Line-Based Protocols

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### Line-Based Protocols

Many commonly-used internet protocols are line-based, which means that they have protocol elements that are delimited by the character sequence `"\r\n"`. Examples include HTTP, SMTP and FTP.

To more easily permit the implementation of line-based protocols, as well as other protocols that use delimiters, Boost.Asio includes the functions `read_until()` and `async_read_until()`.

The following example illustrates the use of `async_read_until()` in an HTTP server, to receive the first line of an HTTP request from a client:

    class http_connection
    {
      ...

      void start()
      {
        asio::async_read_until(socket_, data_, "\r\n",
            boost::bind(&http_connection::handle_request_line, this, _1));
      }

      void handle_request_line(boost::system::error_code ec)
      {
        if (!ec)
        {
          std::string method, uri, version;
          char sp1, sp2, cr, lf;
          std::istream is(&data_);
          is.unsetf(std::ios_base::skipws);
          is >> method >> sp1 >> uri >> sp2 >> version >> cr >> lf;
          ...
        }
      }

      ...

      asio::ip::tcp::socket socket_;
      asio::streambuf data_;
    };