Tcp/ip Protocol Architecture

1. The TCP/IP communications suite was designed with modularity in mind. This means that instead of develop­ing a solution which integrates all aspects of communi­cations in one single piece of code, the designers wisely chose to break the puzzle into its constituent compo­nents and deal with them individually while recognizing the interdependence tying the pieces together. Thus, TCP/IP evolved into a suite of protocols specifying interdependent solutions to the different pieces of the communications puzzle. This approach to problem solving is normally referred to as the layering approach. Conse­quently, hereafter, reference will be made to the TCP/IP suite as a layered suite of communications.

2. Figure 20.1 shows the four-layer model of the TCP/ IP communications architecture. As shown in the dia­gram, the model is based on an understanding of data communications that involves four sets of interdepend­ent processes: application representative processes, host representative processes, network representative processes, and media access and delivery representa­tive process. Each set of processes takes care of the needs of entities it represents whenever an application engages in the exchange of data with its counterpart on the network. These process sets are grouped into the following four layers: application layer, host-to-host (also known as transport) layer, internet layer, and network access layer. Each of these layers may be im­plemented in separate, yet interdependent, pieces of software code.

Application Layer

(Application Representative Process)

Hostio-Host Layer (Host Representative Processes)

Figure 20.1. TCP/IP layered communications architecture.

3. Application Layer

Application representative processes take care of reconciling differences in the data syntax between the platforms on which the communicating applications are running. Communicating with an IBM mainframe, for ex­ample, might involve character translation between the EBCDIC and ASCII character sets. "While performing the translation task the application layer (for instance, application representative process) need not have (and shouldn't care to have) any understanding of how the underlying protocols (for instance, at the host-to- host l^ayer) handles the transmission of translated characters between hosts. Examples of protocols sup­ported at the application layer include FTP, TELNET, NFS, and DNS.

3. Host-to-Host Transport Layer

Host representative processes (for example, the host- to-host, or transport, layer) take care of communicat­ing data reliably between applications running on hosts across the network. It is the responsibility of the host representative process to guarantee the reliabil­ity and integrity of the data being exchanged, without confusing the identities of the communication applica­tions. For this reason the host-to-host layer is pro­vided with the mechanism necessary to allow it to make the distinction between the applications on whose be­half it is making data deliveries. In other words, assume that two hosts, tenor and alto, are connected to the same network, as shown in Figure 20.2. Further­more, assume that a user on host alto is logged in to FTP on host tenor. Also, while using FTP to transfer files, the user is utilizing TELNET to login in to host tenor to edit a document.

3. In this scenario, data exchanged between both hosts could be due to TELNET, FTP, or both. It is the responsibility of the host-to-host layer, hereafter called the transport layer, to make sure that data is sent and delivered to its intended party. What origi­nates from FTP at either end of the connection should be delivered to FTP at the other end. Likewise, TELNET­generated traffic should be delivered to TELNET at the other end, not to FTP. To achieve this, as will be discussed later, the transport layer at both ends of the connection must cooperate in clearly marking data packets so that the nature of the communicating appli­cations is easily identifiable. Protocols operating at the transport layer include both UDP (User Datagram Protocol) and TCP (Transmission Control Protocol). Later sections will cover the characteristics of both protocols.

Host Alto HwlT*ner

И— TELNET data pastels t 1 FTPtfaUjMctott

Figure 20.2. Host-to-host (transport layer) is responsible for connecting applications and for delivering data to its destined process.

Упражнение 8. Translate the questions and answer them. Try not to look in the text. Переведите вопросы к Тексту и ответьте на них. Постарайтесь сделать это, не заглядывая в Текст.

Каков был выбор у разработчиков при постановке задачи по коммуникационным протоколам?

Понимание чего легло в основу создания модели архитектуры TCP/IP коммуникаций?

Какова иерархия элементов данной модели?

За что отвечает уровень процессов, представляющих приложения?

За что отвечает уровень процессов, представляющих компьютерные сети? ’

Что включают в себя протоколы, работающие в транспортном уровне?

Упражнение 9. Пробуем говорить.

Using the text write down the verbs which help to describe the schemes. Выпишите из Текста глаголы, которые помогли бы описать схемы Текста.

Пользуясь этими глаголами и Текстом, представьте схемы какому-либо профессиональ­ному собранию.

Model: is based on — основывается на, involves — включает ...

Figure 20.1:

Figure 20.2:

Грамматика

Прошедшее время

Необходимость отдельно говорить о прошедшем времени не так уж велика.

§ 1. Напомним, что показатель прошедшего времени — это суффикс -ed или вторая форма неправильного глагола, которая даже называется Past Simple. Если посмотреть на группу Perfect внимательно, то мы увидим тот же показатель, в сущ­ности: к формуле Perfect to have + V-ed (V-3), к изменяемой ее части to have, присо­единяем показатель прошедшего времени — суффикс -ed или вторую форму не­правильного глагола — получаем had + V-ed (V-3).