tyt01fi.htm

快速学习TCP/IP协议

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<FONT SIZE=4 COLOR="#FF0000"><B>The Physical Layer</B></FONT></CENTER></H4>

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<P>The physical layer is the lowest layer of the OSI model and deals with the &quot;mechanical, electrical, functional, and procedural means&quot; required for transmission of data, according to the OSI definition. This is really the wiring or other transmission form.

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<P>When the OSI model was being developed, a lot of concern dealt with the lower two layers, because they are, in most cases, inseparable. The real world treats the data link layer and the physical layer as one combined layer, but the formal OSI definition stipulates different purposes for each. (TCP/IP includes the data link and physical layers as one layer, recognizing that the division is more academic than practical.)

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<FONT SIZE=5 COLOR="#FF0000"><B>Terminology and Notations</B></FONT></CENTER></H3>

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<P>Both OSI and TCP/IP are rooted in formal descriptions, presented as a series of complex documents that define all aspects of the protocols. To define OSI and TCP/IP, several new terms were developed and introduced into use; some (mostly OSI terms) are rather unusual. You might find the term <I>OSI-speak</I> used to refer to some of these rather grotesque definitions, much as <I>legalese</I> refers to legal terms.

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<P>To better understand the details of TCP/IP, it is necessary to deal with these terms now. You won't see all these terms in this book, but you might encounter them when reading manuals or online documentation. Therefore, all the major terms are covered here.

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>Many of the terms used by both OSI and TCP/IP might seem to have multiple meanings, but there is a definite attempt to provide a single, consistent definition for each word. Unfortunately, the user community is slow to adopt new terminology, so there is a considerable amount of confusion.</NOTE>

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<FONT SIZE=4 COLOR="#FF0000"><B>Packets</B></FONT></CENTER></H4>

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<P>To transfer data effectively, many experiments have shown that creating a uniform chunk of data is better than sending characters singly or in widely varying sized groups. Usually these chunks of data have some information ahead of them (the <I>header</I>) and sometimes an indicator at the end (the <I>trailer</I>). These chunks of data are called <I>packets</I> in most synchronous communications systems.

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<P>The amount of data in a packet and the composition of the header can change depending on the communications protocol as well as some system limitations, but the concept of a packet always refers to the entire set (including header and trailer). The term <I>packet</I> is used often in the computer industry, sometimes when it shouldn't be.

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<P>You often see the word <I>packet</I> used as a generic reference to any group of data packaged for transmission. As an application's data passes through the layers of the architecture, each adds more information. The term <I>packet</I> is frequently used at each stage. Treat the term <I>packet</I> as a generalization for any data with additional information, instead of the specific result of only one layer's addition of header and trailer. This goes against the efforts of both OSI and the TCP governing bodies, but it helps keep your sanity intact!

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<FONT SIZE=4 COLOR="#FF0000"><B>Subsystems</B></FONT></CENTER></H4>

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<P>A <I>subsystem</I> is the collective of a particular layer across a network. For example, if 10 machines are connected together, each running the seven-layer OSI model, all 10 application layers are the application subsystem, all 10 data link layers are the data link subsystem, and so on. As you might have already deduced, with the OSI Reference Model there are seven subsystems.

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<P>It is entirely possible (and even likely) that all the individual components in a subsystem will not be active at one time. Using the 10-machine example again, only three might have the data link layer actually active at any moment in time, but the cumulative of all the machines makes up the subsystem.

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<FONT SIZE=4 COLOR="#FF0000"><B>Entities</B></FONT></CENTER></H4>

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<P>A layer can have more than one part to it. For example, the transport layer can have routines that verify checksums as well as routines that handle resending packets that didn't transfer correctly. Not all these routines are active at once, because they might not be required at any moment. The active routines, though, are called entities. The word <I>entity</I> was adopted in order to find a single term that could not be confused with another computer term such as module, process, or task.

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<FONT SIZE=4 COLOR="#FF0000"><B>N Notation</B></FONT></CENTER></H4>

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<P>The notations N, N+1, N+2, and so on are used to identify a layer and the layers that are related to it. Referring to Figure 1.7, if the transport layer is layer N, the physical layer is N&#150;3 and the presentation layer is N+2. With OSI, N always has a value of 1 through 7 inclusive.

