Computer Network Time Synchronization

Related Technology (Computer Network Time Synchronization)

NTP is not the only network timekeeping technology. Other mechanisms have been specified in the Internet protocol suite to record and transmit the time at which an event takes place, including the Daytime protocol [1], Time protocol [2], ICMP (Internet Control Message Protocol) Timestamp message [3], and IP (Internet Protocol) Timestamp option [4]. Other synchronization […]

Terms and Notation (Computer Network Time Synchronization)

Recall that t represents the epoch according to the tick counter, called process time, while T(t) represents the real time displayed by a clock at that epoch. Then, whereis some epoch in process time whenis the UTC time,is the frequency,is the drift (first derivative of frequency), and x(t) is some stochastic noise process yet to […]

Process Flow (Computer Network Time Synchronization)

The NTP daemon itself is an intricate, complex, real-time system program. It usually operates simultaneously with multiple servers and may have multiple clients of its own. FIGURE 3.1 Process organization. The overall organization of the processes is illustrated in Figure 3.1. For every server, there are two processes a peer process that receives and processes […]

Peer Process (Computer Network Time Synchronization)

Received packets are first checked for correct format and version and discarded if invalid. While currently not required by the specification, as a configuration option the IP source address must match an entry in the access control list, where each entry contains an IP address, mask, and capability bits. Also not required by the specification, […]

Poll Process (Computer Network Time Synchronization)

The poll process generates output packets at regular poll intervals and determines the state of the association as reachable, unreachable, or time-out. In the normal course of operation, it sometimes happens that a server becomes erratic or unreachable over the Internet. NTP deals with this using good Internet engineering principles developed from the ARPANET (Advanced […]

On-Wire Protocol (Computer Network Time Synchronization)

Packet processing continues to update the peer variables included in the association and run the on-wire protocol that computes the clock offset and round-trip delay. Figure 3.2 shows the basic on-wire protocol and how time-stamps are numbered and exchanged between hosts A and B. The state variables appear in lowercase, while the timestamps appear in […]

Clock Filter Algorithm (Computer Network Time Synchronization)

The NTP clock filter algorithm is designed to select the best sample data while rejecting noise spikes due to packet collisions and network congestion. Recall that the clock offset 8 and round-trip delay 5 samples are computed from the four most recent timestamps. Without making any assumptions about the delay distributions but assuming the frequency […]

Select Algorithm (Computer Network Time Synchronization)

To provide reliable synchronization, NTP uses multiple redundant servers and multiple disjoint network paths whenever possible. When a number of associations are mobilized, it is not clear beforehand which are truechimers and which are falsetickers. Crucial to the success of this approach is a robust algorithm that finds and discards the falsetickers from the selectable […]

Cluster Algorithm (Computer Network Time Synchronization)

NTP configurations usually include several servers to provide sufficient redundancy for the select algorithm to determine which are truechimers and which are not. When a sizable number of servers are present, the individual clock offsets for each are not usually the same, even if each server is closely synchronized to UTC by one means or […]

Combine Algorithm (Computer Network Time Synchronization)

The select and cluster algorithms described previously operate to select a single system peer based on stratum and root distance. The result is that the NTP subnet forms a forest of trees with the primary servers at the root and other servers at increasing stratum levels toward the leaves. However, since each server on the […]

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