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Internet Protocol IP based networks are quickly evolving from the traditional best effort delivery model to a model where performance and reliability need to be quantified and, in many cases, guaranteed with Service Level Agreements SLAs. The need for greater insight into network characteristics has led to significant research efforts being targeted at defining metrics and measurement capabilities to characterize network behavior.

The foundation of many metric methodologies is the measurement of time. Network time synchronization, to the degree required for modern performance analysis, is an essential exercise. Depending on the business models, and the services being provided, the characterization of network performance can be considered an important competitive service differentiator.

In these cases, great expense may be incurred deploying network management systems and directing engineering resources towards analyzing the collected performance data. However, if proper attention is not given to the often-overlooked principle of time synchronization, those efforts may be rendered useless.

This document describes a hypothetical process definition for conducting network management functions for the Network Time Protocol NTP. It is intended that this hypothetical procedure be used as an informational example and customized by an organization to assist in meeting internal objectives. The information provided by this paper is presented in several major sections, which are described below.

The Terminology section provides general definitions of terms concerning time synchronization. The Overview section provides background information on network element hardware related to system time, a technological overview of NTP, and key design aspects for the NTP architecture.

The Process Definitions section provides an overview of the process definitions used to accomplish NTP management. The process details are described in terms of goals, performance indicators, inputs, outputs, and individual tasks. The Task Definitions section provides detailed process task definitions. Each task is described in terms of objectives, task inputs, task outputs, resources required to accomplish the ntp conf options trading, and job skills needed for a task implementer.

Data identification considers the source of the information. The collection of the data is closely related to the location of the data. Data maintained by internal data structures is collected by automatic scripts or by a user manually logging into the system to issue the CLI command and recording the output.

The Data Presentation section provides report format examples of how the data may be presented. Drift —The measurement in the variation of skew, or the second derivation of the clock's offset with respect to time. Joint resolution —When comparing clocks, this is the sum of the resolutions of C1 and C2. The joint resolution then indicates a conservative lower bound on the accuracy of any time intervals computed by subtracting time stamps generated by one clock from those generated by the other.

Node —Refers to an instantiation of the NTP protocol on a local processor. A node can also be referred to as a device. If the clock reports a time Tc and the true time is Tt, then the clock's offset is Tc - Tt. Peer —Refers to an instantiation of ntp conf options trading NTP protocol on a remote processor connected by a network path from the local node.

Relative offset —The notion of true time is replaced by the time as reported by clock C1, when comparing how two clocks, C1 and C2, compare. For example, ntp conf options trading C2's offset relative to C1 at a particular moment is Tc2 - Tc1, the instantaneous difference in time reported by C2 and C1. Resolution —The smallest unit by which a clock's time is updated. Resolution is defined in terms of seconds. However, resolution is relative to the clock's reported time and not to true time. For example, a resolution of 10 milliseconds means that the clock updates its notion ntp conf options trading time in 0.

Skew —A clock's frequency difference, or first derivative of its offset with respect to time. Synchronize ntp conf options trading two clocks are accurate with respect to one another relative ntp conf options trading is zerothey are synchronized. Clocks can be synchronized and still inaccurate in terms of how well they tell true time. The heart of the time service is the system clock. The system clock runs from the moment the system starts and keeps track of the current date and time.

The system clock ntp conf options trading be set from a number of sources and, in turn, can be used to distribute the current time through various mechanisms to other systems. Some routers contain a battery-powered calendar system that tracks the date and time across system restarts and power outages.

This calendar system is always used to initialize the system clock when the system is restarted. It can also be considered as an authoritative source of time and redistributed through NTP if no other source is available. Furthermore, if NTP is running, the calendar can be periodically updated from NTP, compensating for the inherent drift in the calendar time.

When a router with a system calendar is initialized, the system clock is set based on the time in its internal battery-powered calendar. On models without a calendar, the system clock is set to a predetermined time constant. The system clock can be set from the sources listed below. SNTP typically provides time within milliseconds of the accurate time.

In addition, Ntp conf options trading does not authenticate traffic, although you can configure extended access lists to provide some protection. An SNTP client is more vulnerable to misbehaving servers than an NTP client and should only be used in situations where strong authentication is not required. You can configure information about the local time zone and daylight savings time so that the time is displayed correctly relative to the local time zone.

The system clock keeps track of whether ntp conf options trading time is authoritative or not. If it is not authoritative, the time will be available only for display purposes and will not be redistributed.

NTP is designed to synchronize the time on a network of machines. A set of nodes on a network are identified and configured with NTP and the nodes form a synchronization subnet, sometimes referred to as an overlay network. While multiple masters primary servers may exist, there is no requirement for an election protocol.

