As we have seen, IP is implemented in every one of the millions of
routers that make up the Internet -- the (so-called) "network
core". It's obvious, however, that IP alone isn't useful for
reliable data transfer.
We introduce the concept of edge systems, or (to
use the traditional term) hosts. These are (in
general) computers which are connected to the Internet -- in other
words, everything that isn't a router. Your desktop computer is an
edge system, our main departmental server ironbark is an edge
system as are most other servers which you could name.
Big Idea #5
A second protocol, the Transmission Control Protocol
(TCP) is implemented in edge systems. It is TCP's task to
transform the unreliable delivery service provided by IP into a
reliable data transmission system, suitable for building network
applications. This is called a transport
service.
TCP builds the "payload" of IP packets, by slicing
application data into chunks small enough to fit,
with a little extra administrative overhead (ie, a TCP header),
into a single IP packet. These are called TCP
segments[1], thus:
[1] more formally
segments are called "Transport Protocol Data Units" or TPDUs. No
one ever uses this term in relation to the Internet, though.
