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Some Important Glossary in Computer Networks

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LANE: Local area network emulation. Adding functionality to ATM to make it behave like a shared-media (i.e., Ethernet-like) LAN.


LAN switch: Another term for a bridge, usually applied to a bridge with many ports. Also called an Ethernet switch if the link technology it supports is Ethernet.


latency: A measure of how long it takes a single bit to propagate from one end of a link or channel to the other. Latency is measured strictly in terms of time.


LDAP: Lightweight Directory Access Protocol. A subset of the X.500 directory service that has recently become a popular directory service for information about users.

LER: Label edge router. A router at the edge of an MPLS cloud. Performs a complete IP lookup on arriving IP packets, and then applies labels to them as a result of the lookup. link: A physical connection between two nodes of a network. It may be implemented over copper or fiber-optic cable or it may be a wireless link (e.g., a satellite).


link-level protocol: A protocol that is responsible for delivering frames over a directly connected network (e.g., an Ethernet, token ring, or point-to-point link). Also called link-layer protocol.

link state: A lowest-cost-path algorithm used in routing. Information on directly connected neighbors and current link costs are flooded to all routers; each router uses this information to build a view of the network on which to base forwarding decisions. The OSPF routing protocol uses a link-state algorithm. Contrast with distance vector.


LSR: Label-switching router. A router that runs IP control protocols, but uses the label switching forwarding algorithm of MPLS.


MAC: Media access control. Algorithms used to control access to shared-media networks like Ethernet and FDDI.


MACA: Multiple access with collision avoidance. Distributed algorithm used to mediate access to a shared media.


MACAW: Multiple access with collision avoidance for wireless. Enhancement of the general MACA algorithm to better support wireless networks. Used by 802.11.


MAN: Metropolitan area network. A network based on any of several new network technologies that operate at high speeds (up to several Gbps) and across distances wide


enough to span a metropolitan area. Contrast with SAN, LAN, and WAN. Manchester: A bit-encoding scheme that transmits the exclusive-OR of the clock and the NRZ-encoded data. Used on the Ethernet.


MBone: Multicast backbone. A logical network imposed over the top of the Internet, in which multicast-enhanced routers use tunneling to forward multicast datagrams across the Internet.


MD5: Message Digest version 5. An efficient cryptographic checksum algorithm commonly used to verify that the contents of a message are unaltered.


MIB: Management information base. Defines the set of network-related variables that may be read or written on a network node. The MIB is used in conjunctionwith SNMP. MIME: Multipurpose InternetMail Extensions. Specifications for converting binary data (such as image files) to ASCII text, which allows it to be sent via email.


Mosaic: A once-popular and free graphical World Wide Web browser developed at the National Center for Supercomputing Applications at the University of Illinois.


MP3: MPEG Layer 3. Audio compression standard used with MPEG.


MPEG: Moving Picture Experts Group. Typically used to refer to an algorithm for compressing video streams developed by the MPEG.


MPLS: Multiprotocol Label Switching. A collection of techniques used to effectively implement IP routers on top of level 2 (e.g., ATM) switches.


MSAU: Multistation access unit. A device used in token ring networks to connect several stations to the ring and remove them in the event of failure.


MSDP: Multicast Source Discovery Protocol. A protocol used to facilitate interdomain multicast.


MTU: Maximum transmission unit. The size of the largest packet that can be sent over a physical network.


multicast: A special form of broadcast in which packets are delivered to a specified subgroup of network hosts.


multiplexing: Combining distinct channels into a single, lower-level channel. For example, separate TCP and UDP channels are multiplexed into a single host-to-host IP


channel. The inverse operation, demultiplexing, takes place on the receiving host.

name resolution: The action of resolving host names (which are easy for humans to read) into their corresponding addresses (which machines can read). See DNS.

NAT: Network address translation. A technique for extending the IP address space that involves translating between globally understood IP addresses and local-only addresses at the edge of a network or site.


NDR: Network Data Representation. The data-encoding standard used in the Distributed Computing Environment (DCE), as defined by the Open Software Foundation.


NDR uses a receiver-makes-right strategy and inserts an architecture tag at the front of each message.


network-level protocol: A protocol that runs over switched networks, directly above the link level.


NFS: Network File System. A popular distributed file system developed by SunMicrosystems. NFS is based on SunRPC, an RPC protocol developed by Sun.


NIST: National Institute for Standards and Technology. The official U.S. standardization body.


node: A generic term used for individual computers that make up a network. Nodes include general-purpose computers, switches, and routers.


