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An ITU-T standard for packet-based multimedia communications, enabling voice and video calls over IP networks with signaling and control protocols.

Plain language ITU-T standard for packet-based multimedia communications enabling voice and video calls over IP.

Used in RFC 3508 RFC 4123 RFC 7205

When a NAT correctly handles traffic from one internal host to another internal host that is addressed by its external address and port.

Plain language NAT correctly handling traffic from one internal host to another using external address.

Used in RFC 4787 RFC 5128 RFC 5382 RFC 5508 RFC 5597 RFC 6081 RFC 6146 RFC 7596 RFC 7757 RFC 7857

A naming service for digital objects. A handle is a permanent identifier made of a naming authority and a string of characters, and a handle server turns that identifier into the location where the object is stored.

Plain language A permanent name for a digital object. A handle server takes that name and gives back the address where the object is kept.

Used in RFC 1807 RFC 3650

The process of a mobile node transferring its connection from one foreign agent or access point to another while maintaining network connectivity.

Plain language Mobile node switching from one foreign agent or access point to another.

Used in RFC 4857 RFC 4866 RFC 4881 RFC 4907

The process of a mobile device switching from one access network to another while maintaining ongoing communications.

Plain language Mobile device switching access networks while maintaining ongoing communications.

Used in RFC 3374 RFC 3583 RFC 3753 RFC 4066 RFC 4260 RFC 4831 RFC 5169 RFC 5181 RFC 5184 RFC 5268 RFC 5568 RFC 5677 RFC 5679 RFC 5757 RFC 5836 RFC 5873 RFC 5980 RFC 6058 RFC 6252 RFC 6301 RFC 6696 RFC 6697 RFC 8869

The opening exchange of a secure connection in which the parties agree on parameters, authenticate and derive shared keys.

Plain language The opening exchange where connection parties agree on parameters and derive shared keys.

Used in RFC 60 RFC 171 RFC 264 RFC 745 RFC 962 RFC 1644 RFC 1987 RFC 2188 RFC 2246 RFC 2408 RFC 2487 RFC 2747 RFC 3080 RFC 3292 RFC 3373 RFC 3749 RFC 4347 RFC 4366 RFC 4680 RFC 5169 RFC 5238 RFC 5246 RFC 5303 RFC 6012 and 32 more

An algorithm that helps dual-stack clients avoid connection delay by trying both IPv4 and IPv6 and using whichever works first. It is described in RFC 6555.

Plain language Algorithm helping dual-stack clients avoid delay by trying IPv4 and IPv6 simultaneously.

Used in RFC 7556 RFC 7984 RFC 8305 RFC 8777 RFC 9132

A tamper-resistant device that stores private keys and performs signing inside itself. The key never has to be exposed to ordinary computers.

Plain language Tamper-resistant device storing private keys and performing signatures without exposing keys.

Used in RFC 5485 RFC 6781 RFC 8937

Recording the time a packet is sent or received in the network interface itself, so software, processing and queueing delays do not affect the measurement.

Plain language Recording packet timing in the network interface itself, avoiding software delays.

Used in RFC 7821 RFC 10030

An attack where someone records encrypted traffic today in the hope of decrypting it once a powerful enough quantum computer exists. It makes long-lived secrets urgent to protect.

Plain language Attack recording encrypted traffic today hoping to decrypt it with future quantum computers.

Used in RFC 8784 RFC 9954 RFC 9958

A one-way function that turns input of any length into a fixed-size value. SHA-256, for example, produces a 256-bit result.

Plain language One-way function turning any input into a fixed-size value (like SHA-256 producing 256 bits).

Used in RFC 1319 RFC 2945 RFC 2992 RFC 3074 RFC 3112 RFC 3174 RFC 3447 RFC 3874 RFC 4231 RFC 4270 RFC 4754 RFC 5475 RFC 5476 RFC 5702 RFC 5758 RFC 5926 RFC 6150 RFC 6234 RFC 6437 RFC 6507 RFC 6508 RFC 6920 RFC 6986 RFC 7182 and 10 more

A structure in which data items are hashed individually and those hashes are hashed again into a single root hash. Comparing roots quickly shows whether two sets of data are identical.

