Glossary
H
56 terms.
- H.323
An ITU-T standard for packet-based multimedia communications, enabling voice and video calls over IP networks with signaling and control protocols.
Like I'm five A set of rules for making audio and video calls over the internet.
- Hairpinning
When a NAT correctly handles traffic from one internal host to another internal host that is addressed by its external address and port.
Like I'm five Mail that goes out to the post office and comes straight back to a neighbor in the same building.
Used in RFC 4787 RFC 5128 RFC 5382 RFC 5508 RFC 6081 RFC 6146 RFC 7757 RFC 7857
- Handoff
The process of a mobile node transferring its connection from one foreign agent or access point to another while maintaining network connectivity.
Like I'm five Switching your connection from one Wi-Fi router to another.
- Handover
The process of a mobile device switching from one access network to another while maintaining ongoing communications.
Like I'm five Switching from one Wi-Fi hotspot to another while keeping your video call going.
Used in RFC 3374 RFC 3583 RFC 3753 RFC 4066 RFC 4831 RFC 5181 RFC 5184 RFC 5268 RFC 5568 RFC 5677 RFC 5873 RFC 5980 RFC 6058 RFC 6252 RFC 6301 RFC 6697
- Handshake
The opening exchange of a secure connection in which the parties agree on parameters, authenticate and derive shared keys.
Like I'm five The hello chat where both sides agree on the secret code.
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 4680 RFC 5169 RFC 5238 RFC 5246 RFC 5303 RFC 6101 RFC 6460 and 28 more
- Happy Eyeballs
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.
Like I'm five Trying two roads to a friend's house and taking whichever one is open first.
- Hardware security module
A tamper-resistant device that stores private keys and performs signing inside itself. The key never has to be exposed to ordinary computers.
Like I'm five A locked safe that can sign things without ever letting anyone take the key out.
- Hardware timestamping
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.
Like I'm five Stamping the time with a machine at the door instead of guessing later.
- Harvest now, decrypt later
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.
Like I'm five A thief copies your locked diary now and waits until they can pick the lock.
- Hash function
A one-way function that turns input of any length into a fixed-size value. SHA-256, for example, produces a 256-bit result.
Like I'm five A machine that squishes any message into a short fingerprint you cannot turn back into the message.
Used in RFC 3074 RFC 3174 RFC 4270 RFC 5476 RFC 5758 RFC 6986 RFC 7427 RFC 7693 RFC 8017 RFC 9215
- Hash tree
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.
Like I'm five A single fingerprint made from lots of smaller fingerprints, so you can compare everything at once.
- Hash-based signature
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.
Like I'm five A signature made only from scrambling machines, so there is no clever maths to break.
- HDLC
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.
Like I'm five A way of marking where each message starts and stops on a wire.
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 2363 and 14 more
- Head-of-line blocking
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.
Like I'm five One slow person at the front of a line makes everyone behind wait.
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 8239 RFC 8260 RFC 8323 RFC 8895 RFC 9000 RFC 9112 RFC 9113 RFC 9114 RFC 9174 and 5 more
- Header compression
A technique that shrinks the protocol headers of packets on a link, often by sending only what changed since the last packet.
Like I'm five Instead of writing the full address on every postcard, you agree once and then write only a tiny note.
Used in RFC 914 RFC 1144 RFC 1332 RFC 1553 RFC 2507 RFC 2508 RFC 3006 RFC 3095 RFC 3096 RFC 3241 RFC 3243 RFC 3409 RFC 3449 RFC 3843 RFC 4019 RFC 4170 RFC 4224 RFC 4362 RFC 4815 RFC 4840 RFC 4901 RFC 4944 RFC 5225 RFC 5795 and 4 more
- Header field
A line at the top of a message written as
Field-Name: Field-Value, such asSubject: Hello. Long values can be folded onto continuation lines.Like I'm five A labelled blank on the form at the top of the letter.
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 1505 RFC 1522 RFC 1622 RFC 1753 RFC 1806 RFC 1867 RFC 1945 RFC 2017 RFC 2047 RFC 2068 RFC 2076 RFC 2109 and 211 more
- Header Protection
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.
