Load Balancer Glossary

A
ACK
A control bit (acknowledge) occupying no sequence space, which indicates that the acknowledgment field of this segment specifies the next sequence number the sender of this segment is expecting to receive, hence acknowledging receipt of all previous sequence numbers.

address
The unique code assigned to each device or workstation connected to a network. A standard IP address is a 32-bit address field. This field contains two parts. The first part is the network address; the second part is the host number.

advisor
The advisors are a function of the Load Balancer components. Advisors collect and analyze feedback from individual servers and inform the manager component.

agent
(1) In systems management, a user that, for a particular interaction, has assumed an agent role.
(2) An entity that represents one or more managed objects by (a) emitting notifications regarding the objects and (b) handling requests from managers for management operations to modify or query the objects.

alias
An additional name assigned to a server. The alias makes the server independent of the name of its host machine. The alias must be defined in the domain name server.

API (Application programming interface)
The interface (calling conventions) by which an application program accesses operating system and other services. An API is defined at source code level and provides a level of abstraction between the application and the kernel (or other privileged utilities) to ensure the portability of the code.

B

backup
In high availability for the Dispatcher, the partner of the primary machine. It monitors the status of the primary machine and takes over if necessary. See also high availability, primary.

bandwidth
The difference between the highest and lowest frequencies of a transmission channel; the amount of data that can be sent through a given communication circuit per second.

begin range
In rules-based load balancing, a lower value specified on a rule. The default for this value depends on the type of rule.

binary logging
Allows server information to be stored in binary files, and then be processed to analyze the server information that is gathered over time.

C

caching proxy
A caching proxy server that can help speed up end-user response time through highly-efficient caching schemes. Flexible PICS filtering helps network administrators control access to Web-based information at one central location.

CBR (Content Based Routing)
A component of Load Balancer. CBR works with Caching Proxy to load balance incoming client requests, based on Web page content using specified rule types, to HTTP or HTTPS servers.

cbrcontrol
Provides the interface to the Content Based Routing component of Load Balancer.

cbrserver
In Content Based Routing, handles the requests from the command line to the executor, manager and advisors.

ccocontrol
In Cisco CSS Controller, provides the interface to the Cisco CSS switch.

ccoserver
In Cisco CSS Controller, handles the requests from the command line to the consultants.

CGI (Common Gateway Interface)
A standard for the exchange of information between a Web server and an external program. The external program can be written in any language supported by the operating system, and performs tasks not usually done by the server, such as forms processing.

CGI script
A CGI program written in a scripting language such as Perl or REXX that uses the Common Gateway Interface to perform tasks not usually done by the server, such as forms processing.

Cisco CSS Controller (CCO)
A component of Load Balancer. Cisco CSS Controller uses Load Balancer technology to provide real-time load balancing information to the Cisco Content Services Switch.

Cisco CSS Switch
Any of Cisco's CSS 11000 series switches, used for packet forwarding and content routing.

client
A computer system or process that requests a service of another computer system or process. For example, a workstation or personal computer requesting HTML documents from a Lotus Domino Go Webserver is a client of that server.

Cluster
The cluster name or address to which clients connect. The cluster value is either a symbolic name or in dotted-decimal format. A cluster value of 0.0.0.0 can be used to specify a wildcard cluster.

Cluster Address
The address to which clients connect. The cluster address can be either the dotted-decimal IP address or the host name of the machine. If the cluster value is not resolvable, you must provide this IP address of the physical machine.

clustered server
A server that the Dispatcher groups with other servers into a single, virtual server. Load Balancer balances TCP or UDP traffic among these clustered servers.

collocate
When Load Balancer is installed on the same machine it is load balancing. Collocate only applies to AIX, Red Hat Linux, SuSE Linux and Solaris operating systems.

consultant
Collects server metrics from the servers (which are being load balanced) and sends server weight information to the switch that performs the load balancing.

controller
A collection of one or more consultants.

cross port affinity
The affinity (sticky) feature expanded to cover across multiple ports.

