U

A domain name label in Unicode (native character) form, containing one or more non-ASCII characters from an internationalized domain name.

Like I'm five A domain name in a non-English language, before it is converted to ASCII.

Used in RFC 5890 RFC 5891 RFC 5894 RFC 6055 RFC 6531 RFC 6912 RFC 8222 RFC 8399 RFC 8616 RFC 9549

A profile of the Dynamic Delegation Discovery Service that takes a domain name and a pair of service and protocol tags and uses DNS NAPTR records to produce a URI.

Like I'm five A way of looking up a name in the internet phone book and getting back a web address.

Used in RFC 5986 RFC 7286

Unmanned Aircraft System Remote Identification and tracking: a system that lets arbitrary Observers identify a drone while it is in flight. It can work through Internet servers or by local broadcast.

Like I'm five A name tag for a drone that anyone nearby can read.

Used in RFC 9153 RFC 9434

The User Datagram Protocol (RFC 768). A minimal transport that sends independent datagrams between ports with no connection setup, no delivery guarantee and no ordering.

Like I'm five Sending postcards: each one goes by itself and nobody promises it arrives.

Used in RFC 768 RFC 776 RFC 783 RFC 800 RFC 814 RFC 830 RFC 840 RFC 862 RFC 863 RFC 864 RFC 865 RFC 866 RFC 867 RFC 868 RFC 872 RFC 880 RFC 887 RFC 901 RFC 906 RFC 924 RFC 929 RFC 944 RFC 947 RFC 951 and 422 more

A variant of UDP whose checksum can cover only part of a datagram, such as the headers. It uses its own protocol number, so many middleboxes do not recognize it.

Like I'm five A version of UDP that only double-checks the address part of the letter, not all of it.

Used in RFC 4019 RFC 5097 RFC 6936 RFC 8304

The Universal Decompressor Virtual Machine is the small virtual machine defined by SigComp. It runs bytecode sent by the compressor to decompress a message.

Like I'm five A tiny computer that follows the sender's instructions to unpack a squeezed message.

Used in RFC 3320 RFC 4464 RFC 4896

The iCalendar property holding a persistent, globally unique identifier for a component, used to recognise updates to the same event.

Like I'm five A serial number so the same plan is not mistaken for a new one.

Used in RFC 2060 RFC 2359 RFC 4315 RFC 4466 RFC 4731 RFC 5182 RFC 5256 RFC 5423 RFC 5465 RFC 5545 RFC 5546 RFC 6237 RFC 6851 RFC 7162 RFC 7377 RFC 8000 RFC 8474 RFC 8508 RFC 8984 RFC 9073 RFC 9074 RFC 9253 RFC 9394 RFC 9555 and 3 more

An IMAP value that, together with a mailbox name, tells a client whether the message UIDs it remembers are still valid. A mailbox name plus UIDVALIDITY identifies a mailbox.

Like I'm five A number that tells you whether your old list of letter numbers can still be trusted.

Used in RFC 2683 RFC 4549 RFC 7162 RFC 8474 RFC 9051

A network that another network is layered on top of. The upper network, called an overlay, maps its nodes and links onto nodes and links of the underlay.

Like I'm five The roads that a bus route is built on.

Used in RFC 7365 RFC 8345

A DNS node name that starts with an underscore, such as _tcp. The underscore keeps it from colliding with ordinary DNS labels and marks a place where a particular kind of record is scoped.

Like I'm five A special folder name that starts with an underscore so it never gets mixed up with normal names.

Used in RFC 8552 RFC 8553

A message sent to exactly one recipient, as opposed to a broadcast to everyone or a multicast to a group.

Like I'm five Talking to one person instead of the whole room.

