This is an application-level DoS that consumes server resources by maintaining open connections for an extended period of time by slowly sending traffic to the server. If the server maintains too many connections open at once, then it may not be able to respond to new, legitimate connections.
The attack holds serverconnections open by sending properly crafted HTTP POST headers that contain a Content-Length header with a large value to inform the web server how much of data to expect. After the HTTP POST headers are fully sent, the HTTP POST message body is sent at slow speeds to prolong the completion of the
connection and lock up server resources.
All other services remain intact but the web server itself becomes inaccessible.