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<P>One reason this notation was adopted was to enable writers to refer to other layers without having to write out their names every time. It also makes flow charts and diagrams of interactions a little easier to draw. The terms N+1 and N&#150;1 are commonly used in both OSI and TCP for the layers above and below the current layer, respectively, as you will see.

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<P>To make things even more confusing, many OSI standards refer to a layer by the first letter of its name. This can lead to a real mess for the casual reader, because &quot;S-entity,&quot; &quot;5-entity,&quot; and &quot;layer 5&quot; all refer to the session layer.

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<FONT SIZE=4 COLOR="#FF0000"><B>N-Functions</B></FONT></CENTER></H4>

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<P>Each layer performs N-functions. The functions are the different things the layer does. Therefore, the functions of the transport layer are the different tasks that the layer provides. For most purposes in this book, functions and entities mean the same thing.

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<FONT SIZE=4 COLOR="#FF0000"><B>N-Facilities</B></FONT></CENTER></H4>

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<P>This uses the hierarchical layer structure to express the idea that one layer provides a set of facilities to the next higher layer. This is sensible, because the application layer expects the presentation layer to provide a robust, well-defined set of facilities to it. In OSI-speak, the (N+1)-entities assume a defined set of N-facilities from the N-entity.

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<FONT SIZE=4 COLOR="#FF0000"><B>Services</B></FONT></CENTER></H4>

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<P>The entire set of N-facilities provided to the (N+1)-entities is called the N-service. In other words, the service is the entire set of N-functions provided to the next higher layer. Services might seem like functions, but there is a formal difference between the two. The OSI documents go to great lengths to provide detailed descriptions of services, with a &quot;service definition standard&quot; for each layer. This was necessary during the development of the OSI standard so that the different tasks involved in the communications protocol could be assigned to different layers, and so that the functions of each layer are both well-defined and isolated from other layers.

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<P>The service definitions are formally developed from the bottom layer (physical) upward to the top layer. The advantage of this approach is that the design of the N+1 layer can be based on the functions performed in the N layer, avoiding two functions that accomplish the same task in two adjacent layers.

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<P>An entire set of variations on the service name has been developed to apply these definitions, some of which are in regular use:

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<P>An N-service user is a user of a service provided by the N layer to the next higher (N+1) layer.

<P>An N-service provider is the set of N-entities that are involved in providing the N layer service.

<P>An N-service access point (often abbreviated to N-SAP) is where an N-service is provided to an (N+1)-entity by the N-service provider.

<P>N-service data is the packet of data exchanged at an N-SAP.

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<P>N-service data units (N-SDUs) are the individual units of data exchanged at an N-SAP (so that N-service data is made up of N-SDUs).

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<P>These terms are shown in Figure 1.8. Another common term is <I>encapsulation,</I> which is the addition of control information to a packet of data. The control data contains addressing details, checksums for error detection, and protocol control functions.

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<P><B><A HREF="01tyt08.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/01tyt08.gif">Figure 1.8. Service providers and service users </B><B>communicate through service access points.</A></B>

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<FONT SIZE=4 COLOR="#FF0000"><B>Making Sense of the Jargon</B></FONT></CENTER></H4>

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<P>It is important to remember that all these terms are used in a formal description, because a formal language is usually the only method to adequately describe something as complex as a communications protocol. It is possible, though, to fit these terms together so that they make a little more sense when you encounter them. An example should help.

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<P>The session layer has a set of session functions. It provides a set of session facilities to the layer above it, the presentation layer. The session layer is made up of session entities. The presentation layer is a user of the services provided by the session layer (layer 5). A presentation entity is a user of the services provided by the session layer and is called a presentation service user.

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<P>The session service provider is the collection of session entities that are actively involved in providing the presentation layer with the session's services. The point at which the session service is provided to the presentation layer is the session service access point, where the session service data is sent. The individual bits of data in the session service data are called session service data units.