NRZ: Nonreturn to zero. A bit-encoding scheme that encodes a 1 as the high signal and a 0 as the low signal.


NRZI: Nonreturn to zero inverted. A bit-encoding scheme that makes a transition from the current signal to encode a 1 and stays at the current signal to encode a 0.


NSF: National Science Foundation. An agency of the U.S. government that funds scientific research in the United States, including research on networks and on the Internet infrastructure.


nv: Network video. A videoconferencing application.

OC: Optical carrier. The prefix for various rates of SONET optical transmission. For example, OC-1 refers to the SONET standard for 51.84-Mbps transmission over fiber. An OC-n signal differs from an STS-n signal only in that the OC-n signal is scrambled for optical transmission.


ONC: Open Network Computing. A version of SunRPC that is being standardized for the Internet.


optical switch: A switching device that forwards optical lightwaves from input port to output port without converting to electrical format.


OSF: Open Software Foundation. A consortium of computer vendors that have defined standards for distributed computing, including the NDR presentation format.


OSI: Open Systems Interconnection. The seven-layer network reference model developed by the ISO. Guides the design of ISO and ITU-T protocol standards.


OSPF: Open Shortest Path First. A routing protocol developed by the IETF for the Internet architecture. OSPF is based on a link-state algorithm, in which every node constructs


a topography of the Internet and uses it to make forwarding decisions. Today known as Open Group.


overlay: A virtual (logical) network running on top of an existing physical network. Overlay nodes communicate with each other through tunnels rather than over physical links. Overlays are often used to deploy new network services since they do not require the cooperation of the existing network infrastructure.


packet: A data unit sent over a packet-switched network. Also see frame and segment. packet switching: A general strategy for switching data through a network. Packet switching uses store-and-forward switching of discrete data units called packets, and implies statistical multiplexing.


participants: A generic term used to denote the processes, protocols, or hosts that are sending messages to each other.


PAWS: Protection against wrapped sequence numbers. Engineering transport protocol with a large enough sequence number space to protect against the numbers wrapping around on a network where packets can be delayed for a long period of time.


PCS: Personal Communication Services. New digital cellular phone system being deployed throughout the United States and Canada. Similar to GSM, which is being deployed throughout the rest of the world.

PDU: Protocol data unit. Another name for a packet or frame.


peer: A counterpart on another machine that a protocol module interoperates with to implement some communication service.


peer-to-peer networks: A general class of applications that integrate application logic (e.g., file storage) with routing. Popular examples include Napster and Gnutella. Research prototypes often use distributed hash tables.


PEM: Privacy Enhanced Mail. Extensions to Internet email that support privacy and integrity protection. See also PGP.


PGP: Pretty Good Privacy. A collection of public domain software that provides privacy and authentication capabilities using RSA and that uses a mesh of trust for public key distribution.


PHB: Per-hop behavior. Behavior of individual routers in the Differentiated Services architecture.


AF and EF are two proposed PHBs.

physical-level protocol: The lowest layer of the OSI protocol stack. Its main function is to encode bits onto the signals that are propagated across the physical transmission media.


piconet: Wireless network spanning short distances (e.g., 10m). Used to connect office computers (laptops, printers, PDAs, workstations, etc.) without cables.


PIM: Protocol Independent Multicast. A multicast routing protocol that can be built on top of different unicast routing protocols.


Ping: A Unix utility used to test the RTT to various hosts over the Internet. Ping sends an ICMP ECHO_REQUEST message, and the remote host sends an ECHO_ RESPONSE message back.


PIO: Programmed input/output. An approach to connecting hosts to I/O devices, in which the CPU reads data from and writes data to the I/O device. Also see DMA. poison reverse: Used in conjunction with split horizon. A heuristic technique to avoid routing loops in distance-vector routing protocols.


port: A generic term usually used to mean the point at which a network user attaches to the network. On a switch, a port denotes the input or output on which packets are received and sent.


POTS: Plain old telephone service. Used to specify the existing phone service, in contrast to ISDN, ATM, or other technologies that the telephone companies offer now or may offer in the future.


