One way to do that is by restricting clients to a certain number of connections in a given time period. Since the code is designed to prevent a DoS condition, it must be extremely fast and efficient because it may be run thousands of times per minute (during a DoS attack for example).

I designed the connection limit system around a collection of connection records, a prune timer, and a test when the server accepts a client connection.

Since I needed to check two variables (timestamp and ip) in each connection record to determine the connection count for a given IP address in a given time period, I decided to use LINQ, which worked nicely:

// Get this client's connection count
int connCount = (from conn in _connectionList
                 where conn.Ip == clientIp &&
                 conn.Timestamp >
                   DateTime.Now.AddMinutes(-howFarBackInMinutesToCheck)
                 select conn.Ip).Count();

The other query I needed was for pruning the connection list:

 // Get the connections to prune
var connQuery = from conn in _connections
                where conn.Timestamp <
                  DateTime.Now.AddMinutes(-howFarBackInMinutesToCheck)
                select conn;

After checking the number of records to prune was greater than zero (connQuery.Count()), I pruned them by iterating through the query (LINQ queries implement IEnumerable).