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The primary objective of this chapter is to obtain an appreciation for the composition of the TCP/IP protocol suite. We first examine the International Organization for Standardization (ISO) Open Systems Interconnection (OSI) Reference Model, which was briefly men-tioned in Chapter 1. Although the TCP/IP protocol suite predates the ISO’s Reference Model, by examining the layering concept asso-ciated with communications defined by that model we can obtain a better appreciation for the functioning of the TCP/IP protocol suite.

Let’s examine the ISO Reference Model.

2.1 ISO Reference Model

During the 1970s, which was approximately a dozen years after the development of several popular communications protocols, to include TCP/IP, the ISO established a framework for standardizing com-munications systems. This framework was called the OSI Reference Model. One of the key goals behind the development of the OSI Reference Model was to define an architecture in which commu-nications functions could be divided into seven distinct layers, with specific functions becoming the responsibility of a particular layer.

By breaking communications into layers, it became easier for soft-ware and hardsoft-ware developers to work on communications projects, as teams could be assigned to work on different layers of a protocol, which could then be linked together in a standardized manner.

Figure 2.1 illustrates the seven layers of the ISO Reference Model.

Note that each layer, with the exception of the lowest, covers a lower layer, effectively isolating each layer from higher-layer func-tions. Similarly, with the exception of the topmost layer, which is the seventh in the model, each layer interconnects with a higher layer,

facilitating interoperability. Layer isolation is an important aspect of the ISO Reference Model, as it allows the given characteristics of one layer to change without affecting the remaining layers of the model, provided that support services remain the same. This becomes pos-sible since well-known interface points in a layered model enable one layer to communicate with another even though one or both layers may change. In addition, the layering process permits end users to mix and match OSI or other layered protocol-conforming commu-nications products to tailor their commucommu-nications system to satisfy a particular networking requirement. Thus the OSI Reference Model, as well as protocol suites that employ a layered architecture, provides the potential to directly interconnect networks based upon the use of different vendor products. This architecture, which is referred to as an open architecture when its specifications are licensed or placed in the public domain, can be of substantial benefit to both users and vendors.

An open architecture removes users from dependence upon a par-ticular vendor, and may also be considered to be economically advanta-geous, as it fosters competition among vendors. For vendors, this open architecture enables products to reach the market and possibly gain widespread acceptance in comparison to proprietary systems. Thus the ability for vendors to easily interconnect their products with the prod-ucts produced by other vendors opens up a wider market. This in turn fosters competition, which as many students of economics understand should result in a lowering of costs in a competitive environment, unless a nefarious activity such as price fixing occurs. Hopefully this is why governments exist and we can ignore such activities from consideration.

Figure 2.1 The ISO’s Open Systems Interconnection Reference Model.

Now that we have an appreciation for the value of a layered archi-tecture, let us turn our attention to the functions of the seven layers of the OSI Reference Model.

2.1.1 OSI Reference Model Layers

As noted, the OSI Reference Model consists of seven layers, with spe-cific functions occurring at each layer. Here we turn our attention to obtaining an understanding of the functions performed at each layer in the OSI Reference Model. Once this is accomplished, we use the information gained in the next section to better understand the com-ponents of the TCP/IP protocol suite.

2.1.1.1 Layer 1: The Physical Layer The physical layer, as shown in Figure 2.1, represents the lowest layer in the ISO Reference Model.

Because the physical layer involves the connection of a communi-cations system to a communicommuni-cations medium, the physical layer is responsible for specifying the electrical and physical connection between communications devices that connect to different types of media. At this layer such information as cable connectors and the electrical rules necessary to transfer data between devices is specified.

A few examples of physical layer standards include the well-known RS-232, V.24, and V.35 interfaces.

2.1.1.2 Layer 2: The Data Link Layer Moving up the ISO’s OSI model we come to the data link layer, which is layer 2 in the model. The data link layer is responsible for defining the manner by which a device gains access to the medium specified in the physical layer. In addi-tion, the data link layer is also responsible for defining data formats, to include the entity by which information is transported, error control procedures, and other link control procedures.

