teardown (connection termination)

  1. FIN — the closing side signals it has no more data to send.

  2. ACK — the other side acknowledges.

  3. FIN — the other side sends its own FIN when it is done.

  4. ACK — the initiating side acknowledges; both sides close.

Connections enter a TIME_WAIT state after closing to absorb any delayed packets that might otherwise corrupt a new connection reusing the same port pair.

TCP vs UDP

TCP’s reliability comes at a cost: the handshake, acknowledgements, and retransmissions all add latency and overhead. For applications where timeliness matters more than guaranteed delivery — real-time audio/video, gaming, DNS lookups — UDP is preferred.

See Also