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Windows Server 2003 : Using Network Load Balancing (part 1) - Planning a Network Load Balancing Deployment

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3/28/2011 9:11:12 PM
Understanding Network Load Balancing

A Network Load Balancing cluster consists of up to 32 servers, referred to as hosts, each of which is running a duplicate copy of the application you want the cluster to provide to clients. Network Load Balancing works by creating on each host a virtual network adapter that represents the cluster as a single entity. The virtual adapter has its own IP and media access control (MAC) addresses, independent of the addresses assigned to the physical network interface adapters in the computers. Clients address their application requests to the cluster IP address, instead of an individual server’s IP addresses.

Off the Record

In an Ethernet or Token Ring network interface adapter, the MAC address, also known as the adapter’s hardware address, is a unique six-byte hexadecimal value hardcoded into the adapter by the manufacturer. Three bytes of the address contain a code identifying the manufacturer, and three bytes identify the adapter itself.


NLB Clustering and DNS

Directing clients to the IP address of the cluster is a task left to the name resolution mechanism that provides clients with IP addresses. For example, if you are currently running an individual Web server on the Internet, the DNS server hosting your domain has a record associating your Web server’s name with the Web server computer’s IP address. If you change from the single Web server to a Network Load Balancing cluster to host your Web site, you must modify the DNS resource record for the Web site’s name so that it supplies clients with the cluster IP address, not your original Web server’s IP address.


When an incoming client request addressed to the cluster IP address arrives, all the hosts in the cluster receive and process the message. On each host in an NLB cluster, a Network Load Balancing service functions as a filter between the cluster adapter and the computer’s TCP/IP stacks. This filter enables NLB to calculate which host in the cluster should be responsible for resolving the request. No communication between the hosts is required for this purpose. Each host performs the same calculations independently and decides whether it should process that request or not. The algorithm the hosts use to perform these calculations changes only when hosts are added to or removed from the cluster.

Planning a Network Load Balancing Deployment

Before you deploy a Network Load Balancing cluster, you must create a plan for the network infrastructure that will support your cluster servers. The high availability provided by NLB will do you no good if your users can’t access the servers due to a failure in a router, switch, or Internet connection. In addition, because many NLB installations provide Web and other services to Internet users, you must consider the security of your cluster servers and the rest of your internal network.

Real World: NLB Network Design

For a high-traffic Web site with high availability requirements, a typical network infrastructure design would consist of a Web server farm located on a perimeter network, as shown in the following figure. The perimeter network has redundant connections to the Internet, preferably with different Internet service providers (ISPs) or with one ISP that has connections to multiple Internet backbones. A firewall at each Internet access router protects the perimeter network from Internet intruders, and another firewall isolates the perimeter network from the internal network.


Important

Deploying a Network Load Balancing cluster is not a task to undertake casually or haphazardly. As with any major network service, the NLB deployment process must be planned carefully, tested thoroughly on a lab network, and then implemented in a pilot program before proceeding with the full production deployment.


NLB Operational Modes

The servers that are going to be the hosts in your NLB cluster do not require any special hardware. There is no shared data store as in a server cluster, for example, so you do not have to build a storage area network. However, NLB imposes certain limitations on a server with a single network interface adapter in a standard configuration, and in some cases, you can benefit from installing a second network interface adapter in each of your servers.

Windows Server 2003 Network Load Balancing has two operational modes: unicast mode and multicast mode. In unicast mode, Network Load Balancing replaces the MAC address of the physical network interface adapter in each server with the MAC address of the virtual adapter representing the cluster. The server does not use the computer’s original MAC address at all, effectively transforming the computer’s physical network interface adapter into a virtual cluster adapter. The Address Resolution Protocol (ARP) resolves both of the server’s IP addresses (the IP address originally assigned to the network interface adapter and the cluster IP address) to the single MAC address for the cluster.

Off the Record

NLB does not actually modify the MAC address in the network interface adapter itself; the address assigned to the adapter by the manufacturer is permanent and cannot be changed. NLB only replaces the MAC address in the computer’s memory, substituting a virtual cluster address for the physical address the system reads from the network adapter card.


NLB and ARP

ARP is a TCP/IP protocol that resolves IP addresses into MAC or hardware addresses. To transmit to a particular IP address, a TCP/IP computer must first discover the MAC address associated with that IP address, so that it can build a datalink layer protocol frame. ARP functions by transmitting a broadcast message containing an IP address to the local network. The computer using that IP address is responsible for replying with a message containing its MAC address.

In the case of an NLB cluster in unicast mode, each server in the cluster replies to ARP requests that contain either its original IP address or the cluster IP address by sending a response containing the cluster MAC address. Therefore, no computer on the network can transmit to the MAC address assigned for NLB server’s physical network interface adapter.


Because the network interface adapters of all the servers in the cluster have the same MAC address, the cluster servers cannot communicate among themselves in the normal way, using their individual MAC addresses. The servers can, however, communicate with other computers on the same subnet, and with computers on other subnets, as long as the IP datagrams don’t contain the cluster MAC address.

Note

When you configure the servers in an NLB cluster to use unicast mode with a single network interface adapter, you cannot use the Network Load Balancing Manager application on one of the servers to manage the other servers in the cluster.


