U

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

Plain language A domain name label in Unicode form containing non-ASCII characters from an internationalized domain name.

Used in RFC 3490 RFC 3491 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.

Plain language A profile of Dynamic Delegation Discovery Service producing a URI from DNS NAPTR records.

Used in RFC 5986 RFC 7051 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.

Plain language A system letting observers identify a drone in flight through Internet servers or local broadcast.

Used in RFC 9153 RFC 9374 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.

Plain language A minimal transport protocol sending independent datagrams with no connection setup, delivery guarantee or ordering.

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 942 RFC 944 RFC 947 and 450 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.

Plain language A variant of UDP whose checksum can cover only part of a datagram.

Used in RFC 4019 RFC 4867 RFC 5097 RFC 6935 RFC 6936 RFC 8303 RFC 8304 RFC 8923 RFC 9868

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

Plain language The Universal Decompressor Virtual Machine running bytecode sent by a compressor to decompress a 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.

Plain language The iCalendar property holding a persistent, globally unique identifier for a component.

Used in RFC 2060 RFC 2359 RFC 4315 RFC 4466 RFC 4549 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 and 4 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.

Plain language An IMAP value indicating whether the message UIDs a client remembers are still valid.

Used in RFC 2683 RFC 4315 RFC 4549 RFC 5182 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.

Plain language A network that another network is layered on top of.

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.

Plain language A DNS node name starting with an underscore to avoid colliding with ordinary DNS labels.

Used in RFC 8145 RFC 8552 RFC 8553

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

Plain language A message sent to exactly one recipient, as opposed to broadcast or multicast.

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 1787 RFC 1788 RFC 1819 RFC 2073 RFC 2102 RFC 2114 RFC 2131 RFC 2165 RFC 2205 RFC 2207 RFC 2326 RFC 2333 RFC 2373 and 123 more

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

Plain language A standard assigning a code point to characters from the world's writing systems.

Used in RFC 1345 RFC 1502 RFC 1641 RFC 1642 RFC 2044 RFC 2130 RFC 2152 RFC 2218 RFC 2277 RFC 2279 RFC 2319 RFC 2352 RFC 2482 RFC 2596 RFC 2614 RFC 2640 RFC 2759 RFC 2781 RFC 2872 RFC 3454 RFC 3470 RFC 3490 RFC 3491 RFC 3492 and 103 more

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

Plain language Rewriting text into a standard form so different encodings of the same visible characters become identical.

Used in RFC 3454 RFC 3629 RFC 3722 RFC 4013 RFC 4518 RFC 5895 RFC 6885 RFC 7542 RFC 7564 RFC 8264 RFC 8265 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.

Plain language A persistent location-independent identifier for a resource like an ISBN for a book.

Used in RFC 2141 RFC 3541 RFC 4688 RFC 4854 RFC 8141

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

Plain language An IPv6 address from the fc00::/7 range used inside a site and not globally reachable.

Used in RFC 4193 RFC 4852 RFC 4864 RFC 4941 RFC 5220 RFC 5375 RFC 5558 RFC 6177 RFC 6296 RFC 6724 RFC 7084 RFC 7381 RFC 8190 RFC 8368 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.

Plain language A server-chosen string identifying a message in a maildrop and staying the same across sessions.

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.

Plain language The number of seconds elapsed since midnight UTC on 1 January 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.

Plain language A privacy property where an observer cannot determine if two events come from the same user or different users.

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.

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

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.

Plain language A point-to-point interface with no IP address of its own, borrowing from another interface.

Used in RFC 1634 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.

Plain language A point-to-point link with no IP address of its own, identified by locally assigned numbers.

Used in RFC 3477 RFC 3480 RFC 4202

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.

Plain language A bit in an IS-IS prefix advertisement set when the prefix was leaked downwards and must not climb back up.

Used in RFC 3784 RFC 5302 RFC 5305 RFC 7775

The neighbor or PE toward the source of a multicast flow, chosen by a receiving PE as the place to join the flow. Also called UMH.

