Acronyms A–Z
Networking is full of initials. You do not have to remember them — look them up here.
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| Acronym | Stands for | What it does | Ch. |
|---|---|---|---|
| AAA | Authentication, Authorization and Accounting | One of OpenDaylight's enhanced services, sitting beside the basic network functions in Figure 5.17. | 5 |
| ABR | Available Bit Rate — or, in §5.3, Area Border Router | An ATM service in which a router tells the sender the maximum rate it can support. An early example of network-assisted congestion control (§3.6.2). The book prints "Available Bite Rate"; this is the correct expansion. Chapter 5 uses the same three letters for something else entirely: an OSPF area border router, drawn as ABR-1 and ABR-2 in §5.3's area diagram. Both are the book's own notation, so neither can be renamed — read the letters in the chapter you are in. | 3 |
| ACK | acknowledgement | A message, or a header field, saying that data was received correctly. Written ACK in diagrams and code, "acknowledgement" in prose. | 3 |
| AES | Advanced Encryption Standard | The current US block cipher: 128-bit blocks, keys of 128, 192 or 256 bits. | 8 |
| AH | Authentication Header | The IPsec protocol giving source authentication and integrity but no confidentiality. Rarely used. | 8 |
| AIMD | additive-increase, multiplicative-decrease | TCP’s congestion-control shape: grow the window by one MSS each round trip, halve it on a loss event. It draws the sawtooth (§3.7.1). | 3 |
| AKA | Authentication and Key Agreement | The 4G/5G mutual authentication and key generation procedure. | 8 |
| AON | Active Optical Network | An FTTH distribution architecture; essentially switched Ethernet. | 1 |
| AP | access point | 802.11's word for a base station: the central device of a basic service set, which relays between its associated stations and the wired network behind it. | 7 |
| API | Application Programming Interface | The published interface through which one program can use another program's services. | 1 |
| AQM | Active Queue Management | Dropping or marking packets before the buffer is full, to keep the standing queue short and fight bufferbloat. | 4 |
| ARP | Address Resolution Protocol | Translates an IP address into the MAC address of an interface on the same subnet. It works only within one subnet, which is the whole point. | 6 |
| ARPA | Advanced Research Projects Agency | The US agency that funded and built the ARPAnet, the first packet-switched computer network. | 1 |
| ARQ | Automatic Repeat reQuest | The family of reliable protocols that recover from errors by retransmitting. Needs error detection, receiver feedback and retransmission (§3.4.1). | 3 |
| AS | Autonomous System | A group of routers under the same administrative control, identified by a globally unique number. The unit the Internet is divided into for routing. | 5 |
| ASCII | American Standard Code for Information Interchange | The 7-bit text encoding to which SMTP restricts mail bodies. | 2 |
| ASN | Autonomous System Number | The globally unique number identifying an autonomous system. | 5 |
| ATM | Asynchronous Transfer Mode | An older network architecture, cited here for its network-assisted congestion control (§3.6.2). | 3 |
| BBR | Bottleneck Bandwidth and Round-trip propagation time | A delay-based congestion-control protocol [Cardwell 2017] that Google uses on its B4 network and public servers (§3.7.2). | 3 |
| BER | bit error rate | The fraction of received bits that are wrong. On a radio link it is not a small constant — it varies with distance, interference and multipath. | 7 |
| BGP | Border Gateway Protocol | The protocol ISPs use to tell each other which destinations they can reach; its rules follow the customer-provider and peering relationships. | 5 |
| BIND | Berkeley Internet Name Domain | The software most DNS servers run. | 2 |
| BITNET | Because It's Time Network | A 1980s network providing e-mail and file transfer among universities in the US Northeast. | 1 |
| BSS | basic service set | One access point and the wireless stations associated with it — 802.11’s building block. | 7 |
| CBC | Cipher Block Chaining | Chaining each block to the previous ciphertext so identical blocks do not encrypt alike. | 8 |
| CDMA | Code Division Multiple Access | A channel-partitioning protocol that gives each node a different code rather than a time slot or a frequency, so several may transmit at once. Chapter 7 covers it. | 6 |
