The Applications Layer

The TCP/IP applications sit on top of the transport layer.

TCP/IP provides two types of transport:

  TCP  Reliable
  UDP  Unreliable

The applications are free to choose which layer they run over.

telnet and ftp both run over TCP
SNMP and tftp run over UDP and therefore these applications must do their own error checking.

TCP/IP Services

The services are applications, this is what the user sees of TCP/IP.

There are three main types of service

  DARPA commands
  Work on any operating system
  Some commands are:
    telnet
    ftp

  BSD r* services
  Designed for UNIX
  Some commands are:
    rlogin
    rsh
    rcp

Third party commands

These are services that have been designed to work over the TCP/IP transport protocols but are not typically shipped with the TCP/IP package. The most common example would be Suns NFS.

Hostnames

All the TCP/IP services require connection to hosts. Hosts are machines that you can communicate with. Each host has a unique IP address. IP addresses are not user friendly. Users like to use names.

An example name is:
  sales

To make the names unique there is a naming scheme which would make the above name something like:
  sales.paragon.co.uk

See the chapter on more applications for details of this naming scheme. Although the user will use hostnames these names still have to be converted into IP addresses. The hosts file performs this task.

In UNIX the full pathname of the hosts file is:

/etc/hosts

So to find the hosts you can communicate with

$ cat /etc/hosts
127.0.0.1   localhost
128.48.200.1 sales.paragon.co.uk  sales
128.48.200.2 dev.paragon.co.uk   dev

To find your host name

$ hostname

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