UDP Considerations

User Datagram Protocol (UDP) is a connectionless transport protocol that offers low‑latency communication at the cost of reliability. This guide outlines key considerations when designing, implementing, and troubleshooting UDP‑based applications on Windows.

• Packet loss: UDP provides no guarantees that packets will arrive, arrive in order, or arrive only once. Implement application‑level acknowledgements or use a higher‑level protocol (e.g., RTP, QUIC) if reliability is required.
• Message size: The IPv4 MTU is typically 1500 bytes; UDP payloads larger than this may be fragmented, increasing loss probability. Prefer payloads < 1472 bytes (including UDP/IPv4 headers).
• Checksum validation: Windows validates UDP checksums by default; corruption results in the packet being dropped silently.

Because UDP is stateless, it is susceptible to reflection attacks and spoofing. Follow these practices:

• Use IPsec policies to encrypt and authenticate traffic.
• Validate source addresses and implement rate‑limiting on inbound datagrams.
• Avoid exposing unfiltered UDP services directly to the Internet.

Key socket options for optimal UDP throughput on Windows:

// Example (C++)  
socket = ::socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
int rcvs = 2*1024*1024; // 2 MiB receive buffer
::setsockopt(socket, SOL_SOCKET, SO_RCVBUF, (char*)&rcvs, sizeof(rcvs));
::setsockopt(socket, SOL_SOCKET, SO_REUSEADDR, (char*)&opt, sizeof(opt));

Copy

Enable UDP_SEGMENT offload on supported NICs for large payloads and set WSA_FLAG_OVERLAPPED for asynchronous I/O.

• Assuming recvfrom() will always fill the buffer – it returns the actual size of the datagram.
• Neglecting to handle WSAEWOULDBLOCK when using non‑blocking sockets.
• Using a single thread for both sending and receiving without proper synchronization may cause race conditions.
• Misinterpreting network byte order – always use htons/ntohs for port numbers.

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;

class UdpEchoClient
{
    static void Main()
    {
        var endpoint = new IPEndPoint(IPAddress.Loopback, 12345);
        using var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
        socket.ReceiveTimeout = 2000;

        string message = "Hello from C# UDP client";
        byte[] data = Encoding.UTF8.GetBytes(message);
        socket.SendTo(data, endpoint);

        var buffer = new byte[1024];
        EndPoint remote = new IPEndPoint(IPAddress.Any, 0);
        int received = socket.ReceiveFrom(buffer, ref remote);
        Console.WriteLine($"Received: {Encoding.UTF8.GetString(buffer,0,received)}");
    }
}

Copy

• Winsock2 Socket API
• RFC 768 – UDP Specification
• UDP Best Practices