Glossary
S
234 terms.
- S-box
A substitution box, a lookup table that replaces small groups of bits with other groups of bits. It adds non-linear mixing to a cipher.
Like I'm five A table that swaps little pieces of a message for other pieces.
- S-NAPTR
Straightforward-Naming Authority Pointer, a DNS lookup method in which NAPTR records, chosen by an application service tag and a protocol tag, lead step by step to the server for a service in a domain.
Like I'm five A chain of phone book pages where each page tells you which page to read next, until you find the number.
- S/MIME
Secure/Multipurpose Internet Mail Extensions, a way of signing and encrypting email that uses CMS to hold the protected data and certificates to identify the people involved.
Like I'm five A way to put a seal and a lock on email.
Used in RFC 2311 RFC 2312 RFC 2316 RFC 2480 RFC 2630 RFC 2633 RFC 2634 RFC 2649 RFC 2785 RFC 2798 RFC 2984 RFC 2985 RFC 3114 RFC 3157 RFC 3218 RFC 3274 RFC 3327 RFC 3335 RFC 3369 RFC 3370 RFC 3372 RFC 3420 RFC 3428 RFC 3524 and 91 more
- Safe method
A request method whose defined semantics are essentially read-only, so a client does not request or expect any state change on the origin server. GET, HEAD, OPTIONS and TRACE are safe.
Like I'm five A question that does not change anything, only looks.
Used in RFC 2310 RFC 2976 RFC 5789 RFC 7231 RFC 7237 RFC 7252 RFC 8132 RFC 8470 RFC 9110 RFC 9875 RFC 10008
- Safe prime
A prime number p such that p = 2q + 1, where q is also prime. Diffie-Hellman groups built on safe primes are the kind adopted in this RFC.
Like I'm five A special kind of big number that is picked because it makes the secret trick harder to break.
- Salt
A random value mixed in before hashing a secret so that each stored hash is unique to its own value.
Like I'm five A pinch of random extra stuff added to a secret before scrambling it.
Used in RFC 2898 RFC 2945 RFC 3112 RFC 3961 RFC 3962 RFC 5054 RFC 5802 RFC 5869 RFC 7292 RFC 8018 RFC 8146 RFC 9106 RFC 9154 RFC 9901
- Same-origin policy
A browser rule that stops a script from reading responses from a server other than the origin it came from. It keeps each site's scripts in their own isolated sandbox.
Like I'm five Each website gets its own playground and cannot peek into the others.
- SAML
Security Assertion Markup Language, an XML-based framework that allows organizations to exchange identity and authentication information securely.
Like I'm five A standard way for organizations to share who people are.
- SASL
The Simple Authentication and Security Layer, a framework in RFC 4422 that lets protocols plug in authentication mechanisms such as SCRAM or PLAIN. XMPP carries SASL exchanges as XML elements.
Like I'm five A standard way to prove who you are, with different ways to do it to choose from.
Used in RFC 1731 RFC 1734 RFC 2222 RFC 2244 RFC 2245 RFC 2251 RFC 2384 RFC 2444 RFC 2449 RFC 2554 RFC 2595 RFC 2636 RFC 2645 RFC 2808 RFC 2829 RFC 2830 RFC 2831 RFC 3062 RFC 3080 RFC 3117 RFC 3163 RFC 3195 RFC 3207 RFC 3365 and 70 more
- Scalable Congestion Control
A congestion control where the average time from one congestion signal to the next stays the same as the flow rate grows. DCTCP is an example.
Like I'm five A way of slowing down in many small steps that works just as well when traffic gets much bigger.
- SCHC
Static Context Header Compression, a compression and fragmentation mechanism for constrained networks. Two ends share a static context made of Rules.
Like I'm five A shared shorthand that lets small devices send short messages.
- SCIM
System for Cross-domain Identity Management, a standard way for systems to create, read, update and delete user and group records over HTTP using JSON.
Like I'm five A common language that lets different systems keep the same list of people up to date.
- Scope
In OAuth, a value naming a kind or amount of access, such as reading a profile. Scope values are defined by the authorization server and a token carries only the scopes it was granted.
Like I'm five A label on your pass that says which rooms it opens.
Used in RFC 6749 RFC 6750 RFC 6755 RFC 6819 RFC 7522 RFC 7523 RFC 7591 RFC 7628 RFC 7635 RFC 7662 RFC 7853 RFC 8414 RFC 8628 RFC 8693 RFC 8707 RFC 8898 RFC 9068 RFC 9126 RFC 9200 RFC 9396 RFC 9431 RFC 9560 RFC 9635 RFC 9701 and 2 more
- Scoreboard
A data structure maintained by a TCP sender that tracks which octets or ranges of octets have been cumulatively acknowledged or selectively acknowledged.
Like I'm five A list of which pieces the receiver says it has gotten.
- SCRAM
Salted Challenge Response Authentication Mechanism (RFC 5802), a password-based SASL mechanism that does not send the password. SCRAM-SHA-1 and SCRAM-SHA-1-PLUS are mandatory to implement in XMPP.
Like I'm five A way to prove you know a password without ever saying it out loud.
Used in RFC 5802 RFC 5803 RFC 6120 RFC 6331 RFC 7677 RFC 9266
- Script
A set of graphic characters used for the written form of one or more languages.
Like I'm five The set of letters and symbols used to write a language, like the Latin alphabet or Arabic script.
- SCSI
Small Computer Systems Interface, a standard for communicating with storage devices such as disk drives and tape drives.
Like I'm five A protocol for talking to storage devices.
- SCTP
The Stream Control Transmission Protocol, a reliable, message-oriented transport protocol with multiple streams per connection and support for several IP addresses per endpoint.
Like I'm five A delivery service that keeps each letter whole and can use several roads at once.
Used in RFC 2960 RFC 3057 RFC 3127 RFC 3257 RFC 3286 RFC 3309 RFC 3320 RFC 3331 RFC 3332 RFC 3347 RFC 3419 RFC 3423 RFC 3436 RFC 3530 RFC 3539 RFC 3554 RFC 3588 RFC 3708 RFC 3758 RFC 3788 RFC 3807 RFC 3816 RFC 3836 RFC 3868 and 112 more
- SCTP chunk
A unit of information inside an SCTP packet, either control information or user data. A packet can bundle several chunks after one common header.
Like I'm five One labelled piece in a bundle of pieces that travel together in one envelope.
- SDID
Signing Domain Identifier: the domain named in the
d=tag of a DKIM signature. It is the domain that takes responsibility for the message and the one whose DNS holds the public key. It does not have to match the address in the From header.Like I'm five The name of the mail room that put its seal on the letter.
- SDNV
A Self-Delimiting Numeric Value is a number encoded in as many octets as needed. The top bit of each octet says whether more follow, and the other 7 bits hold the value.
Like I'm five A number written in little chunks, where each chunk says whether another chunk is coming.
- SDP
Session Description Protocol. A text format that describes a media session, such as which codecs and addresses to use. SIP messages commonly carry it as an offer and an answer.
Like I'm five A note that says what kind of sound and picture you can handle and where to send them.
Used in RFC 2198 RFC 2326 RFC 2327 RFC 2543 RFC 2705 RFC 2848 RFC 2974 RFC 3015 RFC 3016 RFC 3108 RFC 3190 RFC 3261 RFC 3262 RFC 3264 RFC 3266 RFC 3303 RFC 3311 RFC 3312 RFC 3323 RFC 3372 RFC 3388 RFC 3389 RFC 3398 RFC 3407 and 318 more
- SDP offer/answer
A negotiation model in which one party sends an SDP description as an offer and the other replies with an answer describing what it accepts. Both then set up the session accordingly.
Like I'm five One person proposes a plan and the other says which parts they agree to.
- SDP precondition
A condition that must be met for a media stream before session establishment or modification can proceed. Progress is delayed until it is satisfied or the session fails.
Like I'm five A rule that says wait until this is ready before starting.
A cookie carrying the Secure attribute, which tells the browser to send it only over a secure transport such as HTTPS.
Like I'm five A note that the browser promises to hand over only through the locked door.
- Secure Neighbor Discovery
SEND, a set of protections for IPv6 Neighbor Discovery that uses cryptographically generated addresses and signatures to prove who sent a message.
Like I'm five A way of putting a signature on network announcements so fakes can be spotted.
- Security Association (SA)
The shared state two peers keep for protecting traffic: the algorithms, keys and other parameters. IKE has an IKE SA for its own messages and creates Child SAs for ESP or AH traffic.
Like I'm five A written-down agreement about which code you two are using right now.
Used in RFC 1135 RFC 1777 RFC 1825 RFC 1826 RFC 1829 RFC 2002 RFC 2006 RFC 2084 RFC 2085 RFC 2207 RFC 2228 RFC 2230 RFC 2262 RFC 2344 RFC 2356 RFC 2367 RFC 2394 RFC 2395 RFC 2401 RFC 2402 RFC 2403 RFC 2404 RFC 2405 RFC 2406 and 164 more
- Security context
The shared state that two peers build by exchanging tokens, after which they can protect messages to each other. Several contexts can exist at once between the same peers.
Like I'm five A private line two friends set up before they start talking.
Used in RFC 2078 RFC 2743 RFC 2744 RFC 7546 RFC 8353 RFC 8613 RFC 9172 RFC 9173
- Security Event Token
A signed JSON Web Token that describes a security-related event, such as an account being disabled. It is defined in RFC 8417 and abbreviated SET.
