Tag: UDP stands for

UDP Protocol | UDP Header | UDP Header Format

Computer Networks

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-

 

Note-01:

 

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.

 

Note-02:

 

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.

 

Note-03:

 

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.

 

Note-04:

 

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.

 

Note-05:

 

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.

 

Note-06:

 

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

 

PRACTICE PROBLEMS BASED ON UDP HEADER-

 

Problem-01:

 

Which field is optional in UDP?

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

 

Solution-

 

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

 

Problem-02:

 

The pseudo header of IP is used in-

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

 

Solution-

 

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

 

Problem-03:

 

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

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

 

Solution-

 

  • 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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