TCP/IP (21) layer protocols provide services to the application (22) running on a computer. The application layer does not define the application itself, but rather it defines
(23) that applications need — like the ability to transfer a file in the case of HTTP. In short, the application layer provides an (24) between software running on a computer and the network itself. The TCP/IP application layer includes a relatively large number of protocols, with HTTP being only one of those. The TCP/IP (25) layer consists of two main protocol options — the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

A. application B. sessionC. physical D. transport

瀑布模型在有些情况下允许返工。