PPP: Point-to-Point Protocol. Data link protocol typically used to connect computers over a dial-up line.

process: An abstraction provided by an operating system to enable different operations to take place concurrently. For example, each user application usually runs inside its own process, while various operating system functions take place in other processes. promiscuous mode: A mode of operation for a network adaptor in which it receives all frames transmitted on the network, not just those addressed to it.


protocol: A specification of an interface between modules running on different machines, as well as the communication service that those modules implement. The term is


also used to refer to an implementation of the module that meets this specification. To distinguish between these two uses, the interface is often called a protocol specification.


proxy: An agent sitting between a client and server that intercepts messages and provides some service. For example, a proxy can “stand in” for a server by responding to client

requests, perhaps using data it has cached, without contacting the server.

pseudoheader: A subset of fields fromthe IP header that are passed up to transport protocols TCP and UDP for use in their checksum calculation. The pseudoheader contains


source and destination IP addresses and IP datagram length, thus enabling detection of corruption of these fields or delivery of a packet to an incorrect address.

public key encryption: Any of several encryption algorithms (e.g., RSA) in which each


participant has a private key (shared with no one else) and a public key (available to everyone). A secure message is sent to a user by encrypting the data with that user’s

public key; possession of the private key is required to decrypt the message, and so only the receiver can read it.


QoS: Quality of service. Packet delivery guarantees provided by a network architecture. Usually related to performance guarantees, such as bandwidth and delay. The Internet offers a best-effort delivery service, meaning that every effort is made to deliver a packet but delivery is not guaranteed.


RED: Random early detection. A queuing discipline for routers in which, when congestion is anticipated, packets are randomly dropped to alert the senders to slow down. rendezvous point: A router used by PIM to allow receivers to learn about senders. repeater: A device that propagates electrical signals from one Ethernet cable to another. There can be a maximum of two repeaters between any two hosts in an Ethernet. Repeaters forward signals, whereas bridges forward frames, and routers and switches forward packets.


REST: Representational State Transfer. An approach to building web services that uses HTTP as the generic application protocol.


reverse-path broadcast (RPB): A technique used to eliminate duplicate broadcast packets.


RFC: Request for Comments. Internet reports that contain, among other things, specifications for protocols like TCP and IP.


RIO: RED with In and Out. A packet drop policy based on RED, but involving two drop curves: one for packets that have been marked as being “in” profile and one for packets that have been marked “out” of profile. Designed to be used to implement differentiated



RIP: Routing Information Protocol. An intradomain routing protocol supplied with Berkeley Unix. Each router running RIP dynamically builds its forwarding table based on a distance-vector algorithm.


router: A network node connected to two or more networks that forwards packets from one network to another. Contrast with bridge, repeater, and switch.

routing: The process by which nodes exchange topological information to build correct forwarding tables. See forwarding, link state, and distance vector.


routing table: See forwarding table.

RPC: Remote Procedure Call. Synchronous request/reply transport protocol used in many client/server interactions.


RPR: Resilient Packet Ring. A type of ring network that is mostly used in metropolitan area networks. See 802.17.


RSA: A public-key encryption algorithm named after its inventors: Rivest, Shamir, and Adleman.


RSVP: Resource Reservation Protocol. A protocol for reserving resources in the network. RSVP uses the concept of soft state in routers and puts responsibility for making reservations on receivers instead of on senders.


RTCP: Real-time Transport Control Protocol. Control protocol associated with RTP.

RTP: Real-time Transport Protocol. An end-to-end protocol used by multimedia applications that have real-time constraints.


RTT: Round-trip time. The time it takes for a bit of information to propagate from one end of a link or channel to the other and back again; in other words, double the latency of the channel.


SAN: Storage area network. A network that spans the components of a computer system (e.g., display, camera, disk). Includes interfaces like HiPPI and Fibre Channel. Contrast with LAN, MAN, and WAN.


schema: A specification of how to structure and interpret a set of data. Schema are defined for XML documents.


scrambling: The process of XORing a signal with a pseudorandom bitstream before transmission to cause enough signal transitions to allow clock recovery. Scrambling is used in SONET.


SDP: Session Description Protocol. An application layer protocol used to learn about the available audio/video channels. It reports the name and purpose of the session, start


and end times for the session, the media types (e.g., audio, video) that comprise the session, and detailed information needed to receive the session (e.g., the multicast address, transport protocol, and port numbers to be used).


segment: A TCP packet. A segment contains a portion of the byte stream that is being sent by means of TCP.


semaphore: A variable used to support synchronization between processes. Typically a process blocks on a semaphore while it waits for some other process to signal the semaphore.


server: The provider of a service in a client/server distributed system.


SHA: Secure Hash Algorithm. A family of cryptographic hash algorithms.

signalling: At the physical level, denotes the transmission of a signal over some physical medium. In ATM, signalling refers to the process of establishing a virtual circuit.

silly window syndrome: A condition occurring in TCP that may arise if each time the receiver opens its receive window a small amount, the sender sends a small segment to fill the window. The result is many small segments and an inefficient use of bandwidth.


