User Datagram Protocol (UDP)

USER DATAGRAM PROTOCOL (UDP)

The User Datagram Protocol (UDP) is called a connectionless, unreliable transport protocol. It does not add anything to the services of IP except to provide process-to process communication instead of host-to-host communication. Also, it performs very limited error checking.

UDP is a very simple protocol using a minimum of overhead. If a process wants to send a small message and does not care much about reliability, it can use UDP. Sending a small message by using UDP takes much less interaction between the sender and receiver than using TCP or SCTP.

1. Well-Known Ports for UDP:

Table 4.1 shows some well-known port numbers used by UDP. Some port numbers can be used by both UDP and TCP1

2. User Datagram

UDP packets, called user datagrams, have a fixed-size header of 8 bytes. Figure 4.9 shows the format of a user datagram.

The fields are as follows:

·           Source port number. This is the port number used by the process running on the sourcehost. It is 16 bits long, which means that the port number can range from 0 to 65,535.

·           Destination port number. This is the port number used by the process running on thedestination host. It is also 16 bits long.

·           Length. This is a 16-bit field that defines the total length of the user datagram, headerplus data. The 16 bits can define a total length of 0 to 65,535 bytes.

UDP length = IP length - IP header's length

·           Checksum. This field is used to detect errors over the entire user datagram (header plusdata).

3. Checksum

The UDP checksum calculation is different from the one for IP and ICMP. Here the checksum includes three sections: a pseudoheader, the UDP header, and the data coming from the application layer

Figure 4.10 Pseudoheader for checksum calculation

The pseudoheader is the part of the header of the IP packet in which the user datagram is to be encapsulated with some fields filled with Os (see Figure 4.10).

Example 4.1

Figure 4.11 shows the checksum calculation for a very small user datagram with only 7 bytes of data. Because the number of bytes of data is odd, padding is added for checksum calculation. The pseudoheader as well as the padding will be dropped when the user datagram is delivered to IP.

Optional use of the checksum:

The calculation of the checksum and its inclusion in a user datagram are optional. If the checksum is not calculated, the field is filled with 1s. Note that a calculated checksum can never be all 1s because this implies that the sum is all 0s, which is impossible because it requires that the value of fields to be 0s.

4. UDP Operation:

UDP uses concepts common to the transport layer.

Connectionless Services

UDP provides a connectionless service. This means that each user datagram sent by UDP is an independent datagram. There is no relationship between the different user datagrams even if they are coming from the same source process and going to the same destination program. The user datagrams are not numbered. Also, there is no connection establishment and no connection termination. This means that each user datagram can travel on a different path.

Flow and Error Control

UDP is a very simple, unreliable transport protocol. There is no flow control and hence no window mechanism. The receiver may overflow with incoming messages. There is no error control mechanism in UDP except for the checksum. This means that the sender does not know if a message has been lost or duplicated. When the receiver detects an error through the checksum, the user datagram is silently discarded. The lack of flow control and error control means that the process using UDP should provide these mechanisms.

Encapsulation and Decapsulation

To send a message from one process to another, the UDP protocol encapsulates and decapsulates messages in an IP datagram.

Queuing

In UDP, queues are associated with ports. (Figure 4.12).

At the client site, when a process starts, it requests a port number from the operating system. Some implementations create both an incoming and an outgoing queue associated with each process. Other implementations create only an incoming queue associated with each process.

Note that even if a process wants to communicate with multiple processes, it obtains only one port number and eventually one outgoing and one incoming queue. The queues opened by the client are, in most cases, identified by ephemeral port numbers. The queues function as long as the process is running. When the process terminates, the queues are destroyed.

The client process can send messages to the outgoing queue by using the source port number specified in the request. UDP removes the messages one by one and, after adding the UDP header, delivers them to IP. An outgoing queue can overflow. If this happens, the operating system can ask the client process to wait before sending any more messages.

5. Use of UDP

The following lists some uses of the UDP protocol:

·           UDP is suitable for a process that requires simple request-response communication with little concern for flow and error control. It is not usually used for a process such as FTP that needs to send bulk data.

·           UDP is suitable for a process with internal flow and error control mechanisms. For example, the Trivial File Transfer Protocol (TFTP) process includes flow and error control. It can easily use UDP.

·           UDP is a suitable transport protocol for multicasting. Multicasting capability is embedded in the UDP software but not in the TCP software.

·           UDP is used for management processes such as SNMP.

·           UDP is used for some route updating protocols such as Routing Information Protocol (RIP).