Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity Basics Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next PROTOCOL MODELS Like any modern communications protocol, TCP/IP is a layered protocol and follows its own layering model: the Internet model. This model resembles, but is not the same as, the more familiar Open System Interconnection (OSI) seven layer model. Exhibit 3-1-1 compares the two layering models. Exhibit 3-1-1.  The OSI and Internet Protocol Stacks Working down from the top, the Internet model has a process/application layer. The OSI model has an application layer and a presentation layer where data is transformed and presented in a uniform way to an application. OSI also has a session layer where application sessions are controlled. The Internet model generally incorporates aspects of the OSI presentation and session layers into its process/application layer. The various application protocols are found at this layer. TCP/IP has defined process/application protocols for the following services, among many others: •  Telnet, for remote log in. •  File transfer protocol (FTP), for remote file transfer. •  Simple mail transfer protocol (SMTP), for E-mail transport. •  Hyptertext transfer protocol (HTTP) for World Wide Web access. Below the process/application layer is the transport layer. TCP/IP’s transport layer is equivalent to OSI’s transport layer. This is where end-to-end protocols, which move data from one process to another process, are defined. This refers to processes, not hosts, although processes usually reside on different machines. An end-to-end protocol may or may not be designed with features to improve data communications reliability. It may be packet oriented or byte-stream oriented. A transport layer protocol may incorporate features for end-to-end flow control and multiplexing. The two major TCP/IP transport protocols are TCP and the User Datagram Protocol (UDP). UDP is a datagram or packet oriented protocol that adds little reliability and does not guarantee delivery. Below the transport layer is the internet layer. This generally corresponds to the top half of OSI’s network layer. This is where internetwork addressing and routing issues are handled. In the Internet model a single protocol, IP, is defined at this layer. Below the internet layer is the network layer, corresponding to the bottom half of OSI’s network layer. The network layer deals with the interface to various data link layer protocols (e.g., Ethernet and Token Ring). Essentially, this layer defines how bits are sent from one network interface to another over the same physical medium. Below TCP/IP’s network layer is the link layer. This deals with the lower-level specifications of the various network protocols (e.g., Ethernet and Token Ring), and is equivalent to OSI’s data link layer. The lowest layer in both models is the physical layer, which deals with the physical and electrical (or optical for fiber) specifications of the various network technologies. The Internet model places an emphasis on internetworking software and not hardware. The various specifications of its network, link, and physical layers are generally borrowed from other standards; for example, the Ethernet specification occupies the physical, link, and network layers of the Internet model. The Internet model simply uses the standard Ethernet specifications and builds an interface specification on top of them for the encapsulation of IP packets over an Ethernet. It does the same for Fiber Distributed Data Interface (FDDI) and Token Ring technologies. These interfaces are specified as a part of the Internet model’s network layer. Protocol layering is not an exact science. There is nothing inherently correct in OSI’s seven layer division or the Internet model’s six layers. The general idea is to rationalize a complex communications system and present it in a way that simplifies understanding. An actual implementation may or may not follow the layering model exactly. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.