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Kepware Knowledge Base: Solution

Configuration Fails to Connect or Reconnect to the Runtime on Windows XP SP2

Last Update: 2018/11/07

This issue occurs when there are many Ethernet-based devices in a project running on an XP SP2 operating system. If the devices are unreachable, users may not be able to connect to the server Runtime with the Configuration.

Windows XP SP2 introduced a security measure that limits the number of simultaneous incomplete outbound TCP connection attempts. The limit (which is set by Microsoft) is 10 simultaneous failed connections in 1 second. Once the limit has been reached, subsequent connection attempts will be put in a queue and resolved at a fixed rate. Under normal operation, applications that connect to available hosts at valid IP addresses will not cause connection rate-limiting to occur. When it does, a new event with the ID 4226 will appear in the system's Event Log. The revised rate of connections for the queued requests is 1 per second.This change helps limit the speed at which malicious programs (such as viruses and worms) spread to uninfected computers. Malicious programs often attempt to reach uninfected computers by opening simultaneous connections to random IP addresses. Most of these random addresses result in a failed connection; as such, a burst of such activity is a signal that the computer may have been infected by a malicious program.

Projects that have optimized the server project for performance may have varied effects depending on the version.

  • KEPServerEX V4 will run more slowly, and may appear to be hung or stalled.
  • KEPServerEX V5 will run more slowly, and may appear to be hung or stalled. The configuration (which uses TCP to connect) may not be able to connect to the runtime because of the queued connection requests.

To resolve the issue, attempt the following:

  1. Identify the reason that the devices are unreachable and then correct it.
  2. In cases where the devices' availability cannot be controlled, place them in a single channel. Then, configure auto-demotion on each device to remove them from the poll cycle. This will cause the server to attempt a connection to each device one at a time on a channel, rather than requesting them simultaneously.
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