D

daemon (Disk And Execution MONitor)
A program that is not involved explicitly, but lies dormant waiting for some condition(s) to occur. The idea is that the perpetrator of the condition need not be aware that a daemon is lurking (though often a program will commit an action only because it knows that it will implicitly invoke a daemon).

default
A value, attribute, or option that is assumed when none is explicitly specified.

destination address
The address of the high availability partner machine to which heartbeats and responses are sent.

Dispatcher
A component of Load Balancer that efficiently balances TCP or UDP traffic among groups of individual linked servers. The Dispatcher machine is the server running the Dispatcher code. Dispatcher, CBR, Site Selector, Cisco CSS Controller and Nortel Alteon Controller are all called components of Load Balancer. Each of these components have functions.

DNS (Domain Name Server)
A general-purpose distributed, replicated, data query service chiefly used on Internet for translating hostnames into Internet addresses. Also, the style of hostname used on the Internet, though such a name is properly called a fully qualified domain name. DNS can be configured to use a sequence of name servers, based on the domains in the name being looked for, until a match is found.

dotted-decimal notation
The syntactical representation for a 32-bit integer that consists of four 8-bit numbers, written in base 10 and separated by periods (dots). It is used to represent IP addresses.

dscontrol
Provides the interface to the Dispatcher component of Load Balancer.

dsserver
In Dispatcher, handles the requests from the command line to the executor, manager, and advisors.

E

end range
In rules-based load balancing, a higher value specified on a rule. The default for this value depends on the type of rule.

Ethernet
A standard type of local area network (lan). It allows multiple stations to access the transmission medium at will without prior coordination, avoids contention by using carrier sense and deference, and resolves contention by using collision detection and transmission. Software protocols used by Ethernet systems vary, but include TCP/IP.

executor
One of several Load Balancer functions. The executor routes requests to the TCP or UDP servers, and also monitors the number of new, active, and finished connections and does garbage collection of completed or reset connections. The executor supplies the new and active connections to the manager component. For Cisco CSS Controller or Nortel Alteon Controller, the executor holds the configuration information and contains the information required to connect to the Cisco CSS Switch or the Nortel Alteon Switch.

F

FIN
A control bit (finis) occupying one sequence number, which indicates that the sender will send no more data or control occupying sequence space.

FIN state
The status of a transaction that has finished. Once a transaction is in FIN state, the Load Balancer garbage collector can clear the memory reserved for the connection.

Firewall
A computer that connects a private network, such as a business, to a public network, such as the Internet. It contains programs that limit the access between two networks. See also proxy gateway.

FQDN (fully qualified domain name)
The full name of a system, consisting of its local hostname and its domain name, including a top-level domain (tld). For example, "venera" is a hostname and "venera.isi.edu" is an FQDN. An FQDN should be sufficient to determine a unique Internet address for any host on the Internet. This process, called "name resolution", uses the Domain Name System (DNS).

FTP (File Transfer Protocol)
An application protocol used for transferring files to and from network computers. FTP requires a user ID and sometimes a password to allow access to files on a remote host system.

G

gateway
A functional unit that interconnects two computer networks with different architectures.

GRE (Generic Routing Encapsulation)
A protocol which allows an arbitrary network protocol A to be transmitted over any other arbitrary protocol B, by encapsulating the packets of A within GRE packets, which in turn are contained within packets of B.

H

heartbeat
A simple packet sent between two Load Balancer machines in high availability mode used by the standby Load Balancer to monitor the health of the active Dispatcher.

high availability
A Load Balancer feature in which one Load Balancer can take over the function of another, should that part fail.

host
A computer, connected to a network, that provides an access point to that network. A host can be a client, a server, or both simultaneously.

host name
The symbolic name assigned to a host. Host names are resolved to IP addresses through a domain name server.

HTML (Hypertext Markup Language)
The language used to create hypertext documents. Hypertext documents include links to other documents that contain additional information about the highlighted term or subject. HTML controls the format of text and position of form input areas, for example, as well as the navigable links.

HTTP (Hypertext Transfer Protocol)
The protocol used to transfer and display hypertext documents.

HTTPS (Hypertext Transfer Protocol, Secure)
The protocol used to transfer and display hypertext documents using SSL

I

Internet
The worldwide collection of interconnected networks that use the Internet suite of protocols and permit public access.