| CDN | Content Distribution Network | Geographically distributed servers holding copies of content close to users. | 2 |
| CIDR | Classless Interdomain Routing | Address form a.b.c.d/x, where x says how many leading bits are the network prefix. | 4 |
| CLI | Command Line Interface | Typing commands directly at a device, on its console or over SSH. Vendor-specific, error-prone, and hard to scale — but still how much real configuration is done. | 5 |
| CMTS | Cable Modem Termination System | The head-end device that turns the analog signals of many cable modems back into digital data. | 1 |
| CNAME | Canonical Name | The DNS record type that maps an alias hostname to the real one. | 2 |
| CO | Central Office | The telephone company's local building where DSL lines terminate. | 1 |
| CPU | Central Processing Unit | The general-purpose processor. In a router it runs the control plane at millisecond timescales, never the per-packet data path. | 4 |
| CRC | Cyclic Redundancy Check | The error-detection code used in link-layer adapters. Sender and receiver agree on a generator G; the sender appends bits so the whole pattern divides by G with no remainder under modulo-2 arithmetic. | 6 |
| CRLF | Carriage Return, Line Feed | The two characters that end every line in HTTP and SMTP. A lone dot between two CRLFs ends an SMTP message body. | 2 |
| CSMA | Carrier Sense Multiple Access | Listen before you speak: a node senses the channel and transmits only when it is idle. Collisions still happen, because propagation takes time. | 6 |
| CSMA/CA | Carrier Sense Multiple Access with Collision Avoidance | 802.11’s MAC protocol. It avoids collisions rather than detecting them, because a radio adapter cannot listen while it transmits. | 7 |
| CSMA/CD | Carrier Sense Multiple Access with Collision Detection | CSMA plus listening while you speak: a node that detects a collision aborts immediately rather than finishing a frame nobody can read. Ethernet's protocol. | 6 |
| CSNET | Computer Science Network | A 1980s network linking university researchers who had no access to the ARPAnet. | 1 |
| CSS | Cascading Style Sheets | A style sheet file is one of the objects a Web page is built from. | 2 |
| CTCP | Compound TCP | A TCP variant for high-speed, long-distance paths [Tan 2006]. The book writes "CTPC" once in §3.7.2 and "CTCP" in §3.8; CTCP is correct. | 3 |
| CTS | Clear to Send | The access point’s reply granting a reservation, heard by every station in range. | 7 |
| CWR | Congestion Window Reduced | The TCP header bit a sender sets to confirm it has reacted to an ECN Echo by halving its congestion window (§3.7.2). | 3 |
| DARPA | Defense Advanced Research Projects Agency | The later name of ARPA; it sponsored Cerf and Kahn's work on interconnecting networks. | 1 |
| DASH | Dynamic Adaptive Streaming over HTTP | Chunk-by-chunk quality selection driven by the throughput the client measures. | 2 |
| DCCP | Datagram Congestion Control Protocol | A low-overhead, congestion-controlled, UDP-like unreliable transport protocol that uses ECN [RFC 4340] (§3.7.2). | 3 |
| DCTCP | Data Center TCP | A TCP variant designed for data-centre networks, which also uses ECN [Alizadeh 2010, RFC 8257] (§3.7.2). | 3 |
| DDoS | Distributed Denial of Service | A denial-of-service attack launched from many controlled machines at once. | 1 |
| DES | Data Encryption Standard | The older US block cipher: 64-bit blocks, a 56-bit key, now too small to be safe. | 8 |
| DHCP | Dynamic Host Configuration Protocol | Gives a host its IP address and the address of its local DNS server when it joins a network. Built properly in §4.3.2. | 2 |
| DHT | Distributed Hash Table | A simple database whose records are spread over the peers of a P2P system. | 2 |
| DIFS | Distributed Inter-Frame Space | The longer of 802.11’s two waiting intervals, used before an ordinary transmission. | 7 |
| DMZ | demilitarized zone | The region of an organisation's network holding servers that must talk to the outside world — behind the packet filter, but not behind the application gateway. | 8 |
| DNS | Domain Name System | Translates a human-friendly name such as www.ietf.org into a network address. | 1 |
| DNSSEC | DNS Security Extensions | A secured version of DNS, designed against man-in-the-middle and poisoning attacks. | 2 |
| DOCSIS | Data-Over-Cable Service Interface Specifications | The link-layer protocol of cable Internet access; version 3.0 defines 1.2 Gbps downstream. | 1 |
