Softpanorama (slightly skeptical) Open Source Software Educational Society

May the source be with you, but remember the KISS principle ;-) Softpanorama Search

User Datagram Protocol (UDP)

UDP is defined in RFC 768. UDP is used by applications that only need a connectionless, best effort transport service. The service provided by UDP is an unreliable service that provides no guarantees for delivery and no protection from duplication of data  (if this arises due to software errors). The simplicity of UDP reduces the overhead  and may be adequate in many cases where good connectivity is expected.

A variant of UDP, UDP-Lite [RFC3828] was introduced into the Internet Protocol Suite in 2004. This has been designed to provide more optimized transport for unicast/multicast multimedia data. The header format closely follows that of UDP. The Datagram Congestion Control Protocol (DCCP) provides a more sophisticated datagram service, particularly suited to unicast streaming services.

UDP packet has very simple structure:

• Source Port. 16 bits. The port number of the sender. Cleared to zero if not used. Like in TCP, this field indicates which application sent the data contained within the IP packet.
• Destination Port. 16 bits. Again, indicates which application is to receive the data contained within the IP packet.
• Length. 16 bits. The length in bytes of the UDP header and the encapsulated data. The minimum value for this field is 8. Since all UDP headers are 8 bytes long, if you subtract 8 from this number you'll find out the size of the data being sent.
• Checksum. 16 bits. Since UDP is not responsible for ensuring that the data actually arrives, a checksum is optional. If computed it is set to 16-bit one's complement of the one's complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded as needed with zero bytes at the end to make a multiple of two bytes. If the checksum is cleared to zero, then checksuming is disabled. If the computed checksum is zero, then this field must be set to 0xFFFF.
• Data. Again, this will be the actual data being sent and will not include any additional headers.

These fields are decoded by all sniffers including tcpdump  and Solaris snoop.

In order to use UDP, the application must supply the IP address and port number of the destination application. A port is an abstraction (application number that is somewhat similar to UID) to allow transport protocols like UDP and TCP the capability of handling communications between multiple hosts. It allows a communication to be uniquely identified. Like UID port number is always a positive integer. Among the most commonly used UDP ports are:

• 53 DOMAIN Domain Name Server
• 43 NICNAME Whois
• 69 TFTP Trivial File Transfer
• 137 NETBIOS-NS NetBIOS Name Service
• 138 NETBIOS-DGM NetBIOS DataGrams
• 139 NETBIOS-SSN NetBIOS Session
• 514 Syslog  The BSD Syslog Protocol.  See RFC 3164 (rfc3164)

UDP demultiplexes the ports, keeping track of which data is available for or sent to a port.

Recommended Links

---

In case of broken links please try to use Google search. If you find the page please notify us about new location

• J. Postel. User Datagram Protocol, RFC 768.
• R.T. Braden, D.A. Borman, C. Partridge, Computing the Internet Checksum, RFC 1071
• List of Assigned UDP Port Numbers
• UDP Packet Reflector - Forwarder [Paper] UDP Performance Characterization in a Large-Scale SMP System
• UDP Buffer Sizing

  ---

  Copyright © 1996-2009 by Dr. Nikolai Bezroukov. www.softpanorama.org was created as a service to the UN Sustainable Development Networking Programme (SDNP) in the author free time. Submit comments This document is an industrial compilation designed and created exclusively for educational use and is placed under the copyright of the Open Content License(OPL). Site uses AdSense so you need to be aware of Google privacy policy. Original materials copyright belong to respective owners. Quotes are made for educational purposes only in compliance with the fair use doctrine.

  Disclaimer:

  • The statements, views and opinions presented on this web page are those of the author and are not endorsed by, nor do they necessarily reflect, the opinions of the author present and former employers, SDNP or any other organization the author may be associated with.
  • We do not warrant the correctness of the information provided or its fitness for any purpose
  • In no way this site is associated with or endorse  cybersquatters using the term "softpanorama" with other main or country domains (e.g. softpanorama.com) with bad faith intent to profit from the goodwill belonging to someone else.