[#tcoabbr]: TCO
“total cost of ownership”

In America, this may most often refer to an abbreviation for “total cost of ownership”. This refers to not only the initial costs of parts and/or software licensing, but also the costs of hardware maintenance, labor, license renewals, and any other costs related to a possible solution.

“Tjänstemännens Centralorganisation”

The Swedish name “Tjänstemännens Centralorganisation” is abbreviated as TCO. The English translation of the game is “Swedish Confederation of Professional Employees”. This organization's presense may be noticable in some technology sectors, as noted by Wikipedia's section on “TCO labeling”.

[#tkip]: “Temporary Key Integrity Protocol” (“TKIP”)

Wikipedia's article on “Wired Equivalent Privacy”: citation note 14 on the Feburary 5, 2013 revision states, “TKIP (the basis of WPA) has reached the end of its designed lifetime, has been broken, and has been deprecated”.

Actually, that statement may be a bit harsh against WPA. However, the harshness against TKIP is entirely warranted. IEEE 802.11-09/0601r2 (Word Document) states, “The use of WEP for confidentiality, authentication, or access control is deprecated. The WEP algorithm is unsuitable for the purposes of this standard.” The next paragraph goes on to state, “The use of TKIP is deprecated. The TKIP algorithm is unsuitable for the purposes of this standard.”

TKIP's unsuitability might not be a terribly huge surprise: Wikipedia's article on “Temporary Key Integrity Protocol”: section called “Security” notes, “TKIP uses the same underlying mechanism as WEP, and consequently is vulnerable to a number of similar attacks.” The introduction to the Wikipedia page notes, “ TKIP was designed” ... “as a solution to replace WEP without requiring the replacement of legacy hardware.”

Wikipedia's article on “Temporal Key Integrity Protocol”: “Security” section provides details on some attacks on TKIP.

[#thrfngsl]: three finger salute

(This may be a bit redundant with text at: user interface basics: the three finger salute.)

The three-finger salute refers to pressing Alt-Ctrl-Delete. People with large enough hands could hold the Right-Alt key with their right thumb, hold down the Right-Ctrl, and press the Delete key with one hand. Compared to today's standardized keyboards, this was easier to do back in the days of 101-key “Enhanced” keyboard layouts, because the right side of the Alt key was just below the left side of the right Shift key. (In the 101-key keyboard layout which was the standard of the early 1990s, the Alt key was located below the ? key. The Alt key often got moved further from the Delete key when the 104-key “Windows 95” layout started getting used: the space bar was shortened and the right alt key is now further left so as to make room for the right Start/Windows key and the Shortcut/Context Menu key. Also, newer keyboards may have been more prone to making that key a bit thinner, starting at that same time when the key moved.)

Many people may be more familiar with the keyboard combination being phrased as “Ctrl-Alt-Delete”. This may be because Microsoft Windows has been known to use that order. Actually, the Ctrl-Alt-Delete order probably makes more sense, if people are using two hands and are using the left Ctrl key and the left Alt key (since the Ctrl key is further left), for people who are most familiar with a language that reads from left to right. The order of saying Alt-Ctrl-Delete may be more logical for people with a large enough right hand to do the whole motion using just one hand.

This phrase came about when this combination was typically used to reboot a computer (which would typically be either running MS-DOS, or in the BIOS setup or some other portion of the system startup sequence: in any of these environments, the three finger salute would result in a reboot). Holding Ctrl and Alt, and pressing period on the numpad (even if NumLock is off) might have the same effect on some systems (particularly those using MS-DOS).

Microsoft must have noticed that no standard program tended to use this key combination (since the MS-DOS operating system would cause the computer to reboot). Later on, Microsoft Windows versions might use this as a sort of backdoor method of backgrounding a program, or running code that provides an option to stop a program. Microsoft decided to use this as an authentication feature of their newer graphical operating systems: Pressing this key combination would cause some code from the operating system to interact with the user, hiding any other pixels on the screen that might be coming from other programs. The idea, then, was that the user would be more likely to be interacting with trusted code when performing an action such as changing a password.

The term might be almost derogotory, with the implication that the computer may have misbehaved to the point that it is no longer useful, and requires the rude punishment of being rebooted.

ArsTechnica article: “If Bill Gates really thinks ctrl-alt-del was a mistake, he should have fixed it himself”

What is typically done when pressing Alt-Ctrl-Del is that the computer runs some specific code. Initially this is handled by code on the system's boot ROM. When the three finger salute is pressed, the code that gets run might be part of the software code that is called the BIOS. An operating system can change what happens when the three finger salute happens. For instance, a Linux-based operating system may have a file called /etc/inittab that specifies what to do. A server may have the three finger salute run /bin/false, which has extremely little impact whatsoever, while a computer designed for an end user might run a different program that will cause the operating system to restart.

The secure attention key (“SAK”) may also have purpose/intent similar to how the three finger salute has been used.

[#ttl]: “time to live” (“TTL”)

A way of keeping track of progress so that repetitive actions do not loop infinitely. The purpose is so that incorrect routing will not allow data to be uselessly routed in a loop forever. A packet that theoretically exists forever, because of such a routing loop, is sometimes referred to as an “immortal”. Assigning a TTL value is specifically designed to ensure that such data does eventually cease to exist, so that such data does not continue to consume the resources of network infrastructure equipment.

TTL with IPv4

With IPv4, this is implemented with the 65th through 72nd bits of the IPv4 packet. Every time a packet passes through a router (which is called a “hop”), the value of the TTL is reduced by one. RFC 791: IPv4 (page 14) notes that every “module” (meaning “device” serving as a “router”) “that processes a datagram must decrease the TTL by at least one even if it process the datagram in less than a second”.