Like I'm five A small sealed note that says something important about security just happened.
- Security label
A set of security attributes attached to a subject or object, such as a sensitivity level and compartment or an SELinux identity, role and type.
Like I'm five A badge on something that says how secret it is and who may use it.
- Security policy
A formal statement of the rules that people given access to an organization's technology and information assets must follow.
Like I'm five The rulebook everyone agrees to follow to keep the computers and information safe.
- Security Policy Database
The ordered IPsec database that matches packets by selectors and chooses whether each one is protected, discarded or allowed to bypass IPsec.
Like I'm five The guard's rulebook that says what to do with each visitor.
Used in RFC 2401 RFC 3585 RFC 4301 RFC 4945 RFC 5858 RFC 9061
- Security Token Service
A service that validates security tokens it is given and issues new security tokens in response. In OAuth token exchange, the authorization server plays this role.
Like I'm five A desk that checks your ticket and hands you a different ticket that works somewhere else.
- Seed
The starting value fed to a pseudo-random generator. All the unpredictability of its output comes from the seed, so a seed with only a few bits of entropy gives a small set of possible outputs no matter how complex the generator is.
Like I'm five The first secret roll of the dice that a long list of numbers grows from.
Used in RFC 1760 RFC 2243 RFC 2289 RFC 4086 RFC 4162 RFC 4777 RFC 5669 RFC 5748 RFC 8479 RFC 8682 RFC 9881 RFC 9935 RFC 9964 RFC 9986
- Segment Identifier (SID)
A value that identifies a segment in Segment Routing, for example a prefix or an adjacency.
Like I'm five A number that stands for one stop or one hop on a route.
Used in RFC 8402 RFC 8476 RFC 8491 RFC 8660 RFC 8665 RFC 8666 RFC 8667 RFC 8669 RFC 8754 RFC 9020 RFC 9085 RFC 9086 RFC 9256 RFC 9259 RFC 9513 RFC 9524 RFC 9602 RFC 9631 RFC 9716 RFC 9800 RFC 9902 RFC 10038
- Segment Routing
A way of steering traffic in which the sender lists the segments, or steps, a packet should follow through the network.
Like I'm five Giving a packet a list of stops to visit on its way.
Used in RFC 8287 RFC 8402 RFC 8403 RFC 8426 RFC 8604 RFC 8660 RFC 8661 RFC 8662 RFC 8663 RFC 8664 RFC 8666 RFC 8669 RFC 8754 RFC 8814 RFC 8986 RFC 9085 RFC 9087 RFC 9125 RFC 9252 RFC 9256 RFC 9350 RFC 9491 RFC 9503 RFC 9513 and 17 more
- Segment Routing Global Block (SRGB)
The label ranges a router sets aside for global SIDs. A SID index is turned into an actual label by counting through the ranges in the order they were advertised.
Like I'm five The block of numbers a router promises to use for its name tags.
- Segment Routing Header
An IPv6 routing header used by SRv6 that carries the list of segments a packet must visit and can also hold TLVs with extra metadata.
Like I'm five The list of stops written on a parcel, plus room for extra notes.
- Selective acknowledgement
An acknowledgement that names individual segments received out of order, instead of only the last one received in sequence. It lets a sender resend just the missing segments.
Like I'm five Telling the sender exactly which postcards arrived, so it only resends the ones that did not.
- Selective acknowledgment
A TCP option that lets a receiver tell the sender exactly which blocks of data arrived. The sender can then retransmit only the missing segments.
Like I'm five Telling a friend exactly which pages you got, so they only resend the missing ones.
Used in RFC 2018 RFC 3155 RFC 4653 RFC 6675 RFC 7414 RFC 8985 RFC 9937
- Sender-constrained token
A token that can be used only by a client that proves it holds a particular key, unlike a bearer token that works for anyone who has it. DPoP is one way to do this.
Like I'm five A ticket that only works for the person who can show the matching secret.
- SenML
Sensor Measurement Lists, a data format for carrying sensor readings as a list of records. It is defined in RFC 8428.
Like I'm five A tidy way to write down a list of sensor readings.
- Sequence number
A 32-bit counter that numbers every byte TCP sends, so the receiver can put data in order, detect duplicates and acknowledge exactly what it has.
Like I'm five A number written on every letter of a story so nothing can get shuffled or lost without being noticed.
Used in RFC 442 RFC 512 RFC 516 RFC 675 RFC 683 RFC 700 RFC 761 RFC 793 RFC 889 RFC 905 RFC 908 RFC 916 RFC 938 RFC 964 RFC 1072 RFC 1110 RFC 1144 RFC 1151 RFC 1201 RFC 1216 RFC 1263 RFC 1323 RFC 1582 RFC 1717 and 184 more
- Sequence Number Extension
A 32-bit counter that TCP-AO keeps beside TCP's 32-bit sequence number to form a 64-bit space. It tells apart segments that repeat the same sequence number after rollover.
Like I'm five A counter of how many times the page numbers have started over.
- Serial number arithmetic
A way of comparing counters that wrap around, so that half of the values count as newer than a given number and half as older. It lets a small counter be reused safely.
Like I'm five Comparing numbers on a clock face, where after the biggest number you start again at the beginning.
- Server Name Indication (SNI)
The
server_nameextension (type 0), in which a TLS client states the DNS host name it wants to reach, before the server picks a certificate.Like I'm five Saying which person you want to see when you walk into a building with many offices.
Used in RFC 1013 RFC 3958 RFC 6066 RFC 6186 RFC 6353 RFC 7525 RFC 7590 RFC 7671 RFC 7673 RFC 7712 RFC 7838 RFC 7925 RFC 8143 RFC 8323 RFC 8336 RFC 8461 RFC 8737 RFC 8744 RFC 8765 RFC 9174 RFC 9325 RFC 9460 RFC 9505 RFC 9525 and 7 more
- Server push
An optional HTTP/2 feature in which a server sends a PUSH_PROMISE and a response for something the client has not yet asked for. A client can disable it with SETTINGS_ENABLE_PUSH set to 0.
Like I'm five A waiter bringing the salt before you ask, hoping you will want it.
Used in RFC 2177 RFC 3995 RFC 5465 RFC 5551 RFC 6202 RFC 7540 RFC 8030 RFC 8297 RFC 8336 RFC 8620 RFC 8764 RFC 8887 RFC 8895 RFC 9113 RFC 9114 RFC 9218 RFC 9569 RFC 9749
- Server-Sent Events
A way for a server to keep an HTTP connection open and push a stream of text events to a client, using the media type
text/event-stream.Like I'm five Leaving a phone line open so the other side can keep telling you news.
- Service Access Point (SAP)
An identifier (typically one byte) that specifies which process or service on a station should handle a particular frame, similar to a port number in IP.
Like I'm five A number that tells the station which program should receive the message.
- Service Attachment Point
An abstraction of a network reference point where network services can be, or already are, delivered to customers. It can be bound to one or more attachment circuits.
Like I'm five A named spot on the provider's network where a customer can plug in.
- Service boundary
The region within which all locations map to the same service URI or set of URIs for a given service. It may consist of several non-contiguous shapes.
Like I'm five The edge of the area a particular helper is responsible for.
- Service Code
A field in the DCCP connection handshake that describes the application-level service the client wants to reach. It helps middleboxes and reduces reliance on well-known ports.
Like I'm five A label on a connection that says what it is for.
- Service Demarcation Point (SDP)
The logical point where slice-specific treatment begins or ends, typically where an Attachment Circuit connects to the provider network at a PE.
Like I'm five The gate between what the customer controls and what the network provider controls.
- Service discovery
Finding where a service is running, meaning which hosts and ports, without hard-coding them in the client. DNS SRV records are a classic mechanism for it.
Like I'm five Asking where to find something instead of already knowing.
Used in RFC 462 RFC 620 RFC 623 RFC 830 RFC 887 RFC 1491 RFC 1789 RFC 2052 RFC 2165 RFC 2248 RFC 2413 RFC 2552 RFC 2608 RFC 2609 RFC 2610 RFC 2614 RFC 2782 RFC 2843 RFC 2915 RFC 2968 RFC 2969 RFC 3059 RFC 3082 RFC 3088 and 33 more
- Service Function Chain
A sequence of network functions (such as firewall, then load balancer, then DPI) through which traffic for a service must pass.
Like I'm five An ordered list of virtual network boxes that a packet travels through.
- Service Function Chaining
An architecture in which packets are steered through an ordered set of service functions, such as filters or NAT, so each can apply its own processing.
Like I'm five Sending a parcel through a line of stations that each do one job on it.
Used in RFC 8300 RFC 8459 RFC 8595 RFC 8677 RFC 8979 RFC 9015 RFC 9192 RFC 9263 RFC 9451 RFC 9491 RFC 9516
- Service Function Forwarder (SFF)
The component responsible for reading service path information and forwarding packets to the next service in the chain, then to the next SFF.
Like I'm five The worker who directs packages to the next station in the assembly line.
- Service Function Path
The specific sequence of Service Functions that packets of a given classification are sent through. In the NSH it is identified by the Service Path Identifier.
Like I'm five The actual route a package takes through the helpers.
- Service identification
The process of determining what user-level communication service is being invoked in a SIP session, such as a voice call, a video conference, gaming, or media streaming.
Like I'm five Figuring out whether a phone call is a regular call, a video chat, a game, or something else.
- Service Level Objective
A target value or range for a measurable aspect of network performance, such as bandwidth or latency. A customer can check it by measuring traffic.
Like I'm five A speed target you can check with a stopwatch.
- Service Level Specification
A set of parameters that define the service (such as bandwidth, latency and loss rate) offered to a traffic stream by a Differentiated Services domain.
Like I'm five The technical promises made about how a network will treat your data.
- Service Location Protocol
A protocol in which User Agents find services through Service Agents and Directory Agents. Version 2 is defined in RFC 2608.
Like I'm five A way for your computer to ask the network, 'who can do this job for me?'
- Service Model
A YANG data model that describes a network service from the customer perspective, including requirements like bandwidth, latency, and security, without specifying underlying technology.
Like I'm five A description of what a customer wants from a network service (speed, reliability, who can connect) without saying how to build it.
- Service-level agreement
A specification of performance requirements for a network service, including bandwidth, maximum end-to-end delay, delay variation, and maximum loss ratio.
Like I'm five A promise about how fast and reliably the network will work.
Used in RFC 2475 RFC 2758 RFC 3387 RFC 7536 RFC 9439 RFC 9732 RFC 9938
- Session Border Controller
A signaling intermediary, commonly used by SIP domains, that sits between networks and handles signaling and media, including relaying media for devices behind NATs.
Like I'm five A front desk that passes every call through itself.
- Session Establishment Data (SED)
The set of parameters (ingress point, protocol, IP address, port, and routing policies) that a SIP service provider needs to route a call to another provider's network.
Like I'm five The information telling a phone company where and how to send a call to another company's network.
- Session identifier
A random value, often stored in a cookie, that a server uses as a key to look up information about one visitor. Keeping data on the server and only the identifier in the cookie limits the damage if the cookie leaks.
>Like I'm five A number on your ticket that lets the website find your page in its big notebook.
Used in RFC 2059 RFC 2246 RFC 2326 RFC 2741 RFC 2964 RFC 3018 RFC 3033 RFC 3436 RFC 3451 RFC 3576 RFC 3652 RFC 3749 RFC 3783 RFC 3817 RFC 4081 RFC 4252 RFC 4346 RFC 4454 RFC 4462 RFC 4656 RFC 4817 RFC 5077 RFC 5191 RFC 5247 and 25 more
- Session Initiation Protocol (SIP)
An application-layer protocol for establishing, modifying, and terminating real-time sessions such as phone calls and video conferences. SIP messages carry addressing, routing, and other call information.
Like I'm five The language that internet phones use to say 'hello, I want to call you' and to set up the call.
- Session key
A fresh symmetric key generated by the KDC for a client and a service to share. It appears in the ticket and in the reply to the client, and is used for only a limited time.
Like I'm five A one-visit secret code the helper makes for two people to share.
Used in RFC 1510 RFC 1731 RFC 1851 RFC 1949 RFC 1964 RFC 1991 RFC 2093 RFC 2440 RFC 2451 RFC 2522 RFC 2548 RFC 2695 RFC 2716 RFC 2930 RFC 2941 RFC 2942 RFC 2944 RFC 2945 RFC 2946 RFC 2947 RFC 2948 RFC 3078 RFC 3079 RFC 3217 and 56 more
- Session Recording Client (SRC)
A SIP user agent function that acts as the source of recorded media and metadata, sending them to a recording server. It may be a phone, a gateway, a border controller or a media server.
Like I'm five The part that makes a copy of a call and sends it to be saved.
- Session Recording Server
The party in SIP-based recording that receives the copied media and the metadata describing the recorded communication session.
Like I'm five The machine that stores the recording and its notes.
- Session ticket
A blob a TLS server gives a client after a handshake. The client can present it later to resume the session as a PSK. In TLS 1.3 the NewSessionTicket message carries it.
Like I'm five A stamp on your hand that lets you back in quickly.
Used in RFC 4217 RFC 4507 RFC 5077 RFC 5422 RFC 5723 RFC 5868 RFC 7170 RFC 7301 RFC 7525 RFC 7925 RFC 8472 RFC 9001 RFC 9149 RFC 9190 RFC 9325 RFC 9369 RFC 9427 RFC 9813 RFC 9846 RFC 9887 RFC 9930
- Session Traversal Utilities for NAT (STUN)
A protocol that helps a client behind a NAT discover its external IP address and port by querying a STUN server.
Like I'm five A service that tells you what address you appear as from the outside of your NAT.
Used in RFC 5245 RFC 5766 RFC 6156 RFC 6314 RFC 6679 RFC 7064 RFC 7350 RFC 7443 RFC 7584 RFC 7675 RFC 7825 RFC 7982 RFC 7983 RFC 8016 RFC 8445 RFC 8656 RFC 8835 RFC 9443
- Session-Reflector
The TWAMP role that receives each test packet from the Session-Sender and sends a test packet back in response.
Like I'm five The helper that echoes your message back to you.
- Severely Errored Second
A one-second interval during which a large number of line coding violations occur, indicating serious signal degradation. For example, a P-bit PSES is a second with 44 or more coding violations.
Like I'm five A one-second period when the signal has so many errors that something is badly wrong.
- SGML
Standard Generalized Markup Language, a system for defining markup languages by specifying rules for how tags describe document structure and meaning.
Like I'm five A set of rules for creating languages that use tags to describe what different parts of a document mean.
- SHA-1
The Secure Hash Algorithm 1, a hash function that turns any input into a fixed-size digest. It produces a larger hash than MD5.
Like I'm five A machine that squashes any message into a short fingerprint.
- Sham link
A manually configured, unnumbered OSPF link between two PE routers that makes the route across the VPN backbone look like an intra-area link. It is optional.
Like I'm five A pretend hallway drawn on the map so a path looks like it stays inside the building.
A recovery scheme in which preconfigured protecting paths for several disjoint working paths share resources, and one is activated by the APS protocol when needed.
Like I'm five Several roads sharing one spare bridge that only opens when needed.
A set of links that can fail together because they share some underlying resource, such as the same physical port or line card.
Like I'm five A bunch of roads that all close at once because they cross the same bridge.
Used in RFC 4202 RFC 4203 RFC 4220 RFC 5286 RFC 5298 RFC 5307 RFC 5521 RFC 6119 RFC 6825 RFC 6981 RFC 7446 RFC 8001 RFC 8390 RFC 8800 RFC 9479 RFC 9492
- Shared Risk Link Group (SRLG)
A set of network links that share a common risk of failure, such as links in the same fiber optic cable or sharing the same physical path.
Like I'm five Links that might all break at the same time, like fiber cables bundled together in one duct.
A key known to both sides of an authentication exchange. In TOTP the prover and the verifier each hold the same secret key, and it is never sent along with the code.
Like I'm five A secret that you and a friend both know but never say out loud in front of others.
Used in RFC 1472 RFC 1852 RFC 1994 RFC 2058 RFC 2085 RFC 2104 RFC 2138 RFC 2139 RFC 2195 RFC 2243 RFC 2274 RFC 2403 RFC 2404 RFC 2522 RFC 2539 RFC 2548 RFC 2574 RFC 2607 RFC 2631 RFC 2661 RFC 2759 RFC 2841 RFC 2845 RFC 2865 and 82 more
A multicast distribution tree rooted at the Rendezvous Point and used by all sources of a group, written (*,G). Source-specific groups avoid it.
Like I'm five One big delivery route that every sender of a group uses.
- Shim header
A tunnel header that is not sufficient on its own to forward the packet as an outer header. It sits between two IP headers or between an IP header and a Layer 2 header.
Like I'm five A small extra label tucked between two layers of wrapping.
- Shortest Path First (SPF)
Dijkstra's algorithm as OSPF uses it: each router builds a tree of shortest paths with itself at the root and derives its routing table from the tree.
Like I'm five Always walking to the closest place not yet visited, until you know the best way to everywhere.
Used in RFC 981 RFC 1074 RFC 1131 RFC 1245 RFC 1247 RFC 1583 RFC 1584 RFC 2178 RFC 2328 RFC 2362 RFC 2702 RFC 3626 RFC 3684 RFC 3784 RFC 3847 RFC 3945 RFC 4061 RFC 4062 RFC 4063 RFC 5286 RFC 5302 RFC 5305 RFC 5309 RFC 5311 and 34 more
- Side-channel attack
An attack that learns secrets from how a system behaves, such as how long a calculation takes, rather than by breaking the math itself.
Like I'm five Guessing a secret by watching how long someone takes to do something.
- Sieve
A language for writing email filtering scripts, defined in RFC 5228. Extensions add new tests and actions to it.
Like I'm five A set of sorting rules for your mail, like a helper who puts each letter in the right pile.
Used in RFC 3431 RFC 3685 RFC 3894 RFC 5173 RFC 5228 RFC 5229 RFC 5230 RFC 5232 RFC 5260 RFC 5429 RFC 5435 RFC 5436 RFC 5437 RFC 5463 RFC 5550 RFC 5551 RFC 5703 RFC 5784 RFC 5804 RFC 6009 RFC 6134 RFC 6468 RFC 6609 RFC 6785 and 6 more
- SigComp
Signaling Compression, a scheme for compressing signaling messages such as SIP. It is defined in RFC 3320.
Like I'm five A way of squeezing messages smaller so they travel faster.
- Signaling Gateway
A signaling agent at the edge of an IP network that receives and sends native signaling from a switched circuit network. It may relay, translate or terminate SS7 signaling.
Like I'm five A translator at the door between the phone network and the Internet who passes along the control notes.
- Signaling System 7 (SS7)
A system of signaling protocols used by the public switched telephone network (PSTN) to control call setup, routing, and management. SS7 is separate from the voice traffic itself.
Like I'm five The control signals that tell the telephone network how to connect a phone call, separate from the actual conversation.
- Signed Certificate Timestamp
A digitally signed record, issued by a Certificate Transparency log, that proves a certificate was submitted to the log at a specific time.
Like I'm five A receipt that the log gave you when you added your certificate to the public book.
- SIIT
The Stateless IP/ICMP Translation Algorithm, which converts IPv4 packets to IPv6 and back without keeping per-connection state.
Like I'm five A translator that rewrites each packet's envelope without remembering anything.
- Simultaneous open
A TCP technique where both endpoints try to open a connection to each other at the same time. In ICE TCP it is signaled with the S-O candidate type.
Like I'm five Both sides knock at the same moment and meet in the middle.
- SIP
Session Initiation Protocol. A text-based application-layer protocol for creating, modifying and terminating sessions such as voice and video calls.
Like I'm five The ringing-and-answering part of a phone call.
Used in RFC 1209 RFC 2458 RFC 2543 RFC 2824 RFC 2848 RFC 2871 RFC 2916 RFC 2976 RFC 2995 RFC 3002 RFC 3027 RFC 3050 RFC 3087 RFC 3108 RFC 3204 RFC 3219 RFC 3261 RFC 3262 RFC 3263 RFC 3264 RFC 3265 RFC 3298 RFC 3303 RFC 3310 and 453 more
- SIP dialog
A lasting peer-to-peer SIP relationship between two user agents, such as an ongoing call, identified by Call-ID and the From and To tags.
Like I'm five The whole conversation between two phones, from pick-up to hang-up.
Used in RFC 2543 RFC 3261 RFC 3262 RFC 3265 RFC 3311 RFC 3323 RFC 3326 RFC 3428 RFC 3486 RFC 3515 RFC 3578 RFC 3608 RFC 3665 RFC 3680 RFC 3725 RFC 3840 RFC 3856 RFC 3857 RFC 3891 RFC 3893 RFC 3910 RFC 3911 RFC 3959 RFC 4028 and 84 more
- SIP event package
A concrete use of the SIP event notification framework for one class of events. It names the event and says what subscriptions and notifications carry.
Like I'm five A ready-made topic you can subscribe to, with rules for what the updates look like.
- SIP Info Package
A named definition of what a particular kind of SIP INFO request carries and how it is used, as set out in the INFO framework of RFC 6086.
Like I'm five A label on a note that says what kind of message is inside.
- SIP proxy server
A server that forwards SIP requests and responses on behalf of other user agents, and may apply routing and policy.
Like I'm five A helper in the middle who passes your call along.
Used in RFC 2543 RFC 2824 RFC 3050 RFC 3087 RFC 3261 RFC 3262 RFC 3263 RFC 3265 RFC 3311 RFC 3319 RFC 3323 RFC 3324 RFC 3325 RFC 3326 RFC 3327 RFC 3329 RFC 3361 RFC 3372 RFC 3398 RFC 3428 RFC 3455 RFC 3486 RFC 3487 RFC 3515 and 132 more
- SIP registrar
A server that accepts REGISTER requests and stores where a user can currently be reached.
Like I'm five The office that keeps a list of where everyone is right now.
Used in RFC 2543 RFC 2848 RFC 3261 RFC 3327 RFC 3455 RFC 3515 RFC 3581 RFC 3608 RFC 3665 RFC 3680 RFC 3764 RFC 3840 RFC 3841 RFC 3880 RFC 4083 RFC 4457 RFC 4740 RFC 4780 RFC 5039 RFC 5049 RFC 5360 RFC 5373 RFC 5626 RFC 5627 and 27 more
- SIP Service Provider (SSP)
A carrier or operator that provides VoIP services to customers, using the SIP protocol for call setup and management.
Like I'm five A phone company that lets customers make calls over the internet using SIP.
- SIP transaction
A single request and all the responses to it, up to and including a final response.
Like I'm five One question and all the answers it gets.
Used in RFC 2543 RFC 3261 RFC 3262 RFC 3263 RFC 3265 RFC 3311 RFC 3312 RFC 3428 RFC 3515 RFC 3578 RFC 3581 RFC 3665 RFC 3725 RFC 3891 RFC 3911 RFC 4028 RFC 4168 RFC 4320 RFC 4321 RFC 4475 RFC 5057 RFC 5359 RFC 5393 RFC 5407 and 15 more
- SIP user agent
An endpoint that sends or receives SIP requests, such as a phone or softphone. It acts as a client when it sends a request and as a server when it answers one.
Like I'm five The phone itself.
Used in RFC 2543 RFC 2848 RFC 3261 RFC 3262 RFC 3263 RFC 3265 RFC 3311 RFC 3312 RFC 3323 RFC 3324 RFC 3325 RFC 3327 RFC 3329 RFC 3351 RFC 3372 RFC 3398 RFC 3428 RFC 3486 RFC 3487 RFC 3515 RFC 3524 RFC 3578 RFC 3581 RFC 3608 and 154 more
- SLAAC
Stateless Address Autoconfiguration. A host builds its own IPv6 address from a prefix in a Router Advertisement (with the A flag set) plus an interface identifier, without a DHCP server holding state.
Like I'm five Getting your house address by taking the street name from a sign and adding your own door number.
Used in RFC 2461 RFC 2462 RFC 2464 RFC 2467 RFC 2497 RFC 2529 RFC 2894 RFC 3146 RFC 3315 RFC 3484 RFC 3590 RFC 3756 RFC 3927 RFC 3971 RFC 4242 RFC 4293 RFC 4294 RFC 4338 RFC 4429 RFC 4489 RFC 4861 RFC 4862 RFC 4919 RFC 4941 and 80 more
- SLH-DSA
The Stateless Hash-Based Digital Signature Algorithm, a quantum-resistant signature scheme standardized by NIST in FIPS 205. It was previously known as SPHINCS+.
Like I'm five A way of signing things that is built only from scrambling functions, so it should stay safe even against future quantum computers.
- Sliding window
The most recent stretch of data that a compressor or decompressor can point back into. In DEFLATE it is the previous 32K bytes, and it moves forward as new data is processed.
Like I'm five A short memory of the last few pages you wrote, which moves along as you keep writing.
Used in RFC 700 RFC 714 RFC 813 RFC 935 RFC 998 RFC 1323 RFC 1381 RFC 1663 RFC 1951 RFC 1967 RFC 1974 RFC 1979 RFC 2018 RFC 2118 RFC 2203 RFC 2395 RFC 2402 RFC 2488 RFC 2562 RFC 2637 RFC 2661 RFC 3081 RFC 3095 RFC 3124 and 23 more
- Slotframe
A group of timeslots that repeats over and over in a TSCH network. Each cell in it is a timeslot and channel offset pair that can be scheduled for sending or receiving.
Like I'm five A weekly timetable that starts over every week, telling each device when it may talk.
- Slow Start
The initial phase of a congestion control algorithm, in which the sender raises its sending rate step by step to find how much the path can carry.
Like I'm five Starting a bike ride slowly and pedaling harder bit by bit.
Used in RFC 2914 RFC 3155 RFC 3390 RFC 3465 RFC 3742 RFC 4341 RFC 5681 RFC 5783 RFC 6928 RFC 8337 RFC 9002 RFC 9406 RFC 9959
- Slow-Start Threshold
A TCP control variable that marks the boundary between slow-start (exponential growth) and congestion avoidance (linear growth). It serves as a memory of recent network conditions.
Like I'm five A record of about how fast you could go before, used to help you speed up safely after a slow period.
- Small subgroup attack
An attack on static Diffie-Hellman keys in which a bad public key lets an attacker learn information about the private key. Validating the received public key helps to prevent it.
Like I'm five A trick where someone hands you a rigged number to learn bits of your secret.
- Smart object
A small, resource-constrained device with limited memory, processing power, and battery capacity that connects to the Internet and may sense, process or control physical systems.
Like I'm five A tiny internet-connected device like a smart light bulb or sensor.
- SMI (Structure of Management Information)
A language and framework for defining management information used by SNMP and MIB (Management Information Base) specifications.
Like I'm five A way to describe what network stuff you can check on and control using SNMP.
- SMIv2
The Structure of Management Information Version 2, which defines the data types used by SNMP management information bases. Many YANG types have an equivalent SMIv2 type.
Like I'm five The older rulebook for describing network information, which YANG types can be matched against.
- SMTP
The Simple Mail Transfer Protocol. A sender-SMTP and a receiver-SMTP exchange commands and replies to pass a message and its envelope addresses from one host to the next.
Like I'm five The way mail computers hand letters to each other along the road to your mailbox.
Used in RFC 469 RFC 543 RFC 751 RFC 759 RFC 771 RFC 772 RFC 773 RFC 784 RFC 788 RFC 800 RFC 801 RFC 808 RFC 821 RFC 822 RFC 832 RFC 833 RFC 834 RFC 835 RFC 836 RFC 837 RFC 838 RFC 839 RFC 840 RFC 842 and 245 more
- SMTP AUTH
An SMTP extension, defined in RFC 4954, that lets a client prove who it is to the server before sending mail. RFC 6409 requires submission servers to support it and to require it by default.
Like I'm five Showing your ID to the mail clerk before they accept your letter.
Used in RFC 2476 RFC 2505 RFC 2554 RFC 2645 RFC 2821 RFC 3013 RFC 3207 RFC 3848 RFC 3865 RFC 4405 RFC 4409 RFC 4468 RFC 4616 RFC 4870 RFC 4954 RFC 5068 RFC 5248 RFC 5550 RFC 6409 RFC 6710 RFC 7001 RFC 7601 RFC 8601 RFC 9057 and 1 more
- SMTP reply code
A three-digit number that starts every SMTP server reply. The first digit gives the class: 2 for success, 3 for a request for more input, 4 for a temporary failure and 5 for a permanent failure.
Like I'm five A number the other computer says to tell you yes, keep going, try later or no.
Used in RFC 821 RFC 1211 RFC 1651 RFC 1652 RFC 1653 RFC 1845 RFC 1846 RFC 1861 RFC 1869 RFC 1870 RFC 1893 RFC 1894 RFC 1985 RFC 2033 RFC 2034 RFC 2476 RFC 2505 RFC 2789 RFC 2821 RFC 2852 RFC 2920 RFC 3030 RFC 3207 RFC 3461 and 16 more
- SMTPUTF8
The SMTP extension for internationalized email. It allows UTF-8 characters in the local part of an address and U-labels in the domain.
Like I'm five A way to write email addresses with letters from any alphabet.
- SNA
Systems Network Architecture, IBM's networking architecture for connecting terminals, controllers and hosts.
Like I'm five IBM's own set of rules that its computers and terminals used to talk to each other.
- SNAP (Sub-Network Access Protocol)
A protocol that extends the LLC layer to carry multiple protocol types by including an organization code and protocol type identifier.
Like I'm five A header that says what kind of packet is inside, so the network knows if it is an Internet packet or something else.
- SNAP header
The Sub-Network Access Protocol header, which follows the 802.2 LLC header and carries an organization code and a protocol type such as an EtherType.
Like I'm five A small label on a message that says what kind of message is inside.
- SNMP
The Simple Network Management Protocol. It lets a management program read and set values on a remote device by talking to an agent on it.
Like I'm five A way for an administrator to ask a machine how it is doing and, if needed, tweak its settings from afar.
Used in RFC 1021 RFC 1023 RFC 1052 RFC 1060 RFC 1065 RFC 1066 RFC 1067 RFC 1089 RFC 1098 RFC 1109 RFC 1123 RFC 1147 RFC 1155 RFC 1156 RFC 1157 RFC 1158 RFC 1161 RFC 1162 RFC 1187 RFC 1189 RFC 1207 RFC 1208 RFC 1212 RFC 1213 and 580 more
- SNMP Agent
Software running on a network device that receives SNMP requests from managers and responds with information from the device's MIB.
Like I'm five A helper program on a device that answers a manager's questions and carries out its instructions.
- SNMP context
A collection of management information accessible by an SNMP entity, identified by a contextEngineID and a contextName. The same item of information can exist in more than one context.
Like I'm five A labeled shelf in a big store that tells you which set of items you are asking about.
- SNMP engine
The part of an SNMP entity that sends and receives messages, authenticates and encrypts them, and controls access to managed objects. It is identified by an snmpEngineID.
Like I'm five The machine room of the post office that handles every letter, so the apps do not have to.
- SNMP notification
A message an SNMP entity sends to report that an event has occurred or a condition is present. It is either an unconfirmed trap or a confirmed inform request.
Like I'm five An alarm that a device rings to tell a manager something has happened.
- SNMP trap
An unsolicited notification an SNMP agent sends to a manager when something notable happens, instead of waiting to be polled.
Like I'm five A device tapping you on the shoulder to say something happened, instead of you having to keep asking.
- SNMPv2 Party
A conceptual execution context in SNMPv2 with defined security policies, authentication and privacy protocols, and permissions to access specific managed objects.
Like I'm five A user account with its own security rules for what information it can read or change.
- SNTP
The Simple Network Time Protocol, a simplified way for a client to synchronize its system clock with a time server.
Like I'm five A simple way for a computer to ask another computer what time it is.
- SOA record
The Start of Authority record at the top of a DNS zone. It holds the zone's primary server, contact, serial number and timers, and its last field sets the negative-answer TTL.
Like I'm five The zone's ID card, with a note on how long to trust a "no."
Used in RFC 883 RFC 1033 RFC 1183 RFC 1537 RFC 1713 RFC 1912 RFC 1982 RFC 1995 RFC 1996 RFC 2136 RFC 2181 RFC 2182 RFC 2308 RFC 3007 RFC 5936 RFC 6303 RFC 7477 RFC 7534 RFC 7706 RFC 8020 RFC 8765 RFC 8976 RFC 9103 RFC 9432 and 1 more
- SOAP
Simple Object Access Protocol, a protocol for exchanging structured information in distributed environments using XML as its message format.
Like I'm five A standard way for computers to send messages to each other using XML.
- Socket
An IP address together with a port number, identifying one end of a TCP connection. A connection is identified by the pair of sockets at its two ends.
Like I'm five A building's street address plus the apartment number: together they name exactly one door.
Used in RFC 1 RFC 33 RFC 36 RFC 39 RFC 44 RFC 46 RFC 48 RFC 49 RFC 54 RFC 55 RFC 56 RFC 57 RFC 60 RFC 65 RFC 66 RFC 74 RFC 76 RFC 80 RFC 93 RFC 97 RFC 98 RFC 100 RFC 101 RFC 105 and 124 more
- Soft permanent connection
A type of circuit that combines aspects of permanent and switched connections, with a pre-provisioned permanent segment that can be associated with temporary switched segments dynamically.
Like I'm five A circuit that is partly permanent and partly temporary, like a permanent trunk line connected to temporary access circuits.
- Soft state
State that has a limited lifetime and disappears unless it is refreshed in time. If the owner goes quiet, the state cleans itself up.
Like I'm five A chalk note that fades away unless you rewrite it.
Used in RFC 1102 RFC 1633 RFC 2205 RFC 2961 RFC 3903 RFC 4080 RFC 4094 RFC 4733 RFC 5978 RFC 9176
- Software-Defined Networking
An approach that separates control elements from forwarding components, so software running in a centralized controller can program the devices in the network to behave in specific ways.
Like I'm five One brain decides what all the network's devices should do, and the devices just follow.
- Softwire
A tunnel encapsulating one protocol family within another, for example IPv4 packets within IPv6.
Like I'm five A tunnel through which old-style address mail travels inside new-style envelopes.
Used in RFC 5512 RFC 5565 RFC 5571 RFC 5619 RFC 6674 RFC 6908 RFC 7040 RFC 7596 RFC 7785 RFC 8026 RFC 8513 RFC 8539 RFC 8658 RFC 8676
- Solicited-node multicast address
A link-local multicast address computed from the low-order bits of a unicast address. Neighbor Solicitations for that address are sent to it, so only the few nodes that share those bits need to listen.
Like I'm five A small group chat that only the houses with matching number endings join, so you do not have to bother the whole street.
Used in RFC 2373 RFC 2461 RFC 2462 RFC 2470 RFC 2491 RFC 2529 RFC 3146 RFC 3513 RFC 3590 RFC 4861 RFC 4862 RFC 5798 RFC 6059 RFC 6164 RFC 6820 RFC 6957 RFC 7048 RFC 7219 RFC 8929 RFC 8981 RFC 9131 RFC 10028
- SONET
Synchronous Optical Network, a standard for transmitting data over fiber-optic lines at high speeds, developed in North America.
Like I'm five A very fast way to send data through fiber-optic cables.
Used in RFC 1595 RFC 1619 RFC 2173 RFC 2174 RFC 2823 RFC 3592
- SONET/SDH
Synchronous Optical Network and Synchronous Digital Hierarchy, standards for sending digital data over optical links, which MAPOS uses as its underlying transmission.
Like I'm five Standard fiber-optic lines that carry data very fast.
- Source Active message
An MSDP message that tells peers a source is actively sending to a multicast group, so other domains can learn about it. It is abbreviated SA.
Like I'm five A note passed between networks saying this sender is broadcasting right now.
- Source address spoofing
Forging the source IP address of a packet to impersonate another host or network.
Like I'm five Sending a message and lying about where it came from.
- Source address validation
Checking that the source address of a packet is one that the sender is actually allowed to use. It helps stop address spoofing.
Like I'm five Checking that the return address on a letter really belongs to the person who sent it.
- Source block
A group of consecutive source packets that are protected together as a unit by an FEC scheme, assigned a unique source block number for identification.
Like I'm five A batch of packets grouped together to share the same spare pieces for recovery if any are lost.
- Source filtering
The ability for a listener to ask for multicast traffic only from named senders, or from every sender except named ones.
Like I'm five Choosing to hear only certain speakers, or everyone but a few.
- Source quench
An ICMP message that a gateway or host sends to a datagram's source to ask it to cut back the rate at which it sends traffic to that destination.
Like I'm five A message that says "slow down, you are sending too fast".
- Source route
An IP option in which the sender lists the routers a packet should pass through. It can be strict, where each hop must be adjacent, or loose.
Like I'm five Written directions on a package telling it which towns to pass through.
- Source route bridging
A bridging method in which the sender places the path to the destination, a list of bridge and LAN segment numbers, in a routing information field of the frame.
Like I'm five The sender writes the whole route on the envelope so each bridge knows where to pass it.
- Source Routing
Specifying the path that a message should take through multiple hosts to reach its destination, including intermediate relay points.
Like I'm five Telling the post office the exact route to take to deliver the letter.
Used in RFC 780 RFC 805 RFC 1383 RFC 1454 RFC 1525 RFC 4728 RFC 6554 RFC 7855 RFC 8218 RFC 9914
- Source-specific multicast
A form of multicast where a receiver subscribes to a channel written (S,G): traffic from one specific source S sent to a group address G.
Like I'm five Choosing a show by both the channel and who is making it.
Used in RFC 3678 RFC 3913 RFC 4604 RFC 4607 RFC 4608 RFC 4609 RFC 5132 RFC 5135 RFC 5757 RFC 5760 RFC 5790 RFC 6051 RFC 6128 RFC 6450 RFC 7450 RFC 7667 RFC 8114 RFC 8313 RFC 8777 RFC 8815 RFC 9706 RFC 9777
- Southbound Interface
The API or protocol a controller uses to communicate with and control network devices (switches, routers, etc.). Common examples are OpenFlow and NETCONF.
Like I'm five The communication link from the brain (controller) down to the arms and legs (network devices) telling them what to do.
- Spanning Tree
The Layer 2 protocol used by 802.1 bridges to avoid loops by blocking redundant ports, so frames follow a single tree of active links.
Like I'm five A way of closing extra roads so there is only one route between any two places.
- Spanning Tree Protocol
A bridge protocol that forms a tree over connected bridges by blocking some links, so that forwarding loops are avoided.
Like I'm five A way for a web of roads to close just enough streets so cars cannot drive in circles.
Used in RFC 1220 RFC 1493 RFC 1638 RFC 3518 RFC 4188 RFC 4318 RFC 5171 RFC 7727
- Special-use domain name
A domain name reserved for a particular purpose, which software may handle differently from ordinary names. The name resolver.arpa is one example.
Like I'm five A pretend address that always means one particular thing.
- Special-use mailbox
An IMAP mailbox with an attribute that marks its purpose, such as drafts, sent messages or junk mail. Clients find it by attribute rather than by name.
Like I'm five A folder with a label saying what it is for, like Junk or Sent.
- SPF
Sender Policy Framework. A way for a domain to say which hosts may send mail using it, checked against the SMTP MailFrom or HELO/EHLO domain.
Like I'm five A list a domain posts of which post offices may send mail for it.
Used in RFC 6652 RFC 6686 RFC 7960 RFC 8601 RFC 8617 RFC 9989
- SPF Record
A DNS TXT or SPF record published by a domain that lists the IP addresses and hostnames of mail servers authorized to send mail from that domain.
Like I'm five A list in DNS that says which mail servers a company allows.
- SPI
Security Parameter Index. A number that identifies a particular Security Association. IKE headers carry two 8-byte SPIs, and ESP and AH use 4-byte SPIs.
Like I'm five A ticket number that says which agreement a packet belongs to.
Used in RFC 1829 RFC 1851 RFC 2006 RFC 2085 RFC 2205 RFC 2207 RFC 2332 RFC 2367 RFC 2401 RFC 2402 RFC 2406 RFC 2521 RFC 2522 RFC 3102 RFC 3173 RFC 3554 RFC 3585 RFC 3715 RFC 3753 RFC 3957 RFC 4004 RFC 4106 RFC 4110 RFC 4285 and 29 more
- SPKI pin
A fingerprint, usually a SHA-256 hash, of a server's Subject Public Key Info. A client that holds a set of pins accepts only servers whose keys match one of them.
Like I'm five A photo of the right helper's face that you check before you trust them.
- Split horizon
A rule that a router does not advertise a route back out the interface from which it learned it. With poisoned reverse it does advertise it, but with an infinite metric (16 in RIP).
Like I'm five Do not tell someone the way they just told you.
Used in RFC 1058 RFC 1075 RFC 1582 RFC 1918 RFC 2080 RFC 2091 RFC 2453 RFC 4762 RFC 6147 RFC 6368 RFC 6550 RFC 6957 RFC 7432 RFC 7623 RFC 7782 RFC 8317 RFC 8361 RFC 8365 RFC 8388 RFC 8560 RFC 8598 RFC 8695 RFC 9014 RFC 9464 and 8 more
- Split MAC
A CAPWAP mode in which the 802.11 MAC functions are divided between the WTP and the AC. In Local MAC mode, by contrast, the WTP handles encryption itself.
Like I'm five Sharing the Wi-Fi jobs between the access point and its boss box.
- Split tunnel
A VPN setup that sends only traffic for chosen destinations through the tunnel. All other traffic goes out directly and is not carried by the VPN.
Like I'm five Only the packages for the office go in the company van, and everything else goes by regular mail.
- Sponsoring client
The registry client, usually a registrar, that is responsible for an object and is allowed to change it.
Like I'm five The company that looks after an entry and is allowed to edit it.
- Spurious Timeout
A retransmission timeout that fires even though the original data was not actually lost, only delayed in transit.
Like I'm five Resending a message that was never lost, just running late.
- SR Policy
An ordered list of segments that represents a source-routed policy, identified by headend, color and endpoint. It holds one or more candidate paths, of which one is active.
Like I'm five A named travel plan that tells a router which stops to write on the package.
- SRGB
The Segment Routing Global Block, a block of labels configured on a node and used for global segments. A node's block is a local property and can differ from other nodes.
Like I'm five The range of numbers a town sets aside for its signposts.
- SRTP
The Secure Real-time Transport Protocol. A profile of RTP that encrypts and authenticates media packets and protects them against replay.
Like I'm five RTP with a lock and a tamper-proof seal on every packet.
Used in RFC 3323 RFC 3550 RFC 3711 RFC 3830 RFC 3951 RFC 3959 RFC 4040 RFC 4046 RFC 4102 RFC 4103 RFC 4170 RFC 4184 RFC 4348 RFC 4352 RFC 4383 RFC 4396 RFC 4425 RFC 4442 RFC 4563 RFC 4566 RFC 4567 RFC 4568 RFC 4573 RFC 4587 and 135 more
- SRV record
A DNS record type (RFC 2782) that tells a client which hosts and ports provide a given service for a domain, with priorities and weights. XMPP uses
_xmpp-client._tcpand_xmpp-server._tcp.Like I'm five A phone-book entry that says which number to call for a particular service.
Used in RFC 2168 RFC 2219 RFC 2782 RFC 3088 RFC 3123 RFC 3263 RFC 3319 RFC 3361 RFC 3403 RFC 3404 RFC 3486 RFC 3529 RFC 3596 RFC 3620 RFC 3832 RFC 3861 RFC 3887 RFC 3920 RFC 3958 RFC 4038 RFC 4077 RFC 4085 RFC 4168 RFC 4227 and 57 more
- SRv6 locator
A prefix configured on a node that covers the set of SRv6 SIDs provisioned there. It is advertised as a Prefix NLRI with the SRv6 Locator TLV.
Like I'm five A street name that covers all the house numbers on it.
- SRv6 SID
A 128-bit identifier for a segment in Segment Routing over IPv6. It has locator, function and argument parts, and a behavior is bound to it on the node that owns it.
Like I'm five A special address that tells a router to do one particular job.
- SSH
Secure Shell, a protocol for secure remote login and other secure network services over an insecure network. It combines a transport layer, user authentication and a connection layer that carries channels.
Like I'm five A locked, soundproof tube for talking to a faraway computer.
Used in RFC 2179 RFC 2504 RFC 3535 RFC 3871 RFC 4111 RFC 4250 RFC 4251 RFC 4252 RFC 4253 RFC 4255 RFC 4256 RFC 4335 RFC 4344 RFC 4345 RFC 4419 RFC 4432 RFC 4462 RFC 4716 RFC 4742 RFC 4819 RFC 5114 RFC 5277 RFC 5590 RFC 5591 and 59 more
- SSH channel
A numbered, flow-controlled stream inside an SSH connection. Shells, commands, X11 windows and forwarded TCP connections each get their own channel, and many channels share one encrypted connection.
Like I'm five One of many separate conversations that share a single private phone line.
Used in RFC 4250 RFC 4254 RFC 4256 RFC 4335 RFC 4742 RFC 4819 RFC 5592 RFC 6242 RFC 8160 RFC 8308 RFC 9987
- SSH subsystem
A predefined service with a host-independent name that a server can start on a session channel, such as a file transfer service, instead of a shell or an arbitrary command.
Like I'm five A special helper room on the server that you ask for by name instead of walking into the general lounge.
Used in RFC 3411 RFC 4254 RFC 4742 RFC 4819 RFC 5592 RFC 6242
- SSH_MSG_KEXINIT
The SSH message, number 20, with which each side starts key exchange. It carries a 16-byte random cookie and comma-separated, preference-ordered lists of the key exchange, host key, cipher, MAC and compression algorithms the sender supports.
Like I'm five Each computer hands over its list of favourite secret codes so they can pick one they both know.
- SSRC
Synchronization source identifier. A 32-bit number in each RTP header that names the sender of a stream. SRTP mixes it into the encryption IV.
Like I'm five A number that says which speaker a packet came from.
Used in RFC 1889 RFC 2508 RFC 2658 RFC 2762 RFC 2862 RFC 2959 RFC 3158 RFC 3550 RFC 3611 RFC 3711 RFC 4568 RFC 4571 RFC 4585 RFC 4588 RFC 4695 RFC 4696 RFC 5104 RFC 5109 RFC 5135 RFC 5404 RFC 5506 RFC 5576 RFC 5725 RFC 5760 and 78 more
- Stable identifier
Any property a client discloses that does not change over time, or changes very infrequently, and is unique for that client in a given context. A MAC address, a client-id and a hostname are examples.
Like I'm five Something about your computer that stays the same, so people can recognize it again and again.
- Stale Route
A route that a Receiving Speaker has kept from a peer whose session ended but has not yet deleted. It is still used for forwarding until the peer returns or the stale timer runs out.
Like I'm five An old direction you keep using until you get a fresh one.
- Standard (Internet Standard)
A protocol specification that has completed the Internet standardization process and is recommended for use on the Internet.
Like I'm five A rule that the Internet community has approved and everyone should use.
- Standardization process
The multi-stage procedure by which the IETF moves a technical specification from Proposed Standard through Draft Standard to Internet Standard, based on implementation experience and consensus.
Like I'm five The steps Internet rules go through to move from experiment to official standard.
- Standards Development Organization
An international, open, voluntary organization that develops technical standards and specifications for use on the public Internet, such as the IETF, ITU, W3C, or ETSI.
Like I'm five A group of people who write down the rules for how Internet technology should work.
- Standards track
The path of maturity levels a specification follows on the way to becoming an Internet Standard.
Like I'm five The staircase an idea climbs to become an official rule.
Used in RFC 72 RFC 804 RFC 1007 RFC 1100 RFC 1120 RFC 1140 RFC 1199 RFC 1200 RFC 1250 RFC 1264 RFC 1280 RFC 1310 RFC 1311 RFC 1360 RFC 1396 RFC 1399 RFC 1410 RFC 1500 RFC 1540 RFC 1599 RFC 1600 RFC 1601 RFC 1602 RFC 1610 and 152 more
- Stanza
A top-level XML element sent over an established XMPP stream. There are three kinds: message, presence and iq.
Like I'm five One note passed over the connection.
Used in RFC 3920 RFC 3921 RFC 3922 RFC 3923 RFC 5122 RFC 5437 RFC 6120 RFC 6121 RFC 7395 RFC 7572 RFC 7702 RFC 8600
- STAR certificate
A Short-Term, Automatically Renewed X.509 certificate. It has a short lifetime and is renewed automatically through ACME instead of being revoked when it is no longer needed.
Like I'm five A badge that expires quickly but gets replaced with a new one by itself.
- STARTTLS
A command that upgrades an existing, unencrypted connection to TLS in place, instead of using a separate port for encrypted traffic. In XMPP it is the
<starttls/>element.Like I'm five Asking to lock the door after you have already knocked and been let in.
Used in RFC 821 RFC 2222 RFC 2595 RFC 2829 RFC 2830 RFC 3062 RFC 3207 RFC 3501 RFC 3502 RFC 3656 RFC 3788 RFC 3848 RFC 3887 RFC 3909 RFC 3920 RFC 4217 RFC 4261 RFC 4314 RFC 4409 RFC 4422 RFC 4468 RFC 4511 RFC 4513 RFC 4521 and 50 more
Data the SCTP server puts in its INIT ACK so it need not keep any state until the client echoes it back in COOKIE ECHO. This protects against SYN-flood-style attacks.
Like I'm five A claim ticket the server hands out, so it only has to remember you once you show it back.
Used in RFC 2960 RFC 4732 RFC 4895 RFC 4960 RFC 6013 RFC 9260
- Stateful PCE
A path computation element that remembers the label switched paths in the network, so messages can refer to a tunnel by an identifier instead of describing it again.
Like I'm five A route planner that remembers every route it has already drawn.
- Stateid
An NFSv4 identifier the server returns to the client to reference the state of a file, such as an open or a delegation.
Like I'm five A ticket that proves what you are allowed to do with a file.
Used in RFC 3010 RFC 3530 RFC 5661 RFC 7530 RFC 7531 RFC 7862 RFC 7931 RFC 8881 RFC 9754
- Stateless Address Autoconfiguration
A method for IPv6 hosts to automatically configure their own addresses using information from routers, without requiring a DHCP server.
Like I'm five Hosts picking their own addresses by listening to router announcements.
- Stateless protocol
A protocol in which each request is handled on its own, and the server keeps no memory of earlier requests from the same client. HTTP is stateless, so sessions have to be built on top.
Like I'm five A clerk who forgets you the moment you walk away from the counter.
Used in RFC 1094 RFC 1305 RFC 1436 RFC 1546 RFC 1769 RFC 1813 RFC 1945 RFC 2056 RFC 2639 RFC 2765 RFC 2965 RFC 3041 RFC 3091 RFC 3181 RFC 3263 RFC 3320 RFC 3321 RFC 4241 RFC 5969 RFC 6026 RFC 6434 RFC 6436 RFC 6437 RFC 6503 and 18 more
- Stateless translation
A translation method that needs no per-connection table. It works out the result from the packet itself and from pre-configured information in the translator.
Like I'm five A translator that follows a fixed rule and does not keep a notebook of conversations.
- Static dictionary
A fixed list of common strings that a compressor and decompressor both already know. It lets even the first message be compressed against it.
Like I'm five A shared cheat sheet of common words that never changes.
- Steering Tag (STag)
An identifier of a Tagged Buffer on a node. A peer advertises it, together with an offset and length, so the remote side can place data directly into that buffer.
Like I'm five A label on a shelf that tells the sender exactly where to put things.
- STIR
Secure Telephone Identity Revisited, the IETF framework for cryptographically protecting the identity of callers in real-time communications such as SIP calls.
Like I'm five A way to prove a phone caller is who they say.
- Storage area network
A dedicated network that connects servers to storage devices such as disks and tape drives. Its job is to give servers fast access to shared storage.
Like I'm five A private road system just for computers to reach their big shelves of saved files.
- Store and forward
A delivery method in which each host keeps a complete copy of the message and passes it on to the next host, so the sender and the final recipient do not need to be online at the same time.
Like I'm five Each friend in a line keeps your note safe until they can hand it to the next friend.
Used in RFC 187 RFC 821 RFC 1935 RFC 2542 RFC 2821 RFC 3528 RFC 3997 RFC 4141 RFC 4838 RFC 5050 RFC 5546 RFC 5598 RFC 6257 RFC 7242 RFC 8238 RFC 9052 RFC 9171 RFC 9172 RFC 9174 RFC 9265 RFC 10036
- Stratum
A server's distance, in NTP hops, from a reference clock. Stratum 1 servers are synchronized directly to a reference clock; each step down adds one, up to 15, and 16 means unsynchronized.
Like I'm five How many steps away you are from the boss clock.
Used in RFC 1059 RFC 1305 RFC 1769 RFC 2030 RFC 4149 RFC 4330 RFC 5905 RFC 5906 RFC 5907 RFC 7384 RFC 8633 RFC 9249 RFC 9327 RFC 9769
- Stream (HTTP/2)
An independent, bidirectional sequence of frames inside an HTTP/2 connection, identified by a 31-bit integer. Client-initiated streams are odd-numbered, server-initiated even.
Like I'm five One conversation among many that share the same pipe.
- Stream cipher
A cipher that encrypts data by combining it with a continuous stream of pseudorandom bits called a keystream. RC4 is an example.
Like I'm five A way of scrambling a message by mixing it with a long string of random-looking noise, one piece at a time.
Used in RFC 2628 RFC 3686 RFC 4503 RFC 6229 RFC 7008 RFC 7465 RFC 7539 RFC 8439 RFC 8758
- Stream multiplexing
Sending frames from many streams interleaved on a single connection, so that many requests and responses are in progress at once.
Like I'm five Many conversations taking turns on one phone line.
Used in RFC 4666 RFC 6084 RFC 8441 RFC 8859 RFC 9113 RFC 9297 RFC 9308 RFC 9621 RFC 9622
- Stretch ACK
A TCP acknowledgment that covers more than two segments of previously unacknowledged data. It can happen by design, because of implementation bugs, or when ACKs are lost.
Like I'm five One thank-you note that says thanks for lots of packages at once.
- String-to-key (S2K)
A method that converts a passphrase into a symmetric key. OpenPGP defines several, and recommends Argon2 because it is memory hard.
Like I'm five A recipe that turns your password into a proper key for a lock.
- Stringprep
An older technology for preparing and comparing non-ASCII strings, defined in RFC 3454. XMPP's Nodeprep and Resourceprep were profiles of it.
Like I'm five The old spelling checker that this document replaces.
Used in RFC 3454 RFC 3722 RFC 4013 RFC 4518 RFC 4690 RFC 6122 RFC 6885 RFC 7564 RFC 7622
- Striping
Spreading a file across several storage components in fixed-size pieces, taken in turn. It allows several devices to serve one file in parallel.
Like I'm five Dealing a file out like cards, one piece to each box in turn.
- Structured Data
A part of a syslog message made of named elements, each with a set of name and value parameters. It lets programs attach machine-readable data to a message.
Like I'm five A neat labeled box of facts attached to a message, made for computers to read.
