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-

 

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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