9.7 UDP Communication Flashcards
UDP does not establish a connection. UDP provides low overhead data transport because it has a small datagram header and no network management traffic.
The four fields in the UDP header are as follows:
Source Port - A 16-bit field used to identify the source application by port number.
Destination Port - A 16-bit field used to identify the destination application by port number.
Length - A 16-bit field that indicates the length of the UDP datagram header.
Checksum - A 16-bit field used for error checking of the datagram header and data.
UDP Datagram Reassembly
Like segments with TCP, when UDP datagrams are sent to a destination, they often take different paths and arrive in the wrong order. UDP does not track sequence numbers the way TCP does. UDP has no way to reorder the datagrams into their transmission order, as shown in the figure.
Therefore, UDP simply reassembles the data in the order that it was received and forwards it to the application. If the data sequence is important to the application, the application must identify the proper sequence and determine how the data should be processed.
UDP Server Processes and Requests
Like TCP-based applications, UDP-based server applications are assigned well-known or registered port numbers, as shown in the figure. When these applications or processes are running on a server, they accept the data matched with the assigned port number. When UDP receives a datagram destined for one of these ports, it forwards the application data to the appropriate application based on its port number.
UDP Client Processes
As with TCP, client-server communication is initiated by a client application that requests data from a server process. The UDP client process dynamically selects a port number from the range of port numbers and uses this as the source port for the conversation. The destination port is usually the well-known or registered port number assigned to the server process.
After a client has selected the source and destination ports, the same pair of ports are used in the header of all datagrams in the transaction. For the data returning to the client from the server, the source and destination port numbers in the datagram header are reversed.
