boost::asio::async_read_until reads all data instead of just some

I'm modify the Boost Asio echo example to use async_read_until to read the input word by word. Even though I am using async_read_until, all the data sent seems to be read from the socket. Could someone please advise:

#include <cstdlib>
#include <iostream>
#include <boost/bind.hpp>
#include <boost/asio.hpp>

using boost::asio::ip::tcp;

class session
{
public:
  session(boost::asio::io_service& io_service)
    : socket_(io_service)
  {
  }

  tcp::socket& socket()
  {
    return socket_;
  }

  void start()
  {
 std::cout<<"starting"<<std::endl;
  boost::asio::async_read_until(socket_, buffer, ' ',
        boost::bind(&session::handle_read, this,
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));
  }

  void handle_read(const boost::system::error_code& error,
      size_t bytes_transferred)
  {

 std::ostringstream ss;
 ss<<&buffer;
 std::string s = ss.str();
 std::cout<<s<<std::endl;

 if (!error)
    {
      boost::asio::async_write(socket_,
          boost::asio::buffer(s),
          boost::bind(&session::handle_write, this,
            boost::asio::placeholders::error));
    }
    else
    {
      delete this;
    }
  }

  void handle_write(const boost::system::error_code& error)
  {
 std::cout<<"handling write"<<std::endl;
    if (!error)
    {
    }
    else
    {
      delete this;
    }
  }

private:
  tcp::socket socket_;
  boost::asio::streambuf buffer;
};

class server
{
public:
  server(boost::asio::io_service& io_service, short port)
    : io_service_(io_service),
      acceptor_(io_service, tcp::endpoint(tcp::v4(), port))
  {
    session* new_session = new session(io_service_);
    acceptor_.async_accept(new_session->socket(),
        boost::bind(&server::handle_accept, this, new_session,
          boost::asio::placeholders::error));
  }

  void handle_a