Transmission Control Protocol

Definition:

• TCP is a transport-layer protocol providing reliable, ordered, connection-oriented byte-stream delivery between two hosts
• Runs on top of IP (the “TCP/IP” stack)
• Used when correctness matters more than speed: HTTP(s), email (SMTP), file transfer (FTP/SFTP), SSH

Connection-oriented:

• A connection must be established before any data is sent — 3-way handshake:
  • SYN → client requests to open, sends its initial sequence number
  • SYN-ACK → server acknowledges and sends its own sequence number
  • ACK → client acknowledges; connection established
• Tear-down uses a 4-way handshake (FIN/ACK in each direction)

Reliability mechanisms:

• Sequence numbers: every byte is numbered so the receiver can reorder
• Acknowledgements (ACK): receiver confirms receipt; sender retransmits on timeout / missing ACK
• Checksum: detects corrupted segments
• Retransmission: lost segments are resent

Flow & congestion control:

• Flow control: a sliding window stops a fast sender from overwhelming a slow receiver
• Congestion control: slow start, congestion avoidance, fast retransmit/recovery throttle sending when the network is congested

Overhead:

• Header is 20–60 bytes (larger than UDP’s 8)
• Handshake + ACKs + ordering add latency and head-of-line blocking
• See User Datagram Protocol for the lightweight, connectionless alternative
• QUIC layers TCP-like reliability over UDP to cut handshake latency