 local echo Login Accepted. Welcome! Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols Another responsibility of the Session Layer that is particularly impor- tant in mainframe environments is a process called checkpoint restart. This is a process that is analogous to the autosave function that is avail- able on many PC-based applications today. I call it the Hansel and Gre- tel 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 lay- ers, 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 compre- hensive 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 software- based and, in many cases, a function of the operating system. The Transport Layer's job is simple: to guarantee end-to-end, error- free 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 under