Glossary
F
75 terms.
- Facsimile
A technology for transmitting documents over telephone lines, reproducing them on paper at the receiving end. G3-facsimile is a standardized group of devices defined for use on standard telephone networks.
Like I'm five A fax machine reads a paper document and converts it to signals you can send over a phone line, where another fax machine prints it out.
- Failover
Switching to a backup server or connection when the primary one stops responding, so service continues.
Like I'm five Using the spare key when the first one breaks.
- Fast handover
A set of mobility extensions that reduce the delay when a mobile node moves between access routers. The previous router passes context to the new router ahead of or just after the move.
Like I'm five Telling the next stop about a traveler before they arrive, so there is no waiting at the door.
- Fast Recovery
The part of TCP congestion control that reduces the congestion window after packet loss is detected, using feedback from returning acknowledgments to decide when to send more data.
Like I'm five How a sender slows down and then catches up after it notices something got lost.
- Fast Reroute
Precomputing a backup path so traffic can switch to it immediately when a link or node fails, instead of waiting for the network to recompute routes.
Like I'm five Having a spare route ready so you switch the second the road is blocked.
Used in RFC 5714 RFC 5824 RFC 6894 RFC 7490 RFC 7812 RFC 7855 RFC 8271 RFC 8320 RFC 8333 RFC 8355 RFC 8424 RFC 8537 RFC 9855
- Fast Retransmit
A TCP algorithm that resends a segment after three duplicate ACKs arrive, instead of waiting for the retransmission timeout to expire.
Like I'm five Resending the missing piece as soon as you hear the same reminder three times.
- Fat Tree
A folded spine-and-leaf network topology, also called a Clos, that may have several Points of Delivery and one or more Top of Fabric planes. RIFT is designed for such networks.
Like I'm five A network built in layers, with many small switches at the bottom linking up to a few big ones at the top.
- Fate sharing
Keeping the state of a conversation only in the endpoints, so it can be lost only if an endpoint itself fails. RFC 1958 uses this to explain why datagrams are better than classical virtual circuits.
Like I'm five Your notes about a chat are kept by the two people chatting, so they are lost only if one of them leaves.
Used in RFC 1144 RFC 1958 RFC 2775 RFC 3234 RFC 3439 RFC 3724 RFC 5505 RFC 5534 RFC 5659 RFC 5860 RFC 5880 RFC 5882 RFC 5921 RFC 6182 RFC 6372 RFC 6897 RFC 7211 RFC 7276 RFC 7926 RFC 8800 RFC 9455 RFC 9516 RFC 10014
- FDDI (Fiber Distributed Data Interface)
A high-speed local area network standard using fiber-optic cables in a token-passing ring topology, providing speeds of 100 Mbps.
Like I'm five A very fast network that uses glass cables arranged in a ring, with a special token passed around to control who gets to send data.
- Feasibility condition
A test a router applies to a received route update. The update is accepted only if it can be shown that accepting it cannot cause a routing loop.
Like I'm five A rule that says only trust a road tip if it cannot possibly lead you back to where you started.
- Feature Interaction
Unexpected or ambiguous behavior that arises when two or more telephone features are combined, such as 'call forwarding on busy' combined with 'call waiting.'
Like I'm five When two phone features do not work well together and confuse the phone system (or you).
- Feature tag
A name that identifies one feature of a device or resource, such as the SIP methods it supports. In SIP, feature tags are carried as Contact header field parameters.
Like I'm five A small label that says one thing a device can do or be.
- Federated authentication
An arrangement in which several servers accept the same credential, so a client does not need a separate login for each one. Examples named include OAuth, OpenID, SAML and CA-based certificates.
Like I'm five One ID card that is accepted at many different doors.
- Federation
A group of independent organizations that have agreed to trust each other's identity information using a common standard like SAML.
Like I'm five A group of schools or organizations that trust each other.
- Feedback implosion
The flood of replies that can overwhelm a sender and the network when a large group of receivers all send feedback at the same time.
Like I'm five Everyone in a stadium shouting an answer at once, so nobody can be heard.
- Feedback suppression
A way to keep many receivers from all answering at once. Each waits a random time and stays silent if it hears that someone else already sent the same request.
Like I'm five If a classmate already asked your question, you do not need to ask it again.
