I would say, there is no such thing as "HTTP runtime". There is a protocol stack:
http://en.wikipedia.org/wiki/Protocol_stack[
^].
The protocol stack should be implemented on the OS and used by applications. Even though .NET is a platform, it is based on OS and, in particular, uses the protocol stack provided by the OS. Application use this stack: one application is an HTTP server, on the server host, and another application is a Web browser (and possibly a lot more). In addition of HTTP and HTTPS, FTP and other protocols are implemented in these applications or plug-ins (streaming, and so one). HTTP, HTTPS and FTP, etc. are
application-level protocol, all bases on the
transport-level protocol TCP:
http://en.wikipedia.org/wiki/Transmission_Control_Protocol[
^],
http://en.wikipedia.org/wiki/Transport_Layer[
^],
http://en.wikipedia.org/wiki/Application_Layer[
^].
—SA