問84
頻出度★★☆
PC で電子メールを読むときに, PC にメールをサーバからダウンロードするのではなくサーバ上で保管し管理する。未読管理やメールの削除やフォルダの振分け状態などが会社や自宅にあるどの PC からも同一に見えるようにできるメールプロトコルはどれか。
平成25年度 春期 ITパスポート試験 問84
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 84
Incorrectア
APOP (Authenticated POP) is an extension that makes user authentication safer when receiving mail with POP3. With plain POP3 the password travels over the network in clear text, so APOP uses a challenge-response method: it returns a hash value computed from the password combined with a random string (the challenge) sent by the server, confirming the user’s identity without sending the password itself. However, only the authentication part is strengthened; the mechanism for receiving mail is still the same download type as POP3. The mail is therefore not stored and managed on the server, and the unread status and folder filing are held in the mail software on each PC, so the same state cannot be seen from several PCs. What the question asks about is not the safety of authentication but the way mail is stored and managed, so this choice misses the point.
- イ
正解 The correct answer is イ, “IMAP4.” IMAP4 (Internet Message Access Protocol version 4) is a receiving protocol used to read and manage mail while the mail stays on the mail server. The mail software on the user’s PC only fetches and displays the header or body of the messages it needs, one at a time, and the mail itself keeps sitting on the server. Moreover, with IMAP4 not only the mail itself but also the read/unread flags showing which messages have been read, the deleted status, and the structure of the folders created under the inbox together with the information on which message was filed into which folder are all managed centrally on the server side. Because of this, mail read on the office PC also appears as read on the home PC, and the folder organization created at the office is reproduced just as it is at home. IMAP4 is the only protocol that satisfies both conditions given in the question, namely “storing and managing mail on the server instead of downloading it” and “making the unread status, mail deletions, folder organization, and so on look identical from any PC at the office or at home,” and this is why it is the correct answer. Note as well that even when IMAP4 is used for receiving, sending the mail itself still uses SMTP separately.
Incorrectウ
POP3 (Post Office Protocol version 3) is a receiving protocol used to retrieve (download) mail that has arrived on the server to the user’s PC. In the default way of using it, the mail on the server is deleted the moment it is downloaded, and after that the message exists only inside the PC that retrieved it. Managing read/unread status and creating folders to file mail into are both done by the mail software on each PC within itself, so the results of reading or organizing mail on the office PC are not reflected on the home PC. Even if it is set to leave mail on the server, only the mail itself remains; the unread status and the folder structure are not shared, and what each PC shows ends up differing. The question describes exactly the situation of wanting to remove this weakness of POP3, so POP3 is wrong.
Incorrectエ
SMTP (Simple Mail Transfer Protocol) is the protocol used when sending mail and when mail servers forward mail to each other. Handing mail from the user’s mail software to their own mail server, and delivering it from there to the other party’s mail server, is the role of SMTP; it is not used in the scene where the recipient reads the mail that has arrived. Reading on the receiving side uses a receiving protocol such as POP3 or IMAP4. The question asks about the way mail is stored and managed when reading it on a PC, not about the sending side, so SMTP does not apply. It is good to remember the division of roles as SMTP for sending, and POP3 or IMAP4 for receiving.
ここがポイント
- The point being tested is how to tell the receiving protocols apart. POP3 downloads mail to the PC and manages it on the device side, so the unread status and folder filing end up separate on each PC. IMAP4, by contrast, keeps mail stored on the server and manages it there; in addition to the mail itself, read/unread status, deletions, and folder structure are all managed centrally on the server, so the same state can be seen from several devices such as the office, home, or on the road. Whenever you see wording like “wanting it to look the same from any device” or “wanting to use it while leaving it on the server,” you may judge the answer to be IMAP4.
- Also learn the division of roles among the mail-related protocols together with this. Sending, and forwarding between servers, is handled by SMTP, while receiving (reading) is handled by POP3 or IMAP4. Note that APOP is an extension that turns POP3’s authentication into a challenge-response method so that the password is not sent in clear text; it only strengthens authentication, and the way mail is stored and managed stays that of POP3, unchanged.
用語