- Feistel cipher
A cipher built from repeated rounds in which half of the data is combined with the output of a round function applied to the other half. ARIA uses a 3-round Feistel cipher inside its key schedule.
Like I'm five A scrambling recipe that repeats the same mixing step over and over, swapping sides each time.
- Feistel network
A cipher design that splits a block into two halves and repeats rounds in which one half is XORed with a function of the other half and a subkey. The same structure can be run backwards to decrypt.
Like I'm five Two friends take turns scrambling each other's half of a note, and the same steps in reverse unscramble it.
- Fencing
The process by which a metadata server stops storage devices from processing I/O from a specific client to a specific file. It is used when a layout is revoked.
Like I'm five Putting up a fence so one visitor can no longer touch a particular file.
- Fibre Channel
A family of protocols used for high-speed storage networking, typically in Storage Area Networks (SANs).
Like I'm five A fast way to move data between servers and storage devices.
Used in RFC 2625 RFC 3821 RFC 3831 RFC 4044 RFC 4172 RFC 4338 RFC 4369 RFC 4595 RFC 4625 RFC 4626 RFC 5324 RFC 6847
- File handle
An opaque value chosen by an NFS server to identify a file or directory. In NFSv4.0 it can be at most 128 bytes long.
Like I'm five A label the server gives a file so you can ask for it again without using its name.
- Filehandle
An opaque, server-assigned identifier for a file or directory. Clients store and send it back but must not interpret its contents.
Like I'm five A ticket number the server gave you so it can find your file again.
Used in RFC 2055 RFC 2224 RFC 3010 RFC 3530 RFC 7530 RFC 8881
- Fileset
A set of files together with the directory tree that contains them. It is the basic unit of data management in a federated file system.
Like I'm five One folder of files with everything inside it, treated as a single package.
- Filtering Database
A table maintained by a bridge that maps learned MAC addresses to the ports on which they were seen, used to decide which port to forward each incoming frame to.
Like I'm five The bridge's memory of where each computer's address lives on the network.
- Fingerprint
A short value computed by hashing a piece of data, used to identify that data without sending all of it. If the data changes, the fingerprint changes too.
Like I'm five A short nickname made from a big document, so you can point at it without carrying it.
- Finite field
A mathematical structure used in network coding where addition and multiplication are defined with a limited set of elements, enabling efficient computation on computers.
Like I'm five A special system of math with a limited set of numbers where addition and multiplication wrap around, like arithmetic modulo a prime number.
- Finite State Entropy
An entropy codec based on Asymmetric Numeral Systems that uses a state value to track position in a probability distribution table, enabling efficient compression of symbols with non-uniform frequencies.
Like I'm five A clever math trick that uses a state machine to compress symbols efficiently.
- Finite-state machine
A model with a fixed set of states, where events such as receiving a packet or a timeout cause actions and a move to a new state. PPP uses one to negotiate options.
Like I'm five A traffic light that decides what to do next based on which colour it is and what just happened.
Used in RFC 689 RFC 904 RFC 908 RFC 916 RFC 934 RFC 963 RFC 964 RFC 998 RFC 1267 RFC 1307 RFC 1331 RFC 1434 RFC 1541 RFC 1548 RFC 1661 RFC 1771 RFC 1934 RFC 1987 RFC 2281 RFC 2360 RFC 2490 RFC 2814 RFC 2909 RFC 3083 and 50 more
- Firewall
A gate between two networks that limits which machines on one side may contact machines on the other.
Like I'm five A locked gate that only lets approved visitors through.
Used in RFC 1636 RFC 1678 RFC 1775 RFC 1928 RFC 2196 RFC 2356 RFC 2647 RFC 2979 RFC 3303 RFC 3514 RFC 3620 RFC 6887 RFC 7754 RFC 8329
- First Come First Served
An IANA allocation procedure where values are assigned to requesters in the order the requests are received, without review, as long as they meet basic requirements.
Like I'm five Whoever asks first gets the next number.
- Five-tuple
The five values that identify one transport-protocol instance: protocol, source address, source port, destination address and destination port.
Like I'm five The five facts that together say exactly which conversation a message belongs to.
Used in RFC 2693 RFC 5766 RFC 6056 RFC 7675 RFC 8016 RFC 8859
- Flag Day
A scheduled or unscheduled event in which a system or network is changed suddenly, typically requiring a service outage or planned downtime.