In some cases, this is not a problem. Dedicated Web servers hosting the same site, for example, don’t often need to communicate with each other under normal conditions. However, if you determine that it is necessary for the servers in your NLB cluster to communicate with each other, there are two possible solutions:

  • Configure the cluster servers to operate in NLB multicast mode— In multicast mode, NLB assigns a cluster MAC address to the physical network interface adapter, but also retains the adapter’s original MAC address. The cluster IP address resolves to the cluster MAC address and the server’s original IP address resolves to the original MAC address. For this configuration to function properly, the routers on the network must support the use of multicast MAC addresses.

  • Install a second network interface adapter in each server— One of the adapters becomes the cluster adapter, with its original MAC address replaced by the cluster MAC address. Both the cluster IP address and the adapter’s original IP address resolve to the cluster MAC address. The system does not use this adapter’s original MAC address. Like a single adapter in unicast mode, the cluster adapter cannot communicate with the other servers in the cluster. The second adapter retains its original MAC address and assigned IP address and handles all noncluster network communications.

Tip

In a Windows Server 2003 Network Load Balancing cluster, you must configure all the servers to operate in either unicast or multicast mode. You cannot mix unicast and multicast servers in the same cluster. However, you can mix network interface adapter configurations, installing two network interface adapters in some of a cluster’s servers, while leaving a single adapter in others. In the case of a unicast cluster, only the servers with multiple adapters are able to communicate with the other servers.


In summary, a server in an NLB cluster can have either one network interface adapter or multiple adapters, and it can run in either unicast or multicast mode. By combining these options, you can use four possible NLB configurations, each of which has advantages and disadvantages, as shown in Table 1.

Table 1. NLB Configuration Advantages and Disadvantages
NLB ConfigurationAdvantagesDisadvantages
Single network interface adapter in unicast mode
  • Requires no special hardware

  • No router incompatibility problems

  • Ordinary communications with other servers in the cluster are not possible

  • Network performance might degrade when one network interface adapter is handling both ordinary traffic and cluster traffic

Single network interface adapter in multicast mode
  • Requires no special hardware

  • Permits ordinary communications among cluster servers

  • Some routers cannot support multicast MAC addresses

  • Network performance might degrade when one network interface adapter is handling both ordinary traffic and cluster traffic

Multiple network interface adapters in unicast mode
  • No router incompatibility problems

  • Permits ordinary communications among cluster servers

  • Network performance enhanced, because cluster traffic and ordinary network traffic use different network interface adapters

  • Requires installation of second network interface adapter

Multiple network interface adapters in multicast mode
  • Permits ordinary communications among cluster servers

  • Network performance enhanced, because cluster traffic and ordinary network traffic use different network interface adapters

  • Requires installation of second network interface adapter

  • Some routers cannot support multicast MAC addresses


The most popular configuration for large NLB installations is to install two network interface adapters in each server and run them in unicast mode. This enables the servers to function as normal participants on the network, in addition to performing their NLB server duties. There are also no problems with routers handling multicast MAC addresses and no bottlenecks caused by cluster traffic and ordinary network traffic sharing a single network interface adapter.

NLB Networking

Although the servers in a Network Load Balancing cluster do not share a single data store, as in a server cluster, and perform their own independent calculations to determine which server will service an incoming request, the servers do communicate with each other. The cluster servers must exchange information to know many servers are in the cluster, and to determine when a server has been added or removed from the cluster. This communication enables the cluster to compensate for a failed server and to take advantage of new servers in the cluster by redistributing the traffic load.

Important

A single computer running Windows Server 2003 cannot be a member of a Network Load Balancing cluster and a server cluster at the same time, because these two clustering solutions use network interface adapters in different ways. If you want to deploy both an NLB cluster and a server cluster on your network, you must use separate servers for each cluster.


The cluster traffic between NLB servers takes the form of a heartbeat message that each server transmits once per second to the other servers in the cluster. If one cluster server fails, it stops transmitting its heartbeat messages, and the other servers detect the absence of the heartbeats. Once the other servers in the cluster miss five consecutive heartbeat messages from a server, they begin a process called convergence, in which they recalculate their traffic distribution algorithm to compensate for the missing server. In the same way, adding a new server to an NLB cluster introduces a new heartbeat to the network, which triggers a convergence in the other servers, enabling them to redistribute the traffic so that the new server receives an equal share of the load.

Note

Because all the servers in the cluster are using the same cluster MAC address, transmitting the heartbeats is simply a matter of directing the packets to that address. The servers don’t need to broadcast the heartbeat messages, reducing the impact of the cluster traffic on the network.


When you deploy NLB cluster servers with a single network interface adapter in each computer, obviously all the cluster-related traffic must travel over the same network as your ordinary traffic. This is usually not a major burden, because the heartbeat packets are small, less than 1,500 bytes, so they fit into a single Ethernet packet. If you decide to install multiple network interface adapters in each cluster server, you can connect both adapters to the same local area network (LAN) or construct a separate network for the cluster traffic.

Planning

If your NLB cluster consists of servers that are already isolated on a perimeter network, there is probably no need to create a separate LAN for cluster traffic. However, if you are deploying an NLB cluster on a heavily trafficked internal network, you might benefit from installing a dedicated cluster LAN.

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