RFC 792: ICMP, Page 6 (about the “Time Exceeded Message” of ICMP) starts to say (at the bottom of the page), “If the gateway processing a datagram finds the time to live field is zero it must discard the datagram. The gateway may also notify the source host via the time exceeded message.” So packet discarding is required, and informing (via ICMP's “Time Exceeded Message”) is an optional task (as the standard notes that it “may” be done).

In RFC 791 (IPv4, TTL values are supposed to be measured in actual time (measured in seconds). That idea is widely ignored. Instead, popular TCP/IP network stack implementations take the approach from part of the description at RFC 792 (ICMP): page 2, which notes, “as this field is decremented at each machine in which the datagram is processed, the value in this field should be at least as great as the number of gateways which this datagram will traverse.”

TTL with IPv6

This operates in the same way as how TTL was generally handled by popular TCP/IPv4 network stacks. RFC 2460: IPv6 identifies a field called “Hop Count”. Wikipedia's article on “Time to live”: section called “IP packets” notes that the reason for the name change is “to reflect” the common approach of completely ignoring time, and simply tracking the number of hops.

There may also be other examples of TTL protocols. (Perhaps in a routing protocol such as RIP?)

The phrase “Time To Live” may also be used in some other contexts, such as how long data is allowed to remain active/valid. DNS uses that concept when determining the maximum length of time that information should be cached.

[#token]: token

Something that gives permission. A classic example is when some video game arcade machines accepted coins called “tokens”. Inserting the token gave a person permission to interact with the game. However, note that often a “token” is not necessarily a physical thing: it could simply be a data arrangement inside a computer's memory that is designed to remember what permissions a person may have.

When logging into a network, sometimes a “token” may be created. A computer may be able to remember the “token”, which may be accepted for authorization. This way, a user can achieve authorization without needing to re-authenticate. (Such tokens may be routinely created when people log into some operating systems, and they may also be routinely deleted whenever the person logs off.)

For another example, see token ring.

[#toknring]: token ring

Wikipedia article on “Token ring” says “Initially used only in IBM computers, it was eventually standardized with protocol IEEE 802.5. Other vendors have been known to use a form of “Token Ring” which may have used the same sort of generalized concepts, but these implementations were not necessarily compatible with other implementations (including IBM's).

A standard for network communications. This standard competed with another standard called Ethernet. Many people considered the protocol to be a communication design that was superior to the rival technology called Ethernet, as Token Ring did not have the problems with collisions that Ethernet had. (Since no two devices tried to talk on a wire at the same time, there were no collisions.) However, when Ethernet switches replaced Ethernet hubs, Ethernet collisions became much less of a problem. The lower cost of the slower Ethernet technology was compelling, and eventually Ethernet became the mainstream standard.

Newer networks may use a “Media Access Unit” (a networking device abbreviated as “MAU”, but not the same as another networking device which was called “Medium Attachment Unit” and which was also used the “MAU” abbreviation). An MAU provided the same basic role as an Ethernet “hub” or “switch”: it allowed multiple devices to be easily connected to a network using a centralized piece. However, earlier Token Ring designs actually involved a physical “ring” topology. Wikipedia's page for “Media Access Unit” notes, “The loop that used to make up the ring of the Token ring is now integrated into the chip.” (That page further discusses the device, and the advantages of using such a device.)

The name “Token Ring” comes from two things. One is the concept that computers were hooked up in a ring topology. (This may not necessarily have been a reality: Wikipedia's article on “Network topology” states, “Token Ring is a logical ring topology, but is wired a physical star from the Media Access Unit.”. However, that is only after the invention of the Media Access Unit “MAU” emulated the earlier designs.)

The other concept that led to the term “Token Ring” is that devices used a “token”, which was simply permission to communicate. Every device would have a “token” only during certain times. The device would then lose the token, while other devices had the token. (This describes a single-token design. There (might have been) some multiple-token designs that could be used for larger networks. As long as each token moved in the same direction around the ring, this shouldn't have been a problem.

Note that the “token” was not necessarily any sort of physical object. In theory, this type of permission scheme wouldn't even have required network communication to be passed. (If each device was allocated certain specific time slots on a routine basis, the devices wouldn't necessarily require constant regular communication to synchronize the detail of which device had a token at any one specific time.) However, in practice, Token Ring did transmit a “token frame”.

It might have been true that, at least with early/simple implementations, traffic only flowed in one direction around the ring.

[#tpyo]: Tpyo
This has been known to be seen in some “commit logs”, indicating that the update was to fix a typo. The anagram works in part because trying to pronounce it results in a sound that does actually sound quite similar to the correctly-spelled word.
[#typo]: typo
A misspelling, typically caused by accidental mistyping
[#twain]: TWAIN
Info which may need further review:

This is a standard supported by many scanners (that scan images on paper).

TWAIN FAQ: “What is TWAIN an acronym for?” says the name “was up-cased to TWAIN to make it more distinctive.” As the site says, the upper case “led people to believe it was an acronym”, this suggests that TWAIN wasn't initially meant to be an acronym. However, “Technology Without An Interesting Name” is one expansion that Wikipedia says “is widely known”, and the official TWAIN website's FAQ says that backronym “continues to haunt the standard.”

Wikipedia calls it “open source”. See also Wikipedia's page on “Windows Image Aquisition”: “See also” section. e.g. SANE, ISIS, WIA.