Plain language Structure where individual data hashes are hashed into a single root for quick comparison.

Used in RFC 4998 RFC 6283 RFC 7787

A digital signature scheme built only from a cryptographic hash function. Its security rests on the hash function rather than on a hard mathematical problem.

Plain language Digital signature using only hash functions without mathematical hard problems.

Used in RFC 8240 RFC 8391 RFC 9708 RFC 9814

High-level Data Link Control, a family of bit-oriented link framing methods. PPP's default framing is HDLC-like and is defined in a companion document to RFC 1661.

Plain language High-level Data Link Control, a family of link framing methods used in protocols like PPP.

Used in RFC 891 RFC 935 RFC 1134 RFC 1172 RFC 1232 RFC 1331 RFC 1381 RFC 1547 RFC 1548 RFC 1549 RFC 1570 RFC 1598 RFC 1618 RFC 1619 RFC 1661 RFC 1662 RFC 1663 RFC 1973 RFC 2171 RFC 2172 RFC 2173 RFC 2175 RFC 2176 RFC 2341 and 16 more

A delay in which one stalled item holds up everything queued behind it. HTTP/2 removes it between requests, but a lost TCP segment can still stall all streams on the connection.

Plain language Delay where one stalled item blocks others queued behind it.

Used in RFC 2616 RFC 2889 RFC 2960 RFC 3257 RFC 3436 RFC 3539 RFC 4168 RFC 4838 RFC 4960 RFC 6083 RFC 6084 RFC 6182 RFC 6613 RFC 6614 RFC 7540 RFC 8087 RFC 8239 RFC 8260 RFC 8290 RFC 8323 RFC 8895 RFC 8923 RFC 9000 RFC 9112 and 8 more

A technique that shrinks the protocol headers of packets on a link, often by sending only what changed since the last packet.

Plain language Technique shrinking protocol headers by sending only changes since the last packet.

Used in RFC 914 RFC 1144 RFC 1332 RFC 1553 RFC 2507 RFC 2508 RFC 2686 RFC 2689 RFC 2757 RFC 3006 RFC 3095 RFC 3096 RFC 3241 RFC 3243 RFC 3374 RFC 3409 RFC 3449 RFC 3544 RFC 3843 RFC 4019 RFC 4163 RFC 4170 RFC 4224 RFC 4362 and 12 more

A line at the top of a message written as Field-Name: Field-Value, such as Subject: Hello. Long values can be folded onto continuation lines.

Plain language A line in a message formatted as Field-Name: Field-Value, such as Subject: Hello.

Used in RFC 64 RFC 561 RFC 680 RFC 696 RFC 733 RFC 808 RFC 822 RFC 934 RFC 987 RFC 1049 RFC 1138 RFC 1154 RFC 1327 RFC 1505 RFC 1522 RFC 1622 RFC 1753 RFC 1766 RFC 1806 RFC 1867 RFC 1894 RFC 1945 RFC 2017 RFC 2047 and 224 more

The cryptographic protection of an email's header section, or parts of it, by signatures and encryption. It stops attackers from altering fields such as the Subject without detection.

Plain language Cryptographic protection of email headers by signatures and encryption.

Used in RFC 9001 RFC 9788

A small message sent at regular intervals to show that a peer is still alive and reachable. If several heartbeats are missed, the other side can treat the session as dead.

Plain language Small message sent at intervals showing a peer is still alive and reachable.

Used in RFC 1301 RFC 3528 RFC 4171 RFC 4174 RFC 4765 RFC 5062 RFC 5811 RFC 7121 RFC 8175 RFC 8811

HTTP-Enabled Location Delivery, a protocol in which a device or other requestor asks a Location Information Server for location information over HTTP.

Plain language HTTP-Enabled Location Delivery, a protocol requesting location information over HTTP.

Used in RFC 5986 RFC 6155 RFC 6915 RFC 7378

A TLS 1.3 message in which the server asks the client to send a second ClientHello, for example with a different key share. ECH has special rules for the second hello and for confirming acceptance in the HRR.