Like I'm five A day when everything changes at once and services stop working while the change happens.
- Flash renumbering
An event where a prefix in use on a network suddenly becomes invalid and is replaced by a new prefix, without the planned phase-out of the old one.
Like I'm five When your street suddenly gets a new name overnight and nobody was warned.
- Flexible Algorithm
A way for a link-state protocol to compute paths for a custom topology, chosen by rules such as link colors, alongside the default computation. It is described in RFC 9350.
Like I'm five A second set of directions that only uses certain roads picked for a special purpose.
- Floating time
A DATE-TIME with no Z suffix and no TZID, which means the same wall-clock time in whatever zone the viewer is in.
Like I'm five "Lunch at noon" wherever you happen to be.
- Flooding
The network behavior of sending a packet to all switches in a VLAN when the destination MAC address is unknown or the destination is a broadcast address.
Like I'm five Sending the same message to everyone because you do not know who needs it.
- Flooding scope
The set of routers or circuits that a link state PDU is flooded to. IS-IS originally had area (Level 1) and Level 2 subdomain scopes.
Like I'm five Who gets to hear an announcement: one room, one floor, or the whole school.
- Floor control
A mechanism that gives applications or users safe, mutually exclusive or shared access to a resource in a conference. A floor is the temporary permission to use that resource.
Like I'm five Taking turns so only the right person uses the microphone.
- Flow
A sequence of packets with a common source, destination, and purpose, such as all packets in a TCP connection or UDP stream.
Like I'm five All the packets going between the same two machines or addresses for the same reason.
- Flow cache
A stateful data structure in intermediate devices that remembers the classification (encrypted vs. ESP-NULL) of each IPsec security association, so heuristics need only run once per flow.
Like I'm five A notebook that remembers which conversations are secret and which ones aren't.
- Flow control
A way for a receiver to limit how much data a sender may send before it must wait. HTTP/2 uses credit-based windows, per stream and for the whole connection, topped up with WINDOW_UPDATE frames.
Like I'm five The listener saying, "slow down, I can only hold this much at a time."
Used in RFC 33 RFC 36 RFC 47 RFC 54 RFC 55 RFC 59 RFC 62 RFC 68 RFC 71 RFC 102 RFC 107 RFC 132 RFC 210 RFC 442 RFC 449 RFC 492 RFC 626 RFC 632 RFC 635 RFC 647 RFC 675 RFC 714 RFC 793 RFC 813 and 67 more
- Flow Identifier
A field in the packet header that identifies which flow the packet belongs to, allowing routers to associate the packet with pre-installed state about its service requirements and routing.
Like I'm five A tag on the packet saying 'this belongs to flow number 42,' so routers know what to do with it.
- Flow Key
The set of packet fields used to decide which flow a packet belongs to. Packets that share the same key values are counted as one flow.
Like I'm five The label that decides which pile a packet goes in.
- Flow label
A 20-bit IPv6 header field that a source can set to mark packets of one flow so routers can handle them together. Zero means no flow.
Like I'm five A sticker put on every packet in one stream.
Used in RFC 1752 RFC 1809 RFC 1883 RFC 1953 RFC 2170 RFC 2460 RFC 2491 RFC 3595 RFC 3697 RFC 4149 RFC 4413 RFC 5548 RFC 5673 RFC 5979 RFC 5982 RFC 6088 RFC 6282 RFC 6294 RFC 6391 RFC 6436 RFC 6437 RFC 6438 RFC 6674 RFC 6936 and 16 more
- Flow Record
A record in IPFIX that holds measured values about a single traffic flow, such as its packet count, as exported by a device.
Like I'm five A line on a report that sums up one conversation between two computers.
- Flow Specification
A BGP-distributed rule that describes a traffic flow, such as by protocol or port, so that BGP speakers can filter it. It is defined in RFC 8955.
Like I'm five A note that tells routers which kind of traffic to stop or handle specially.
- Flowspec
The part of an RSVP reservation request that describes the quality of service wanted. It is used to configure the packet scheduler and is opaque to RSVP itself.
Like I'm five The order form that says how much of the road you want to reserve.
- Focus
The SIP user agent identified by a conference URI that makes sure all users in the conference can communicate with each other. It also knows the conference state and serves it to subscribers.
