Multi-homed Systems
RPC Manager will take advantage of multiple network interfaces on a machine. Each network interface must have its own IP address.
There are two reasons why you might want to use a multi-homed system:
- To improve the tolerance of the distributed system to network failures.
- To separate the SCADA system network usage from a network used by business systems. The usage of a network by business systems can be quite unpredictable and dedicating a network to the SCADA and associated control systems may be prudent.
These two justifications have an area of overlap, in that the VTScada distributed system can see all networks connected to it, regardless of purpose.
Through configuration parameters, you can configure RPC Manager to use the available networks in two ways:
- Prioritized. The available networks are arranged in an order of priority. RPC Manager always uses the highest priority available network. This could be used for either of the above two scenarios.
- Round-Robin. All network interfaces are treated equal and RPC Manager traffic is sent using each interface in turn. This could only be used for scenario 1 and is the default behavior.
The two methods can be combined, where there are three or more networks, such that one of the networks could be prioritized and the other two left as round robin. RPC Manager traffic will always be sent over the prioritized network first, using the remaining two networks in round-robin mode if the prioritized network link to a machine fails.
For example:
[RPCManager-NetPriority]
IP = 192.168.1.0/24
IP = 192.168.2.0/24
Each line of the "RPCManager-NetPriority" section specifies an IP mask, of the form:
<IP address>/<number of bits>
such that the number of bits specified is applied to the IP address from the most significant end. A mask value of 24 specifies the first three numeric parts of the IP address (8-bits per number), a mask of 16 the first two numeric parts and so on.
The example above specifies that IPs on subnet 192.168.1 are to be given priority over IPs to the same machine on all other subnets. Likewise, IPs on subnet 192.168.2 are to be given priority over IPs to the same machine on all other subnets, except subnet 192.168.1. If an IP on subnet 192.168.1 becomes unusable, the corresponding one on subnet 192.168.2 will be used. If that also fails, any remaining IPs on other subnets will have RPCs transmitted to them in round-robin fashion.