An NTP network usually gets its time from an authoritative time source, such as a radio clock or an atomic clock attached to a time server. NTP then distributes this time across the network. An NTP client makes a transaction with its server over its polling interval from 64 to seconds which dynamically changes over time ntp conf options trading on the network conditions between the NTP server and the client. The other situation occurs when the router communicates to a bad NTP server for example, NTP server with large dispersion ; the router also increases the poll interval.

No more than one NTP transaction per minute is needed to synchronize two machines. It is not possible to ntp conf options trading the NTP poll interval on a router. NTP uses the concept of a stratum to describe how ntp conf options trading NTP hops away a machine is from an authoritative time source.

For example, a stratum 1 time server has a radio or atomic clock directly attached to it. It then sends its time to a stratum 2 time server through NTP, and so on. A machine running NTP automatically chooses the machine with the lowest stratum number that it is configured to communicate with using NTP as its time source. This strategy effectively builds a self-organizing tree of NTP speakers. NTP performs well over the non-deterministic path lengths of packet-switched networks, because it makes robust estimates ntp conf options trading the following three key variables in the relationship between a client and a time server.

Clock synchronization at the 10 millisecond level over long distance wide-area networks WANs kmand at the 1 millisecond level for local-area networks LANsis routinely ntp conf options trading. NTP ntp conf options trading synchronizing to a machine whose time may not be accurate in two ways. First of all, NTP never synchronizes to a machine that is not synchronized itself. Secondly, NTP compares the time reported by several machines, and will not synchronize to a machine whose time is significantly different than the others, even if its stratum is lower.

The communications between machines running NTP associations are usually statically configured. Each machine is given the IP address of all machines ntp conf options trading which it should form associations. Accurate timekeeping is made possible by exchanging NTP messages between each pair of machines with an association. This alternative reduces configuration complexity because each machine can be configured to send or receive broadcast messages.

However, the accuracy of timekeeping is marginally reduced because the information flow is one-way only. The time ntp conf options trading on a machine is a critical resource and it is strongly recommend that you use the security features of NTP to avoid the accidental or malicious setting of incorrect time. The two security features available are an access list-based restriction scheme and an encrypted authentication mechanism. If a release supports the ntp refclock command, it is possible to connect a radio or atomic clock.

If the network uses the public time servers on the Internet and the network is isolated from the Internet, Cisco's implementation of NTP allows a machine to be configured so that it acts as though it is synchronized through NTP, when in ntp conf options trading it has determined the time using other means. Ntp conf options trading machines then synchronize to that machine through NTP.

Each client in the synchronization subnet, which may also be a server for higher stratum clients, chooses one of the available servers to synchronize to. This is usually from among the lowest stratum servers it has access to. However, this is not always an optimal configuration, because NTP also operates under the premise that each server's time should be viewed with a certain amount of distrust.

NTP prefers to have access to several sources ntp conf options trading lower stratum time at least three since it can then apply an agreement algorithm to detect insanity on the part of any one of these. Normally, when all ntp conf options trading are in agreement, NTP chooses the best server in terms of lowest stratum, closest in terms of network delayand claimed precision.

The implication is that, while one should aim to provide each client with three or more sources of lower stratum time, several of these will only be providing backup service and may be of lesser quality in terms of network delay and stratum. For example, a same-stratum peer that receives time from lower stratum sources the local server doesn't access directly, can also provide good backup service. NTP generally prefers lower stratum servers to higher stratum servers unless the lower stratum server's time is significantly different.

The algorithm is able to detect when a time source is likely to be extremely inaccurate, or insane, and to prevent synchronization in these cases, even if the inaccurate clock is at a lower stratum level.

And it will never synchronize a device to another server that is not synchronized itself. Implementations should include sanity timeouts which prevent trap transmissions if the monitoring program does not renew this information after a lengthy interval.

Additional sanity checks are included for authentication, range bounds, and to avoid use of very old ntp conf options trading. Checks have been added to warn that the oscillator has gone too long without update from a reference source. The following sections describe the associating modes used by NTP servers to associate with each other. This provides protection against malfunctions or protocol attacks.

It operates in the classic remote-procedure-call RPC paradigm with stateless servers. In this mode, a client sends a request to the server and expects a reply at some future time. In some contexts, this would be described as a ntp conf options trading operation, in that the client polls the time and authentication data from the server.

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Want to link to this manual page? Skip site navigation 1 Skip section navigation 2 Header And Logo. The file format is similar to other UNIX configuration files.

Configuration commands consist of an initial keyword fol- lowed by a list of arguments, some of which may be optional, separated by whitespace. Commands may not be continued over multiple lines. Argu- ments may be host names, host addresses written in numeric, dotted-quad form, integers, floating point numbers when specifying times in seconds and text strings.

