問85
頻出度★★☆
TCP/IP ネットワークで使用される NTP (Network Time Protocol) の役割として, 適切なものはどれか。
平成23年度 特別試験 ITパスポート試験 問85
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 85
Incorrectア
Distributing video and audio in real time describes streaming delivery, which is handled by protocols such as RTP (Real-time Transport Protocol) and RTSP. NTP only exchanges time information and has no function for carrying video or audio data.
- イ
正解 The correct answer is イ. NTP (Network Time Protocol) is a protocol for adjusting (synchronizing) the internal clock of a device to an accurate reference time over a TCP/IP network. With an upper-level time server that has an accurate time source such as an atomic clock or GPS at the top, the time is distributed down to lower-level devices through a hierarchy called stratum, and each device also takes the round-trip time of the communication into account to correct the error of its own clock. The communication uses UDP port 123. If the clocks of the servers and PCs in an organization are all different, problems occur: logs from multiple devices cannot be matched in chronological order, so the course of a failure or of unauthorized access cannot be traced; judgments about the expiration dates of digital certificates and passwords go wrong; and decisions about which backup or file update is newer become mistaken. Aligning the time of all devices in order to prevent such situations is the role of NTP, so イ, which states that it “synchronizes the time between devices connected to the network”, is appropriate.
Incorrectウ
Issuing a certificate that guarantees a file existed before a certain point in time describes a time stamp (time authentication) service: a Time Stamping Authority (TSA) issues a time stamp token, which is used to prove the existence of an electronic document and to prove that it has not been altered since that time. NTP only adjusts the clock of a device and has no function for issuing certificates or tokens. A time stamping authority may use NTP to keep its own time accurate, but the roles of the two are different things.
Incorrectエ
Automatically changing the encryption key after a certain period of time in a wireless LAN describes TKIP, adopted in WPA, and the key renewal mechanism of WPA2 and later. It is a wireless LAN security feature that makes it hard to work out the key from intercepted communication. NTP is a protocol that distributes time over both wired and wireless connections, and it does not generate or renew encryption keys.
ここがポイント
- NTP (Network Time Protocol) = a protocol that distributes time from a time server over a TCP/IP network and synchronizes the clocks of the connected devices (it uses UDP port 123 and a hierarchy called stratum). The purpose of aligning the time is to make it possible to match the logs of multiple devices in chronological order and trace failures and unauthorized access, to judge the expiration dates of certificates and passwords correctly, and to judge correctly which files are newer or older.
- Sort out the confusing terms related to “time” by their roles. Adjusting the time = NTP; proving that a document existed at a certain time and has not been altered since = time stamp (Time Stamping Authority, TSA); carrying video and audio in real time = streaming protocols such as RTP; periodically renewing the encryption key of a wireless LAN = wireless LAN security schemes such as TKIP.
用語