Application Level
The application layer defines how certain services operate and how they can be used. Examples are the FTP service for transferring files, HTTP for serving Web pages and SMTP for e-mail.
These services are defined in a rather abstract manner. Two parties, called the client and the server, set up a connection over which they exchange messages in accordance with a specific protocol. The client starts the protocol by requesting the service. Often the next step is for the server to authenticate the client, for example by asking for a password or by executing a public-key based protocol.
Taking e-mail as an example, the protocol in question is called the Simple Mail Transfer Protocol (SMTP). The client and the server set up an SMTP connection over which they exchange identifying information. The client then tells who the message is from and who the intended recipient is. The server then indicates whether it accepts or refuses the message (for example if it's spam or the intended recipient is unknown). If the message is accepted, the client sends the actual content of the message and the server stores it in the right mailbox.
The most widely known and implemented applications protocols are:
- Telnet - The Network Terminal Protocol, which provides remote login over the network.
- FTP - The File Transfer Protocol, which is used for interactive file transfer.
- SMTP - The Simple Mail Transfer Protocol, which delivers electronic mail.
- HTTP - The Hypertext Transfer Protocol, which delivers Web pages over the network.
While HTTP, FTP, SMTP, and telnet are the most widely implemented TCP/IP applications, you will work with many others as both a user and a system administrator. Some other commonly used TCP/IP applications are:
Domain Name Service (DNS)
Also called name service, this application maps IP addresses to the names assigned to network devices. DNS is discussed in detail in this book.
Domain Name Service (DNS)
Also called name service, this application maps IP addresses to the names assigned to network devices. DNS is discussed in detail in this book.
Open Shortest Path First (OSPF)
Routing is central to the way TCP/IP works. OSPF is used by network devices to exchange routing information. Routing is also a major topic of this book.
Routing is central to the way TCP/IP works. OSPF is used by network devices to exchange routing information. Routing is also a major topic of this book.
Network Filesystem (NFS)
This protocol allows files to be shared by various hosts on the network.
This protocol allows files to be shared by various hosts on the network.
Some protocols, such as telnet and FTP, can only be used if the user has some knowledge of the network. Other protocols, like OSPF, run without the user even knowing that they exist. As system administrator, you are aware of all these applications and all the protocols in the other TCP/IP layers. And you're responsible for configuring them!
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