The rest of this page describes the configuration and control options. In addition to the discussion of general Configuration Options , there are sections describ- ing the following supported functionality and the options used to control it: While there is a rich set of options available, the only required option is one or more pool , server , peer , broadcast or manycastclient commands. These commands have the same basic functions as in NTPv3 and in some cases new functions and new arguments.

There are two classes of commands, configu- ration commands that configure a persistent association with a remote server or peer or reference clock, and auxiliary commands that specify environmental variables that control various related operations.

Configuration Commands The various modes are determined by the command keyword and the type of the required IP address. Addresses are classed by type as s a remote server or peer IPv4 class A, B and C , b the broadcast address of a local interface, m a multicast address IPv4 class D , or r a refer- ence clock address Note that only those options applica- ble to each command are listed below.

Use of options not listed may not be caught as an error, but may result in some weird and even destructive behavior. In a few cases, including the reslist billboard generated by ntpq 8 or ntpdc 8 , IPv6 addresses are automatically generated. IPv6 addresses can be used almost everywhere where IPv4 addresses can be used, with the excep- tion of reference clock addresses, which are always IPv4.

Note that in contexts where a host name is expected, a -4 qualifier pre- ceding the host name forces DNS resolution to the IPv4 namespace, while a -6 qualifier forces DNS resolution to the IPv6 namespace.

See IPv6 ref- erences for the equivalent classes for that address family. In this mode the local clock can synchronized to the remote server, but the remote server can never be synchronized to the local clock. In this mode the local clock can synchronized to the remote server, but the remote server can never be synchro- nized to the local clock.

This command should not be used for type b or m addresses. In this mode the local clock can be synchronized to the remote peer or the remote peer can be synchronized to the local clock. This is useful in a network of servers where, depending on vari- ous failure scenarios, either the local or remote peer may be the better source of time. This command should NOT be used for type b, m or r addresses. Note that local broadcast mes- sages go only to the interface associated with the subnet speci- fied, but multicast messages go to all interfaces.

In broadcast mode the local server sends periodic broadcast messages to a client population at the address specified, which is usually the broadcast address on one of the local network s or a multicast address assigned to NTP. Ordinarily, this specification applies only to the local server operating as a sender; for operation as a broadcast client, see the broadcastclient or multicastclient commands below.

In this case a specific address must be supplied which matches the address used on the manycastserver command for the designated manycast servers. The NTP multicast address The client broadcasts a request message to the group address associated with the specified address and specifically enabled servers respond to these messages. The client selects the servers providing the best time and continues as with the server command.

The remaining servers are discarded as if never heard. The packet spacing is normally 2 s; however, the spacing between the first and second packets can be changed with the calldelay command to allow additional time for a modem or ISDN call to complete. This is designed to improve timekeeping quality with the server command and s addresses.

The packet spacing is normally 2 s; however, the spacing between the first two packets can be changed with the calldelay command to allow additional time for a modem or ISDN call to complete. This is designed to speed the initial synchronization acquisition with the server command and s addresses and when ntpd 8 is started with the -q option.

The default is to include no encryption field. The minimum poll inter- val defaults to 6 64 s , but can be decreased by the minpoll option to a lower limit of 4 16 s. The server is discarded by the selection algroithm. Use this option only for test- ing. All other things being equal, this host will be chosen for synchronization among a set of cor- rectly operating hosts. This option should almost certainly only be used while testing an association.

It specifies the time-to-live ttl to use on broad- cast server and multicast server and the maximum ttl for the expanding ring search with manycast client packets.

Selection of the proper value, which defaults to , is something of a black art and should be coordinated with the network administrator. Versions are the choices, with version 4 the default. Auxiliary Commands broadcastclient This command enables reception of broadcast server messages to any local interface type b address. Note that, in order to avoid accidental or malicious disruption in this mode, both the server and client should operate using symmetric-key or public-key authentication as described in Authentication Options.

This command enables reception of manycast client messages to the multicast group address es type m specified. At least one address is required, but the NTP multicast address Note that, in order to avoid accidental or malicious disruption in this mode, both the server and client should operate using symmetric-key or pub- lic-key authentication as described in Authentication Options.

This command enables reception of multicast server messages to the multicast group address es type m specified. Note that, in order to avoid accidental or malicious disruption in this mode, both the server and client should operate using sym- metric-key or public-key authentication as described in Authentication Options.

If that registration attempt fails, we try again at one minute intervals for up to mdnstries times. After all, ntpd may be starting before mDNS. The default value for mdnstries is 5. Authentication Support Authentication support allows the NTP client to verify that the server is in fact known and trusted and not an intruder intending accidentally or on purpose to masquerade as that server.