Like I'm five The host of the meeting who keeps everyone connected and knows who is in the room.
- Footprint
In CDNI, a description of the clients or locations a downstream CDN can serve. It is expressed as footprint objects that each carry a type and a value.
Like I'm five The list of places a delivery company can reach.
- ForCES
Forwarding and Control Element Separation, an IETF approach in which the control elements of a network device configure and manage separate forwarding elements using a standard protocol.
Like I'm five A router split into a boss that decides and a worker that moves the traffic, talking by a standard language.
- Foreign agent
A router on the network a mobile node is visiting that offers it a care-of address and helps deliver tunneled packets to it.
Like I'm five The helper at the place you are visiting who takes care of your mail while you are there.
Used in RFC 1688 RFC 2002 RFC 2006 RFC 2290 RFC 2794 RFC 3024 RFC 3344 RFC 3543 RFC 3957 RFC 4004 RFC 4093 RFC 4636 RFC 4857 RFC 5944 RFC 6098 RFC 6245
- Forking
What a SIP proxy does when it forwards one incoming request to several remote destinations at the same time.
Like I'm five Ringing several phones at once for one call.
- Forward error correction
Adding repair data to a stream so that a receiver can rebuild lost packets without asking the sender to resend them.
Like I'm five Sending a spare piece along with the real ones so a missing piece can be rebuilt without asking again.
Used in RFC 2354 RFC 2728 RFC 2733 RFC 2736 RFC 3048 RFC 3208 RFC 3451 RFC 3452 RFC 3453 RFC 3819 RFC 4352 RFC 4837 RFC 5052 RFC 5053 RFC 5109 RFC 5170 RFC 5401 RFC 5445 RFC 5510 RFC 5651 RFC 5725 RFC 5740 RFC 5956 RFC 5993 and 15 more
- Forwarder
A name server that other resolvers use to look up names for them, often because it has better access to the Internet or a larger shared cache.
Like I'm five A friend who looks things up for you and remembers the answers.
Used in RFC 799 RFC 823 RFC 1668 RFC 2117 RFC 2308 RFC 2845 RFC 3654 RFC 5625 RFC 7121 RFC 7871 RFC 8945 RFC 9269 RFC 9273 RFC 9462 RFC 9466 RFC 9499 RFC 9507 RFC 9508 RFC 9531 RFC 9631
- Forwarding adjacency
A label switched path that is advertised in the routing protocol as if it were a link, so that other paths can be routed over it.
Like I'm five A tunnel that the map shows as if it were an ordinary road.
Used in RFC 3477 RFC 3480 RFC 3945 RFC 4206 RFC 6001 RFC 6107
- Forwarding Element (FE)
A component in a ForCES network element responsible for data-plane functions such as packet forwarding and queuing.
Like I'm five The part that does the actual work of sending packets.
Used in RFC 3746 RFC 5810 RFC 5812 RFC 6041 RFC 6369 RFC 6956 RFC 7729 RFC 8013
- Forwarding Equivalence Class
A group of packets that are forwarded the same way, such as all packets headed to one address prefix. In LDP a label is bound to a FEC.
Like I'm five A pile of packages that all get sent down the same road.
- Forwarding Equivalence Class
A group of packets that are forwarded the same way and so share one label. Label switching binds labels to these classes.
Like I'm five A group of parcels all going to the same place, so they share one tag.
Used in RFC 3031 RFC 3034 RFC 3035 RFC 3036 RFC 3037 RFC 3063 RFC 3478 RFC 3988 RFC 4182 RFC 4378 RFC 4379 RFC 4447 RFC 5036 RFC 5283 RFC 5331 RFC 5884 RFC 5918 RFC 5919 RFC 6178 RFC 6388 RFC 6424 RFC 6425 RFC 6512 RFC 6667 and 15 more
- Forwarding Information Base
A table on a router that maps destination addresses to outgoing interfaces and next hops, used to forward packets.
Like I'm five A router's address book that says which way to send packets.
- Forwarding Information Base
The table a forwarder uses to decide where to send a request. In CCNx it maps name prefixes to next hops, local applications or caches.
Like I'm five A signpost list that says which way to go for each kind of name.
- Forwarding plane
The part of a network device that handles the data path, doing per-packet processing and forwarding. It is often built from ASICs or network processors.
