AI News, Difference between revisions of "Portal talk:Computer networks"

Difference between revisions of "Portal talk:Computer networks"

Here is a some of my favorite networking topics --Mattwj2002 17:39, 9 June 2007 (UTC)

I'd like to move the list of certifications and so on to a sub page, and talk about what we are/want to teach here.

I simply propose that we contact as many of the contributors as possible and try to get all of this organized into a logical whole.

Here in the Topic: namespace is where people come together to develop learning resources in the (main) namespace.

would suggest reorganizing this page as Computer Networks is a very huge field need to be discussed in a better way rather than what have been done currently.

After the merger performed not too long ago it is not hard to see that there are swathes of information hanging around related to various introductions to the topic of networking.

We need to come together and determine a formal structure for teaching courses related to the department of computer networks.

Information that will definitely fit into this area beneath what I have listed includes LAN, wireless networks, security, cryptography etc.

Hope to see people add to my list here and throw in some comments about the direction of this topic, doing this kind of feels like I'm talking to myself on a desert island.

We've all got commitments in life but it also heartwarming to know that your knowledge will benefit some student who can't afford higher education but is willing to learn and help himself in self improvement.

Use your best judgement on where it should go, and try to not to disturb the layout too too much (unless you feel like trying to give it all a bit of a facelift - again, very few people are actually maintaining the CS pages).

NIST Internet time service

of the public files on the time servers, including example source code for programmers who want to write their own applications, have been moved to these public ftp sites:ftp.nist.gov/pub/time and ftp.boulder.nist.gov/pub/time.

Windows, Mac, Linux) have an option to automatically synchronize the system clock periodically using an NTP (network time protocol) server:Windows: Double-click the system clock and then click on the Internet Time tab. Mac:

leap second is announced in advanced in Bulletin C of the International Earth Rotation and Reference Service (www.iers.org).  The leap second can be either positive or negative, although only positive leap seconds have ever been used, and it is very unlikely that negative leap seconds will ever be required.

The leap second event is linked to the UTC time scale (not local time as with daylight saving time), and therefore occurs at different local times in different time zones.

The name of a positive leap second is 23:59:60, but systems that represent the current time as the number of seconds that have elapsed since some origin (NTP, for example) generally cannot represent that time.

For example, it can be difficult to establish the time-ordering of events in the vicinity of a leap second, since the time 23:59:59.2 in the leap second occurred after 23:59:59.5 in the first second with that name.

There are two ways of realizing the leap second that we see as incorrect: 1) Some systems implement the leap second by repeating second 0 of the next day instead of second 23:59:59 of the leap second day.

2) Some systems implement the leap second by a frequency adjustment that smears the leap second out over some longer interval.

Therefore, any application that requires time that is legally traceable to national standards and uses these methods to realize the leap second, will have a time error on the order of 0.5 - 1 s in the vicinity of the leap second event.

The accuracy of the time stamps as seen by a user will usually be determined largely by the stability and reciprocity of the network connection between the server and the user's systems.

The client software ignores responses from servers that appear to be sending the wrong time, and averages the results from those that appear to be correct.

The data packet includes a 64-bit timestamp containing the time in UTC seconds since January 1, 1900 with a resolution of 200 ps.

In addition to this standard NTP service (which will not be modified), we have begun testing an authenticated version of NTP using a single time server that implements the symmetric key encryption method defined in the NTP documentation.

NIST chose a time code format similar to the one used by its dial-up Automated Computer Time Service (ACTS), as shown below: JJJJJ YR-MO-DA HH:MM:SS TT L H msADV UTC(NIST) OTM where: This simple protocol is now used by only about 1% of ITS customers.

Since many computers keep time internally as the number of seconds since January 1, 1970 (or another date), converting the received time to the necessary format is often a simple matter of binary arithmetic.

However, the format does not allow any additional information to be transmitted, such as advance notification of leap seconds or daylight saving time, or information about the health of the server.

How the Windows Time Service Works

Applies to: Windows Server 2016, Windows Server 2012 R2, Windows Server 2012, Windows 10 or later In this section Although the Windows Time service is not an exact implementation of the Network Time Protocol (NTP), it uses the complex suite of algorithms that is defined in the NTP specifications to ensure that clocks on computers throughout a network are as accurate as possible.

The following factors often affect the accuracy of synchronization in AD DS: Computers that synchronize their time less frequently or are not joined to a domain are configured, by default, to synchronize with time.windows.com.

If more than one time source is configured on a computer, Windows Time uses NTP algorithms to select the best time source from the configured sources based on the computer's ability to synchronize with that time source.

