Denial-of-service attacks, such as distributed denial-of-service attacks, are disruptive to electronic commerce.  In a distributed denial-of-service attack, computer systems, which may be compromised, are used to overwhelm servers, such as web servers and mail servers, among others, so as to cause the servers to become unavailable for access by legitimate users.  The computer systems may “flood” a server with data including, for example, web service requests.  When the server is facing a high volume of traffic, backlogs may occur, whereby the server may not be timely processing user requests and may become at least temporarily unavailable to process the requests.  That results in adverse economic impacts on various computer systems such as those supporting various organizations' operations.  For example, electronic access to banks, retailers and government services may become unavailable due to the attacks. 
It is often challenging to mitigate denial-of-service attacks and distributed denial-of-service attacks by selectively blocking participating computer systems and permitting traffic of legitimate users.  Furthermore, it is challenging to ensure that if legitimate user traffic is mistakenly blocked, for example, due to a “false positive” identification, a computer system may later be permitted access to a server.