Intenet Security & Acceleration Server - NLB Support

ISA server integrated with NLBIntroduction

Microsoft® Internet Security and Acceleration (ISA) Server 2004 Enterprise Edition introduces a multi-networking model, which allows you to configure how policy should be applied between multiple networks. With this multi-networking model, ISA Server integrates Network Load Balancing (NLB) functionality, so that you can balance the load across all the array members on one or more networks.

You can use ISA Server to configure and manage the NLB functionality of Microsoft Windows Server™ 2003 running on ISA Server arrays. When you configure NLB through ISA Server, NLB is integrated with ISA Server functionality. This provides important functionality that is not available in Windows NLB alone.

In addition, ISA Server monitors NLB configuration, and discontinues NLB on a particular computer as necessitated by its status. This prevents the continued functioning of NLB when the state of the computer does not allow the passage of traffic. For example, if there is a failure of the network adapter on the computer, or if you stop the Microsoft Firewall service, ISA Server stops NLB-directed traffic from passing through that computer. When the issue is resolved, ISA Server will again allow traffic to pass through that computer.

What is Network Load Balancing?

Network Load Balancing is a clustering technology offered by Microsoft as part of all Windows 2000 Server and Windows Server 2003 family operating systems. NLB uses a distributed algorithm to load balance network traffic across a number of hosts, helping to enhance the scalability and availability of mission critical, IP-based services, such as Web, Virtual Private Networking, Streaming Media, Terminal Services, Proxy, etc. It also provides high availability by detecting host failures and automatically redistributing traffic to operational hosts.

Network Load Balancing (NLB) clusters dynamically distribute the flow of incoming TCP and UDP traffic among the cluster nodes according to a set of traffic-handling rules. NLB clusters provide a highly available and scalable platform for applications such as IIS, ISA server, etc. NLB is used for stateless applications; i.e. those that do not build any state as a result of a request.

NLB has a kernel component called wlbs.sys. This is an intermediate NDIS driver. NLB also has user-mode components for management purposes.

NLB does not respond to changes in the server load (such as CPU usage or memory utilization) or the health of an application


NLB Working :

NLB employs a fully distributed filtering algorithm to map incoming clients to the cluster hosts. This algorithm enables cluster hosts to independently and quickly make a load balancing decision for each incoming packet. It is optimized to deliver statistically even load balance for a large client population making numerous, relatively small requests, such as those typically made to Web servers. When the client population is small and/or the client connections produce widely varying loads on the server, the load-balancing algorithm is less effective. However, the simplicity and speed of NLBs algorithm allows it to deliver very high performance, including both high throughput and low response time, in a wide range of useful client/server applications. If No Affinity is set, NLB load balances incoming client requests so as to direct a selected percentage of new requests to each cluster host; the load percentage for each host is set in the NLB Properties dialog for each port range to be load balanced. The algorithm does not dynamically respond to changes in the load on each cluster host (such as the CPU load or memory usage). However, the load distribution is modified when the cluster membership changes, and load percentages are renormalized accordingly.

When inspecting an arriving packet, all hosts simultaneously perform a mapping to quickly determine which host should handle the packet. The mapping uses a randomization function that calculates a host priority based on their IP address, port, and other information. The corresponding host forwards the packet up the network stack to TCP/IP, and the other cluster hosts discard it. The mapping remains unchanged unless the membership of cluster hosts changes, ensuring that a given clients IP address and port will always map to the same cluster host. However, the particular cluster host to which the clients IP address and port map cannot be predetermined since the randomization function takes into account the current and past clusters membership to minimize remappings.

In general, the quality of load balance is statistically determined by the number of clients making requests. This behavior is analogous to dice throws where the number of cluster hosts determines the number of sides of a die, and the number of client requests corresponds to the number of throws. The load distribution improves with the number of client requests just as the fraction of throws of an N-sided die resulting in a given face approaches 1/N with an increasing number of throws. As a rule of thumb, with client affinity set, there must be at least five times more clients than cluster hosts to begin to observe even load balance.

The Network Load Balancing client affinity settings are implemented by modifying the statistical mapping algorithms input data. When client affinity is selected in the NLB Properties dialog, the clients port information is not used as part of the mapping. Hence, all requests from the same client always map to the same host within the cluster. Note that this constraint has no timeout value and persists until there is a change in cluster membership. When single affinity is selected, the mapping algorithm uses the clients full IP address. However, when Class C affinity is selected, the algorithm uses only the Class C portion (upper 24 bits) of the clients IP address. This ensures that all clients within the same Class C address space map to the same cluster host.