SIP: Session Initiation Protocol. An application layer protocol used in multimedia applications.

It determines the correct device with which to communicate to reach a particular


user, determines if the user is willing or able to take part in a particular communication, determines the choice of media and coding scheme to use, and establishes session parameters (e.g., port numbers).


sliding window: An algorithm that allows the sender to transmit multiple packets (up


to the size of the window) before receiving an acknowledgment. As acknowledgments are returned for those packets in the window that were sent first, the window “slides”


and more packets may be sent. The sliding window algorithm combines reliable delivery with a high throughput. See ARQ.


slow start: A congestion-avoidance algorithm for TCP that attempts to pace outgoing segments. For each ACK that is returned, two additional packets are sent, resulting in an exponential increase in the number of outstanding segments.


SMDS: Switched Multimegabit Data Service. A service supporting LAN-to-WAN connectivity, offered by some telephone companies.


SMTP: Simple Mail Transfer Protocol. The electronic mail protocol of the Internet. See 822.


SNA: System Network Architecture. The proprietary network architecture of IBM. SNMP: Simple Network Management Protocol. An Internet protocol that allows the monitoring of hosts, networks, and routers.


SOAP: A component of the web services framework for specifying and implementing application protocols.


socket: The abstraction provided by Unix that provides the application programming interface (API) to TCP/IP.


soft state: Connection-related information contained in a router that is cached for a limited period of time rather than being explicitly established (and requiring explicit teardown) through a connection setup.


SONET: SynchronousOpticalNetwork. A clock-based framing standard for digital transmission over optical fiber. It defines how telephone companies transmit data over optical



source routing: Routing decisions performed at the source before the packet is sent. The route consists of the list of nodes that the packet should traverse on the way to the destination.


source-specific multicast: A mode of multicast in which a group may have only a single sender.


sparse mode multicast: A mode used in PIM when relatively few hosts or routers need to receive multicast data for a certain group.


split horizon: A method of breaking routing loops in a distance-vector routing algorithm. When a node sends a routing update to its neighbors, it does not send those


routes it learned from each neighbor back to that neighbor. Split horizon is used with poison reverse.


spread spectrum: Encoding technique that involves spreading a signal over a wider frequency than necessary, so as to minimize the impact of interference.


SSL: Secure Socket Layer. A protocol layer that runs over TCP to provide authentication and encryption of connections. Also known as Transport Layer Security (TLS). statistical multiplexing: Demand-based multiplexing of multiple data sources over a shared link or channel.

stop-and-wait: A reliable transmission algorithm in which the sender transmits a packet and waits for an acknowledgment before sending the next packet. Compare with sliding window and concurrent logical channels. See also ARQ.


STS: Synchronous Transport Signal. The prefix for various rates of SONET transmission. For example, STS-1 refers to the SONET standard for 51.84-Mbps transmission. subnetting: The use of a single IP network address to denote multiple physical networks. Routers within the subnetwork use a subnet mask to discover the physical network to which a packet should be forwarded. Subnetting effectively introduces a third level to


the two-level hierarchical IP address.

SunRPC: Remote procedure call protocol developed by Sun Microsystems. SunRPC is used to support NFS. See also ONC.


switch: A network node that forwards packets from inputs to outputs based on header information in each packet. Differs from a router mainly in that it typically does not interconnect networks of different types.


switching fabric: The component of a switch that directs packets from their inputs to the correct outputs.


T1: A standard telephone carrier service equal to 24 ISDN circuits, or 1.544 Mbps. Also called DS1.


T3: A standard telephone carrier service equal to 24 T1 circuits, or 44.736 Mbps. Also called DS3.


TCP: Transmission Control Protocol. Connection-oriented transport protocol of the Internet architecture. TCP provides a reliable, byte-stream delivery service.


TDMA: Time Division Multiple Access. A form of multiplexing used in cellular wireless networks. Also the name of a particular wireless standard.


Telnet: Remote terminal protocol of the Internet architecture. Telnet allows you to interact with a remote system as if your terminal is directly connected to that machine. throughput: The observed rate at which data is sent through a channel. The term is


often used interchangeably with bandwidth.

