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.


In an operating system, the time (clock) is determined by four parts: time value, time standard, time zone, and Daylight Saving Time (DST) if applicable.

The localtime standard is dependent on the current time zone, while UTC is the global time standard and is independent of time zone values.

An OS that uses the UTC standard will generally consider the hardware clock as UTC and make an adjustment to it to set the OS time at boot according to the time zone.

If multiple operating systems are installed on a machine, they will all derive the current time from the same hardware clock: it is recommended to adopt a unique standard for the hardware clock to avoid conflicts across systems and set it to UTC.

Otherwise, if the hardware clock is set to localtime, more than one operating system may adjust it after a DST change for example, thus resulting in an over-correction;

In case you choose to create the link manually (for example during chroot where timedatectl won't work), keep in mind that it must be a symbolic link, as specified in archlinux(7):

To set the timezone automatically based on the IP address location, one can use a geolocation API to retrieve the timezone, for example $ curl, and pass the output to timedatectl set-timezone for automatic setting.

The drift value is calculated by using the difference between the new value set and the hardware clock value just before the set, taking into account the value of the previous drift value and the last time the hardware clock was set.

If the hardware clock keeps losing or gaining time in large increments, it is possible that an invalid drift has been recorded (but only applicable, if the hwclock daemon is running).

The drift value can be removed by removing the file /etc/adjtime, then set the correct hardware clock and system clock time, and check if your time standard is correct.

For example to test applications relying on the time during development or adjusting the system time zone when logging into a server remotely from another zone.

This is different than just setting the time, as for example it allows to test the behavior of a program with positive or negative UTC offset values, or the effects of DST changes when developing on systems in a non-DST time zone.

Another use case is having different time zones set for different users of the same system: this can be accomplished by setting the TZ variable in the shell's configuration file, see Environment variables#Defining variables.

For example, if your hardware clock is running on local time, but timedatectl is set to assume it is in UTC, the result would be that your timezone's offset to UTC effectively gets applied twice, resulting in wrong values for your local time and UTC.

alarm-fake-hwclock designed especially for system without battery backed up RTC, it includes a systemd service which on shutdown saves the current time and on startup restores the saved time, thus avoiding strange time travel errors.

Coordinated Universal Time

The system has been adjusted several times, including a brief period where time coordination radio signals broadcast both UTC and 'Stepped Atomic Time (SAT)' before a new UTC was adopted in 1970 and implemented in 1972.

Thus, in the UTC time scale, the second and all smaller time units (millisecond, microsecond, etc.) are of constant duration, but the minute and all larger time units (hour, day, week, etc.) are of variable duration.

Since 1972, UTC is calculated by subtracting the accumulated leap seconds from International Atomic Time (TAI), which is a coordinate time scale tracking notional proper time on the rotating surface of the Earth (the geoid).

Users who require an approximation in real time must obtain it from a time laboratory, which disseminates an approximation using techniques such as GPS or radio time signals.

later corrections may be applied using the International Bureau of Weights and Measures (BIPM) monthly publication of tables of differences between canonical TAI/UTC and TAI(k)/UTC(k) as estimated in real time by participating laboratories.[21]

It is not possible to compute the exact time interval elapsed between two UTC timestamps without consulting a table that shows how many leap seconds occurred during that interval.

By extension, it is not possible to compute the precise duration of a time interval that ends in the future and may encompass an unknown number of leap seconds (for example, the number of TAI seconds between 'now' and 2099-12-31 23:59:59).

Current civil time in a particular time zone can be determined by adding or subtracting the number of hours and minutes specified by the UTC offset, which ranges from UTC−12:00 in the west to UTC+14:00 in the east (see List of UTC time offsets).

The time zone using UTC is sometimes denoted UTC±00:00 or by the letter Z—a reference to the equivalent nautical time zone (GMT), which has been denoted by a Z since about 1950.

On electronic devices that only allow the current time zone to be configured using maps or city names, UTC can be selected indirectly by selecting Reykjavík, Iceland, which is always on UTC and does not use daylight saving time.[25]

UTC does not change with a change of seasons, but local time or civil time may change if a time zone jurisdiction observes daylight saving time (summer time).

For example, local time on the east coast of the United States is five hours behind UTC during winter, but four hours behind while daylight saving is observed there.[26]

Sir Sandford Fleming promoted worldwide standard time zones, a prime meridian, and the use of the 24-hour clock as key elements in communicating the accurate time.[27]

At the 1884 International Meridian Conference held in Washington, D.C., the local mean solar time at the Royal Observatory, Greenwich in England was chosen to define the Universal day, counted from 0 hours at mean midnight.

In contrast, astronomical GMT began at mean noon, 12 hours after mean midnight of the same date until 1 January 1925, whereas nautical GMT began at mean noon, 12 hours before mean midnight of the same date, at least until 1805 in the Royal Navy, but persisted much later elsewhere because it was mentioned at the 1884 conference.

In 1960, the U.S. Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory coordinated their radio broadcasts so that time steps and frequency changes were coordinated, and the resulting time scale was informally referred to as 'Coordinated Universal Time'.[31]

In a controversial decision, the frequency of the signals was initially set to match the rate of UT, but then kept at the same frequency by the use of atomic clocks and deliberately allowed to drift away from UT.

The ephemeris second is a unit in the system of time that, when used as the independent variable in the laws of motion that govern the movement of the planets and moons in the solar system, enables the laws of motion to accurately predict the observed positions of solar system bodies.

At the end of 1971, there was a final irregular jump of exactly 0.107758 TAI seconds, so that 1 January 1972 00:00:00 UTC was 1 January 1972 00:00:10 TAI exactly, making the difference between UTC and TAI a whole number of seconds.

In earlier centuries, the mean solar day was shorter than 86,400 SI seconds, and in more recent centuries it is longer than 86,400 seconds.

Near the end of the 20th century, the length of the mean solar day (also known simply as 'length of day' or 'LOD') was approximately 86,400.0013 s.[43]

The excess of the LOD over the nominal 86,400 s accumulates over time, causing the UTC day, initially synchronised with the mean sun, to become desynchronised and run ahead of it.

Near the end of the 20th century, with the LOD at 1.3 ms above the nominal value, UTC ran faster than UT by 1.3 ms per day, getting a second ahead roughly every 800 days.

Just as adding a leap day every four years does not mean the year is getting longer by one day every four years, the insertion of a leap second every 800 days does not indicate that the mean solar day is getting longer by a second every 800 days.

It will take about 50,000 years for a mean solar day to lengthen by one second (at a rate of 2 ms/cy, where cy means century).

The correct reason for leap seconds, then, is not the current difference between actual and nominal LOD, but rather the accumulation of this difference over a period of time: Near the end of the 20th century, this difference was about 1/800 of a second per day;

(The slope became shallower in the 2000s (decade), because of a slight acceleration of Earth's crust temporarily shortening the day.) Vertical position on the graph corresponds to the accumulation of this difference over time, and the vertical segments correspond to leap seconds introduced to match this accumulated difference.

The resulting gradual shift of the sun's movements relative to civil time is analogous to the shift of seasons relative to the yearly calendar that results from the calendar year not precisely matching the tropical year length.

Assuming that there are no major events affecting civilisation over the coming centuries, the difference between UTC and UT could reach 0.5 hour after the year 2600 and 6.5 hours around 4600.[49]

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 and

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 (  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.

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