UDP Header Byte Order and Endianness Explained

This article provides a concise overview of the byte order (endianness) used across all fields in the User Datagram Protocol (UDP) header. You will learn the specific endian standard mandated by internet protocols, how it applies to each 16-bit field in the UDP packet structure, and how host systems convert these values to ensure reliable cross-platform communication.

Network Byte Order: Big-Endian

The byte order used for all multi-byte fields in the UDP header is Big-Endian, officially referred to in networking standards as Network Byte Order.

In a big-endian system, the most significant byte (MSB)—the “big end”—is stored and transmitted first across the network, followed by the least significant byte (LSB). This standard is defined by the Internet Engineering Task Force (IETF) in RFC 768 and RFC 791 to ensure interoperability among different computer architectures.

Fields in the UDP Header

A standard UDP header is 8 bytes (64 bits) long and consists of four distinct fields, each 2 bytes (16 bits) in size. Every field adheres strictly to big-endian byte order:

  1. Source Port (16 bits): Identifies the sending application port. Transmitted MSB first.
  2. Destination Port (16 bits): Identifies the receiving application port. Transmitted MSB first.
  3. Length (16 bits): Specifies the total length in bytes of the UDP header and user data payload. Transmitted MSB first.
  4. Checksum (16 bits): Used for error-checking the header and payload. Transmitted MSB first.

Example

If a UDP packet is destined for port 53 (DNS), the value in hexadecimal is 0x0035. In big-endian format, it is placed on the network wire in the following byte sequence:

Host Byte Order vs. Network Byte Order

Most modern consumer and server processors (such as x86, x86-64, and many ARM configurations) use Little-Endian format natively, where the least significant byte is stored first.

To transmit data correctly, applications must convert 16-bit integer values from Host Byte Order to Network Byte Order before sending, and back to Host Byte Order upon receipt. Standard networking socket libraries provide built-in conversion functions for this purpose: