Application Protocols (HTTP, SMTP, etc.)

TCP/IP provides an assortment of application protocols, providing the end-user
applications with access to the data being passed across the transport protocols.
These protocols include the Simple Message Transfer Protocol (SMTP),
which is used by electronic mail systems to move mail messages around the
Internet, and the Hyper-Text Transfer Protocol (HTTP), which is used by web
browsers to access data stored on web servers, among many others.
All of these services get called upon whenever an application wants to exchange
data with another application across the Internet. For example, a mail client will
use the SMTP application protocol whenever a user wants to send a mail message
to a remote mail server, and the SMTP protocol uses rules defined by the NVT
specification whenever it exchanges data with TCP. In turn, TCP provides errorcorrection
and flow-control services back to SMTP. IP is used to move the TCP
segments between the source and destination networks, while hardware-specific
protocols (like Ethernet-specific framing) will be used to move the IP packets
between the various systems on the network itself.