Like I'm five The doing part of a device that actually moves each packet along.
- Fountain code
An error-correcting code where the sender can generate as many encoded symbols as needed from the source data. A receiver can rebuild the data from any sufficiently large set of them.
Like I'm five A fountain of puzzle pieces: catch enough of any of them and you can build the picture.
- Four-tuple
The four values that identify a TCP connection: the local and remote IP addresses and the local and remote port numbers.
Like I'm five The two addresses and two door numbers that say exactly which conversation a message belongs to.
- Fragment identifier
The part of a URI after a hash sign that names a piece of, or a point within, an object. Its meaning is defined by the application or content type, not by the URI syntax.
Like I'm five A bookmark inside a page that points to one spot in it.
- Fragment offset
The IPv4 header field, 13 bits wide, that says where a fragment's data belongs in the original datagram, counted in units of 8 octets.
Like I'm five A page number that tells you where each torn piece goes.
Used in RFC 791 RFC 1858 RFC 2435 RFC 3128 RFC 3514 RFC 4036 RFC 5371 RFC 5722 RFC 6864 RFC 6946 RFC 7112 RFC 7113 RFC 7610 RFC 7739
- Frame
The unit WebSocket sends on the wire: a short header (flags, opcode, length, optional masking key) followed by payload data. A message may be split across several frames.
Like I'm five A small envelope holding one piece of a message.
Used in RFC 935 RFC 1134 RFC 1242 RFC 1286 RFC 1490 RFC 1525 RFC 1605 RFC 1623 RFC 1662 RFC 2029 RFC 2032 RFC 2038 RFC 2043 RFC 2171 RFC 2175 RFC 2285 RFC 2496 RFC 2665 RFC 2823 RFC 2878 RFC 2889 RFC 2955 RFC 3153 RFC 3643 and 14 more
- Frame (HTTP/2)
The smallest unit of HTTP/2 communication: a 9-byte header (length, type, flags, stream identifier) followed by a payload. Types include DATA, HEADERS, SETTINGS and WINDOW_UPDATE.
Like I'm five One labelled piece of a message.
- Frame Check Sequence
A checksum appended to a frame by Ethernet to detect whether it has been corrupted in transmission.
Like I'm five A number at the end of a message that tells you if it got scrambled.
Used in RFC 1284 RFC 1398 RFC 1549 RFC 1570 RFC 1662 RFC 3518 RFC 4349 RFC 4720
- Frame Relay
A packet-switched network service that moves variable-length frames between attached stations over virtual circuits.
Like I'm five A shared road where data travels in labeled envelopes along fixed lanes.
- Frame-block
The time-synchronized set of audio frames in a multi-channel session. In an N-channel session it holds N frames, one per channel, all covering the same time period.
Like I'm five One slice of sound for every channel, bundled together like the left and right speaker parts of the same moment.
- Frequency slot
The range of optical frequency allocated to one channel on a flexible grid. It is unavailable to other channels on that link.
Like I'm five The slice of the rainbow that one signal gets all to itself.
- Front-end processor
A small computer placed between a host and a network that handles network protocol work on the host's behalf. The host then deals only with a simpler interface to the front end.
Like I'm five A helper computer that takes care of the network's rules so the big computer does not have to.
- FTP
The File Transfer Protocol. It uses one TCP connection for commands and replies and separate data connections to move files between hosts.
Like I'm five A way to send and fetch files with a set of polite commands, like a post office counter.
Used in RFC 48 RFC 87 RFC 114 RFC 133 RFC 141 RFC 146 RFC 172 RFC 180 RFC 238 RFC 265 RFC 269 RFC 278 RFC 281 RFC 283 RFC 294 RFC 310 RFC 327 RFC 354 RFC 360 RFC 385 RFC 407 RFC 409 RFC 412 RFC 414 and 141 more
A three-digit number that starts every FTP server reply. The first digit tells the client what happened: 1 means started, 2 success, 3 more input needed, 4 temporary failure and 5 permanent failure.
Like I'm five A number code at the start of each answer that says yes, no or keep going.
Used in RFC 463 RFC 607 RFC 614 RFC 624 RFC 630 RFC 640 RFC 686 RFC 691 RFC 959 RFC 1846 RFC 4217 RFC 7151 RFC 7504