When users think of time, they think about the clock on their host, tablet, phone, or some other mobile device showing the correct time, but there is much more to time in a network than display. For instance:
• Two users have an item in their carts, but only one is available. Both users check out at about the same time. Which one should be allowed to check out, and which one should be notified the item has already been sold?
• Two users place orders to sell stock at about the same time.
Which sell order should be completed first, and what is the final price for the second sale?
• Someone deposits some money into your account. A few moments later, you withdraw the same amount. Should the deposit or withdrawal be counted first?
In thousands of situations, the relative timing of two events can impact. Since the computer network is often the only connection between two users or hosts, the network’s time is often used to “judge” event ordering.
Network Time Protocol (NTP ) is the standard protocol for synchronizing network devices and hosts within a few milliseconds of one another across a global network. Figure 16- 11 illustrates NTP’s operation.
Figure 16-11 Network Time Protocol
Stratums, as shown in Figure 16-11, are the distance between trusted sources of time and the local server. Stratum 1 servers directly query trusted sources of time, while stratum 2 servers query stratum 1 servers, etc.
Stratum 0 devices are atomic clocks, the Global Positioning System (GPS) radio clocks, or devices using Precision Time Protocol (PTP) to obtain sub-microsecond time from some other stratum 0 devices. Stratum 0 devices must be accurate to less than a micro-second and generate a hardware signal every second to connected stratum 1 servers.
Stratum 1 servers may be connected to several stratum 0 devices and may also query other stratum 1 servers.
NTP is a client/server protocol; the client queries a server for the current time. A server can act as a client to other NTP servers and as a server to other servers, hosts, and devices.
To compute the precise local time an NTP client:
• Queries one or more servers for the current time.
• Computes the round-trip time (RTT) to each queried server.
• Removes the RTT from each server’s response and discards all but the three closest responses.
• Calculates the correct time from the three closest responses.
The precise math used to compute the correct local time is outside the scope of this book.
Once the client computes the correct time, it adjusts the speed of the local clock to either catch up or slow down to the correct time.
Chapter Review
Names and time are two important services in the global
Internet—in fact, in any network. Domain names are resolved using DNS, a global, distributed, disaggregated system. DNS separates responsibilities and roles, enabling an entire ecosystem of organizations, and preventing any single organization from dominating the Internet’s name space.
While network engineers do not always support the DNS infrastructure, it is important for network engineers to understand the process of DNS resolution and to know how to troubleshoot and fix domain name resolution problems.
From a user’s perspective, there is no real difference between naming and reachability; it is all just a “part of the network.”
Time is even less obvious to users than names. Network time is hidden behind applications, and does not, at first glance, appear to be very important. Network time, however, is crucial to the proper operation of many services, and the time ecosystem is complex.
The next chapter considers cloud computing—what it is and why it is important. Many engineers seem to think the cloud will eliminate the need for computer networking expertise, but the opposite is true. Connecting to a cloud, and connecting resources within a cloud, is still an important part of a network engineer’s role.
One key to doing well on the exams is to perform repetitive spaced review sessions. Review this chapter’s material using either the tools in the book or interactive tools for the same material found on the book’s companion website. Refer to the online Appendix D, “Study Planner,” element for more details.
Table 16-4 outlines the key review elements and where you can find them. To better track your study progress, record when you completed these activities in the second column.
Table 16-4 Chapter Review Tracking
Review All the Key Topics
Table 16-5 lists the key topics for this chapter.
Table 16-5 Key Topics for Chapter 16
