TCP vs UDP

Two Ways to Send a Message

TCP is a certified letter. You hand it to the post office, they track it, they confirm delivery, and if it gets lost they resend it. You get a receipt. The recipient signs for it. It takes longer, but you know it arrived.

UDP is a flyer dropped from a plane. You print thousands, push them out the door, and hope they land where you intended. Fast, cheap, no tracking, no guarantee, no receipt. Some land in gardens. Some blow away. You do not know which.

Neither is better in absolute terms. The right choice depends entirely on what the application needs.

TCP: Transmission Control Protocol

TCP is connection oriented, which means two parties must establish a connection before any data flows (the three way handshake). Once connected, TCP guarantees:

Reliability: Every segment sent is acknowledged. Missing segments are retransmitted.

Ordering: Segments carry sequence numbers. Even if they arrive out of order, TCP reassembles them correctly at the receiver.

Flow Control: The receiver advertises a window size how many bytes it can accept. The sender never floods a slow receiver.

Congestion Control: TCP monitors packet loss as a signal of network congestion and slows down when the network is struggling (algorithms: Slow Start, Congestion Avoidance, Fast Retransmit, Fast Recovery).

Error Detection: A checksum covers the header and data. Corrupted segments are discarded and retransmitted.

The cost of all this: overhead. TCP headers are 20–60 bytes. Each acknowledged exchange adds round trip latency. For latency sensitive applications, that cost is unacceptable.

UDP: User Datagram Protocol

UDP strips away everything TCP provides except the bare minimum needed to deliver a datagram to the right application on the right machine.

The entire UDP header is 8 bytes:

No handshake. No acknowledgment. No sequence numbers. No retransmission. No flow control.

What you get: Minimal latency, minimal CPU overhead, minimal bandwidth waste. The application itself can implement only the reliability features it needs (some do; most do not).

Port Numbers

Both TCP and UDP use port numbers to direct traffic to the correct application on a host.

Real Port Examples

Why Video Streaming Uses UDP

This is the classic question. Surely reliability matters for a video call?

The answer: for real time media, latency is more important than perfection.

Consider a video call. If a packet carrying 20ms of audio arrives 500ms late (because TCP retransmitted it), inserting it into the stream causes a jarring stutter worse than the brief glitch from a dropped packet. Real time protocols prefer a small gap over a late arrival.

• A dropped video frame: a brief visual artifact, imperceptible at 30fps
• A retransmitted video frame: arrives too late to use, causes buffer underrun, visible freeze

Applications like Zoom, Skype, YouTube Live, DNS, online games use UDP and implement their own strategies: forward error correction, jitter buffers, graceful quality degradation. They accept occasional imperfection to avoid unacceptable delay.

HTTP/3 (QUIC) also uses UDP as its base because QUIC implements its own, smarter reliability layer without TCP's head of line blocking.

Feature Comparison Table

Socket Types

When developers write network applications, they choose between:

SOCK_STREAM (TCP)

• Provides a reliable, ordered, continuous byte stream
• Used for: web servers, databases, file transfers, SSH
• Example: socket(AF_INET, SOCK_STREAM, 0)

SOCK_DGRAM (UDP)

• Sends individual datagrams with no connection setup
• Used for: DNS resolvers, game servers, video conferencing
• Example: socket(AF_INET, SOCK_DGRAM, 0)

The socket type determines which transport protocol the OS uses for that application.

Choosing Between TCP and UDP

Ask three questions:

1. Does lost data invalidate the result?

   • Web page, database query, file download → yes → TCP
   • One video frame, one sensor reading, one DNS query → no → consider UDP

2. Is latency more important than completeness?

   • Real time voice, gaming, live streaming → yes → UDP
   • Accuracy critical transactions → no → TCP

3. Can the application tolerate or handle loss itself?

   • If yes, UDP lets you build exactly the reliability you need
   • If no, TCP gives you everything out of the box

Most internet traffic is TCP. Most real time and latency sensitive traffic is UDP. Understanding why is the foundation of network application design.