Plain language TLS 1.3 message asking the client to send a second ClientHello with different parameters.

Used in RFC 9001 RFC 9147 RFC 9149 RFC 9846 RFC 9849

A 128-bit Host Identity Tag with extra hierarchy information built in. It is a valid but non-routable IPv6 address derived from a public key and a registry hierarchy.

Plain language Hierarchical Host Identity Tag, a 128-bit HIT with hierarchy information derived from public keys.

Used in RFC 9374 RFC 9434

The status given to protocols that are no longer recommended for use, typically because they have been superseded by better alternatives.

Plain language Status for protocols no longer recommended, typically superseded by better alternatives.

Used in RFC 3000 RFC 3300 RFC 4450 RFC 7841

The HMAC-based Extract-and-Expand Key Derivation Function. TLS 1.3 uses it to turn shared secrets into many separate keys.

Plain language HMAC-based Extract-and-Expand Key Derivation Function turning shared secrets into separate keys.

Used in RFC 5869 RFC 6234 RFC 6476 RFC 7401 RFC 7978 RFC 8110 RFC 8188 RFC 8291 RFC 8418 RFC 8447 RFC 8548 RFC 8613 RFC 8619 RFC 8645 RFC 8696 RFC 8915 RFC 8937 RFC 9001 RFC 9053 RFC 9150 RFC 9180 RFC 9190 RFC 9203 RFC 9258 and 16 more

Keyed-hash Message Authentication Code (RFC 2104). It mixes a secret key with a message through a hash function such as SHA-1, SHA-256 or SHA-512, so only someone with the key can produce or check the result.

Plain language Keyed-hash Message Authentication Code mixing a secret key with a message through hashing.

Used in RFC 1828 RFC 2085 RFC 2104 RFC 2195 RFC 2202 RFC 2246 RFC 2274 RFC 2286 RFC 2316 RFC 2332 RFC 2334 RFC 2402 RFC 2403 RFC 2404 RFC 2406 RFC 2409 RFC 2574 RFC 2660 RFC 2747 RFC 2748 RFC 2841 RFC 2845 RFC 2847 RFC 2857 and 265 more

A BGP session timer. If no KEEPALIVE or UPDATE arrives before it expires, the sender is presumed gone, a NOTIFICATION is sent and the session is closed. The value used is the smaller of the two Hold Times in the OPEN messages; the suggested default is 90 seconds.

Plain language BGP timer closing the session if no message arrives before expiry (default 90 seconds).

Used in RFC 891 RFC 1267 RFC 1582 RFC 1771 RFC 1772 RFC 2091 RFC 2281 RFC 2909 RFC 3036 RFC 3219 RFC 3478 RFC 4271 RFC 4273 RFC 4724 RFC 5306 RFC 6439 RFC 7349 RFC 7412 RFC 8327 RFC 8538 RFC 8916 RFC 9384 RFC 9687

Creating soft state in a NAT or firewall by sending traffic outward, so that a well-behaved device later lets matching traffic back in to the host behind it.

Plain language Creating soft state in NAT/firewall by sending outbound traffic so replies are permitted back.

Used in RFC 5128 RFC 5596 RFC 5766 RFC 7604 RFC 8656

The permanent IP address assigned to a mobile node on its home network, which remains unchanged regardless of where the node moves.

Plain language The permanent IP address assigned to a mobile node on its home network.

Used in RFC 1688 RFC 2794 RFC 3220 RFC 3775 RFC 4283 RFC 5026 RFC 5149 RFC 5177 RFC 5944 RFC 6089 RFC 6275 RFC 6612

The Mobile IPv4 agent on a mobile node's home network. It answers registration requests and authenticates its Registration Reply.

Plain language Mobile IPv4 router on the home network answering registration requests.

Used in RFC 2002 RFC 2006 RFC 2290 RFC 2356 RFC 2794 RFC 3024 RFC 3115 RFC 3344 RFC 3519 RFC 3543 RFC 3753 RFC 3775 RFC 3776 RFC 3846 RFC 3957 RFC 3963 RFC 4004 RFC 4093 RFC 4225 RFC 4260 RFC 4283 RFC 4295 RFC 4332 RFC 4433 and 33 more

A residential home network. In RFC 8375 the term means any home network, regardless of which features it implements.

