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CCNA / HCIA Basic Network Engineer Basic Knowledge NTP Network Time Protocol Analysis

Network time protocol, English name: Network Time Protocol (NTP) is a protocol used to synchronize the time of the computer, it can synchronize the computer to its server or clock source, it can provide highly accurate time correction ( The difference between LAN and standard is less than 1 millisecond, and tens of milliseconds on WAN), and can be used to prevent malicious protocol attacks through encryption confirmation. The purpose of NTP is to provide accurate and robust time services in a disordered Internet environment. Used for time synchronization between distributed time servers and clients. NTP is transmitted based on UDP packets, and the UDP port number used is 123.

For each device in the network, it is impossible to modify the system clock by manually inputting commands from the administrator, which not only has a huge workload, but also cannot guarantee the accuracy of the clock. Through NTP, you can quickly synchronize the clocks of the devices in the network, while also ensuring high accuracy. NTP is mainly used in the following occasions:

1. In network management, when analyzing log information and debugging information collected from different devices, time needs to be used as a reference basis.

2. The billing system requires that the clocks of all devices be consistent.

3. Complete certain functions, such as restarting all the devices in the network regularly. At this time, the clocks of all the devices must be consistent.

4. When multiple systems cooperate to process the same complex event, in order to ensure the correct execution sequence, multiple systems must refer to the same clock.

5. When performing incremental backup between the backup server and the client, clock synchronization between the backup server and all clients is required.

NTP provides accurate time, we must first have an accurate time source, this time should be UTC. The time source for NTP to obtain UTC can be an atomic clock, observatory, satellite, or it can be obtained from the Internet. This has an accurate and reliable time source. The time is spread according to the level of the NTP server. All servers are grouped into different Stratums according to their distance from external UTC sources. Stratum-1 is on the top layer and has external UTC access, while Stratum-2 gets time from Stratum-1, Stratum-3 gets time from Stratum-2, and so on, but the total number of Stratum layers is limited to 15. All these servers are logically connected in a ladder-like architecture, and the Stratum-1 time server is the foundation of the entire system. The computer host is generally connected to multiple time servers, using statistical algorithms to filter the time from different servers to select the best path and source to correct the host time. Even if the host cannot contact a certain time server for a long time, the NTP service still operates effectively. To prevent malicious damage to the time server, NTP uses an authentication mechanism to check whether the information coming from the time is really from the claimed server and check the return path of the data to provide a protection mechanism against interference. The time contained in the NTP time synchronization message is Greenwich Mean Time, which is the number of seconds since 1900.

UTC time is obtained using many different methods, including radio and satellite systems. Special receivers can be used for advanced services in some countries, including GPS. However, it is not practical and economical to install these receivers on every computer. As an alternative, the receiver is installed on the computer of the designated time server and uses a protocol such as NTP to synchronize the time. The degree of separation from UTC is defined as a layer, a radio receiver (from a designated transmitter or satellite navigation device) The upper receiving information) is layer 0, the layer directly connected to the radio clock is layer 1, and the layer receiving the time from the layer 1 computer is layer 2, and so on.

The first implementation of Network Time Protocol (NTP) is documented in the Internet Engineering Note, and its accuracy is hundreds of milliseconds. Later, the specification of the first time protocol appeared, namely RFC-778, which was named DCNET Internet time service, and it provided this service by means of Internet control Message Protocol (ICMP), which is the time in the Internet control message protocol. The timestamp and timestamp response message serves as NTP.

The name of NTP first appeared in RFC-958. This version is also called NTP v0, and its purpose is to provide time synchronization for the ARPA network. It has completely separated from ICMP and is used as an independent protocol to complete the more demanding time synchronization. It performs basic operations such as error estimation and precision of local clocks, characteristics of reference clocks, packet data packets on the network and their message formats. Described. However, it does not compensate for any frequency error, nor does it specify filtering and synchronization algorithms.

As of June 2010, the latest NTP version is version 4 (NTPv4), whose standardized document is RFC 5905, which inherits from NTP v3 described in RFC 1305. Network time synchronization technology will also develop towards higher precision, stronger compatibility and adaptability to multiple platforms. Network time protocol NTP is one of the standards used for time synchronization in the Internet. Its purpose is to synchronize the computer’s clock to UTC in the world. Its accuracy can reach 0.1ms in the local area network. Accuracy can reach 1- 50ms. It is worth mentioning that the simple NTP (SNTP) version 4 has been described in RFC2030.

To adapt to the hierarchical structure of the Internet, NTP uses a hierarchical time distribution model. The network structure of the NTP system includes a master time server, a slave time server, and a transmission path between the client and the node. Main time server and high-precision synchronization clock source. Obtain synchronization from the time server via the master server or from other servers. Under normal circumstances, nodes (including slave time servers and clients) only use the most reliable and accurate server and transmission path for synchronization, so the usual synchronization path presents a hierarchical structure. In this hierarchical structure, the main server is located at the root node, and the other servers are located on the layer that gradually approaches the leaf node as the synchronization accuracy increases.

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