A. Glossary

This appendix gives a glossary of technical terms used throughout this thesis. Where terms are taken from another source, the source is cited in the definition of the term. Italicised words represent terms that are defined elsewhere in this glossary.

Acknowledgment

A response sent by a Receiver to indicate successful reception of information. Acknowledgments may be implemented at any level [Comer 88].

ACK

Abbreviation for Acknowledgment [Comer 88].

Application

A computer program.

Bandwidth

The difference between the limiting frequencies of a continuous frequency spectrum [Stallings 91]. Used colloquially in this thesis to mean the overall Bit Rate capacity of a Link.

Binary Feedback Mechanism

A mechanism which returns a single bit of data as its feedback. DecBit [Jain et al 87] and PRCA [Hluchyj 94] are binary feedback congestion control mechanisms.

Bit Rate

The rate at which bits are transmitted, in bits per unit time. See also Data Rate.

Bottleneck

The network resource which is limiting the Bit Rate of a transmission.

Buffer

An amount of computer memory used to store information. Routers have a finite amount of buffers (also known as Buffer Space) in which to hold incoming Packets for forwarding. Hosts also have a finite amount of buffer space to hold incoming packets and packets which they have generated but not yet transmitted.

Buffer Occupancy

The current level of Buffer use, either in terms of Packets queued, or bits queued.

Byte

A colloquial term for Octet.

Capacity

The highest Data Rate that can be sustained through a network from a Source to a Destination, or through part of a network. See also Sustainable Rate.

Congestion

The state when the resource demands on a network (or a part of a network) exceeds its Capacity [Jain & Ramakrishnan 88].

Congestion Avoidance

A congestion control scheme allows a network to operate in the region of low Delay and high Throughput, where Power is at its optimum
[Jain & Ramakrishnan 88]. See Section 1.3 for more details.

Congestion Control

A congestion control scheme tries to bring a network back into an operating state, when demand has already exceeded Capacity
[Mankin & Ramakrishnan 91].

Connection

The path between two Protocol modules that provides reliable delivery service [Comer 88].

Connection Setup

The stage of a Connection-Oriented Service where the Connection is established between two Hosts.

Connection Teardown

The stage of a Connection-Oriented Service where the Connection between two Hosts is broken.

Connectionless Service

Characteristic of the Packet delivery service offered by most hardware and the Internet Protocol. The connectionless service treats each packet as a separate entity that contains the Source and Destination address. Usually, connectionless services can drop packets or deliver them out of sequence [Comer 88].

Connection-Oriented Service

A communication service which is based upon the concept of a Connection.

Data Rate

The rate of data in bits per unit time. See also Bit Rate.

Datagram

The basic unit of information passed by a Network Layer across a network. It contains a Source and Destination address along with data [Comer 88]. See also Frame, Packet and Segment.

Delay

The amount of time it takes for data (or a quantum of data such as a Packet) to leave a Source and reach its Destination. See also End-to-End Time.

Destination

The Host which is the intended destination of a flow of data across a network.

End-to-End

Something which is performed by a Source and Destination, of which the intermediate components of the network have no knowledge. Flow Control is an end-to-end mechanism.

End-to-End Time

The time between the transmission of a Packet at the Source and its reception at the Destination. See also Delay.

Field

An individual item of data within a Header.

Flow Control

Control of the rate at which Hosts or Routers inject Packets into a network. Flow control mechanisms can be implemented at various levels [Comer 88].

Frame

Literally, a packet as it is transmitted across a serial line [Comer 88]. In this thesis, it refers to a packet as it travels across a Link between two peer Link Layers.

Gateway

A dedicated computer that is attached to two or more networks and routes Packets from one to the other. In particular, an Internet gateway routes IP datagrams among the networks to which it connects [Comer 88]. Although the term has a completely different meaning within the OSI Reference Model, its meaning as defined here is widely used in the Internet community, and is used as such throughout this thesis. See also Node, Router and Packet Switch.

Go Back N Acknowledgment

A method of data acknowledgment where the first loss of data is indicated. Upon receipt of this acknowledgment, a Source must retransmit all data from the point of first loss onwards. Compare Selective Acknowledgment and Selective Retransmission.

Header

System defined control data that precedes user data [Stallings 91].

Host

A computer attached to a network that is not a Router. It is usually a Source or Destination (or both) of data flows.

Interface

The physical connection of a Host or Node to a network. A network-connected computer may have one or more interfaces.