Either algorithm computes a message digest, or one-way hash, which can be used to verify the server has the correct private key and key identifier. NTPv4 retains the NTPv3 scheme, properly described as symmetric key cryp- tography and, in addition, provides a new Autokey scheme based on public key cryptography. Public key cryptography is generally considered more secure than symmetric key cryptography, since the security is based on a private value which is generated by each server and never revealed.

With Autokey all key distribution and management functions involve only public values, which considerably simplifies key distribution and storage. Pub- lic key management is based on X. While the algorithms for symmetric key cryptography are included in the NTPv4 distribution, public key cryptography requires the OpenSSL software library to be installed before building the NTP distribution.

Directions for doing that are on the Building and Installing the Distribution page. Authentication is configured separately for each association using the key or autokey subcommand on the peer , server , broadcast and manycastclient configuration commands as described in Configuration Options page.

The authentication options described below specify the locations of the key files, if other than default, which symmetric keys are trusted and the interval between various operations, if other than default.

Authentication is always enabled, although ineffective if not configured as described below. If a NTP packet arrives including a message authen- tication code MAC , it is accepted only if it passes all cryptographic checks.

The checks require correct key ID, key value and message digest. If the packet has been modified in any way or replayed by an intruder, it will fail one or more of these checks and be discarded. Furthermore, the Autokey scheme requires a preliminary protocol exchange to obtain the server certificate, verify its credentials and initialize the protocol The auth flag controls whether new associations or remote configuration commands require cryptographic authentication.

This flag can be set or reset by the enable and disable commands and also by remote configuration commands sent by a ntpdc 8 program running on another machine. If this flag is enabled, which is the default case, new broadcast client and sym- metric passive associations and remote configuration commands must be cryptographically authenticated using either symmetric key or public key cryptography.

If this flag is disabled, these operations are effective even if not cryptographic authenticated. It should be understood that operating with the auth flag disabled invites a significant vulnerability where a rogue hacker can masquerade as a falseticker and seriously dis- rupt system timekeeping. It is important to note that this flag has no purpose other than to allow or disallow a new association in response to new broadcast and symmetric active messages and remote configuration com- mands and, in particular, the flag has no effect on the authentication process itself.

An attractive alternative where multicast support is available is many- cast mode, in which clients periodically troll for servers as described in the Automatic NTP Configuration Options page. Either symmetric key or public key cryptographic authentication can be used in this mode.

The principle advantage of manycast mode is that potential servers need not be configured in advance, since the client finds them during regular operation, and the configuration files for all clients can be identical.

The security model and protocol schemes for both symmetric key and public key cryptography are summarized below; further details are in the brief- ings, papers and reports at the NTP project page linked from http: Symmetric-Key Cryptography The original RFC specification allows any one of possibly 65, keys, each distinguished by a bit key identifier, to authenticate an association.

The servers and clients involved must agree on the key and key identifier to authenticate NTP packets. Keys and related information are specified in a key file, usually called ntp.

Besides the keys used for ordinary NTP associations, addi- tional keys can be used as passwords for the ntpq 8 and ntpdc 8 utility programs. When ntpd 8 is first started, it reads the key file specified in the keys configuration command and installs the keys in the key cache. How- ever, individual keys must be activated with the trusted command before use.

This allows, for instance, the installation of possibly several batches of keys and then activating or deactivating each batch remotely using ntpdc 8. This also provides a revocation capability that can be used if a key becomes compromised. The requestkey command selects the key used as the password for the ntpdc 8 utility, while the controlkey command selects the key used as the password for the ntpq 8 utility. Using all of these schemes provides strong security against replay with or without modification, spoofing, masquer- ade and most forms of clogging attacks.

The Autokey protocol has several modes of operation corresponding to the various NTP modes supported. Most modes use a special cookie which can be computed independently by the client and server, but encrypted in transmission. All modes use in addition a variant of the S-KEY scheme, in which a pseudo-random key list is generated and used in reverse order.

These schemes are described along with an executive summary, current sta- tus, briefing slides and reading list on the Autonomous Authentication page. The specific cryptographic environment used by Autokey servers and clients is determined by a set of files and soft links generated by the ntp-keygen 1ntpkeygenmdoc program. This includes a required host key file, required certificate file and optional sign key file, leapsecond file and identity scheme files.

NTP secure groups can be used to define cryptographic compartments and security hierarchies. It is important that every host in the group be able to construct a certificate trail to one or more trusted hosts in the same group. Each group host runs the Autokey protocol to obtain the cer- tificates for all hosts along the trail to one or more trusted hosts.