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   Strong Host Model, Weak Host Model and Load Balancing

RFC 1122 introduced strong host model for IPV4. With the strong host send model, a computer with more than one network interfaces will only transmit the data packets with its source IP address matching the NIC IP address. With strong host receive model, a computer will only receive data packet addressed to the NIC′ address on the receiving NIC.

In the weak host model, as long as the destination address is one of the multiple addresses on the computer, the computer will receive it. If the IP address on one of the NIC (including loopback adapter), the computer will send it through any NIC on that computer.

By default most computer OSs use the weak host model, so that as long as the IP address is on the computer, it will be sent or received. Starting in 2007, Microsoft® has tightened the security on its operating system, and so in Windows 2008 Servers the default host model for both IPV4 and IPV6 is Strong Host Model.

However, in certain situation, server configuration requires that the NIC accepting data packets addressed to the loopback adapter or sending data packet out on the behalf the process running on another IP address on the server. For example, if a server participates in the Out-of-path (OOP) mode load balancing, the server NIC must accept the data packets for the loop back adapter address (which is the FARM address on the load balancer), and the NIC also must send the reply back to the clients on behalf the loop back adapter. This Out-of-path mode is useful when huge amount data replies need to be send back to the clients without going through load balancer.

To enable the Weak Host Model on Windows 2008, you will need to run command line command on Windows 2008 server with the administrator privilege. The following commands allow administrator to switch back and forth the host model on an interface:

netsh interface ipv4 set interface [InterfaceNameOrIndex] weakhostsend=enabled|disabled
netsh interface ipv4 set interface [InterfaceNameOrIndex] weakhostreceive=enabled|disabled
netsh interface ipv6 set interface [InterfaceNameOrIndex] weakhostsend=enabled|disabled
netsh interface ipv6 set interface [InterfaceNameOrIndex] weakhostreceive=enabled|disabled

In OOP mode, administrator needs to enable "weakhostreceive" on the server NIC, and enable both "weakhostreceive" and "weakhostsend" on the server loopback adapter.

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