問65
頻出度★★☆
ネットワークを介したアプリケーション間の通信を実現するために，数多くのプロトコルが階層的に使用されている。次の記述中の a，b に入れるプロトコル名の適切な組合せはどれか。 インターネットで Web ページを閲覧する場合，ブラウザと Web サーバは，[ a ] というプロトコルを使用する。この [ a ] による通信は，その下層の [ b ] と，さらにその下層の IP というプロトコルを使用する。
平成22年度 秋期 ITパスポート試験 問65
テクノロジ系ネットワーク › 通信プロトコル
This question's explanation and your answers so far.
解説
問 65
Incorrectア
Both a and b are wrong. FTP (File Transfer Protocol) is an application-layer protocol used for transferring files, not for browsing web pages. Ethernet, for its part, is a standard that defines the cabling of a LAN and the way signals are sent (the data link layer and the physical layer), so it sits in a layer below IP. The question describes b as the layer directly above IP, so Ethernet, which is below IP, cannot be placed in b.
- イ
正解 The correct answer is イ (a: HTTP / b: TCP). When you browse a web page on the Internet, what is exchanged between the browser (the client) and the web server is HTTP (HyperText Transfer Protocol), a protocol of the application layer, the topmost layer of the TCP/IP layered model. HTTP only sets the rules for requests and responses — “please give me the page” and “here it is” — and does not itself do the work of delivering that data reliably to the other side. For that it uses TCP (Transmission Control Protocol) in the transport layer below it. TCP splits the data up and puts it back in the correct order, and resends anything that is missing or corrupted, thereby guaranteeing the reliability of the connection. One layer further down, IP in the internet layer takes charge of the routing that delivers packets to the destination host, using the IP address as its clue. The description in the question follows exactly this top-to-bottom stacking of HTTP → TCP → IP, so イ, which puts HTTP in a and TCP in b, is appropriate.
Incorrectウ
Both a and b are wrong. SMTP (Simple Mail Transfer Protocol) is used to send email, and POP3 (Post Office Protocol version 3) is used to receive email from a mail server; both are for email and have nothing to do with browsing web pages. Moreover, SMTP and POP3 both belong to the application layer and are peers of each other, so the layered relationship in which POP3 sits below SMTP does not hold either.
Incorrectエ
Neither a nor b is a protocol in the first place. WWW (World Wide Web) is the name of a mechanism (a service) for linking documents to one another on the Internet so that they can be published and viewed; it is not a protocol that defines a communication procedure. HTML (HyperText Markup Language) is a markup language for describing the structure of a web page, such as its headings and links; it too is not a communication procedure, nor does it sit in a layer below HTTP.
ここがポイント
- The protocol stack used when browsing the web is stacked from the top down as HTTP (application layer) → TCP (transport layer) → IP (internet layer) → Ethernet and the like (data link layer and physical layer). Grasp it as a division of labor: the upper layers handle what is exchanged, and the lower layers handle how it is delivered reliably.
- Tell confusable terms apart by two points: what they are used for, and whether they are protocols at all. HTTP = browsing the web, FTP = file transfer, SMTP = sending mail, POP3 = receiving mail, TCP = reliable transfer, IP = routing. WWW, on the other hand, is the name of a service, HTML is a language for describing pages, and Ethernet is a LAN standard below IP; none of them can be a lower-layer protocol under HTTP.
用語