Used in RFC 7368 RFC 8375

The number of routers or switches a packet must pass through to reach its destination.

Plain language The number of routers a packet passes through to reach its destination.

Used in RFC 975 RFC 1058 RFC 1118 RFC 3036 RFC 3860 RFC 5497 RFC 5883 RFC 6439 RFC 6551 RFC 6847 RFC 7455

An IPv6 header field that a router lowers by one each time it forwards a packet. When it reaches zero the packet is discarded and an ICMPv6 Time Exceeded message is sent back.

Plain language IPv6 header field decremented at each hop, discarding packets when it reaches zero.

Used in RFC 1574 RFC 1622 RFC 1716 RFC 1752 RFC 1883 RFC 2460 RFC 2461 RFC 2463 RFC 2466 RFC 2473 RFC 2553 RFC 2598 RFC 2710 RFC 2893 RFC 3443 RFC 3493 RFC 3542 RFC 3626 RFC 3682 RFC 3810 RFC 4213 RFC 4286 RFC 4293 RFC 4443 and 45 more

A feature agreed between each directly connected pair of hosts, not carried end to end.

Plain language A feature agreed between each directly connected pair, not carried end to end.

Used in RFC 1164 RFC 2205 RFC 2227 RFC 2333 RFC 2460 RFC 2634 RFC 2675 RFC 2711 RFC 2745 RFC 2747 RFC 2817 RFC 3542 RFC 3810 RFC 4072 RFC 4727 RFC 5001 RFC 5425 RFC 5570 RFC 5749 RFC 6028 RFC 6398 RFC 6553 RFC 6564 RFC 6621 and 31 more

A computer or device connected to the Internet, identified by a name and IP address.

Plain language A computer or device connected to the Internet identified by name and IP address.

Used in RFC 547 RFC 548 RFC 952 RFC 1296 RFC 1413

The set of hosts, possibly zero, that share one IP multicast address. Membership is dynamic and can span many networks.

Plain language The set of hosts sharing one IP multicast address with dynamic membership spanning networks.

Used in RFC 229 RFC 469 RFC 471 RFC 1054 RFC 1112 RFC 1768 RFC 2022 RFC 2236 RFC 2710 RFC 3376 RFC 4604 RFC 4607 RFC 5135 RFC 5374 RFC 9776

A cryptographic public key that uniquely identifies a computing platform. Unlike IP addresses, a Host Identity remains constant even when the host moves to a different network location.

Plain language Cryptographic public key uniquely identifying a computing platform independent of location.

Used in RFC 4423 RFC 5201 RFC 5204 RFC 7401 RFC 8003 RFC 8005 RFC 8046 RFC 9063

A protocol in which each host's identity is derived from a public key, giving hosts a cryptographic namespace of their own. It is also known as HIP.

Plain language Protocol where host identity derives from public keys, providing cryptographic host identification.

Used in RFC 4830 RFC 5201 RFC 5205 RFC 5207 RFC 6261 RFC 6537 RFC 6967 RFC 7401 RFC 7402 RFC 8002 RFC 8003 RFC 8004 RFC 8005 RFC 8046 RFC 8047 RFC 9063

A short identifier for a HIP host, derived by hashing its public key. Applications use it in place of an IP address.

Plain language Short identifier for a HIP host derived from hashing its public key.

Used in RFC 5201 RFC 5205 RFC 5338 RFC 5770 RFC 6317 RFC 6537 RFC 6538 RFC 7401 RFC 7402 RFC 8002 RFC 8004 RFC 8005 RFC 8046 RFC 9063 RFC 9374

The public key that identifies an SSH server. During key exchange the client uses it to check that it is talking to the right server, either by comparing against a stored key or by trusting a certification authority.

Plain language Public key identifying an SSH server, checked by clients against known keys or certification.