Inter-packet Delay

The interval between successive Packet transmissions from a Source to a particular Destination.

Latency

The propagation time of a bit or Frame across a Link.

Leaky Bucket

A single-server queueing system with constant service time [Turner 86].

Link

A physical connection between two Nodes on a network. Data flows between the Nodes on a network via links.

Link Layer

The communications layer that provides for the reliable transfer of information across a Link [Stallings 91].

Load

The Data Rate being offered to a network, or part of a network. See also Capacity.

Network Layer

The communications layer that provides for the delivery of information across a number of Links.

Node

The computers in a network which transfer data between Links. In a Packet-Switched Network, a Node is also known as a Packet Switch or Gateway. See also Router.

Octet

A group of eight bits. Often colloquially known as a Byte.

Output Interface

The Interface used by a Host or Node to transmit on a Link.

Packet

The basic unit of data flow in a Packet-Switched Network. Packets can vary in size between a fixed range of octets. See also Datagram, Frame and Segment.

Packet Admission

The insertion of a Packet into a network via an Output Interface.

Packet Switch

A dedicated computer that is attached to two or more networks and routes Packets from one to the other. See also Gateway, Node and Router.

Packet Switched Network

A network which uses Packet Switching as its method of data flow.

Packet Switching

A method of transmitting data through a network, in which long messages are subdivided into Packets, which are transmitted one at a time across the network via Packet Switches [Stallings 91].

Packet Loss

The loss of Packets at a Node due to the lack of available Buffer space to queue them.

Path Length

The length of the communications path between the Source and Destination. This may be measured in distance, delay, or by the number of intermediate Nodes. See also End-to-End Time.

Power

The ratio of Throughput to Delay [Mankin & Ramakrishnan 91].

Protocol

A set of rules that govern the operation of functional units to achieve communication [Stallings 91].

Queue

A first-in, first-out Buffer.

Queue Length

An alternative term for Buffer Occupancy.

Receiver

An alternative term for Destination.

Response Time

An alternative term for Delay.

Round Trip Time

The Delay for data or a Packet to reach a Destination and the Acknowledgment for that packet to return to the Source from the destination.

Route

The path that network traffic takes from its Source to its Destination.

Router

A device used to connect two networks that may or may not be similar. The router employs an internet Protocol present in each router and each Host of the network. The router operates at the Network Layer (layer 3) of the OSI Reference Model
[Stallings 91].

Segment

The unit of transfer sent from TCP on one machine to TCP on another. Each segment contains part of a stream of bytes being sent between the machines as well as additional fields that identify the current position in the stream and contains a checksum to ensure validity of received data [Comer 88]. See also Datagram, Frame and Packet.

Selective Acknowledgment

A method of data acknowledgment where only the data that has been lost is indicated as such. Compare Go Back N Acknowledgment.

Selective Retransmission

A method of retransmission in the face of network errors where only that data which has been lost is retransmitted.

Sender

An alternative term for Source.

Sequence Number

A Field within a Header that uniquely identifies the location of a Packet's contents within a Traffic Flow.

Sliding Window

Characteristic of Protocols that allow a Sender to transmit more than one Packet of data before receiving an Acknowledgment. After receiving an acknowledgment for the first packet, the sender `slides' the packet window and sends another. The number of outstanding packets or Octets is known as the Window Size [Comer 95].

Source

The Host which is the source of a flow of data across a network to a Destination.

Sustainable Rate

The maximum Bit Rate at which a Source can transmit data to a Destination without causing congestion in the intermediate Nodes in the network. See also Capacity, Load.

Throughput

The ratio of Data Rate being offered to a network by a Source or sources to the data rate being received by the corresponding Destinations.

Traffic Flow

A flow of data over a Connection between a Source and its Destination.

Transmission Rate

The rate of data transmission in bits per unit time. See also Bit Rate, Data Rate.

Transport Layer

The communications layer that provides reliable, transparent transfer of data between end points; it provides End-to-End error recovery and Flow Control.

Utilisation

The use of a Link for data transmission, averaged over a time interval. The value is often normalised, so that the value of 1 indicates that the link was completely utilised over the time interval.

Variance

The non-uniformity of a set of samples from a constant value. Mathematically, the square of the set's standard deviation.

Window

See Sliding Window.

Window Size

The number of outstanding Packets or Octets in a Sliding Window.