Used in RFC 826 RFC 903 RFC 1001 RFC 1045 RFC 1112 RFC 1201 RFC 1541 RFC 1542 RFC 1546 RFC 1633 RFC 1671 RFC 1788 RFC 1819 RFC 2073 RFC 2102 RFC 2114 RFC 2131 RFC 2165 RFC 2205 RFC 2207 RFC 2326 RFC 2333 RFC 2373 RFC 2379 and 118 more

A standard that assigns a code point to characters from the world's writing systems. Its code points range from 0 to 10FFFF.

Like I'm five One big list that gives every letter in every language its own number.

Used in RFC 1345 RFC 1502 RFC 1641 RFC 1642 RFC 2044 RFC 2152 RFC 2218 RFC 2277 RFC 2279 RFC 2319 RFC 2482 RFC 2596 RFC 2614 RFC 2640 RFC 2759 RFC 2781 RFC 2872 RFC 3454 RFC 3470 RFC 3490 RFC 3491 RFC 3492 RFC 3536 RFC 3629 and 97 more

Rewriting text into a standard form so that different encodings of the same visible characters become identical. Form KC is one such form.

Like I'm five Writing the same word the same way every time.

Used in RFC 3722 RFC 4013 RFC 4518 RFC 5895 RFC 7542 RFC 7564 RFC 8264 RFC 8266

A persistent identifier for a resource that is location-independent, unlike a URL. Examples include ISBN numbers for books and DOI numbers for academic papers.

Like I'm five A name that says what something is, not where to find it. Like an ISBN for a book: the same ISBN works whether the book is in a library, a store, or online.

Used in RFC 3541 RFC 4688 RFC 4854

An IPv6 address from the fc00::/7 range, used inside a site and not expected to be globally reachable. It is abbreviated ULA.

Like I'm five A home address that works inside your neighborhood but not across the whole world.

Used in RFC 4193 RFC 4864 RFC 5220 RFC 5375 RFC 6296 RFC 6724 RFC 7084 RFC 7381 RFC 8190 RFC 8994 RFC 9099

A server-chosen string of 1 to 70 printable characters that identifies a message in a maildrop and stays the same across sessions. The UIDL command lists them.

Like I'm five A permanent sticker on each letter so you can tell which ones you have already seen.

Used in RFC 1939 RFC 2734 RFC 3501

The number of seconds elapsed since midnight UTC on 1 January 1970 (the Unix epoch). TOTP uses it as its clock so that both sides can agree on the time without negotiation.

Like I'm five A running count of every second that has passed since New Year's Day 1970.

Used in RFC 868 RFC 2626 RFC 2783 RFC 4049 RFC 6019 RFC 6238 RFC 6396 RFC 6872 RFC 6873 RFC 7071 RFC 7164 RFC 7183 RFC 8428 RFC 8536 RFC 8943 RFC 9581 RFC 9636 RFC 9651 RFC 9880 RFC 10049

A privacy property where an observer cannot determine whether two events (like multiple requests) come from the same user or different users.

Like I'm five When something is unlinkable, you cannot tell if two events came from the same person or different people.

Used in RFC 8915 RFC 9576

A network such as a home or small office with no dedicated IT staff or professional network administrator to manage it.

Like I'm five A home network, not a data center.

Used in RFC 3750 RFC 3904

A point-to-point interface that has no IP address of its own and borrows the address of another interface on the same router.

Like I'm five A line that has no street address of its own and uses a neighbor's.

Used in RFC 4205 RFC 5309

A point-to-point link that has no IP address of its own. Each end identifies it with a locally assigned number instead.

Like I'm five A road with no street address, where each end just gives it a number.

Used in RFC 3477 RFC 3480

A bit in an IS-IS prefix advertisement that is set to 1 when the prefix has been leaked downwards in the level hierarchy. Such a prefix must not be leaked back upwards.

Like I'm five A flag on a route that says it already came down the ladder and must not climb back up.

Used in RFC 5302 RFC 5305 RFC 7775

A label chosen by the upstream router rather than the downstream one. It lets one label stand for an LSP that is carried inside a shared multicast tunnel.

Like I'm five A ticket number handed out by the sender instead of by each receiver.