Like I'm five Locking the words on the envelope as well as the letter inside.
- Heartbeat
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.
Like I'm five A regular 'I am still here' signal, like tapping someone on the shoulder every minute.
- HELD
HTTP-Enabled Location Delivery, a protocol in which a device or other requestor asks a Location Information Server for location information over HTTP.
Like I'm five A way to ask a server over the web where something is.
- HelloRetryRequest (HRR)
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.
Like I'm five The office saying, "That form wasn't quite right, please fill in a new one."
- Hierarchical Host Identity Tag (HHIT)
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.
Like I'm five An ID made from your secret key that also says which office registered it.
- Historic Status
The status given to protocols that are no longer recommended for use, typically because they have been superseded by better alternatives.
Like I'm five An old rule that nobody uses anymore because better rules replaced it.
- HKDF
The HMAC-based Extract-and-Expand Key Derivation Function. TLS 1.3 uses it to turn shared secrets into many separate keys.
Like I'm five A recipe that turns one secret into lots of different keys.
Used in RFC 5869 RFC 6234 RFC 6476 RFC 7401 RFC 7978 RFC 8110 RFC 8188 RFC 8291 RFC 8418 RFC 8447 RFC 8613 RFC 8619 RFC 8696 RFC 8915 RFC 8937 RFC 9001 RFC 9053 RFC 9180 RFC 9190 RFC 9203 RFC 9258 RFC 9303 RFC 9369 RFC 9420 and 12 more
- HMAC
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.
Like I'm five A way to blend a secret with a message so only people who know the secret can get the same scrambled answer.
Used in RFC 1828 RFC 2085 RFC 2104 RFC 2195 RFC 2202 RFC 2246 RFC 2274 RFC 2286 RFC 2316 RFC 2332 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 RFC 2869 and 255 more
- Hold Timer
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.
Like I'm five An egg timer that goes off if your friend stops talking for too long.
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
- Hole punching
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.
Like I'm five Knocking from the inside so the gate stays open a crack for your friend to walk in.
- Home Address
The permanent IP address assigned to a mobile node on its home network, which remains unchanged regardless of where the node moves.
Like I'm five Your permanent address that you keep no matter where you go.
Used in RFC 1688 RFC 3220 RFC 3775 RFC 5026 RFC 6275 RFC 6612
- Home Agent
The Mobile IPv4 agent on a mobile node's home network. It answers registration requests and authenticates its Registration Reply.
Like I'm five The helper back home who keeps track of where you are.
Used in RFC 2002 RFC 2006 RFC 2290 RFC 2794 RFC 3024 RFC 3344 RFC 3543 RFC 3753 RFC 3775 RFC 3776 RFC 3846 RFC 3957 RFC 3963 RFC 4004 RFC 4093 RFC 4225 RFC 4283 RFC 4295 RFC 4332 RFC 4433 RFC 4487 RFC 4636 RFC 4640 RFC 4886 and 24 more
- Hop count
The number of routers or switches a packet must pass through to reach its destination.
Like I'm five How many stops a packet makes on its way to the destination.
- Hop Limit
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.
Like I'm five A number of stamps on a ticket. Each stop uses one, and when they run out the ticket is thrown away.
Used in 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 4213 RFC 4286 RFC 4293 RFC 4443 RFC 4890 RFC 5082 and 42 more
- Hop-by-hop
A feature agreed between each directly connected pair of hosts, not carried end to end.
Like I'm five Each pair of friends in a chain agrees on its own way to talk.
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 5749 RFC 6028 RFC 6398 RFC 6553 RFC 6564 RFC 6621 RFC 6891 and 29 more
- Host
A computer or device connected to the Internet, identified by a name and IP address.
Like I'm five A computer connected to the Internet.
- Host group
The set of hosts, possibly zero, that share one IP multicast address. Membership is dynamic and can span many networks.
Like I'm five The members of one club.
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
- Host Identity
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.
