User Datagram Protocol (UDP)

The User Datagram Protocol (UDP) provides a connectionless, unreliable transport service for delivering datagrams (packets) across an IP network. In the Windows API, UDP is accessed through Winsock functions that operate on sockets.

Key Characteristics

• Connectionless – no handshake before sending data.
• Unreliable – packets may be lost, duplicated, or arrive out of order.
• Low overhead – minimal protocol header (8 bytes).
• Suitable for real‑time applications, streaming, DNS, VoIP, etc.

Typical Workflow

1. Create a socket with socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP).
2. Optionally bind the socket to a local port using bind().
3. Send data with sendto() or WSASendTo().
4. Receive data with recvfrom() or WSARecvFrom().
5. Close the socket with closesocket().

Example Code (C++)

#include <winsock2.h>
#include <ws2tcpip.h>
#pragma comment(lib, "Ws2_32.lib")

int main() {
    WSADATA wsaData;
    WSAStartup(MAKEWORD(2,2), &wsaData);

    SOCKET udpSock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (udpSock == INVALID_SOCKET) return 1;

    sockaddr_in serverAddr{};
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_port = htons(514);
    inet_pton(AF_INET, "192.168.1.100", &serverAddr.sin_addr);

    const char* msg = "Hello, UDP!";
    sendto(udpSock, msg, (int)strlen(msg), 0,
           (sockaddr*)&serverAddr, sizeof(serverAddr));

    char buffer[512];
    sockaddr_in fromAddr{};
    int fromLen = sizeof(fromAddr);
    int recvLen = recvfrom(udpSock, buffer, sizeof(buffer)-1, 0,
                           (sockaddr*)&fromAddr, &fromLen);
    if (recvLen != SOCKET_ERROR) {
        buffer[recvLen] = '\\0';
        printf("Received: %s\\n", buffer);
    }

    closesocket(udpSock);
    WSACleanup();
    return 0;
}

Related Functions

• socket()
• bind()
• sendto()
• recvfrom()
• getsockopt() / setsockopt() (e.g., SO_RCVBUF, SO_SNDBUF)
• closesocket()

Further Reading

• socket function
• sendto function
• recvfrom function
• UDP Overview (Microsoft Docs)