ICMP (Internet Control Message Protocol)
A message control and error-reporting protocol between a host server and a gateway to the Internet.

IMAP (Internet Message Access Protocol)
A protocol allowing a client to access and manipulate electronic mail messages on a server. It permits manipulation of remote message folders (mailboxes), in a way that is functionally equivalent to local mailboxes.

intranet
A secure, private network that integrates Internet standards and applications (such as Web browsers) with an organization's existing computer networking infrastructure.

IP (Internet Protocol)
A connectionless protocol that routes data through a network or interconnected networks. IP acts as an intermediary between the higher protocol layers and the physical layer.

IP address
Internet Protocol address. The unique 32-bit address that specifies the actual location of each device or workstation in a network. It is also known as an Internet address.

IPSEC (Internet Protocol Security)
A developing standard for security at the network or packet processing layer of network communication.

L

LAN (Local Area Network)
A computer network of devices connected within a limited geographical area for communication and which can be connected to a larger network.

loopback alias
An alternative IP address associated with the loopback interface. The alternative address has the useful side effect of not advertising on a real interface.

loopback interface
An interface that bypasses unnecessary communications functions when the information is addressed to an entity within the same system.

M

MAC address (Media Access Control address)
The hardware address of a device connected to a shared network medium.

managed node
In Internet communications, a workstation, server, or router that contains a network management agent. In the Internet Protocol (IP), the managed node usually contains a Simple Network Management Protocol (SNMP) agent.

manager
One of several Load Balancer functions existing on all components. The manager sets weights based on internal counters in the executor and feedback provided by the advisors. The executor then uses the weights to perform load balancing.

mark down
To break all active connections to a server and stop any new connections or packets from being sent to that server.

mark up
To allow a server to receive new connections.

metric
A process or command that returns a numeric value that can be used in load balancing on the network; for example, the number of users currently logged on.

metric address
The address where the metric server connects.

metric collector
Resides in the consultant and is responsible for collecting a metric or metrics.

metric server
Formerly known as Server Monitor Agent (SMA). Metric server provides system specific metrics to the manager(s) of Load Balancer components.

MIB
(1) Management Information Base. A collection of objects that can be accessed by means of a network management protocol.
(2) A definition for management information that specifies the information available from a host or gateway and the operations allowed.

multiple address collocation
Multiple address collocation allows the customer to specify the address of the collocated server to be different than the nonforwarding address (NFA) in the configuration.

mutual high availability
Mutual high availability allows two Dispatcher machines to be both primary and backup for each other.

N

nalcontrol
Provides the interface to the Nortel Alteon Controller component of Load Balancer.

nalserver
In Nortel Alteon Controller, a script that starts the Nortel Alteon Controller in a java virtual machine.

netmask
For Internet subnetworking, a 32-bit mask used to identify the subnetwork address bits in the host portion of an IP address.

network
Hardware and software data communication system. Networks are often classified according to their geographical extent, local area network (LAN), metropolitan area network (MAN), wide area network (WAN) and also according to the protocols used.

Network Address Translation (NAT, or Network Address Translator, Virtual LAN)
Allows you to configure multiple server daemons within one physical server to respond to requests to different IP addresses

Network Address Port Translation (NAPT)
Also known as port mapping. This allows you to configure multiple server daemons within one physical server to listen on different port numbers.

network management station
In the Simple Network Management Protocol (SNMP), a station that executes management application programs that monitor and control network elements.

network proximity
The proximity of two networked entities, such as a client and server, which Site Selector determines by measuring round-trip time.

NIC (Network Interface Card)
An adapter circuit board installed in a computer to provide a physical connection to a network.

NNTP (Network News Transfer Protocol)
A TCP/IP protocol for transferring news items.

nonforwarding address (nfa)
The primary IP address of the Load Balancer machine, used for administration and configuration.

Nortel Alteon Controller
A component of IBM Load Balancer. Nortel Alteon Controller uses Load Balancer technology to provide real-time load balancing information to the Nortel Alteon Web Switch.

Nortel Alteon Web Switch
The Nortel Alteon ACE Director Series Switch and the Nortel Alteon 180 Series Switch from the Alteon Web Switching portfolio, used for packet forwarding and content routing.