Like I'm five A secret number that proves who you are, separate from where you are.
- Host Identity Protocol
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.
Like I'm five Instead of being known by where it is, a computer is known by a secret-key name tag that only it can prove.
Used in RFC 5205 RFC 5207 RFC 6261 RFC 6537 RFC 7402 RFC 8002 RFC 8046 RFC 8047
- Host Identity Tag
A short identifier for a HIP host, derived by hashing its public key. Applications use it in place of an IP address.
Like I'm five A short nickname made from your secret-key lock, used instead of your street address.
Used in RFC 5205 RFC 5338 RFC 5770 RFC 6317 RFC 6537 RFC 6538 RFC 7401 RFC 7402 RFC 8002 RFC 8004 RFC 9063
- Host key
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.
Like I'm five A special stamp each computer has, so you can tell it apart from an impostor.
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 9644
- Host requirements
The behaviors that an Internet host must, should or may implement. RFC 1122 defines them for the link, IP and transport layers.
Like I'm five The rulebook for being a good computer on the Internet.
- Host table
A file mapping host names to their network addresses and other properties. Used before DNS for name resolution.
Like I'm five A list of computer names and their addresses.
- HOTP
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.
Like I'm five A code maker that gives a new code each time you press its button, counting presses.
Used in RFC 4226 RFC 6030 RFC 6031 RFC 6063 RFC 6238 RFC 6287 RFC 6560 RFC 7894 RFC 8176
- HPACK
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.
Like I'm five A shorthand where repeated words are replaced by a short code both sides remember.
- HPKE (Hybrid Public Key Encryption)
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.
Like I'm five A padlock anyone can snap shut using a public key, but that only the owner of the private key can open.
- HSTS
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.
Like I'm five A sign on a shop that says, use only the locked front door, and your browser remembers it.
Used in RFC 2818 RFC 6797 RFC 7457 RFC 7469 RFC 7525 RFC 7711 RFC 9325 RFC 9460
- HSTS preload list
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.
Like I'm five A list of shops written into your notebook at the factory, so you already know the rule before your first visit.
- HTTP
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.
Like I'm five The set of rules your browser uses to ask websites for things.
Used in RFC 1689 RFC 1738 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 RFC 2326 RFC 2388 RFC 2518 and 229 more
- HTTP Datagram
An unreliable message associated with an HTTP request stream. IP proxying uses it to carry each IP packet, prefixed by a Context ID.
Like I'm five A loose package sent along with a web request that might get lost and is not resent.
- HTTP method
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.
Like I'm five The action word in a request, like get, send, or brew.
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 36 more
- HTTP status code
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.
Like I'm five A number that tells you how your request turned out.
Used in RFC 1945 RFC 2068 RFC 2069 RFC 2324 RFC 2616 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 RFC 7009 RFC 7168 and 47 more
- HTTP Upgrade
An HTTP/1.1 mechanism where the client asks, with
UpgradeandConnection: Upgradeheaders, to switch the connection to another protocol. The server agrees with101 Switching Protocols.Like I'm five Asking, 'can we talk a different way from now on?'
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
- HTTP/2
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.
Like I'm five The same web questions as before, packed into a smarter shipping container.
Used in RFC 7301 RFC 7540 RFC 7541 RFC 7838 RFC 8030 RFC 8164 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
- HTTP/3
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.
Like I'm five The newest way of sending web requests, built on QUIC.
Used in RFC 9114 RFC 9204 RFC 9218 RFC 9220 RFC 9292 RFC 9297 RFC 9317 RFC 9411 RFC 9412 RFC 9484 RFC 9569
- Huffman coding
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.
Like I'm five Giving the things you say most often the shortest nicknames.
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
- Hunting and Pecking
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.
Like I'm five A method of trying different numbers until finding one that works as a Password Element.
- Hypertext
Text that contains links, so a reader can jump from one document, or one place within a document, to another by selecting the link.
Like I'm five Writing with doors in it that you can click to go somewhere else.
- Hypervisor
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.
Like I'm five The manager that lets one real computer pretend to be several.