UDP (UDP)

Protocol

User Datagram Protocol. A connectionless, lightweight transport protocol with no handshake, ordering, or delivery guarantees. UDP is the foundation for QUIC (HTTP/3), DNS lookups, and real-time streaming.

Updated Mar 9, 2026

Full Explanation

UDP sends datagrams without establishing a connection first. No handshake, no sequence numbers, no automatic retransmission. If a packet is lost, UDP doesn't care—that's the application's problem. This makes UDP faster for use cases where occasional loss is acceptable or where the application implements its own reliability layer.

In the CDN world, UDP is important for two reasons. First, DNS resolution uses UDP for most queries, and every CDN request starts with a DNS lookup. Second, QUIC (the protocol under HTTP/3) runs on top of UDP. QUIC builds its own reliability and congestion control on top of UDP, but gains the ability to do 0-RTT connection establishment—eliminating the TCP handshake overhead entirely.

UDP is also the transport for real-time media protocols like RTP and SRT, which are used in live streaming where low latency matters more than perfect delivery.

Examples

Testing UDP connectivity for DNS:

# DNS query over UDP (default)
dig @8.8.8.8 cdn.example.com A

# Force TCP for comparison
dig @8.8.8.8 cdn.example.com A +tcp

Checking if a server supports HTTP/3 (QUIC over UDP):

# curl with HTTP/3 support
curl --http3 -I https://cdn.example.com/

# Check Alt-Svc header advertising HTTP/3
curl -sI https://cdn.example.com/ | grep -i alt-svc
# alt-svc: h3=":443"; ma=86400

Frequently Asked Questions

User Datagram Protocol. A connectionless, lightweight transport protocol with no handshake, ordering, or delivery guarantees. UDP is the foundation for QUIC (HTTP/3), DNS lookups, and real-time streaming.

Testing UDP connectivity for DNS:

# DNS query over UDP (default)
dig @8.8.8.8 cdn.example.com A

# Force TCP for comparison
dig @8.8.8.8 cdn.example.com A +tcp

Checking if a server supports HTTP/3 (QUIC over UDP):

# curl with HTTP/3 support
curl --http3 -I https://cdn.example.com/

# Check Alt-Svc header advertising HTTP/3
curl -sI https://cdn.example.com/ | grep -i alt-svc
# alt-svc: h3=":443"; ma=86400

Related CDN concepts include:

  • TCP (TCP) — Transmission Control Protocol. The reliable, ordered, connection-oriented transport protocol underneath HTTP/1.1 and HTTP/2. TCP's three-way …
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