UDP Sockets

Overview

UDP (User Datagram Protocol) sockets provide unreliable, unordered, connectionless message-based communication. They’re simpler and faster than TCP but don’t guarantee delivery, ordering, or duplicate protection.

UDP vs TCP

UDP Header

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Source Port          |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Length             |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Only 8 bytes vs TCP’s 20+ bytes.

UDP Socket Lifecycle

sequenceDiagram
    participant S as Server
    participant C as Client
    S->>S: socket(SOCK_DGRAM)
    S->>S: bind(IP, port)
    C->>C: socket(SOCK_DGRAM)
    C->>S: sendto(data, addr)
    S->>C: recvfrom(buf, addr)
    C->>S: sendto(data, addr)
    S->>C: recvfrom(buf, addr)

No handshake, no connection setup — each sendto/recvfrom is independent.

UDP Server Code Example (Python)

import socket

# Create UDP socket
server = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Bind to address
server.bind(('0.0.0.0', 5353))

print("UDP server listening on port 5353...")

while True:
    # Receive data and sender address
    data, client_addr = server.recvfrom(4096)
    print(f"Received from {client_addr}: {data.decode()}")
    
    # Send response
    server.sendto(b"ACK: " + data, client_addr)

UDP Client Code Example (Python)

import socket

# Create UDP socket
client = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Send data (no connect needed)
client.sendto(b"Hello, UDP!", ('127.0.0.1', 5353))

# Receive response
data, server_addr = client.recvfrom(4096)
print(f"Received: {data.decode()}")

client.close()

Key UDP Characteristics

Message Boundaries

UDP preserves message boundaries. Each sendto() creates a separate datagram that must be received with a single recvfrom().

graph LR
    A[sendto 100 bytes] --> B[Datagram 1]
    C[sendto 200 bytes] --> D[Datagram 2]
    B --> E[recvfrom → 100 bytes]
    D --> F[recvfrom → 200 bytes]

With TCP, the receiver might get 50+50 bytes, or 300 bytes — no boundaries.

No Reliability

sequenceDiagram
    participant C as Client
    participant S as Server
    C->>S: sendto(msg1) ✓
    C->>S: sendto(msg2) ✗ (lost!)
    C->>S: sendto(msg3) ✓
    Note over S: Server receives msg1, msg3 only
    Note over S: No indication msg2 was lost

No Ordering

Packets may arrive out of order:

Sent:     msg1, msg2, msg3
Received: msg3, msg1, msg2  (or any order)

When to Use UDP

Adding Reliability to UDP

Since UDP doesn’t provide reliability, applications must implement it themselves:

sequenceDiagram
    participant C as Client
    participant S as Server
    C->>S: seq=1, data
    Note over C: Start timer
    S->>C: ack=1
    C->>S: seq=2, data
    Note over C: Timer expires (timeout)
    C->>S: seq=2, data (retransmit)
    S->>C: ack=2
    C->>S: seq=3, data
    S->>C: ack=3

Application-level reliability includes:

• Sequence numbers (detect ordering/duplicates)
• Acknowledgments (confirm receipt)
• Retransmission (resend lost packets)
• Checksums (detect corruption — UDP has a basic one)

UDP and Broadcast/Multicast

# Broadcast
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
sock.sendto(b"Hello everyone!", ('<broadcast>', 9999))

# Multicast
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP,
    socket.inet_aton('224.0.0.1') + socket.inet_aton('0.0.0.0'))
sock.bind(('0.0.0.0', 9999))

UDP Socket Options

Interview Questions

1. Q: When would you choose UDP over TCP? A: When latency matters more than reliability (VoIP, gaming), for simple request/response protocols (DNS, DHCP), for multicast/broadcast, or when you want to implement custom reliability at the application layer.

2. Q: Does UDP have any reliability? A: UDP itself provides no reliability. However, it has a checksum for error detection (not correction). Some applications (like QUIC/HTTP3) build reliability on top of UDP at the application layer.

3. Q: What are message boundaries in UDP? A: Each sendto() creates a distinct datagram. The receiver gets exactly what was sent in each sendto(). TCP, by contrast, is a byte stream — data from multiple send() calls may be received in a single recv().

4. Q: Why does DNS use UDP? A: DNS queries are small (typically <512 bytes), request/response based, and benefit from minimal overhead. The application handles retries if no response is received. TCP is used for large responses (>512 bytes) or zone transfers.

5. Q: What is QUIC and how does it relate to UDP? A: QUIC (RFC 9000) is a transport protocol built on UDP that provides TCP-like reliability with lower latency. It’s the basis of HTTP/3. It implements: 0-RTT connection establishment, multiplexed streams, and built-in TLS 1.3.

6. Q: Can UDP guarantee ordering? A: No, not by itself. Packets may arrive out of order. If ordering is needed, the application must implement sequence numbers and reordering logic.

Common Mistakes

• Assuming UDP is always faster (for bulk transfers, TCP may be better due to congestion control)
• Not handling lost packets (application must implement retry logic if needed)
• Forgetting that UDP has message boundaries (don’t treat it like a byte stream)
• Not knowing that UDP supports broadcast and multicast (TCP doesn’t)
• Confusing UDP’s checksum (basic error detection) with reliability

Summary

UDP is a simple, fast, connectionless protocol. It preserves message boundaries but doesn’t guarantee delivery or ordering. Use it for real-time applications, DNS, and when you need custom transport logic. QUIC builds reliability on top of UDP for modern web traffic.

Cross-References

• Sockets Overview
• TCP Sockets — Reliable alternative
• Unix Domain Sockets — Local IPC
• Non-blocking I/O — Async UDP

Cross References

• UDP Protocol
• TCP Sockets