Used in RFC 5332 RFC 7060 RFC 8104 RFC 8400 RFC 9573

Uniform Resource Identifier. A string that identifies a resource, either by location (a URL) or by name (a URN). In HTTP the Request-URI says which resource a method applies to.

Like I'm five The address you type to say which page you want.

Used in RFC 1630 RFC 1689 RFC 1727 RFC 1736 RFC 1737 RFC 1738 RFC 1808 RFC 1942 RFC 1945 RFC 1959 RFC 1980 RFC 2016 RFC 2017 RFC 2070 RFC 2079 RFC 2110 RFC 2141 RFC 2168 RFC 2218 RFC 2221 RFC 2244 RFC 2255 RFC 2276 RFC 2288 and 361 more

Rewriting a URI into a standard form, for example by lowercasing the scheme and host, uppercasing percent-encoding hex digits and removing dot-segments, so that equivalent URIs compare as equal.

Like I'm five Tidying two differently written addresses into the same neat form so you can tell they match.

Used in RFC 2141 RFC 2396 RFC 3966 RFC 3986 RFC 3987 RFC 4452 RFC 5017 RFC 5870 RFC 5954 RFC 6943 RFC 7472 RFC 7512 RFC 7565 RFC 8224

Either a full URI or a relative reference, which is a URI with leading parts left off. A relative reference is turned into a full URI by resolving it against a base URI.

Like I'm five Either a whole address, or a short hint like "the next room" that only makes sense from where you are standing.

Used in RFC 469 RFC 1630 RFC 1808 RFC 2396 RFC 3986 RFC 4088 RFC 5092 RFC 5147 RFC 5285 RFC 5988 RFC 6570 RFC 6690 RFC 7111 RFC 7595 RFC 7807 RFC 8288 RFC 8820 RFC 9264 RFC 9457 RFC 9652

The part of a URI before the first colon, such as http or mailto, that says which protocol or naming system the rest of the address uses. HTCPCP defines a coffee scheme.

Like I'm five The first word of an address, like http, which says how to talk to it.

Used in RFC 1630 RFC 1736 RFC 1738 RFC 1808 RFC 2056 RFC 2110 RFC 2111 RFC 2122 RFC 2141 RFC 2192 RFC 2193 RFC 2324 RFC 2369 RFC 2384 RFC 2392 RFC 2396 RFC 2397 RFC 2609 RFC 2718 RFC 2806 RFC 2818 RFC 2838 RFC 2910 RFC 2911 and 107 more

A pattern from RFC 6570 for building a URL from variables, such as https://dnsserver.example.net/dns-query{?dns}. A DoH client is configured with one.

Like I'm five A fill-in-the-blank web address.

Used in RFC 6415 RFC 6570 RFC 6690 RFC 7072 RFC 7320 RFC 7512 RFC 7808 RFC 8075 RFC 8484 RFC 8620 RFC 8820 RFC 9176 RFC 9205 RFC 9298 RFC 9461 RFC 9484 RFC 9569 RFC 9652

A Uniform Resource Locator is a form of URI that expresses an address which maps onto an access algorithm using network protocols.

Like I'm five A web address that tells a computer how and where to fetch something.

Used in RFC 1630 RFC 2664

A Uniform Resource Name, a location-independent name for a resource such as urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6. It names a thing rather than saying where to fetch it.

Like I'm five A name for something that stays the same even if it moves house.

Used in RFC 1630 RFC 1727 RFC 1737 RFC 1807 RFC 2016 RFC 2141 RFC 2168 RFC 2169 RFC 2276 RFC 2288 RFC 2371 RFC 2396 RFC 2483 RFC 2611 RFC 2648 RFC 3001 RFC 3043 RFC 3044 RFC 3061 RFC 3085 RFC 3120 RFC 3151 RFC 3187 RFC 3188 and 97 more

A family of Uniform Resource Names that share a Namespace ID, such as dvb in urn:dvb:... The owner decides how names inside it are assigned and resolved.

