UDP Protocol | UDP Header | UDP Header Format

Transport Layer Protocols-


There are mainly two transport layer protocols that are used on the Internet-



  1. Transmission Control Protocol (TCP)
  2. User Datagram Protocol (UDP)


In this article, we will discuss about User Datagram Protocol (UDP).

Learn about Transmission Control Protocol.


UDP Protocol-


  • UDP is short for User Datagram Protocol.
  • It is the simplest transport layer protocol.
  • It has been designed to send data packets over the Internet.
  • It simply takes the datagram from the network layer, attaches its header and sends it to the user.


Characteristics of UDP-


  • It is a connectionless protocol.
  • It is a stateless protocol.
  • It is an unreliable protocol.
  • It is a fast protocol.
  • It offers the minimal transport service.
  • It is almost a null protocol.
  • It does not guarantee in order delivery.
  • It does not provide congestion control mechanism.
  • It is a good protocol for data flowing in one direction.


Need of UDP-


  • TCP proves to be an overhead for certain kinds of applications.
  • The Connection Establishment Phase, Connection Termination Phase etc of TCP are time consuming.
  • To avoid this overhead, certain applications which require fast speed and less overhead use UDP.


UDP Header-


The following diagram represents the UDP Header Format-



1. Source Port-


  • Source Port is a 16 bit field.
  • It identifies the port of the sending application.


2. Destination Port-


  • Destination Port is a 16 bit field.
  • It identifies the port of the receiving application.


3. Length-


  • Length is a 16 bit field.
  • It identifies the combined length of UDP Header and Encapsulated data.


Length = Length of UDP Header + Length of encapsulated data


4. Checksum-


  • Checksum is a 16 bit field used for error control.
  • It is calculated on UDP Header, encapsulated data and IP pseudo header.
  • Checksum calculation is not mandatory in UDP.


Applications Using UDP-


Following applications use UDP-

  • Applications which require one response for one request use UDP. Example- DNS.
  • Routing Protocols like RIP and OSPF use UDP because they have very small amount of data to be transmitted.
  • Trivial File Transfer Protocol (TFTP) uses UDP to send very small sized files.
  • Broadcasting and multicasting applications use UDP.
  • Streaming applications like multimedia, video conferencing etc use UDP since they require speed over reliability.
  • Real time applications like chatting and online games use UDP.
  • Management protocols like SNMP (Simple Network Management Protocol) use UDP.
  • Bootp / DHCP uses UDP.
  • Other protocols that use UDP are- Kerberos, Network Time Protocol (NTP), Network News Protocol (NNP), Quote of the day protocol etc.


Important Notes-




Size of UDP Header= 8 bytes


  • Unlike TCP header, the size of UDP header is fixed.
  • This is because in UDP header, all the fields are of definite size.
  • Size of UDP Header = Sum of the size of all the fields = 8 bytes.




UDP is almost a null protocol.


This is because-

  • UDP provides very limited services.
  • The only services it provides are checksumming of data and multiplexing by port number.




UDP is an unreliable protocol.


This is because-

  • UDP does not guarantee the delivery of datagram to its respective user (application).
  • The lost datagrams are not retransmitted by UDP.




Checksum calculation is not mandatory in UDP.


This is because-

  • UDP is already an unreliable protocol and error checking does not make much sense.
  • Also, time is saved and transmission becomes faster by avoiding to calculate it.


It may be noted-

  • To disable the checksum, the field value is set to all 0’s.
  • If the computed checksum is zero, the field value is set to all 1’s.




UDP does not guarantee in order delivery.


This is because-

  • UDP allows out of order delivery to ensure better performance.
  • If some data is lost on the way, it does not call for retransmission and keeps transmitting data.




Application layer can perform some tasks through UDP.


Application layer can do the following tasks through UDP-

  1. Trace Route
  2. Record Route
  3. Time stamp


When required,

  • Application layer conveys to the UDP which conveys to the IP datagram.
  • UDP acts like a messenger between the application layer and the IP datagram.


Also Read- TCP Header | IPv4 Header






Which field is optional in UDP?

  1. Checksum
  2. Destination port
  3. Length
  4. None




  • Checksum calculation is not mandatory in UDP.
  • Thus, Option (A) is correct.




The pseudo header of IP is used in-

  1. Only TCP
  2. Only UDP
  3. Both TCP and UDP
  4. None




  • IP Pseudo header is used in both TCP and UDP while calculating checksum.
  • Thus, Option (C) is correct.




Broadcasting applications like WHOD (who daemon on UNIX) uses what transport layer protocol?

  1. TCP
  2. UDP
  3. Either TCP or UDP
  4. IGMP




  • Broadcasting and multicasting applications use UDP.
  • Thus, Option (B) is correct.


To gain better understanding about UDP Header,

Watch this Video Lecture


Next Article- Application Layer Protocols


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UDP Protocol | UDP Header | UDP Header Format
Article Name
UDP Protocol | UDP Header | UDP Header Format
UDP Protocol is the simplest Transport Layer Protocol. UDP stands for User Datagram Protocol. UDP Header consists of 4 fields. UDP Header Format is shown. UDP header size is 8 bytes. UDP Checksum calculation is not mandatory.
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