# Build an Echo Server

URL: /docs/tutorials/echo-server/
Section: tutorials

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What you'll build A minimal QUIC echo server that: Accepts a connection Receives stream data Echoes it back on the same stream Handles loss recovery timers Prerequisites Python 3.14+ Zoomies installed (pip install bengal-zoomies) A TLS certificate and key (self-signed is fine for testing) Generate test certificates openssl req -x509 -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 \ -keyout key.pem -out cert.pem -days 365 -nodes \ -subj "/CN=localhost" The server import socket from zoomies.core import QuicConnection, QuicConfiguration from zoomies.events import HandshakeComplete, StreamDataReceived # Load certs with open("cert.pem", "rb") as f: cert = f.read() with open("key.pem", "rb") as f: key = f.read() config = QuicConfiguration(certificate=cert, private_key=key) conn = QuicConnection(config) # Bind UDP socket sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) sock.bind(("0.0.0.0", 4433)) print("Listening on :4433") while True: data, addr = sock.recvfrom(65535) events = conn.datagram_received(data, addr) for event in events: match event: case HandshakeComplete(): print(f"Handshake complete with {addr}") case StreamDataReceived(stream_id=sid, data=payload): print(f"Stream {sid}: {payload!r}") conn.send_stream_data(sid, payload) # echo for dg in conn.send_datagrams(): sock.sendto(dg, addr) # Handle timers timeout = conn.get_timer() if timeout is not None: sock.settimeout(max(0, timeout - __import__("time").monotonic())) What's happening UDP socket — You own the socket. Zoomies never touches it. datagram_received — Raw UDP bytes go in, protocol events come out. send_datagrams — Zoomies produces response bytes. You send them. Timers — Loss recovery needs periodic wakeups. get_timer() tells you when. Next steps HTTP/3 concepts — add request/response handling Connection lifecycle — understand what happens under the hood

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Metadata:
- Author: lbliii
- Word Count: 232
- Reading Time: 1 minutes