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Unformatted text preview: Computer Networks UDP and TCP Saad Mneimneh Computer Science Hunter College of CUNY New York Im a system programmer specializing in TCP/IP communication protocol on UNIX systems. How can I explain a thing like that to a seven-year-old? 1 Introduction So far, we have studied the DLC layer. The next layer up is the network layer, upon which the transport layer operates. Internet Internet DLC transport Figure 1: DLC, network, transport While it may be natural to discuss the network layer next, DLC and the transport layer share many features in terms of reliable communication (but one is on a single link and the other is over a network). Many of the algorithms and protocols that we have seen for DLC apply for the transport layer. For this reason, we will consider the transport layer next. 1 From the viewpoint of the transport layer, the underlying network has certain limitations in the level of service it can provide. Some of the more typical limitations are: loss, reordering, and duplicate delivery of packets limited packet size (MTU, Maximum Transmission Unit) arbitrary long delays host to host communication Such a network is said to provide best-effort level of service, as exemplified by the Internet. Therefore, it is the role of the transport layer to turn the unreliable host based communication of the underlying network into a reliable communication among application programs. The following list shows some of the common properties that a transport protocol can be expected to provide: reliable message delivery: once, only once, and in order support for arbitrarily large messages synchronization between sender and receiver through flow control support for multiple application processes on each host Two transport protocols are particularly famous for the Internet: User Datagram Protocol (UDP) and Transmission Control Protocol (TCP). 2 User Datagram Protocol (UDP) UDP is a simple transport protocol that extends the host-to-host delivery of packets of the underlying network into a process-to-process communication. Since there are many processes running on a given host (e.g. multiple Internet browsers), UDP needs to add a level of demultiplexing, allowing multiple application processes on each host to share the network. Therefore, the only interesting issue in UDP is the form of address used to identify a process. Although it is possible to directly identify a process with the operating system (OS) assigned id (pid), such an approach is only practical in a close distributed system with one OS that assigns unique ids to all processes (does not work for the entire world!). Instead, a process is indirectly identified using an abstract locator, often called a port . A source process sends a message to a port, and a destination process receives a message from a port. The UDP header contains a 16 bit port number for both the sender (source) and the receiver (destination)....
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This note was uploaded on 03/27/2010 for the course CSCI 415 taught by Professor Saadmneimneh during the Spring '08 term at CUNY Hunter.
- Spring '08
- Computer Networks