TLS: Transport Layer Security. Security services that can be layered on top of a transport protocol like TCP. It is often used by HTTP to perform secure transactions on theWorld Wide Web. Derived from SSL.


token bucket: A way to characterize or police the bandwidth used by a flow. Conceptually, processes accumulate tokens over time, and they must spend a token to transmit a


byte of data and then must stop sending when they have no tokens left. Thus, overall bandwidth is limited, with the accommodation of some burstiness.


token ring: A physical network technology in which hosts are connected in a ring. A token (bit pattern) circulates around the ring. A given node must possess the token before


it is allowed to transmit. 802.5 and FDDI are examples of token ring networks.

TP4: OSI Transport Protocol Class 4. The most powerful OSI transport protocol. TP4 is the ISO equivalent of TCP.


transport protocol: An end-to-end protocol that enables processes on different hosts to communicate. TCP is the canonical example.


TTL: Time to live. Usually a measure of the number of hops (routers) an IP datagram can visit before it is discarded.


tunneling: Encapsulating a packet using a protocol that operates at the same layer as the packet. For example, multicast IP packets are encapsulated inside unicast IP packets to tunnel across the Internet to implement the MBone. Tunneling will also be used during the transition from IPv4 to IPv6.


two-dimensional parity: A parity scheme in which bytes are conceptually stacked as a matrix, and parity is calculated for both rows and columns.


Tymnet: An early network in which a virtual circuit abstraction was maintained across a set of routers.

UBR: Unspecified bit rate. The “no frills” service class in ATM, offering best-effort cell delivery. Contrast with ABR, CBR, and VBR.


UDP: User Datagram Protocol. Transport protocol of the Internet architecture that provides a connectionless datagram service to application-level processes.


UMTS: Universal Mobile Telecommunications System. Cellular wireless standard based on wideband CDMA that offers relatively high data rates.


unicast: Sending a packet to a single destination host. Contrast with broadcast and multicast.


URI: Uniform Resource Identifier. A generalization of the URL. Used for example, in conjunction with SIP to set up audio/visual sessions.


URL: Uniform Resource Locator. A text string used to identify the location of Internet resources. A typical URL looks like http://www.cisco.com. In this URL, http is the protocol to use to access the resource located on host www.cisco.com.


vat: Audioconferencing tool used on the Internet that runs over RTP.

VBR: Variable bit rate. One of the classes of service in ATM, intended for applications with bandwidth requirements that vary with time, such as compressed video. Contrast with ABR, CBR, and UBR.


VCI: Virtual circuit identifier. An identifier in the header of a packet that is used for virtual circuit switching. In the case of ATM, the VPI and VCI together identify the end-to-end connection.


vic: Unix-based videoconferencing tool that uses RTP.


virtual circuit: The abstraction provided by connection-oriented networks such as ATM.


Messages must usually be exchanged between participants to establish a virtual circuit (and perhaps to allocate resources to the circuit) before data can be sent. Contrast with datagram.


virtual clock: A service model that allows the source to reserve resources on routers using a rate-based description of its needs. Virtual clock goes beyond the best-effort delivery service of the current Internet.


VPI: Virtual path identifier. An 8-bit or 12-bit field in the ATM header. VPI can be used to hide multiple virtual connections across a network inside a single virtual “path,” thus

decreasing the amount of connection state that the switches must maintain. See also VCI. VPN: Virtual private network. A logical network overlaid on top of some existing network. For example, a company with sites around the world may build a virtual network


on top of the Internet rather than lease lines between each site.

WAN: Wide area network. Any physical network technology that is capable of spanning long distances (e.g., cross-country). Compare with SAN, LAN, and MAN.


weighted fair queuing (WFQ): A variation of fair queuing in which each flow can be given a different proportion of the network capacity.

well-known port: A port number that is, by convention, dedicated for use by a particular server. For instance, the Domain Name Server receives messages at well-known UDP and TCP port 53 on every host.


WSDL: Web Services Description Language. A component of the web services framework for specifying and implementing application protocols.


WWW: World Wide Web. A hypermedia information service on the Internet. X.25: The ITU packet-switching protocol standard.


X.400: The ITU electronic mail standard. The counterpart to SMTP in the Internet architecture.


X.500: The ITU directory services standard, which defines an attribute-based naming service.

X.509: An ITU standard for digital certificates.


XDR: External Data Representation. SunMicrosystems’ standard for machine-independent data structures. Contrast with ASN.1 and NDR.


XML: Extensible Markup Language. Defines a syntax for describing data that may be


passed between Internet applications.

XSD: XML Schema Definition. A schema language for defining the format and interpretation of XML objects.


zone: A partition of the domain name hierarchy, corresponding to an administrative authority that is responsible for that portion of the hierarchy. Each zone must have at least two name servers to field DNS requests for the zone.


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