| DoS | Denial of Service | An attack that makes a network, host or service unusable by legitimate users. | 1 |
| DPI | Deep Packet Inspection | Looking past the headers into a packet’s payload, usually in a firewall or intrusion detection system. | 4 |
| DRAM | Dynamic Random Access Memory | Dense, comparatively slow memory. Routers embed it on-chip for forwarding tables and cache it with SRAM. | 4 |
| DSL | Digital Subscriber Line | Home Internet access over the existing telephone wire. | 1 |
| DSLAM | Digital Subscriber Line Access Multiplexer | The device in the telco central office that serves hundreds or thousands of DSL lines. | 1 |
| DV | Distance-Vector | A routing algorithm in which no node knows the whole graph; each iterates the Bellman-Ford equation over its neighbours’ distance vectors. | 5 |
| EAP | Extensible Authentication Protocol | The protocol carrying 802.11 authentication messages between device, access point and authentication server. | 8 |
| eBGP | external BGP | A BGP session between routers in two different autonomous systems. | 5 |
| ECE | ECN Echo | The TCP header bit a receiver sets to tell the sender that a datagram arrived carrying an ECN congestion mark (§3.7.2). | 3 |
| ECMP | Equal Cost Multi Path | The simplest multipath scheme: pick the next hop at random among the switches between source and destination. | 6 |
| ECN | Explicit Congestion Notification | Network-assisted congestion control [RFC 3168]: routers mark two bits in the IP header so senders can slow down before loss occurs (§3.7.2). | 3 |
| EIGRP | Enhanced Interior Gateway Routing Protocol | Cisco's intra-AS routing protocol. Its specification was proprietary for about twenty years and was opened only recently. | 5 |
| ESP | Encapsulation Security Payload | The IPsec protocol giving source authentication, integrity and confidentiality. The one actually deployed. | 8 |
| EWMA | exponential weighted moving average | An average that weighs recent samples more heavily, with older ones decaying exponentially. TCP uses one for EstimatedRTT and another for DevRTT (§3.5.3). | 3 |
| FCFS | First-Come-First-Served | The simplest packet-scheduling discipline: packets leave in the order they arrived. Also called FIFO. | 4 |
| FDDI | Fiber Distributed Data Interface | A token-passing standard for a fibre ring, named by the book alongside IEEE 802.5 token ring as one of the many token-passing protocols built — each of which had to solve the same failure and token-loss problems. | 6 |
| FDM | Frequency-Division Multiplexing | Gives each connection its own frequency band on the link, all of the time. | 1 |
| FEC | Forward Error Correction | A receiver both detecting and correcting errors, without asking for a retransmission. Two-dimensional parity is the book's example. | 6 |
| FIFO | First-In-First-Out | Another name for first-come-first-served scheduling. | 4 |
| FIN | finish | The TCP flag bit meaning "I have no more data to send". A full teardown uses two, one from each side (§3.5.6). | 3 |
| FSM | finite-state machine | A protocol drawn as states joined by labelled transitions: the event above the rule, the action below it (§3.4.1). | 3 |
| FTP | File Transfer Protocol | An application-layer protocol for transferring files between two hosts. | 1 |
| FTTH | Fiber To The Home | Access with an optical fibre path running from the central office to the house. | 1 |
| GBN | Go-Back-N | A pipelined protocol with cumulative ACKs and one timer, which resends every unacknowledged packet after a timeout (§3.4.3). | 3 |
| GTP | GPRS Tunneling Protocol | The protocol 4G and 5G use to carry a device's datagrams through a tunnel. | 7 |
| HDLC | High-level Data Link Control | A point-to-point link-layer protocol, named alongside PPP as one of the classic ones. | 6 |
| HFC | Hybrid Fiber Coax | The cable access network: fibre to the neighbourhood junction, coaxial cable to the home. | 1 |
| HMAC | Hashed Message Authentication Code | The most popular MAC standard, running the data and key through the hash function twice. | 8 |
| HOL | Head of Line | The blocking problem where one large object delays the small ones queued behind it. | 2 |
| HSS | Home Subscriber Service | The 4G database holding what the home network knows about each subscriber. | 7 |
| HTML | HyperText Markup Language | The document format of the Web, created by Tim Berners-Lee at CERN between 1989 and 1991. | 1 |
| HTTP | HyperText Transfer Protocol | The application-layer protocol that requests and transfers Web documents. | 1 |
