Congestion control provides a throttling mechanism which is designed to protect the internal AD FS servers from excessive external traffic. The Web Application Proxy monitors the average round trip latency of federation requests. When the latency exceeds the configured congestion control setting, the proxy starts limiting the number of requests which are sent to the federation servers. When the latency returns to normal, the number of requests is increased. The proxy uses a sliding window algorithm, similar to TCP congestion control, to throttle the request pool.
Each time a request is rejected due to a congestion condition, the proxy will write an event ID 230 to the AD FS admin event log. The presence of these events signifies that your AD FS farm is currently or has recently been operating in congestion mode and rejecting requests.