UDP

UDP (User Datagram Protocol) is a communications protocol that offers a limited amount of service when messages are exchanged between computers in a network that uses the Internet Protocol (IP).

UDP is an alternative to the Transmission Control Protocol (TCP) and, together with IP, is sometimes referred to as UDP/IP. Like the Transmission Control Protocol, UDP uses the Internet Protocol to actually get a data unit (called a datagram) from one computer to another. Unlike TCP, however, UDP does not provide the service of dividing a message into packets (datagrams) and reassembling it at the other end. Specifically, UDP doesn't provide sequencing of the packets that the data arrives in.Some of the Protocols are:

• DNS
• SNMP
• BOOTP
• TFTP
• NFS
• RPC
• RIP

UDP Message Format

The UDP header includes:

1. Source port number (16 bits) - An optional field
2. Destination port number (16 bits)
3. UDP length (16 bits)
4. UDP checksum (16 bits)

This is followed by data. The UDP checksum includes UDP data, not just the header as with IP message formats. For UDP and TCP checksum calculation a 12 byte pseudo header is included which contains some fields form the IP message header. This header is not transmitted as part of UDP or TCP, but is only used to help compute the checksum as a means of being sure that the data has arrived at the correct IP address. This is the TCP/UDP pseudo header:

1. Source IP address (32 bits)
2. Destination IP address (32 bits)
3. blank filler(0) (8 bits)
4. Protocol (8 bits)
5. UDP length (16 bits)