| IAB | Internet Architecture Board | The body whose 2002 network-management workshop recorded the shortcomings of SNMP for configuration and for management at scale, which led to NETCONF and YANG. | 5 |
| IANA | Internet Assigned Numbers Authority | Coordinates the DNS root servers and publishes the well-known port numbers. | 2 |
| iBGP | internal BGP | A BGP session between routers inside one autonomous system, distributing what the eBGP sessions learned. | 5 |
| ICANN | Internet Corporation for Assigned Names and Numbers | Accredits the registrars that sell domain names. | 2 |
| ICMP | Internet Control Message Protocol | Carries error and diagnostic messages between hosts and routers. A ping is an ICMP message. Covered in §5.6. | 2 |
| IDS | Intrusion Detection System | A middlebox that matches traffic against known attack patterns and filters accordingly. | 4 |
| IEEE | Institute of Electrical and Electronics Engineers | The body whose 802 committee standardises Ethernet and WiFi. | 1 |
| IETF | Internet Engineering Task Force | The body that develops Internet standards. | 1 |
| IKE | Internet Key Exchange | The IPsec protocol that establishes security associations automatically. | 8 |
| IMAP | Internet Mail Access Protocol | Lets a user agent read and manage a mailbox held on a mail server (RFC 3501). | 2 |
| IMSI | International Mobile Subscriber Identity | The identifier on a SIM card that names the device and names its home network. | 7 |
| IoT | Internet of Things | Everyday devices with network connections — the Mirai botnet was built from about 100,000 of them. | 2 |
| IP | Internet Protocol | The network-layer protocol that defines the datagram format and addressing every Internet device must use. | 1 |
| IPS | Intrusion Prevention System | Like an intrusion detection system, but it blocks the offending packets as well as raising an alert. | 4 |
| IPsec | IP Security | The network-layer security suite, most often used in tunnel mode to build VPNs. | 8 |
| IPv4 | Internet Protocol version 4 | The Internet Protocol with 32-bit addresses and a variable-length header. | 4 |
| IPv6 | Internet Protocol version 6 | The Internet Protocol with 128-bit addresses and a fixed 40-byte header, without fragmentation at routers or a header checksum. | 4 |
| IS-IS | Intermediate System to Intermediate System | A link-state intra-AS routing protocol, closely related to OSPF and widely used in the Internet. | 5 |
| ISN | initial sequence number | The randomly chosen starting sequence number each side of a TCP connection picks during the three-way handshake. Randomising it matters for security (§3.5.6). | 3 |
| ISP | Internet Service Provider | Connects hosts, and other ISPs, to the rest of the Internet. | 1 |
| ITU | International Telecommunication Union | The United Nations body that standardises telecommunications. Its X.509 recommendation defines the certificate format used across the Internet. | 8 |
| IXP | Internet Exchange Point | A building where many ISPs peer with each other; there are over 600 today. | 1 |
| JPEG | Joint Photographic Experts Group | The image format used in the book’s ten-image Web page example. | 2 |
| LAN | Local Area Network | A network covering one building, campus or home. | 1 |
| LDNS | Local DNS Server | The nearby server a host queries first; a CDN identifies a client by its address. | 2 |
| LEO | Low-Earth Orbiting | Satellites placed close to Earth that move relative to the ground, unlike geostationary satellites. | 1 |
| LS | Link-State | A routing algorithm in which every node knows the whole graph and runs Dijkstra locally. | 5 |
| LTE | Long-Term Evolution | The dominant 4G cellular standard. Its architecture is a set of gateways plus a subscriber database. | 7 |
| MAC | Media Access Control | The link-layer address burned into a network interface. Before a host has an IP address it is the only identity it has, which is why DHCP keys on it. Chapter 6 covers it properly. | 4 |
| MD5 | Message Digest 5 | A cryptographic hash function. OSPF can use it, with a shared secret key, to authenticate the messages routers exchange. Chapter 8 covers hashes properly. | 5 |
| MIB | Management Information Base | The collection of managed objects in a device whose values reflect its status, and which SNMP reads and writes. | 5 |
| MIMO | Multiple Input Multiple Output | Using several antennas at each end to raise the rate over one channel. | 7 |