Most trade literature and other technically oriented publications reference the entity by which information is transported at the data link layer as a frame. Depending upon the protocol used, the frame will have a certain header composition with fields that normally indi-cate the destination address and the originator or source address of the frame. In addition, frames will have a trailer with a cyclic redun-dancy check (CRC) field that indicates the value of an error-checking

mechanism algorithm performed by the originator on the contents of the frame. The receiver will apply the same algorithm to an inbound received frame and compare the locally generated CRC to the CRC in the trailer. If the two match, the frame is considered to be received without error, while a mismatch indicates a transmission error occurred, and the receiver will then normally request the originator to retransmit the frame. A key exception to the retransmission of error frames occurs when voice is digitized and transferred as 20 ms of digitized data. Here the retransmission of a frame containing an error would introduce a variable delay that would inhibit the reconstruc-tion of the original conversareconstruc-tion. Thus the receiver’s software would either drop the frame or carefully examine the preceding and suc-ceeding frames in an attempt to smooth out the conversation, with the procedure for handling errors usually differentiating one product from another. Examples of common layer 2 protocols include such local area network (LAN) protocols as Ethernet and Token-Ring, as well as such wide area network (WAN) like High-Level Data Link Control (HDLC).

2.1.1.3 Layer 2 Subdivision The original development of the OSI Reference Model was targeted toward wide area networking. This resulted in its applicability to LANs requiring a degree of modifica-tion. The Institute of Electrical and Electronic Engineers (IEEE), which was delegated the responsibility for developing LAN standards by the American National Standards Institute (ANSI), subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). The LLC layer is responsible for generating and interpreting commands that control the flow of data and performing recovery operations in the event errors are detected.

In comparison, the MAC layer is responsible for providing access to the local area network, which enables a station on the network to transmit information. The subdivision of the data link layer allows a common LLC layer to be used regardless of differences in the method of network access. Thus a common LLC is used for both Ethernet and Token-Ring, although their access methods are dissimilar.

2.1.1.4 Layer 3: The Network Layer Once again let’s move up the ISO Reference Model, this time to the third layer, the network layer.

This layer is responsible for arranging a logical connection between a source and destination on the network. This action includes the selec-tion and management of a route for the flow of informaselec-tion between the source and destination based upon the available paths within a network. Note that the source and destination can be any type of computational device, ranging from a conventional desktop computer to a router, tablet, or even smart phone.

Services or functions provided at the network layer are associ-ated with the movement of data through a network. Thus this can include addressing, routing, switching, sequencing, as well as flow control procedures to enable the orderly flow of data. At the net-work layer units of information are placed into packets that have a header and trailer similar to frames at the data link layer. Thus the network layer packet will contain addressing information as well as a field that facilitates error detection and correction. Between the data link layer and network layer we have frames and packets, each with unique addresses. Thus we need a mechanism to resolve one address to another address at a different layer. In a TCP/IP environment this technique is referred to as address resolution and occurs using the Address Resolution Protocol (ARP), which we discuss later in this chapter and the next.

Returning to our discussion of the network layer, it’s important to note that in a complex network, the source and destination may not be directly connected by a single path. Instead a path may be required to be established through the network that consists of several sub-paths. Thus the routing of packets through the network, as well as the mechanism in the form of routing protocols that provide information about available paths, is an important feature of this layer.

Currently there are over 20 protocols that are standardized for layer 3, which can be subdivided into operating protocols and man-agement protocols. Examples of the former include the International Telecommunications Union Telecommunications (ITU-T) body X.25 packet switching protocol, the ITU-T X.75 gateway proto-col, Internet Protocol versions 4 and 6, IPSec or IP Security, and the Internet Control Message Protocol (ICMP). It should be noted that X.25 governs the flow of information through the packet net-work, whereas X.75 governs the flow of information between packet networks. Examples of the latter include Open Shortest Path First

(OSPF), Routing Information Protocol (RIP), and Border Gateway Protocol (BGP), all of which are routing protocols.

