the layer that provides a wide variety of applications with the ability to access the services of the lower network layers  protocols: dns, dhcp, smtp/imap, http, rtsp
the layer that provides a wide variety of applications with the ability to access the services of the lower network layers  protocols: dns, dhcp, smtp/imap, http, rtsp
(upper layer data)facilitates communication between such programs and lower-layer network services. high-level apis, including resource sharing, remote file access.