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Material Type: Notes; Professor: Bosworth; Class: Computer Networks; Subject: Computer Science; University: Columbus State University; Term: Summer 2006;
Typology: Study notes
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Topics: Chapter 24: UDP – User Datagram Protocol Why Study UDP? Host–to–Host Transport vs. End–to–End Transport UDP Services UDP Datagram Format UDP Encapsulation Advantages of UDP Chapter 25: TCP and Reliable Transport Service The Need for A Reliable Transport Service TCP Services
Most of our work will focus on TCP. So why study UDP?
Page 366 of the textbook characterizes the service provided by UDP End–to–End The UDP can distinguish between processes on a given host computer. IP delivers to the host computer; UDP delivers to the process. Connectionless UDP is another example of a connectionless protocol, as opposed to a virtual circuit protocol such as ATM. Message–Oriented UDP focuses its services on delivery of individual messages. Remember that the application process can interpret the message in any number of ways. In this view, a message is just an identifiable grouping of bytes. Best–Effort Following the IP example, UDP guarantees its best effort at delivery. Operating System Independence UDP and TCP provide means of identifying endpoint processes that is independent of the conventions of the host operating systems.
Each of UDP and TCP must be able to specify the application with which it is communicating. Such an identification must be independent of the computer and operating system implemented on the host system. The abstraction chosen by each of these protocols to represent applications is called “ Protocol Ports ”. These are set by the standard Internet RFC method. The protocol ports are identified by 16–bit unsigned integers. Protocol ports 0 – 1023 are called “ Well Known Ports ” and their use is restricted. These ports are associated with applications that are commonly assumed to be running on any given host computer. Examples are tcp, ftp, remote login, etc. References: [RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, USC/Information Sciences Institute, August 1980. [RFC793] Postel, J., ed., "Transmission Control Protocol - DARPA Internet Program Protocol Specification", STD 7, RFC 793, USC/Information Sciences Institute, September 1981.
Given the services provided by TCP, why do we need UDP? UDP is a small, efficient protocol without the overhead associated with protocols, such as TCP, that guarantee reliable message transport. The most obvious applications for UDP are those in which the overhead of retransmitting a lost data packet is more than is justified by the inconvenience of losing it. Consider Internet broadcast of streaming audio. As he is writing these notes, your instructor is listening to the NPR Program “All Things Considered” over the Internet. The occasional dropped packet in streaming audio is hardly noticeable, appearing as a barely audible click. Bottom Line IP provides an unreliable datagram delivery service, guaranteeing only a “best effort” at delivery. IP delivers datagrams to host computers. UDP extends this unreliable datagram delivery service to end–to–end service. For many applications, this is sufficient. TCP extends the service to provide reliable end–to–end delivery service. Naturally, this comes at the cost of extra overhead.
Some services require reliable transport of messages between the two end–point applications. The list of these is endless and includes financial applications. The U.S. Government realizes the necessity for reliable communication and has devised a pseudo–classification called “EFTO” or “Encrypt For Transmission Only”. An EFTO message does not contain any sensitive or classified data, but must be delivered exactly as transmitted. TCP ( T ransmission C ontrol P rotocol) is the name of the service that provides reliable service over essentially unreliable IP communication links. Originally TCP/IP was devised as a single protocol. This was soon split into the two current protocols (TCP and IP), so that each could evolve separately. As a part of providing reliable transport, TCP builds a connection oriented session on top of the connectionless IP session. The connection is established via the “three way handshake” (studied later) that is necessary to initiate reliable communication. Another “handshake” protocol terminates the session. The service must be a reliable end–to–end service, as reliable communication between host computers will not guarantee the communication between applications.