Used in RFC 634 RFC 637 RFC 4251 RFC 4252 RFC 4253 RFC 4255 RFC 4419 RFC 4432 RFC 4462 RFC 5592 RFC 5656 RFC 6187 RFC 6317 RFC 8332 RFC 8709 RFC 8731 RFC 9644

The behaviors that an Internet host must, should or may implement. RFC 1122 defines them for the link, IP and transport layers.

Plain language Required behaviors for Internet hosts defined in RFC 1122 for link, IP and transport layers.

Used in RFC 1122 RFC 1281 RFC 4294

A file mapping host names to their network addresses and other properties. Used before DNS for name resolution.

Plain language File mapping host names to addresses, used before DNS for name resolution.

Used in RFC 752 RFC 810 RFC 881 RFC 882 RFC 952 RFC 1031 RFC 3467

HMAC-Based One-Time Password, the event-counter algorithm from RFC 4226. It computes a code from a shared key and a counter. TOTP is HOTP with a time-derived value in place of the counter.

Plain language HMAC-Based One-Time Password algorithm from RFC 4226 using shared key and counter.

Used in RFC 4226 RFC 6030 RFC 6031 RFC 6063 RFC 6238 RFC 6287 RFC 6560 RFC 7894 RFC 8176

The header compression format for HTTP/2, defined in RFC 7541. Both ends keep a table of recently used header fields, so repeated fields can be sent as short references.

Plain language Header compression format for HTTP/2 using a shared table for repeated header fields.

Used in RFC 7540 RFC 7541 RFC 9113 RFC 9114 RFC 9204

A standard way, defined in RFC 9180, to encrypt a message to someone's public key by combining a key encapsulation mechanism, a key derivation function and an AEAD cipher. ECH uses it to seal the inner ClientHello.

Plain language Hybrid Public Key Encryption, a standard for encrypting to public keys and verifying origins.

Used in RFC 9180 RFC 9230 RFC 9420 RFC 9458 RFC 9849 RFC 9958

HTTP Strict Transport Security. A policy a website declares with the Strict-Transport-Security header, telling browsers to use only HTTPS with that site for a stated time.

Plain language HTTP Strict Transport Security policy declaring HTTPS-only for a site over a stated duration.

Used in RFC 2818 RFC 6797 RFC 7457 RFC 7469 RFC 7525 RFC 7711 RFC 9325 RFC 9460

A list of HSTS hosts shipped inside the browser itself, so the browser enforces HTTPS even on the very first visit, before any header has been seen.

Plain language An HSTS list shipped in browsers enforcing HTTPS even on first visit.

Used in RFC 6797 RFC 7469

The Hypertext Transfer Protocol, the request-and-response protocol the web runs on. A client sends a method, a target and headers, and a server answers with a status code and an optional body.

Plain language HyperText Transfer Protocol, the request-and-response protocol the web runs on.

Used in RFC 1689 RFC 1727 RFC 1736 RFC 1738 RFC 1855 RFC 1867 RFC 1945 RFC 2039 RFC 2068 RFC 2069 RFC 2084 RFC 2109 RFC 2122 RFC 2145 RFC 2150 RFC 2151 RFC 2169 RFC 2186 RFC 2187 RFC 2227 RFC 2291 RFC 2296 RFC 2310 RFC 2324 and 256 more

An unreliable message associated with an HTTP request stream. IP proxying uses it to carry each IP packet, prefixed by a Context ID.

Plain language Unreliable message associated with HTTP request stream for IP proxying.

Used in RFC 9298 RFC 9484

The verb in an HTTP request, such as GET or POST, that says what the client wants done with the target resource. HTCPCP adds BREW and WHEN.

Plain language The verb in an HTTP request (like GET or POST) specifying the desired action.

Used in RFC 1945 RFC 2068 RFC 2291 RFC 2310 RFC 2324 RFC 2518 RFC 2616 RFC 2817 RFC 3205 RFC 3253 RFC 3648 RFC 3744 RFC 3875 RFC 4331 RFC 4825 RFC 4918 RFC 5023 RFC 5323 RFC 5689 RFC 5789 RFC 5995 RFC 6352 RFC 6578 RFC 6638 and 37 more

A three-digit number at the start of an HTTP response that says how the request went. 4xx codes mean the client should change something, 5xx codes mean a server problem.

