TCP vs UDP

TCP vs UDP Comparison

TCP — Transmission Control Protocol

TCP is a connection-oriented protocol that provides reliable, ordered, and error-checked delivery of data between applications. Before any data is exchanged, TCP establishes a connection via a three-way handshake (SYN → SYN-ACK → ACK). Every packet is acknowledged, and lost packets are retransmitted.

How TCP Works

1. 1.SYN: Client sends connection request to server
2. 2.SYN-ACK: Server acknowledges and responds
3. 3.ACK: Client confirms — connection established
4. 4.Data transfer with sequence numbers for ordering
5. 5.Each packet acknowledged; lost packets retransmitted
6. 6.FIN/FIN-ACK: Graceful connection termination

TCP Use Cases

• ✓HTTP/HTTPS web browsing
• ✓Email (SMTP, IMAP, POP3)
• ✓File Transfer Protocol (FTP/SFTP)
• ✓SSH remote access
• ✓Database queries (MySQL, PostgreSQL)
• ✓API calls requiring complete responses

UDP — User Datagram Protocol

UDP is a connectionless protocol that sends datagrams without establishing a connection or guaranteeing delivery. There's no handshake, no acknowledgment, no retransmission. This minimal overhead makes UDP significantly faster, which is why latency-sensitive applications prefer it.

UDP Use Cases

• ✓DNS queries (speed matters, small packets fit in one datagram)
• ✓Online gaming (low latency critical; dropped frame better than delayed one)
• ✓VoIP and video calls (WebRTC uses UDP)
• ✓Live video streaming (RTSP, HLS over UDP)
• ✓Network monitoring (SNMP, syslog)
• ✓QUIC protocol (HTTP/3 — Google's UDP-based transport)

QUIC — The Future of Fast & Reliable Transport

QUIC is a modern transport protocol developed by Google (now an IETF standard) that runs over UDP but implements TCP-like reliability features in user space. QUIC is the foundation of HTTP/3 and combines UDP's speed with improved connection establishment, multiplexing (no head-of-line blocking), and built-in TLS 1.3 encryption. It reduces connection time from 3 round trips (TCP+TLS) to 0–1 round trips for repeat connections.

Well-Known Port Numbers by Protocol

Port numbers identify specific services on a host. The IANA divides the port space into three ranges: Well-Known Ports (0–1023, reserved for system services), Registered Ports (1024–49151, for applications), and Dynamic/Ephemeral Ports (49152–65535, assigned temporarily by the OS for outgoing connections).

TCP Head-of-Line Blocking: A Real-World Problem

One of TCP’s most significant limitations in modern web applications is head-of-line (HoL) blocking. When a packet is lost in a TCP stream, the receiver must wait for the retransmission before processing any subsequent data — even if that data has already arrived and is ready to use. In HTTP/2 (which multiplexes many requests over a single TCP connection), a single dropped packet stalls every request on that connection simultaneously.

This is exactly the problem QUIC (and by extension HTTP/3) solves. Because QUIC runs over UDP, it can implement its own stream multiplexing. A lost packet in one stream only blocks that stream — not the others. In practical terms, this means a lost image on a web page doesn’t delay loading the article text next to it.

This is why HTTP/3 adoption has accelerated rapidly: as of 2025, roughly 30% of all web traffic uses QUIC/HTTP/3. Major sites like Google, YouTube, Cloudflare, and Facebook have been running HTTP/3 in production for years. You can check which HTTP version a website serves by looking at the response headers or using browser developer tools (Network tab → Protocol column).