| MME | Mobility Management Entity | The 4G element that authenticates a device, tracks where it is, and sets up its data path. | 7 |
| MOSPF | Multicast OSPF | An extension of OSPF for multicast routing. It reuses the existing OSPF link database and adds one new type of link-state advertisement. | 5 |
| MPEG | Moving Picture Experts Group | The video compression family behind the file sizes used in the P2P and streaming examples. | 2 |
| MPLS | Multiprotocol Label Switching | Fixed-length labels carried in a header between the link and network layers, so routers can forward on a label rather than on a longest-prefix match of an IP address. | 6 |
| MSS | maximum segment size | The largest amount of application data TCP puts in one segment. Typically 1,460 bytes. It limits the data, not the whole segment (§3.5.1). | 3 |
| MTU | maximum transmission unit | The largest frame a link will carry — 1,500 bytes for Ethernet and PPP. The MSS is derived from it (§3.5.1). | 3 |
| MX | Mail eXchange | The DNS record type that names the mail server for a domain. | 2 |
| NAK | negative acknowledgement | A message saying that data arrived damaged and must be resent. Used by rdt2.1; removed in rdt2.2; TCP has none, and uses three duplicate ACKs instead. | 3 |
| NAT | Network Address Translation | Letting a whole private network share one public IP address by rewriting addresses and ports. | 4 |
| NCP | Network-Control Protocol | The first host-to-host protocol of the ARPAnet, replaced by TCP/IP on 1 January 1983. | 1 |
| NETCONF | Network Configuration Protocol | A protocol for communicating configuration and query data between a managing server and managed network devices. The modern alternative to polling with SNMP. | 5 |
| NFS | Network File System | A remote file-server protocol, listed in Figure 3.6 as typically running over UDP (§3.3). | 3 |
| NFV | Network Function Virtualization | Running middlebox functions as software on commodity hardware. | 4 |
| NIC | Network Interface Controller | The chip that implements most of the link layer — framing, link access, error detection. Also called a network adapter. | 6 |
| NOC | Network Operations Centre | The centralized station where the managing server runs, typically with human network managers in the loop. | 5 |
| NOX | Network Operating System | The first SDN controller, from 2008. The architecture of Figure 5.15 closely resembles it. | 5 |
| NS | Name Server | The DNS record type that names the authoritative server for a domain. | 2 |
| NSFNET | National Science Foundation Network | The 1986 US research backbone, 56 kbps rising to 1.5 Mbps, decommissioned in 1995. | 1 |
| OC | Optical Carrier | The standard for fibre link speeds; OC-n runs at n × 51.8 Mbps. | 1 |
| ODL | OpenDaylight | One of the two modern SDN controller platforms the book names, alongside ONOS. Both are logically centralized but physically distributed. | 5 |
| OLT | Optical Line Terminator | The device in the central office that converts between optical and electrical signals. | 1 |
| ONOS | Open Network Operating System | An open-source SDN controller platform. | 5 |
| ONT | Optical Network Terminator | The device in the home that terminates the optical fibre. | 1 |
| OSPF | Open Shortest Path First | The Internet’s intra-AS link-state routing protocol. Each router builds a complete map of its autonomous system and runs Dijkstra locally. | 5 |
| OVSDB | Open vSwitch Database Management Protocol | A southbound protocol used to manage data centre switching, which is an important application area for SDN. | 5 |
| P-GW | PDN Gateway | The 4G gateway between the cellular network and the wider Internet. | 7 |
| P2P | Peer-to-Peer | An architecture where users’ own machines serve each other directly. | 2 |
| PARC | Palo Alto Research Center | Xerox's research centre, where Metcalfe and Boggs invented Ethernet in the mid-1970s. | 6 |
| PDU | Protocol Data Unit | The generic name for one message of a protocol. SNMPv3 defines seven PDU types. | 5 |
| PGP | Pretty Good Privacy | Phil Zimmermann's 1991 e-mail encryption scheme, certifying keys through a web of trust rather than a CA. | 8 |
| PON | Passive Optical Network | An FTTH distribution architecture using a passive neighbourhood splitter shared by up to about 100 homes. | 1 |
| PoP | Point of Presence | A place in a provider ISP's network where customer ISPs can connect in. | 1 |
| PPP | Point-to-Point Protocol | A link-layer protocol; a datagram may travel over Ethernet on one link and PPP on the next. | 1 |
