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The Session Layer
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We have now left the Presentation Layer. Our e-mail message is encrypted, compressed, and may have gone through an ASCII-toEBCDIC code conversion before descending into the complexity of the
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Protocols
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Session Layer. As before, the Presentation Layer added a header containing information about the services it employed. For being such an innocuous layer, the Session Layer certainly engenders a lot of attention. Some believe that the Session Layer could be eliminated by incorporating its functions into the layer above or the layer below, thus simplifying the OSI model. Whatever, the bottom line is that it does perform a set of critical functions that cannot be ignored. First of all, the Session Layer ensures that a logical relationship is created between the transmitting and receiving applications. It guarantees, for example, that our PC user in Madrid receives his or her mail and only his or her mail from the mainframe, which is undoubtedly hosting large numbers of other e-mail users. This requires the creation and assignment of a logical session identifier. Many years ago, I recall an instance when I logged into my e-mail account and found, to my horror, that I was actually logged into my vice president s account. Needless to say, I back-pedaled out of there as fast as I could. Today I know that this occurred because of an execution glitch in the Session Layer. Layer five also shares responsibility for security with the Presentation Layer. You may have noticed that when you log in to your e-mail application, the first thing the system does is ask for a login ID, which you dutifully enter. The ID appears in the appropriate field on the screen. When the system asks for your password, however, the password does not appear on the screen the field remains blank or is filled with stars, shown graphically in Figure 2-24. This is because the Session Layer knows that the information should not be displayed. When it receives the correct login ID, it sends a command to the terminal (your PC) asking you to enter your password. It then immediately sends a second message to the terminal telling it to turn off local echo so that your keystrokes are not echoed back onto the screen. As soon as the password has been transmitted, the Session Layer issues a command to turn local echo back on again, allowing you to once again see what you type.
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Figure 2-24 Session Layer turns off echo to protect user.
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Enter Login (Login received) Enter Password Turn off local echo ********* Turn on local echo Login Accepted. Welcome!
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Protocols
Protocols
Another responsibility of the Session Layer that is particularly important in mainframe environments is a process called checkpoint restart. This is a process that is analogous to the autosave function that is available on many PC-based applications today. I call it the Hansel and Gretel function: As the mainframe performs its many tasks during the online day, the Session Layer keeps track of everything that has been done, scattering a trail of digital bread crumbs along the way as processing is performed. Should the mainframe fail for some reason (the dreaded ABEND), the Session Layer will provide a series of recovery checkpoints. As soon as the machine has been rebooted, the Session Layer performs the digital equivalent of walking back along its trail of bread crumbs. It finds the most recent checkpoint and the machine uses that point as its most recent recovery data, thus eliminating the possibility of losing huge amounts of recently processed information. So, the Session Layer may not be the most glamorous of the seven layers, but its functions are clearly important. As far as standards go, the list is fairly sparse; see the ITU-T s X.225 standard for the most comprehensive document on the subject. After adding a header, layer five hands the steadily growing Protocol Data Unit, or PDU, down to the Transport Layer. This is the point where we first enter the network. Until now, all functions have been softwarebased and, in many cases, a function of the operating system. The Transport Layer s job is simple: to guarantee end-to-end, errorfree delivery of the entire transmitted message not bits, not frames or cells, not packets, but the entire message. It does this by taking into account the nature and robustness of the underlying physical network over which the message is being transmitted, including the following characteristics:
Class of service required Data transfer requirements User interface characteristics Connection management requirements Specific security concerns Network management and reporting status data
There are two basic network types: dedicated and switched. We will examine each in turn before discussing Transport Layer protocols. Dedicated networks are exactly what the name implies: an always-on network resource, often dedicated to a specific customer, which provides
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