P

packet
The unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network.

PICS (Platform for Internet Content Selection)
PICS-enabled clients allow the users to determine which rating services they want to use and, for each rating service, which ratings are acceptable and which are unacceptable.

ping
A command that sends Internet Control Message Protocol (ICMP) echo-request packets to a host, gateway, or router with the expectation of receiving a reply.

POP3 (Post Office Protocol 3)
A protocol used for exchanging network mail and accessing mailboxes.

port
A number that identifies an abstracted communication device. Web servers use port 80 by default.

primary
In high availability for the Dispatcher, the machine that starts out as the machine actively routing packets. Its partner, the backup machine, monitors the status of the primary machine and takes over if necessary. See also backup, high availability.

priority
In rules-based load balancing, the level of importance placed upon any given rule. The Dispatcher evaluates rules from the first priority level to the last priority level.

private network
A separate network on which Load Balancer communicates with clustered servers for performance reasons.

protocol
The set of rules governing the operation of functional units of a communication system if communication is to take place. Protocols can determine low-level details of machine-to-machine interfaces, such as the order in which bits from a byte are sent; they can also determine high-level exchanges between application programs, such as file transfer.

Q

Quality of Service (QoS)
The performance properties of a network service, including throughput, transit delay and priority. Some protocols allow packets or streams to include QoS requirements.

quiesce
To end a process by allowing operations to complete normally.

R

reach
In Dispatcher, an advisor that issues pings to a given target and reports whether that target is responding.

reach address
In high availability for the Dispatcher, the address of the target to which the advisor should issue pings to see if the target is responding.

return address
A unique IP address or hostname. It is configured on the Dispatcher machine and used by Dispatcher as its source address when load balancing the client's request to the server.

RMI (Remote Method Invocation)
A method used by a programmer, using the Java programming language and development environment, to write object-oriented programs in which objects on different computers can interact in a distributed network.

root user
The unrestricted authority to access and modify any part of the AIX, Red Hat Linux, SUSE Linux or Solaris operating system, usually associated with the user who manages the system.

route
The path of network traffic from origin to destination.

router
A device which forwards packets between networks. The forwarding decision is based on network layer information and routing tables, often constructed by routing products.

RPM
Red Hat Package Manager.

rule
In rules-based load balancing, a mechanism for grouping servers such that a server can be chosen based on information other than the destination address and port.

rule type
In rules-based load balancing, an indicator of the information that should be evaluated to determine whether a rule is true.

S

scalable
Pertaining to the capability of a system to adapt readily to a greater or lesser intensity of use, volume, or demand. For example, a scalable system can efficiently adapt to work with larger or smaller networks performing tasks of varying complexity.

server
A computer that provides shared services to other computers over a network; for example, a file server, a print server, or a mail server.

server address
The unique code assigned to each computer that provides shared services to other computers over a network; for example, a file server, a print server, or a mail server. A standard IP address is a 32-bit address field. The server address can be either the dotted decimal IP address or the host name.

server machine
A server that the Dispatcher groups with other servers into a single, virtual server. The Dispatcher balances traffic among the server machines. Synonymous with clustered server.

service
A function provided by one or more nodes; for example, HTTP, FTP, Telnet.
For Nortel Alteon Controller, a service is the function or information requested by an end user from a site. It is identified by a virtual IP address and a virtual port number by an end user request. On the switch, it is defined by a virtual server identifier, which is an integer and a virtual port number or service name.
For Cisco CSS Controller, a service is a destination location where a piece of content physically resides. For example, a local or remote server and port.

shell
The software that accepts and processes command lines from a user's workstation. The Bash shell is one of several UNIX shells available.

site name (aka host name)
A site name is an unresolvable host name that the client will request. For example, a web site has 3 servers (1.2.3.4, 1.2.3.5, and 1.2.3.6) configured for site name www.dnsload.com. When a client requests this site name, on of the three server IP addresses will be returned as the resolution. The site name must be a fully qualified domain name, for example, dnsload.com. An unqualified name, for example, dnsload is invalid for a site name.

Site Selector
A component of Load Balancer. Site Selector works in conjunction with a name server to load balance among a group of servers using measurements and weights that are gathered from the Metric Server component running on those servers.

