TCP vs UDP

Also Known As

TL;DR

Explanation

TCP (Transmission Control Protocol): connection-oriented (three-way handshake), guaranteed delivery (retransmission on loss), ordered packets, congestion control. Use for: HTTP/HTTPS, database connections, email, file transfers. UDP (User Datagram Protocol): connectionless, fire-and-forget, no guaranteed delivery, no ordering. Use for: DNS, video streaming, online games, VoIP, QUIC/HTTP3. The trade-off: TCP's reliability costs latency; UDP's speed costs reliability. Applications that can handle loss (real-time audio) prefer UDP; applications that cannot (banking) require TCP.

Common Misconception

Why It Matters

Common Mistakes

• Using TCP for DNS queries — UDP is standard (less overhead for small queries).
• Using UDP for reliable data transfer without implementing your own ACK/retry — data loss is silent.
• Not understanding that QUIC is UDP with reliability — HTTP/3's UDP doesn't mean unreliable.
• PHP stream sockets default to TCP — explicitly specify udp:// for UDP socket programming.

Code Examples

// UDP for order confirmation — data can be silently lost:
$socket = socket_create(AF_INET, SOCK_DGRAM, SOL_UDP);
socket_sendto($socket, $orderConfirmation, strlen($orderConfirmation), 0,
    '10.0.0.1', 9000);
// Fire and forget — confirmation may never arrive
// Customer never gets their order update

// TCP for reliable delivery of business data:
$socket = stream_socket_client('tcp://10.0.0.1:9000');
fwrite($socket, $orderConfirmation);
// TCP guarantees delivery — retransmits on loss

// UDP appropriate for real-time metrics:
$socket = socket_create(AF_INET, SOCK_DGRAM, SOL_UDP);
socket_sendto($socket, $metric, strlen($metric), 0, $statsd_host, 8125);
// StatsD: dropped metrics are fine — stream of data not individual records

References

• ↗ https://www.cloudflare.com/learning/ddos/glossary/tcp-ip/

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