| QAM | Quadrature Amplitude Modulation | A modulation scheme; the higher its order, the more bits per symbol and the more signal quality it needs. | 7 |
| QUIC | Quick UDP Internet Connections | A transport-like protocol built in the application layer over UDP, carrying HTTP/3. The IETF now treats QUIC as a name rather than an abbreviation. | 2 |
| RDT | reliable data transfer | The service of delivering data uncorrupted, complete and in order over a channel that may lose or damage it. The book names its example protocols rdt1.0 to rdt3.0. | 3 |
| RED | Random Early Detection | The most widely studied active queue management algorithm: drop or mark packets at random, with a probability that rises as the average queue grows, so senders back off before the buffer overflows and before they all overflow together. | 4 |
| REST | Representational State Transfer | A request-response style for web APIs. Two popular SDN controllers use a RESTful interface to talk to their network-control applications. | 5 |
| RESTCONF | REST Configuration Protocol | One of the northbound interfaces OpenDaylight offers to network-control applications, alongside REST and NETCONF. | 5 |
| RFC | Request For Comments | The name of an IETF standards document. There are currently nearly 9000 of them. | 1 |
| RIP | Routing Information Protocol | An early intra-AS distance-vector routing protocol. | 5 |
| RPC | Remote Procedure Call | A style in which one machine invokes an operation on another as if calling a local function. NETCONF uses it. | 5 |
| RR | Resource Record | One (Name, Value, Type, TTL) entry in the DNS database. | 2 |
| RSA | Rivest–Shamir–Adleman | The public key algorithm that became almost synonymous with public key cryptography. | 8 |
| RST | reset | The TCP flag bit meaning "there is no socket here". Sent when a segment arrives for a port nothing is listening on (§3.5.6). | 3 |
| RSVP | Resource Reservation Protocol | The protocol whose extension RSVP-TE the IETF chose as the focus of its MPLS signalling work. | 6 |
| RTP | Real-time Transport Protocol | Carries the audio and video of a call once SIP has set it up (RFC 3550). | 2 |
| RTS | Request to Send | A short 802.11 frame reserving the channel, used to defuse the hidden terminal problem. | 7 |
| RTT | Round-Trip Time | How long a small packet takes to travel to the other side and back. The unit almost every §2.2 result is counted in. | 2 |
| S-GW | Serving Gateway | The 4G gateway carrying a device’s traffic within the visited network. | 7 |
| SA | security association | A one-way IPsec agreement holding the keys and algorithms for one direction of traffic. | 8 |
| SACK | selective acknowledgement | A TCP extension [RFC 2018] letting a receiver acknowledge out-of-order segments individually rather than only cumulatively (§3.5.4). | 3 |
| SAL | Service Abstraction Layer | The nerve centre of the OpenDaylight controller. It lets controller components and applications invoke each other's services, reach configuration and operational data, and subscribe to each other's events, and it presents one uniform interface over the several southbound protocols. | 5 |
| SCTP | Stream Control Transmission Protocol | An earlier reliable, message-oriented protocol [RFC 4960] that pioneered multiplexing several application streams over one connection. Used in 4G and 5G control-plane protocols (§3.8). | 3 |
| SDN | Software-Defined Networking | Separating the control plane from the data plane by computing forwarding tables in a remote controller. | 4 |
| SIFS | Short Inter-Frame Space | The shorter interval, used before an acknowledgement so it takes priority. | 7 |
| SIP | Session Initiation Protocol | An application-layer protocol that establishes an Internet telephony call (RFC 3261). | 2 |
| SMI | Structure of Management Information | The data-definition language for MIB objects. | 5 |
| SMTP | Simple Mail Transfer Protocol | The application-layer protocol that transfers e-mail messages. | 1 |
| SNMP | Simple Network Management Protocol | The Internet’s network-management protocol, typically run over UDP so that it still works when the network is under stress (§3.3, built in §5.7). | 3 |
| SNR | signal-to-noise ratio | How far a signal stands above the noise, in decibels. Plotted against bit error rate, it is chapter 7’s central trade-off. | 7 |
| SOHO | Small Office, Home Office | The small private network NAT was invented for: a handful of machines behind one router and one public address. | 4 |