SMTP (Simple Mail Transfer Protocol)
In the Internet suite of protocols, an application protocol for transferring mail among users in the Internet environment. SMTP specifies the mail exchange sequences and message format. It assumes that the Transmission Control Protocol (TCP) is the underlying protocol.

SNMP (Simple Network Management Protocol)
The Internet standard protocol, defined in STD 15, RFC 1157, developed to manage nodes on an IP network. SNMP is not limited to TCP/IP. It can be used to manage and monitor all sorts of equipment including computers, routers, wiring hubs, toasters and jukeboxes.

source address
In high availability for the Dispatcher, the address of the high availability partner machine that sends heartbeats.

SPARC
Scalable processor architecture.

sscontrol
Provides the interface to the Site Selector component of Load Balancer.

SSL (Secure Sockets Layer)
A popular security scheme developed by Netscape Communications Corp. along with RSA Data Security Inc. SSL allows the client to authenticate the server and all data and requests to be encrypted. The URL of a secure server protected by SSL begins with https (rather than http).

ssserver
In Site Selector, handles the requests from the command line to the nameserver, manager and advisors.

sticky time
The interval between the closing of one connection and the opening of a new connection during which a client will be sent back to the same server used during the first connection. After the sticky time, the client may be sent to a server different from the first.

strategy
In high availability for the Dispatcher, a keyword for specifying how recovery takes place following the failure of the active machine.

subnet mask
For Internet subnetworking, a 32-bit mask used to identify the subnetwork address bits in the host portion of an IP address.

SYN
A control bit in the incoming segment, occupying one sequence number, used at the initiation of a connection, to indicate where the sequence numbering will start.

T

TCP (Transmission Control Protocol)
A communications protocol used on the Internet. TCP provides reliable host-to-host exchange of information. It uses IP as the underlying protocol.

TCP/IP (Transmission Control Protocol/Internet Protocol)
A suite of protocols designed to allow communication between networks regardless of the communication technologies used in each network.

TCP server machine
A server that Load Balancer links with other servers into a single, virtual server. Load Balancer balances TCP traffic among the TCP server machines. Synonymous with clustered server.

Telnet
Terminal emulation protocol, a TCP/IP application protocol for remote connection service. Telnet allows a user at one site to gain access to a remote host as if the user's workstation were connected directly to that remote host.

Time to Live (TTL)
A DNS TTL is the number of seconds a client can cache the name resolution response.

timeout
A period of time after which an error is raised if some event has not occurred. A common example is sending a message. If the receiver does not acknowledge the message with some preset timeout period, a transmission error is assumed to have occurred.

TOS (Type of Service)
A one byte field in the IP header of the SYN packet.

U

UDP (User Datagram Protocol)
In the Internet suite of protocols, a protocol that provides unreliable, connectionless datagram service. It enables an application program on one machine or process to send a datagram to an application program on another machine or process. UDP uses the Internet Protocol (IP) to deliver datagrams.

URI (Universal Resource Identifier)
The encoded address for any resource on the Web, such as HTML document, image, video clip, program, and so forth.

URL (Uniform Resource Locator)
A standard way of specifying the location of an object, typically a Web page, on the Internet. URLs are the form of address used on the World-Wide Web. They are used in HTML documents to specify the target of a hyperlink which is often another HTML document (possibly stored on another computer).

V

VPN (Virtual Private Network)
A network comprised of one or more secure IP tunnels connecting two or more networks.

W

WAN (Wide Area Network)
A network that provides communication services to a geographic area larger than that served by a local area network or a metropolitan are network, and that may use or provide public communication facilities.

WAP (Wireless Application Protocol)
An open international standard for applications that use wireless communication, e.g. Internet access from a mobile phone.

WAS
WebSphere Application Server.

Web
The network of HTTP servers that contain programs and files, many of them hypertext documents that contain links to other documents on HTTP servers. Also World Wide Web.

wizard
A dialog within an application that uses step-by-step instructions to guide a user through a specific task.

WLM (Workload Manager)
An advisor provided with Dispatcher. It is designed to work only in conjunction with servers on OS/390 mainframes running the MVS Workload Manager (WLM) component.