End systems, protocols, and end-to-end service models in Java

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End systems, protocols, and end-to-end service models
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Figure 13-1: End system Interaction Computers (eg, a PC or a workstation), operating as clients and servers, are the most prevalent type of end system However, an increasing number of alternative devices, such as so-called network computers and thin clients [Thinworld 1998], Web TV's and set top boxes [Mills 1998], digital cameras, and other devices are being attached to the Internet as end systems An interesting discussion of the continuing evolution of Internet applications is [AT&T 1998]
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132 Connectionless and Connection-Oriented Services
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We have seen that end systems exchange messages with each other according to an application-level protocol in order to accomplish some task The links, routers and other pieces of the Internet provide the means to transport these messages between the end system applications But what are the characteristics of this communication service that is provided The Internet, and more generally TCP/IP networks, provide two types of services to its applications: connectionless service and connection-oriented service A developer creating an Internet application (eg, an email application, a file transfer application, a Web application or an Internet phone application) must program the application to use one of these two services Here, we only briefly describe these two services; we shall discuss them in much more detail in 3, which covers transport layer protocols
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End systems, protocols, and end-to-end service models
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Connection-Oriented Service
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When an application uses the connection-oriented service, the client and the server (residing in different end systems) send control packets to each other before sending packets with real data (such as e-mail messages) This so-called handshaking procedure alerts the client and server, allowing them to prepare for an onslaught of packets It is interesting to note that this initial hand-shaking procedure is similar to the protocol used in human interaction The exchange of "hi's" we saw in Figure 12-1 is an example of a human "handshaking protocol" (even though handshaking is not literally taking place between the two people) The two TCP messages that are exchanged as part of the WWW interaction shown in Figure 12-1 are two of the three messages exchanged when TCP sets up a connection between a sender and receiver The third TCP message (not shown) that forms the final part of the TCP three-way handshake (see Section 37) is contained in the get message shown in Figure 12-1 Once the handshaking procedure is finished, a "connection" is said to be established between the two end systems But the two end systems are connected in a very loose manner, hence the terminology "connection-oriented" In particular, only the end systems themselves are aware of this connection; the packet switches (ie, routers) within the Internet are completely oblivious to the connection This is because a TCP connection is nothing more than allocated resources (buffers) and state variables in the end systems The packet switches do not maintain any connection state information The Internet's connection oriented service comes bundled with several other services, including reliable data transfer, flow control and congestion control By reliable data transfer, we mean that an application can rely on the connection to deliver all of its data without error and in the proper order Reliability in the Internet is achieved through the use of acknowledgments and retransmissions To get a preliminary idea about how the Internet implements the reliable transport service, consider an application that has established a connection between end systems A and B When end system B receives a packet from A, it sends an acknowledgment; when end system A receives the acknowledgment, it knows that the corresponding packet has definitely been received When end system A doesn't receive an acknowledgment, it assumes that the packet it sent was not received by B; it therefore retransmits the packetFlow control makes sure that neither side of a connection overwhelms the other side by sending too many packets too fast Indeed, the application at one one side of the connection may not be able to process information as quickly as it receives the information Therefore, there is a risk of overwhelming either side of an application The flow-control service forces the sending end system to reduce its rate whenever there is such a risk We shall see in 3 that the Internet implements the flow control service by using sender and receiver buffers in the communicating end systems The Internet's congestion control service helps prevent the Internet from entering a state of grid lock When a router becomes congested, its buffers can overflow and packet loss can occur In such circumstances, if every pair of communicating end systems continues to pump packets into the network as fast as they can, gridlock sets in and few packets are delivered to their destinations The Internet avoids this problem by forcing end systems to diminish the rate at which they send packets into the network during periods of congestion End systems are alerted to the existence of severe congestion when they stop receiving acknowledgments for the packets they have sent
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