Boost.Asio HTTP Server Session Closing - c++

Question

I have a http server based on Boost.Asio Example HTTP Server 2 (using an io_service-per-CPU design)
Each Request gets parsed, checked if it's a POST-Request, if Path is correct, if Content-Length Header is present, if one of these conditions is not correct, I generate a Bad-Request Response.
As everything is okay, I send a OK Response.
That's how it gets started.
int main(int argc, char* argv[])
{
try
{
http::server s("127.0.0.1", "88", 1024);
s.run();
}
catch (std::exception& e)
{
std::cerr << "Exception: " << e.what() << "\n";
}
return 0;
}
Now I made a quick C# app which starts 16 Threads and each Thread sends continuously in a while(true)-Loop POST-Requests to this server.
A while it runs smoothly and fast. Then at some point of time the server gets unresponsive (Cannot connect to remote server).
A cause for this could be that the sessions from the server don't get closed properly.
Here's the code for Bad-Request/OK Responses:
void session::handle_request()
{
//Generate OK Response
reply_ = reply::stock_reply(reply::ok);
//Send OK Response
boost::asio::async_write(client_socket_, reply_.to_buffers(),
boost::bind(&session::handle_client_write, shared_from_this(),
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}
void session::send_bad_request()
{
//Generate Bad Request Response
reply_ = reply::stock_reply(reply::bad_request);
//Send Bad Request Response
boost::asio::async_write(client_socket_, reply_.to_buffers(),
boost::bind(&session::handle_client_write, shared_from_this(),
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}
And the handle_client_write which both trigger:
void session::handle_client_write(const boost::system::error_code& err, size_t len)
{
if (!err) {
//Dummy Error Code Var
boost::system::error_code ignored_ec;
//Shutdown socket
client_socket_.shutdown(boost::asio::ip::tcp::socket::shutdown_both, ignored_ec);
client_socket_.close();
//std::cout << "Closed" << ignored_ec.message() << "\n";
}
else {
std::cout << "Error: " << err.message() << "\n";
}
}
Like I said, at some point it gets unresponsive and after little time it responses again.