Used in RFC 7431 RFC 7988 RFC 9026

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.

Plain language A label chosen by the upstream router rather than the downstream one.

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.

Plain language A string identifying a resource either by location (a URL) or by name (a URN).

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 2169 RFC 2218 RFC 2221 RFC 2244 RFC 2255 RFC 2276 and 384 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.

Plain language Rewriting a URI into a standard form so equivalent URIs compare as equal.

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.

Plain language Either a full URI or a relative reference turned into a full URI by resolving against a base.

Used in RFC 469 RFC 1630 RFC 1808 RFC 2396 RFC 3986 RFC 4088 RFC 4151 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.

Plain language The part of a URI before the first colon indicating which protocol or naming system the address uses.

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 110 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.

Plain language A pattern for building a URL from variables, used to configure clients.

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 9230 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.

Plain language A Uniform Resource Locator expressing a network address and access algorithm for a resource.

Used in RFC 1505 RFC 1630 RFC 1727 RFC 1736 RFC 1807 RFC 1808 RFC 1941 RFC 2168 RFC 2169 RFC 2187 RFC 2255 RFC 2326 RFC 2369 RFC 2392 RFC 2396 RFC 2483 RFC 2609 RFC 2614 RFC 2664 RFC 2849 RFC 3253 RFC 4483 RFC 4516 RFC 6983 and 3 more

The first component of a Uniform Resource Locator, appearing before the colon. Examples include http, https, ftp, and mailto. A scheme identifies the protocol or method used to access the resource.

Used in RFC 1808 RFC 2255 RFC 2369 RFC 2717 RFC 4227

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.

Plain language A Uniform Resource Name, a location-independent name for a resource.

Used in RFC 1630 RFC 1727 RFC 1736 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 3121 RFC 3151 and 103 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.

Plain language A family of Uniform Resource Names sharing a Namespace ID, with one owner assigning names.

Used in RFC 2141 RFC 3043 RFC 3121 RFC 3188 RFC 3406 RFC 3613 RFC 3937 RFC 4122 RFC 4729 RFC 5119 RFC 5174 RFC 5328 RFC 6338 RFC 6893 RFC 8141 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.

Plain language The 7-bit character set assigning numbers 0 to 127 to letters, digits, punctuation and control characters.

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 94 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.

Plain language A worldwide system of discussion groups whose articles are copied between news servers.

Used in RFC 1505 RFC 1849 RFC 1855 RFC 1941 RFC 2076 RFC 2980 RFC 4643 RFC 4707 RFC 5536

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.

Plain language The client software making HTTP requests on a person's behalf, usually a web browser.

Used in RFC 841 RFC 989 RFC 1081 RFC 1138 RFC 1431 RFC 1524 RFC 1820 RFC 1844 RFC 1945 RFC 2049 RFC 2291 RFC 2295 RFC 2610 RFC 2614 RFC 2703 RFC 3049 RFC 3059 RFC 3111 RFC 3261 RFC 3351 RFC 3458 RFC 3528 RFC 3832 RFC 5369 and 22 more

A 3-bit priority value, 0 to 7, carried in the QoS Control field of an IEEE 802.11 frame. It selects the access category used to send the frame.

Used in RFC 2816 RFC 8325

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

Plain language A user-defined variable in the Telnet Environment option carrying custom settings.

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.

Plain language Coordinated Universal Time, the world's reference time scale.

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 8877 RFC 8984 RFC 9553 RFC 9557 RFC 9581 RFC 9636 and 1 more

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

Plain language A variable-width encoding of Unicode into bytes.

Used in RFC 1345 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 and 219 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.

Plain language A 128-bit universally unique identifier any system can generate without central coordination.

Used in RFC 2371 RFC 3928 RFC 4122 RFC 4287 RFC 4479 RFC 4530 RFC 4533 RFC 4578 RFC 5049 RFC 5626 RFC 5716 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 and 17 more