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Inserting database records from multiple servers using timestamps requires even more accurate timekeeping.
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were not synchronized with millisecond accuracy, we would never know which balance ($18,475.90 or $17,475.90) is actually correct. What if a leap second was observed on one server and not another Clearly, there is a need for systems to be able to regularly synchronize their clocks to ensure consistency in enterprise applications. One solution that solves the accuracy problem for single systems and for networks is the Network Time Protocol (NTP). The current production version of NTP is v3, specified in RFC 1305, which allows time to be synchronized between all systems on a network by using multicast, and also permits high-precision external hardware clock devices to be supported as authoritative time sources. These two approaches ensure that potential data-consistency problems caused by timestamps do not hamper online transaction processing and other real-time data-processing applications. By using a master-slave approach, one server on the network can be delegated the authority for timekeeping for all systems on that network. Using a master-slave approach ensures that multiple, potentially conflicting sources of authoritative time do not interfere with each other s operation. Note that Simple NTP (SNTP) version 4 is described in RFC 2030; however, it is still under development and is not a formal RFC yet. NTPv3 provides a number of enhancements over previous versions it supports a method for servers to communicate with a set of peer servers to average offsets and achieve a more accurate estimation of current time. This is a similar method used by national measurement laboratories and similar timekeeping organizations. In addition, network bandwidth can be preserved because the interval between client/server synchronizations has been substantially increased. This improvement in efficiency has been achieved because of the improvements to the local-clock algorithm s accuracy. In any case, NTP uses UDP to communicate synchronization data, minimizing any network overhead. In order for clients to access server data, the IP address or hostname of the server must be known there is no mechanism for automatic discovery of a time server defined by NTP.
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While NTP has a simple client/server interface, individual servers also have the ability to act as secondary servers for external, authoritative time sources. For example, a network might have a designated time server from which all clients retrieve the correct time, which the server in turn authoritatively receives from a national measurement laboratory. In addition, hardware clocks can be used as a backup in case of network failure between the local network and the measurement laboratory. When a connection is reestablished, the local server s time can simply be recalibrated with the authoritative time received from the laboratory. This chapter examines how to configure NTP servers and clients to synchronize their timekeeping and examines strategies for maintaining accurate time on the server side.
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The following procedures are used to run DHCP and NTP.
DHCP Operations
The basic DHCP process is a straightforward, two-phase process involving a single DHCP client and at least one DHCP server. When the DHCP client (dhcpagent) is started on a client, it broadcasts a DHCPDISCOVER request for an IP address on the local network, which is received by all available servers running a DHCP server (in.dhcpd). Next, all DHCP servers that have spare IP addresses answer the client s request through a DHCPOFFER message, which contains an IP address, subnet mask, default router name, and DNS server IP address. If multiple DHCP servers have IP addresses available, it is possible that multiple servers will respond to the client request. The client simply accepts the first DHCPOFFER that it receives, upon which it broadcasts a DHCPREQUEST message, indicating that a lease has been obtained. Once the server whose IP is accepted has received this second request, it confirms the lease with a DHCPACK message. After a client has finished using the IP address, a DHCPRELEASE message is sent to the server. In the situation where a server has proposed a lease in the first phase that it is unable to fulfill in the second phase, it must respond with a DHCPNACK message. This means that the client will then broadcast a DHCPDISCOVER message, and the process will start again. A DHCPNACK message is usually sent if a timeout has occurred between the original DHCPDISCOVER request and the subsequent reception at the server side of a DHCPREQUEST message. This is often due to network outages or congestion. The list of all possible DHCP messages is shown in Table 22-1. The DHCPOFFER message specifies the lease period, after which the lease will be deemed to have expired and will be made available to other clients. However, clients also have the option of renewing an existing lease, so that their existing IP address can be retained. DHCP defines fixed intervals prior to actual lease expiry at which time a client should indicate whether or not it wishes to extend the lease. If these renewals are not made in time, a DHCPRELEASE message will be broadcast, and then the lease will be invalid.
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