UDP (User Datagram Protocol) provides a connectionless and unreliable transport mechanism for data transmission across IP networks. Here’s how UDP works:
UDP operates at the transport layer of the TCP/IP protocol stack and is used by applications that do not require guaranteed delivery of data or strict ordering of packets. When an application wants to send data using UDP, it encapsulates the data into a UDP datagram. Each UDP datagram includes headers with source and destination port numbers, along with a checksum for error detection (though error correction is not provided).
Once the UDP datagram is formed, it is handed over to the network layer (IP layer), where it becomes part of an IP packet. The IP packet contains additional headers with source and destination IP addresses, enabling routers to forward it across different networks toward its destination.
Upon arrival at the destination host, the IP packet is passed up to the transport layer, where UDP processes it. UDP extracts the data payload from the IP packet based on the destination port number specified in the UDP header. Unlike TCP, UDP does not establish a connection before transmitting data, nor does it maintain session state or ensure reliable delivery. Instead, UDP simply delivers the data to the specified application or service running on the destination host.
A datagram refers to an independent, self-contained unit of data transmitted over a network.
UDP, a datagram consists of the UDP header followed by the data payload. Each datagram is treated as a separate entity and is transmitted independently of other datagrams. This means that UDP datagrams can arrive out of order or be lost without UDP providing mechanisms for retransmission or sequencing. Applications utilizing UDP must handle these conditions if required for their specific use case.
An example of a UDP-based protocol is DNS (Domain Name System). DNS uses UDP for quick and lightweight transmission of DNS queries and responses between clients (resolvers) and DNS servers. DNS queries, which ask for mappings of domain names to IP addresses, are typically small and benefit from UDP’s low overhead and fast transmission characteristics. DNS responses, providing the requested mappings, are also sent over UDP. DNS servers listen on UDP port 53 for incoming queries and respond with UDP datagrams containing the requested information. DNS employs UDP primarily for its efficiency in resolving domain names and IP addresses without the overhead of establishing and maintaining connections.