Plain language Three-digit response number indicating request outcome (4xx client error, 5xx server error).

Used in RFC 1945 RFC 2068 RFC 2069 RFC 2169 RFC 2324 RFC 2616 RFC 2617 RFC 2660 RFC 2817 RFC 3205 RFC 3229 RFC 3253 RFC 3648 RFC 4331 RFC 5323 RFC 5689 RFC 5789 RFC 5842 RFC 5861 RFC 6546 RFC 6578 RFC 6585 RFC 6712 RFC 6764 and 49 more

An HTTP/1.1 mechanism where the client asks, with Upgrade and Connection: Upgrade headers, to switch the connection to another protocol. The server agrees with 101 Switching Protocols.

Plain language HTTP/1.1 mechanism to switch a connection to another protocol.

Used in RFC 2817 RFC 2910 RFC 6455 RFC 7118 RFC 7395 RFC 7977 RFC 8441 RFC 8857 RFC 8887 RFC 9297 RFC 9298 RFC 9931

Version 2 of HTTP (RFC 7540, later RFC 9113). It keeps HTTP semantics but sends them as binary frames over multiplexed streams on one connection.

Plain language Version 2 of HTTP sending binary frames over multiplexed streams on one connection.

Used in RFC 7301 RFC 7540 RFC 7541 RFC 7838 RFC 8030 RFC 8164 RFC 8170 RFC 8297 RFC 8336 RFC 8441 RFC 8650 RFC 8740 RFC 8895 RFC 9204 RFC 9218 RFC 9220 RFC 9292 RFC 9297 RFC 9411 RFC 9569

The mapping of HTTP semantics onto the QUIC transport protocol, defined in RFC 9114. Each request and response uses its own QUIC stream, and connections are encrypted by default.

Plain language Mapping of HTTP semantics onto QUIC transport with encrypted connections by default.

Used in RFC 9114 RFC 9204 RFC 9218 RFC 9220 RFC 9292 RFC 9297 RFC 9317 RFC 9411 RFC 9412 RFC 9484 RFC 9569

A way to build an optimal prefix code from symbol frequencies, giving common symbols short bit patterns and rare ones long patterns. DEFLATE uses it for literal/length and distance symbols.

Plain language Prefix code giving common symbols short patterns and rare ones long patterns.

Used in RFC 803 RFC 1217 RFC 1951 RFC 2306 RFC 2435 RFC 3284 RFC 3320 RFC 3949 RFC 5215 RFC 5577 RFC 6386 RFC 7541 RFC 7692 RFC 7932 RFC 8054 RFC 8478 RFC 8878 RFC 9204 RFC 9407 RFC 9841

A deterministic algorithm to derive a valid Password Element from a password by iteratively checking candidates until finding one that satisfies the group's mathematical requirements.

Plain language Algorithm deriving a valid password element by iteratively checking candidates.

Used in RFC 6617 RFC 7664 RFC 8492

A key exchange that runs several algorithms at once and combines their outputs. It stays secure as long as at least one of the algorithms remains secure.

Used in RFC 9370 RFC 10024

A network element supporting multiple switching capabilities on at least one interface, allowing traffic of different types to be adapted and switched at a single node.

Used in RFC 5339 RFC 6001

Text that contains links, so a reader can jump from one document, or one place within a document, to another by selecting the link.

Plain language Text containing links so readers can jump to other documents or locations.

Used in RFC 1580 RFC 1689 RFC 1941 RFC 2150

Software, firmware or hardware that creates and runs virtual machines on a physical server. Each virtual machine behaves as if it had its own computer.

Plain language Software creating and running virtual machines on a physical server.

Used in RFC 7666 RFC 8368 RFC 8394

A deliberate reluctance to change a decision unless the difference is large enough, used to stop a system from flipping back and forth over small changes.

Plain language Reluctance to change decisions unless the new choice is clearly better.

Used in RFC 5882 RFC 6719 RFC 9616