When we examine the TCP/IP protocol suite later in this chapter, as well as in succeeding chapters, we note that there are two operating versions of the Internet Protocol (IP), IPv4 and IPv6, both of which represent the network layer protocol used in the TCP/IP protocol suite. IPv4 is the primary mechanism used on the Internet; how-ever, the last IPv4 address was used and there are no longer any IPv4 addresses available. Thus the Internet is gravitating to the use of IPv6.

2.1.1.5 Layer 4: The Transport Layer Continuing our tour of the ISO Reference Model, the transport layer is responsible for governing the transfer of information after a route has been established through the network by the network layer protocol. There are two general types of transport layer protocols: connection oriented and connectionless.

A connection-oriented protocol first requires the establishment of a connection prior to the actual data transfer occurring. This type of transport layer protocol performs error control, sequence checking, and other end-to-end data reliability functions. A second type or cat-egory of transport layer protocol operates as a connectionless, best-effort protocol. This type depends upon higher layers in the protocol suite for error detection and correction. When we examine the TCP/

IP protocol suite we note that TCP represents a layer 4 connection-oriented protocol, while the User Datagram Protocol (UDP) repre-sents a connectionless layer 4 protocol. Other examples of transport layer protocols include Authentication Header (AH) over IP or IPSec, Encapsulating Security Payload (ESP) over IP or IPSec, and Generic Routing Encapsulation (GRE) for tunneling.

2.1.1.6 Layer 5: The Session Layer Continuing our tour of the OSI Reference Model, the fifth layer is the session layer. This layer is responsible for providing a set of rules that govern the establishment and termination of data streams flowing between nodes in a network.

The services that the session layer can provide include establishing and terminating node connections, message flow control, dialog con-trol, and end-to-end data control. In the TCP/IP protocol suite layers 5 through 7 are grouped together as an application layer. Examples of

session layer protocols include the Netware Core Protocol (NCP) and the Network File System (NFS).

2.1.1.7 Layer 6: The Presentation Layer The sixth layer of the OSI Reference Model is the presentation layer. This layer is concerned with the conversion of transmitted data into a display format appro-priate for a receiving device. This conversion can include data codes as well as display placement. Other functions performed at the presenta-tion layer can include data compression and decompression and data encryption and decryption.

2.1.1.8 Layer 7: The Application Layer The top layer of the OSI Reference Model is the application layer. This layer functions as a window through which the application gains access to all of the ser-vices provided by the model. Examples of functions performed at the application layer include electronic mail, file transfers, resource shar-ing, and database access. Although the first four layers of the OSI Reference Model are fairly well defined, the top three layers can vary considerably between networks. As previously mentioned, the TCP/

IP protocol suite, which is a layered protocol that predates the ISO Reference Model, combines layers 5 through 7 into one application layer. Thus while the File Transfer Protocol (FTP), Telnet, the remote access protocol, and the web browsing protocols Hypertext Transfer Protocol (HTTP) and its security version Hypertext Transfer Protocol Secure (HTTPS) are many times listed as layer 7 protocols, in reality they cover the upper three layers of the ISO Reference Model.

2.1.2 Data Flow

The design of an ISO Reference Model-compatible network is such that a series of headers are appended to each data unit as packets are transmitted and delivered at layer 2 by frames. At the receiver, the headers are removed as a data unit flows up the protocol suite, until the headerless data unit is identical to the transmitted data unit. In the next section, we will examine the flow of data in a TCP/IP network that follows the previously described ISO Reference Model data flow.

The ISO Reference Model never lived up to its intended goal, with ISO protocols achieving a less than anticipated level of utilization.

The concept of the model made persons aware of the benefits that could be obtained by a layered open architecture as well as the func-tions that would be performed by different layers of the model. Thus the ISO can be considered as succeeding in making networking per-sonnel aware of the benefits that could be derived from a layered open architecture and more than likely contributed to the success of the acceptance of the TCP/IP protocol suite. We now turn our attention to the TCP/IP protocol suite.