The ultimate goal of data communications is to interconnect
users and applications residing on end systems.
TCP/IP was designed to allow many different kinds of end
systems to interact. TCP/IP has been implemented in
everything from embedded controllers to supercomputers.
End system requirements can be found in
RFC 1122 and
RFC 1123.
Communication Links
Internet hosts are connected by all kinds of communication links
over which data is exchanged.
Any communications link will suffice,
so long as it can transport IP packets. Internet links have been operated
using amateur radio transceivers, between high-voltage experimental
physics apparatus, and over parallel printer ports.
The higher level TCP/IP protocols strive
to make no assumptions about hardware details.
However, there is clearly a need for standardization
for communication over common hardware technologies, such
as Ethernet.
So, while not strictly part of the Internet protocol suite, many RFCs
document standard ways to transport IP packets over various
common hardware types.
RFC 2400 §6.3
has a list of these Network-Specific Standard Protocols.
Ethernet and serial lines are sampled below;
others such as FDDI and Token Ring are not discussed here.
Intermediate Systems
Computers interconnecting communications links together
are refered to as intermediate systems
because they relay traffic between other hosts.
They include routers, bridges, and switches. The distinctions
is not always clear. A router allowing an network administrator
to login via telnet behaves as an end system. An end system
with multiple network connections can act as a router.
The difference between a router and a bridge is only one of
software; systems capable of acting as both as sometimes
called brouters. Cisco Systems manufactures a
Route Switch Processor (RSP) which plugs into a switch
and allows it to act as a router, bridge, and switch, all
more or less simultaneously.
