UDP

UDP
11.1.8 This page will discuss UDP. UDP is the connectionless transport protocol in the TCP/IP protocol stack.


UDP is a simple protocol that exchanges datagrams without guaranteed delivery. It relies on higher-layer protocols to handle errors and retransmit data.

UDP does not use windows or ACKs. Reliability is provided by application layer protocols. UDP is designed for applications that do not need to put sequences of segments together.

The following protocols use UDP:

• TFTP
• SNMP
• DHCP
• DNS

The following are the definitions of the fields in the UDP segment:

• Source port – Number of the port that sends data
• Destination port – Number of the port that receives data
• Length – Number of bytes in header and data
• Checksum – Calculated checksum of the header and data fields
• Data – Upper-layer protocol data

The next page discusses port numbers used by both TCP and UDP.

TCP and UDP port numbers
11.1.9 This page examines port numbers.


Both TCP and UDP use port numbers to pass information to the upper layers. Port numbers are used to keep track of different conversations that cross the network at the same time.

Application software developers agree to use well-known port numbers that are issued by the Internet Assigned Numbers Authority (IANA). Any conversation bound for the FTP application uses the standard port numbers 20 and 21. Port 20 is used for the data portion and Port 21 is used for control. Conversations that do not involve an application with a well-known port number are assigned port numbers randomly from within a specific range above 1023. Some ports are reserved in both TCP and UDP. However, applications might not be written to support them. Port numbers have the following assigned ranges:

• Numbers below 1024 are considered well-known ports numbers.
• Numbers above 1024 are dynamically-assigned ports numbers.
• Registered port numbers are for vendor-specific applications. Most of these are above 1024.

End systems use port numbers to select the proper application. The source host dynamically assigns source port numbers. These numbers are always greater than 1023.

This page concludes this lesson. The next lesson will focus on the application layer. The first page provides an introduction.