Symantec™ Security Gateways Reference Guide - Sawmill

32 Security gateway fundamentalsRoutesLooking at Figure 3-1, the protocols are almost always presented in this format, like a pile of buildingblocks stacked upon one another. Because of this appearance, the structure is often referred to as a stack orprotocol stack. All hosts desiring to communicate using TCP/IP map to this type of network stack. Table 3-1 describes the functions performed at each layer of the OSI reference model network stack.Table 3-1LayerPhysicalData linkNetworkTransportSessionPresentationApplicationOSI reference model layer functionsFunctionDefines the physical characteristics of the hardware needed to carry the data transmission signal. Thislayer encompasses features such as voltage levels, and the number and location of interface pins. Someestablished standards for this layer include RS232C, V.35, and IEEE 802.3.Responsible for handling the reliable delivery of data across the underlying physical network.Manages connections across the network, and isolates the upper layer protocols from the details of theunderlying network.Guarantees that the receiver gets the data exactly as it was sent.Manages the sessions between cooperating applications.Provides standard data presentation routines.The layer where user-accessed network processes reside. This includes all of the processes (applications)that users directly interact with, as well as other processes at this level that the user is unaware of.Each layer of the stack is concerned only with communicating to its peer, the same layer or protocolimplemented on another host, and does not concern itself with what happens in the layers above or below.Each layer understands how to work with data it receives, and also understands that the packet is passed upor down to the next layer after it is finished.TCP/IP basicsThe most widely used protocols for transmitting data from one host to another are grouped into a protocolsuite called TCP/IP. This suite is a combination of different protocols at various layers with each layerresponsible for a different facet of communications. The TCP/IP protocol suite is normally grouped intofour functional areas, with some areas incorporating more than one layer of the stack. Figure 3-2 showshow the network stack layers from the OSI reference model are grouped under the TCP/IP suite.Figure 3-2Comparison of the OSI reference model to the TCP/IP suite