Glossary of Key Terms

ARP Address Resolution Protocol. This is a TCP/IP protocol that resolves IP addresses used by TCP/IP-based software to media access control addresses used by LAN hardware.

availability A measure of the fault tolerance of a computer and its programs. A highly available computer system provides continuous service without interruptions due to software or hardware failures.

client affinity A configuration option for the multiple-host filtering mode within a port rule that specifies whether Network Load Balancing should direct multiple requests from the same client IP address or class C address space to the same cluster host. Three affinity settings are possible: none, single client, and class C.

client request A service request from a client computer to a server computer or a cluster of computers. Network Load Balancing forwards each client request to a specific host within a cluster according to the system administrator's load-balancing policy.

cluster A set of computers that work together to provide a service. The use of a cluster enhances both the availability and scalability of the service. Network Load Balancing provides a software solution for clustering multiple computers running networked client/server applications.

cluster adapter The network adapter in a Network Load Balancing cluster that handles the network traffic for cluster operations (that is, the traffic for all hosts in the cluster). This adapter is assigned one or more virtual IP address and, optionally, a dedicated IP address.

convergence A process by which Network Load Balancing hosts exchange messages to determine a new, consistent state of the cluster and to elect the host with the highest host priority, known as the default host. During convergence, a new load distribution is determined for hosts that share the handling of network traffic for specific TCP or UDP ports.

CPU filtering overhead A performance measure defined here as Network Load Balancing's CPU percentage on a cluster host required to analyze and filter network packets (lower is better).

CPU transfer overhead A performance measure defined here as Network Load Balancing's CPU percentage on a cluster host required for it to transfer network packets through a cluster host.

dedicated IP address A cluster host's unique IP address used for network traffic not associated with the cluster (for example, Telnet access to a specific host within the cluster).

default host The host with the highest host priority and which handles all of the network traffic for TCP and UDP ports that are not otherwise covered by port rules.

filtering mode A configuration setting for a port rule that determines load-balancing policy for the range of ports covered by the port rule. There are three possible filtering modes: multiple host, single host, and disabled.

handling priority A configuration setting in single-host filtering mode that specifies a host's priority for handling all of the cluster's network traffic for that port rule. Handling priority overrides host priority for the range of ports covered by the port rule.

heartbeat message A network packet periodically broadcast by each cluster host to inform other hosts of its health and configuration. Network Load Balancing initiates convergence when it fails to receive heartbeat messages from another host or when it receives a heartbeat message from a new host.

high availability See availability.

host As used here, a computer that participates in a Network Load Balancing cluster. A host is also called a server.

host priority A configuration setting that specifies a cluster host's priority for handling all of the cluster's network traffic not covered by port rules. The host with the highest priority (lowest numerical value in the range of 1 to 32) is called the default host.

latency A performance measure defined here as the non-overlapped portion of Network Load Balancing's CPU overhead (lower is better). Latency adds to the client response time.

load balancing A technique for scaling performance by distributing load among multiple servers. Network Load Balancing distributes load for networked client/server applications in the form of client requests that it partitions across multiple cluster hosts.

load weight A configuration setting for the multiple-host filtering mode within a port rule that specifies the percentage of load-balanced network traffic that this host should handle; allowed values range from 0 (zero) to 100. The actual fraction of traffic handled by each host is computed as the local load weight divided by the sum of all load weights across the cluster.

MAC address A link-layer network address, called a media access control address, that is used to communicate with other network adapters on the same subnet. Each network adapter has an associated MAC address.

multicast MAC address A type of media access control address used by multiple, networked computers to concurrently receive the same incoming network packets. In multicast mode, Network Load Balancing optionally uses multicast MAC addresses to efficiently distribute incoming network traffic to cluster hosts.

multicast mode A configuration setting which instructs Network Load Balancing to add a multicast MAC address to the cluster adapters on all hosts in a cluster. The adapters' existing MAC addresses are not changed. See also unicast mode.

multihomed A computer that has multiple network cards or that has been configured with multiple IP addresses for a single network card. Network Load Balancing supports multihomed servers by allowing multiple virtual IP addresses to be assigned to the cluster adapter.

network adapter A plug-in board that connects a computer to a local area network (LAN). A Network Load Balancing cluster communicates using a cluster adapter over the LAN connecting the cluster's hosts to clients.

overhead A performance measure defined here as the CPU percentage on a cluster host used by Network Load Balancing, consisting primarily of CPU filtering overhead and CPU transfer overhead. A portion of overhead contributes to latency.

port rule A configuration setting within the Network Load Balancing Properties dialog box that specifies how cluster network traffic for a range of ports is to be handled. The method by which a port's network traffic is handled is called its filtering mode.

primary IP address A virtual IP address used by Network Load Balancing to identify the cluster for remote control operations and within heartbeat messages. See also virtual IP address.

response time A performance measure defined as the round-trip delay to process a client request. Response time increases with the non-overlapped portion of CPU overhead, called latency (lower is better).

RRDNS A type of domain name service, called round-robin DNS, which distributes clients to multiple servers in a round-robin manner. RRDNS scales performance but does not provide high availability.

scalability A measure of how well a computer, service, or application can grow to meet increasing performance demands. For clusters, scalability is measured by the ability to incrementally add one or more systems to an existing cluster when the overall load of the cluster exceeds its capabilities.

session In the context of load balancing TCP/IP traffic, a set of client requests directed to a server. These requests can be invoked with multiple, possibly concurrent, TCP connections. The server program sometimes maintains state information between requests. To preserve access to the server state, system administrators can have Network Load Balancing direct all requests within a session to the same cluster host when load balancing by setting client affinity.

state As used here, data maintained by a server application on behalf of its clients. To scale a server application by running multiple instances with Network Load Balancing, client state must be accessible and properly coordinated by all instances of the application.

switch A central network device that forwards packets to specific ports rather than, as in conventional hubs, broadcasting every packet to every port. Switches can deliver higher total bandwidth than hubs can by supporting multiple, simultaneous connections.

switch-flooding A switch behavior induced by Network Load Balancing to simultaneously deliver all incoming client packets to all switch ports. This behavior enables Network Load balancing to deliver very high throughput, but it may cause high switch occupancy.

switch occupancy A performance measure defined here as the fraction of a switch port's bandwidth occupied by Network Load Balancing's incoming client requests (lower is better). Switch occupancy increases with additional client traffic and must not adversely impact port bandwidth.

TCP/IP Transmission Control Protocol/Internet Protocol. A set of networking protocols that is used on the Internet to provide communications among interconnected networks made up of computers with diverse hardware architectures and various operating systems. TCP/IP includes standards for how computers communicate and conventions for connecting networks and routing traffic.

throughput A performance measure defined here as the number of client requests processed by a Network Load Balancing cluster per unit time (higher is better). Throughput increases with additional client traffic that the cluster can handle prior to saturating its hosts.

unicast mode A configuration setting which instructs Network Load Balancing to change the MAC address of the cluster adapters to the same value for all hosts in a cluster. This is the default mode of operation. See also multicast mode.

virtual IP address An IP address that is shared among the hosts of a Network Load Balancing cluster and used by clients to address the cluster as a whole. A Network Load Balancing cluster supports multiple virtual IP addresses, such as in a cluster of multihomed Web servers. One of the virtual IP addresses is the primary IP address, which is used to identify the cluster for remote control operations.