Resources that acquire time from the NTP server are two steps away from the reference clock, and therefore occupy a stratum that is two higher than the most accurate time source, and so on.

When the time service has determined which time sample is best, based on the above criteria, it adjusts the local clock rate to allow it to converge toward the correct time.

If the time difference between the local clock and the selected accurate time sample (also called the time skew) is too large to correct by adjusting the local clock rate, the time service sets the local clock to the correct time.

In addition to providing information about the current system state, such as the current time source or the last time the system clock was updated, the Windows Time Service Manager is also responsible for creating events in the event log.

The time synchronization process involves the following steps: If a computer has been designated as a time server, it can send the time on to any computer requesting time synchronization at any point in this process.

A time protocol is responsible for determining the best available time information and converging the clocks to ensure that a consistent time is maintained on separate systems.

NTP is a fault-tolerant, highly scalable time protocol and is the protocol used most often for synchronizing computer clocks by using a designated time reference.

The clock-filtering algorithm is designed to sift through time samples that are received from queried time sources and determine the best time samples from each source.

This information is then passed to the clock discipline algorithm, which uses the information gathered to correct the local clock of the computer, while compensating for errors due to network latency and computer clock inaccuracy.

The NTP provider follows the standards specified by NTP version 3 for a client and server, and can interact with SNTP clients and servers for backward compatibility with Windows 2000 and other SNTP clients.

The security of NTP packets that are sent between a domain member computer and a local domain controller that is acting as a time server is based on shared key authentication.

When a time server returns an authenticated NTP packet to a client that requests the time, the packet is signed by means of a Kerberos session key defined by an interdomain trust account.

In this way, the domain controller that is configured as reliable in the forest root domain becomes the authenticated time source for all of the domain controllers in both the parent and child domains, and indirectly for all computers located in the domain tree.

To obtain accurate time synchronization from a computer in a different forest, the client needs network access to that computer and the time service must be configured to use a specific time source located in the other forest.

If a client is manually configured to access time from an NTP server outside of its own domain hierarchy, the NTP packets sent between the client and the time server are not authenticated, and therefore are not secure.

By default, the Windows Time service NTP time provider does not support the direct connection of a hardware device to a computer, although it is possible to create a software-based independent time provider that supports this type of connection.

Without the use of an independent time provider, Windows time servers can acquire their time by connecting to an external NTP server, which is connected to a hardware device by means of a telephone or the Internet.

For example, if your domain is configured to synchronize time by using the domain hierarchy-based method of synchronization and you want computers in the domain hierarchy to synchronize time with a Windows NT 4.0 domain controller, you have to configure those computers manually to synchronize with the Windows NT 4.0 domain controllers.

The network time synchronization process, also called time convergence, occurs throughout a network as each computer accesses time from a more accurate time server.

The information provided within a packet indicates whether an adjustment needs to be made to the computer's current clock time so that it is synchronized with the more accurate server.

In a Windows Server 2003 forest, the computer that holds the primary domain controller (PDC) emulator operations master role, located in the forest root domain, holds the position of best time source, unless another reliable time source has been configured.

If a domain controller is configured to be a reliable time source, Net Logon service announces that domain controller as a reliable time source when it logs on to the network.

cycle in the synchronization network occurs when time remains consistent between a group of domain controllers and the same time is shared between them continuously without a resynchronization with another reliable time source.

computer uses one of the following methods to identify a time source to synchronize with: If the computer is a domain controller, it makes up to six queries to locate another domain controller to synchronize with.

Each query is designed to identify a time source with certain attributes, such as a type of domain controller, a particular location, and whether or not it is a reliable time source.

A computer that is a member of a domain is configured by default to synchronize from the domain hierarchy, manually-specified synchronization is most useful for the forest root of the domain or for computers that are not joined to a domain.

However, configuring the authoritative computer for your domain to synchronize with a hardware clock is actually a better solution for providing the most accurate, secure time to your domain.

This method allows synchronization with the domain hierarchy and may also provide an alternate time source if the domain hierarchy becomes unavailable, depending on the configuration.

If the client is unable to synchronize time with the domain hierarchy, the time source automatically falls back to the time source specified by the NtpServer setting.

If a time source is taken off of the network for scheduled maintenance and you do not intend to reconfigure the client to synchronize from another source, you can disable synchronization on the client to prevent it from attempting synchronization while the time server is unavailable.

If the root of the synchronization hierarchy is not set to NoSync and if it is unable to synchronize with another time source, clients do not accept the packet that this computer sends out because its time cannot be trusted.

This is because all NTP servers need access to User Datagram Protocol (UDP) port 123, and as long as the Windows Time service is running on the Windows Server 2003 operating system, port 123 remains reserved by Windows Time.

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