- Structured Field Values
A common syntax for HTTP header fields, defined in RFC 8941, built from types such as List, String, Token, Integer and Boolean. Cache-Status uses it.
Like I'm five A shared set of rules for writing header values so every program reads them the same way.
- Structured syntax suffix
A suffix on a media type name, such as +cbor-seq, that signals the type is built on another media type as its foundation.
Like I'm five A tag at the end of a name that tells you what the thing is made from.
- Stub area
An area into which AS external advertisements are not flooded. Routing to outside destinations uses a default route injected by an area border router.
Like I'm five A quiet neighborhood where everything outside town is just reached by following the one main exit sign.
- Stub Gateway
A gateway that is the only link between a local network and the rest of the internet. It is not meant to carry traffic that neither starts nor ends in that local network.
Like I'm five A small town's only gate to the outside world.
- Stub resolver
The small DNS client built into a device or operating system. It does not look names up itself; it sends queries to a recursive resolver and uses the answer.
Like I'm five A kid who asks a grown-up to look something up instead of searching the whole library alone.
Used in RFC 1034 RFC 1536 RFC 2845 RFC 3833 RFC 4033 RFC 4035 RFC 4074 RFC 4697 RFC 4986 RFC 5358 RFC 5452 RFC 5625 RFC 6055 RFC 6147 RFC 6731 RFC 6975 RFC 7706 RFC 7849 RFC 7858 RFC 7871 RFC 8027 RFC 8094 RFC 8310 RFC 8375 and 15 more
- Sub-TLV
A TLV nested inside another TLV to add extra information to that one TLV. Receivers ignore and skip sub-TLVs they do not know.
Like I'm five A small labeled box packed inside a bigger labeled box.
- Subdomain
A domain name that sits below another one, such as www.example.com or mail.example.com under example.com.
Like I'm five A smaller shop on the same street as the main shop, sharing its name.
Used in RFC 881 RFC 882 RFC 920 RFC 921 RFC 1032 RFC 1480 RFC 1530 RFC 1535 RFC 1591 RFC 1816 RFC 2146 RFC 2240 RFC 2352 RFC 2377 RFC 3982 RFC 6763 RFC 6797 RFC 7469 RFC 7489 RFC 9272 RFC 9444 RFC 9965 RFC 9990
- Subflow
One TCP connection over a single path, used as a part of a Multipath TCP connection. A Multipath TCP connection can have several subflows.
Like I'm five One of the several roads that a single delivery is split across.
Used in RFC 6182 RFC 6356 RFC 6897 RFC 8041 RFC 8684 RFC 9897
- subjectAltName
An X.509 certificate extension that lists the identities the certificate is valid for, such as dNSName host names or iPAddress addresses. RFC 2818 says a dNSName here must be used for the host name check if present.
Like I'm five A list on the name tag of every name the owner answers to.
Used in RFC 2818 RFC 3923 RFC 4043 RFC 4683 RFC 4945 RFC 4976 RFC 4985 RFC 5734 RFC 5922 RFC 6047 RFC 6125 RFC 6187 RFC 6614 RFC 7817 RFC 8310 RFC 9115 RFC 9289 RFC 9310 RFC 9462 RFC 9525 RFC 9549 RFC 9598 RFC 9761 RFC 9799 and 3 more
- Subkey
A key derived from a main secret key rather than chosen separately. AES-CMAC derives two subkeys, K1 and K2, from its one key.
Like I'm five A smaller secret key made from the big secret key.
- Subnamespace IDentifier
The component of a CCSDS URN that categorizes the type of resource (such as "document" or "schema"). Each SNID has its own assignment rules defined by a CCSDS document.
Like I'm five A category label that says what kind of thing the URN points to.
- Subnegotiation
A richer exchange, framed by the Telnet commands SB (250) and SE (240), that carries the details of an option once both sides have agreed with DO/WILL that they understand it.
Like I'm five After both agree to use a feature, a longer chat to settle the small details, like how wide the screen is.
Used in RFC 562 RFC 580 RFC 651 RFC 653 RFC 654 RFC 655 RFC 656 RFC 657 RFC 726 RFC 732 RFC 749 RFC 764 RFC 779 RFC 854 RFC 855 RFC 859 RFC 861 RFC 1041 RFC 1043 RFC 1073 RFC 1079 RFC 1091 RFC 1096 RFC 1184 and 7 more
- Subnet
One of several physical networks, such as LAN cables, that share a single Internet network number. A subnet is identified by a subnet number carried in part of the host bits of the address (RFC 950).
Like I'm five One floor of a big building that has a single street address.
Used in RFC 917 RFC 919 RFC 922 RFC 925 RFC 932 RFC 936 RFC 940 RFC 950 RFC 985 RFC 1009 RFC 1027 RFC 1118 RFC 1209 RFC 1219 RFC 1256 RFC 1366 RFC 1466 RFC 1709 RFC 1724 RFC 1797 RFC 1878 RFC 1879 RFC 1937 RFC 2071 and 42 more
- Subnet mask
A 32-bit value with bits set for the network number and subnet number of an address (the RFC calls it the address mask). A host ANDs it with two addresses to see whether they are on the same subnet. Written 255.255.255.0 or as a prefix length such as /24.
Like I'm five A stencil that shows which part of an address is the neighborhood and which part is the house.
Used in RFC 917 RFC 940 RFC 950 RFC 985 RFC 1195 RFC 1219 RFC 1338 RFC 1395 RFC 1533 RFC 1721 RFC 1722 RFC 1723 RFC 1860 RFC 1878 RFC 2096 RFC 2131 RFC 2226 RFC 2453 RFC 3011 RFC 3021 RFC 3442 RFC 3527 RFC 3531 RFC 4183 and 3 more
- Subnet-Router anycast address
An IPv6 address, formed from the subnet prefix with all zeros in the rest, that reaches any one router on that link. It is defined in RFC 4291.
Like I'm five A shout of "any grown-up here?" that whichever router is nearest will answer.
- Subscription
A request from one user to receive notifications of another user's presence status changes.
Like I'm five Asking to be notified when someone comes online.
- Subsequent Address Family Identifier
A value used with BGP multiprotocol extensions to identify a specific type of network layer reachability information, such as unicast routes, multicast routes, or VPN routes.
Like I'm five A number that tells BGP what kind of route information is being sent.
- Suite B
A set of public cryptographic algorithms for key exchange, digital signatures, hashing and encryption, announced by the US National Security Agency in 2005. It was chosen from algorithms approved by NIST.
Like I'm five A short, approved shopping list of code-making recipes.
- Supplemental data
Application-level information sent inside a TLS handshake message. TLS itself does not process it; it is meant for the protocols and applications above TLS.
Like I'm five An extra note slipped into the handshake for the application, not for the secure connection itself.
- Supplicant
The IEEE 802.1X name for the end of a link that responds to the authenticator and asks for network access. EAP calls the same role the peer.
Like I'm five The visitor who has to show an ID to get in.
- Surrogate
A caching server in a content network that stores copies of content and serves requests from clients, reducing the load on the origin server.
Like I'm five A copy of a website stored on a server near users so it loads faster.
Used in RFC 3040 RFC 3466 RFC 3507 RFC 3568 RFC 3570 RFC 6707 RFC 7337 RFC 7937 RFC 8006
- Surrogate pair
Two UTF-16 code units that together represent a character outside the Basic Multilingual Plane. In JSON strings such a character can be written as two \uXXXX escapes.
Like I'm five Two small pieces that together make one big letter, like an emoji.
Used in RFC 2781 RFC 3629 RFC 5137 RFC 7159 RFC 7493 RFC 8259 RFC 9839
- SVCB record
A DNS record type, Service Binding, that tells a client how to reach a service, such as which protocols, ports and alternate names to use.
Like I'm five A business card in the phone book that says which door and which language to use.
- Swarm
A group of peers participating in the distribution of the same streaming content at a given time.
Like I'm five All the people watching and sharing the same movie right now.
- SWID tag
A Software Identification tag is a record that identifies and describes a software component, patch or installation bundle. It is defined in ISO/IEC 19770-2:2015 as XML.
Like I'm five A label that says exactly which software you have.
- Switching provider edge (S-PE)
A provider edge router in the middle of a multi-segment pseudowire that switches the control and data planes of the segment before it to the segment after it.
Like I'm five The connector that joins two pieces of a long wire.
- Sybil attack
An attack in which one person creates many fake identities in a network to gain disproportionate control or influence.
Like I'm five Pretending to be a thousand different people to vote a thousand times.
- Symmetric encryption
Encryption where the same secret key is used both to encrypt and decrypt; the sender and receiver must share the key beforehand.
Like I'm five Both people have the same secret code to lock and unlock messages.
- Symmetric NAT
A NAT that may map the same internal address and port to a different external address and port for each destination. It accepts packets only from hosts the internal host has already sent to.
Like I'm five A router that gives you a new door number for every person you call.
- SYN flood
A denial-of-service attack that sends many TCP SYN packets to make a server hold state for half-open connections until it has no room left for real ones.
Like I'm five Knocking on a door so many times that the person inside cannot answer real visitors.
- Synonymous Flow Label
An MPLS label that causes the same behavior at the egress router as the label it replaces, but also triggers extra actions agreed in advance, such as counting packets.
Like I'm five A nickname for a label that works the same but also makes the receiver do an extra job, like tallying.