| SPI | Security Parameter Index | The 32-bit identifier naming which security association a datagram belongs to. | 8 |
| SR | Selective Repeat | A pipelined protocol with individual ACKs and per-packet timers, which resends only the packets it believes were lost (§3.4.4). | 3 |
| SRAM | Static Random Access Memory | Faster and less dense than DRAM. Used as a DRAM cache in router lookup hardware, because memory access time is the binding constraint. | 4 |
| SSH | Secure Shell | An encrypted remote-login protocol over TCP, now preferred to Telnet, whose traffic including passwords is unencrypted (§3.5.2). | 3 |
| SSID | Service Set Identifier | The network name an access point announces in its beacon frames. | 7 |
| SSL | Secure Sockets Layer | Netscape's original design; SSL version 3 is the predecessor of TLS. | 8 |
| SYN | synchronize | The TCP flag bit that marks a connection-establishment segment. Set in the first two segments of the three-way handshake (§3.5.6). | 3 |
| TCAM | Ternary Content Addressable Memory | Memory whose stored bits can be 0, 1 or don’t-care. Present an address and it returns the matching forwarding-table entry in essentially constant time, whatever the table size. | 4 |
| TCP | Transmission Control Protocol | The Internet transport protocol that delivers data reliably and in order, with flow control and congestion control. | 1 |
| TCP/IP | Transmission Control Protocol / Internet Protocol | The collective name for the Internet's principal protocols. It became the ARPAnet standard on 1 January 1983. | 1 |
| TDM | Time-Division Multiplexing | Gives each connection the whole link, but only during its own repeating time slot. | 1 |
| TEID | Tunnel Endpoint Identifier | The field in a GTP header that says which tunnel a datagram belongs to. | 7 |
| TLD | Top-Level Domain | The last label of a hostname — com, edu, uk — and the DNS servers responsible for it. | 2 |
| TLS | Transport Layer Security | Encryption, integrity and authentication added above TCP, in the application layer. | 2 |
| TOR | Top of Rack | The switch at the top of a rack of data-centre hosts, connecting every host in that rack and linking upward into the switching hierarchy. | 6 |
| TOS | Type Of Service | An IPv4 header field distinguishing datagrams needing low delay from those needing high throughput. | 4 |
| TPID | Tag Protocol Identifier | The fixed 2-byte value 81-00 that marks an Ethernet frame as carrying an 802.1Q VLAN tag. | 6 |
| TTL | Time To Live | How long a cached DNS record may be kept before it must be fetched again. | 2 |
| UDP | User Datagram Protocol | A simple transport protocol with no reliability, no flow control and no congestion control. | 1 |
| URL | Uniform Resource Locator | The address of a single Web object: hostname plus path name. | 2 |
| UTP | Unshielded Twisted Pair | The copper cable commonly used for LANs inside a building; category 6a reaches 10 Gbps over 100 m. | 1 |
| VLAN | Virtual Local Area Network | Several logical LANs configured over one physical switched infrastructure, so that broadcast traffic and group membership are decided by configuration rather than by cabling. | 6 |
| VoIP | Voice over IP | Carrying telephone calls over a packet-switched network; it suffers an extra packetization delay. | 1 |
| VPN | Virtual Private Network | An ISP joins a customer's separate sites across its own network, isolating that customer's addressing and resources from everyone else's. | 6 |
| WAN | Wide Area Network | The public side of a home or office router — the interface facing the ISP, as opposed to the LAN side facing the home. | 4 |
| WEP | Wired Equivalent Privacy | The original 802.11 security specification, whose flaws left users no safer than with no security at all. | 8 |
| WFQ | Weighted Fair Queueing | Round-robin scheduling where each class gets a guaranteed share of the link rate. | 4 |
| WiFi | IEEE 802.11 wireless LAN | Wireless LAN access; a user must be within a few tens of metres of the access point. WiFi is a trade name, not an abbreviation. | 1 |
| WPA | WiFi Protected Access | The WiFi Alliance's replacement for WEP; WPA2 and WPA3 followed. | 8 |
| XML | Extensible Markup Language | The structured document format NETCONF encodes its messages and configurations in. | 5 |
| YANG | Yet Another Next Generation | The data modelling language used to describe configuration and operational data, from which NETCONF messages are generated. | 5 |