TCP vs UDP Simulator

Overview

TCP and UDP both carry data over IP, but they make opposite promises. This simulator sends packets between two hosts so you can watch the difference. In TCP mode the connection starts with the three-way handshake, SYN, SYN-ACK and ACK, then every data packet is acknowledged and anything lost is sent again, so all data arrives in order at the cost of extra round trips. In UDP mode packets are simply sent, with no handshake, no acknowledgements and no retransmission, so whatever is lost stays lost. You can change the simulation speed, add packet loss and add jitter, which gives each packet its own delay so later packets can overtake earlier ones. UDP hands them over out of order, while TCP holds an early packet until the gap before it is filled, and the counters show packets sent, received, lost, retransmitted and out of order. With loss turned up, TCP slows down as it retransmits, while UDP keeps sending at the same rate and drops data. That is why TCP is used where every byte matters, such as web pages, APIs and file transfers, and UDP where fresh data matters more than complete data, such as DNS queries, video calls, games and QUIC, which builds its own reliability on top of UDP.

Try this

1. Start in TCP mode and watch the three-way handshake before any data is sent.
2. Raise packet loss and compare the retransmit counter in TCP mode with the lost counter in UDP mode.
3. Turn jitter up to 100% and compare the out-of-order counter in UDP mode with the held count in TCP mode, where early packets wait for the missing one.

Frequently asked questions

What is the difference between TCP and UDP?

TCP is connection oriented: it sets up a connection, acknowledges data, retransmits losses and delivers bytes in order. UDP sends independent datagrams with no delivery or ordering guarantees, which makes it lighter and lower latency.

Why does DNS use UDP?

Most DNS queries fit in one small request and response, so a connection setup would double the time. DNS falls back to TCP for large responses and zone transfers.

What is the TCP three-way handshake?

The client sends SYN, the server replies with SYN-ACK and the client answers ACK. After these three messages both sides have agreed on starting sequence numbers and can send data.