homemade udp in rust

udp is simpler than you think

TCP gets all the attention because it's what most applications use. but UDP is where the interesting low-level work happens — game networking, video streaming, DNS, QUIC.

UDP has no handshake, no connection state, no retransmission, no ordering. you send a datagram, it either arrives or it doesn't. that's it.

use std::net::UdpSocket;

fn main() -> std::io::Result<()> {
    let socket = UdpSocket::bind("0.0.0.0:8080")?;
    let mut buf = [0u8; 65535];
    loop {
        let (len, src) = socket.recv_from(&mut buf)?;
        socket.send_to(&buf[..len], src)?; // echo server
    }
}

building reliability on top

if you need reliability over UDP — like QUIC does — you build it yourself. this gives you control over:

• retransmission strategy: exponential backoff vs fixed intervals
• ordering: sequence numbers, reorder buffer size
• congestion control: you choose the algorithm

struct ReliableUdp {
    socket:     UdpSocket,
    send_buf:   HashMap<u32, (Vec<u8>, Instant)>,
    recv_buf:   BTreeMap<u32, Vec<u8>>,
    next_seq:   u32,
    next_ack:   u32,
}

where this is useful

game servers use unreliable UDP for position updates — a stale position is useless anyway, so retransmitting it wastes bandwidth. they use reliable UDP only for events that must arrive: kills, item pickups, chat messages.

this split reliability model is impossible with raw TCP. with UDP, it's straightforward.