Learn network fundamentals to have cutting-edge skills and hands-on experience to excel in any DevOps role.

DevOps for Developers

Learn network fundamentals essential for any DevOps specialist. You'll explore socket programming, command-line tools, OSI layer protocols, and more.

Network Fundamentals

# What is UDP?

UDP, or **User Datagram Protocol**, is a transport layer protocol that works over the network layer's famous **Internet protocol** (which we'll look at in-depth in the [next chapter](https://www.educative.io/courses/grokking-computer-networking/the-internet-protocol-introduction-to-ipv4)). [RFC 768](https://tools.ietf.org/pdf/rfc768.pdf) is the official RFC for UDP.

# How It Works
UDP does not involve any initial handshaking like TCP does, and is hence called a **connectionless** protocol. This means that there are no established 'connections' between hosts. 

UDP prepends the **source and destination ports** to messages from the application layer and hands them off to the network layer. The Internet Protocol of the network layer is a **best-effort** attempt to deliver the message. This means that the message-
1. May or **may not get delivered**.
2. May get **delivered with changes in it**.
3. May get **delivered out of order**.

UDP only adds the **absolute bare minimum** functionality over the network layer. So it...
- Does not ensure that messages get sent.
- It does check, however, if a message got 'corrupted' yet does not take any measures to correct the errors by default.

# Structure of A UDP Datagram

## Header
UDP prepends **four** 2-byte header fields to the data it receives from the application layer. So in total, a UDP header is **8 bytes** long. The fields are:

1. **Source** port number
2. **Destination** port number
3. **Length** of the datagram (header and data in bytes)
4. **Checksum** to detect if errors have been introduced into the message. We'll study this in detail in the [next lesson](https://www.educative.io/courses/grokking-computer-networking/udp-checksum-calculation-why-udp)!

## Data

Other than the headers, a UDP datagram contains a body of data which can be up to **65,528** bytes long. Since the maximum possible length of a UDP datagram is $2^{16} =$ 65,536 bytes which includes the $8$-byte header, we are left with 65,528 bytes available. The nature of the data depends on the overlying application. So if the application is querying a DNS server, it would contain bytes of a zone file.

Here's what a UDP message looks like:

This lesson gives an introduction to one of the protocols at the heart of the transport layer: UDP!

The User Datagram Protocol

What is UDP?