Like I'm five A set of names that all start the same way and are handed out by one owner.

Used in RFC 3043 RFC 3121 RFC 3188 RFC 3613 RFC 3937 RFC 4729 RFC 5119 RFC 5174 RFC 5328 RFC 6338 RFC 6893 RFC 8464 RFC 9676

The 7-bit character set that assigns numbers 0 to 127 to letters, digits, punctuation and control characters. ABNF quoted strings and its core rules are defined in terms of it.

Like I'm five A table that gives every letter and symbol a number so computers can write them down.

Used in RFC 7 RFC 8 RFC 9 RFC 20 RFC 44 RFC 50 RFC 56 RFC 66 RFC 86 RFC 97 RFC 98 RFC 109 RFC 110 RFC 122 RFC 125 RFC 135 RFC 137 RFC 138 RFC 139 RFC 177 RFC 183 RFC 385 RFC 698 RFC 822 and 89 more

A worldwide system of discussion groups, called newsgroups, whose articles are copied between news servers. It is also known as Netnews or network news.

Like I'm five A giant set of public message boards whose posts are copied from computer to computer.

Used in RFC 1849 RFC 4707

The client software that makes HTTP requests on a person's behalf, most often a web browser. In RFC 6265 it is the party that stores and returns cookies.

Like I'm five The program you use to visit websites, like your browser.

Used in RFC 841 RFC 989 RFC 1081 RFC 1138 RFC 1431 RFC 1524 RFC 1820 RFC 1844 RFC 1945 RFC 2049 RFC 2295 RFC 2610 RFC 3049 RFC 3059 RFC 3111 RFC 3261 RFC 3458 RFC 3528 RFC 3832 RFC 5390 RFC 6265 RFC 6454 RFC 6749 RFC 6797 and 13 more

A user-defined variable in the Telnet Environment option, used to carry custom settings that the standard variables do not cover.

Like I'm five A custom note a client can pass to the server, beyond the usual ones.

Used in RFC 2877 RFC 4777

Coordinated Universal Time, the world's reference time scale, maintained by the BIPM. Other time zones are expressed as an offset from it, and a trailing Z in a timestamp means UTC.

Like I'm five The world's main clock that every other clock is measured against.

Used in RFC 1857 RFC 2445 RFC 2626 RFC 2679 RFC 2822 RFC 3060 RFC 3339 RFC 3628 RFC 3659 RFC 4049 RFC 6302 RFC 6557 RFC 7164 RFC 7273 RFC 7680 RFC 7808 RFC 8412 RFC 8536 RFC 8984 RFC 9553 RFC 9557 RFC 9581 RFC 9636 RFC 9922

A variable-width encoding of Unicode into bytes. JSON text exchanged between systems must use it.

Like I'm five A way of turning letters from any language into bytes.

Used in RFC 1641 RFC 1922 RFC 2130 RFC 2141 RFC 2152 RFC 2192 RFC 2218 RFC 2241 RFC 2244 RFC 2251 RFC 2252 RFC 2253 RFC 2254 RFC 2277 RFC 2279 RFC 2327 RFC 2482 RFC 2484 RFC 2566 RFC 2608 RFC 2609 RFC 2610 RFC 2614 RFC 2640 and 208 more

A Universally Unique IDentifier: a 128-bit value, usually written as 32 hex digits in the pattern 8-4-4-4-12, that any system can generate without central coordination. Also called a GUID.

Like I'm five A super-long name tag a computer can invent by itself that nobody else will ever invent.

Used in RFC 2371 RFC 3928 RFC 4122 RFC 4287 RFC 4479 RFC 4530 RFC 4533 RFC 4578 RFC 5049 RFC 5626 RFC 6080 RFC 6350 RFC 6355 RFC 6688 RFC 6925 RFC 6933 RFC 6991 RFC 7022 RFC 7047 RFC 7461 RFC 7532 RFC 7533 RFC 7612 RFC 7865 and 15 more