The Internet simply delivers IP packets. However, for various
reasons, things can go wrong. For example, if a packet gets
corrupted due to (for example) electrical interference then it
won't be possible to deliver it. A more common situation is where a
router is receiving packets faster than they can be processed, and
runs out of memory buffer space to hold the incoming packets. In
these situations, packets can be dropped --
discarded by a router. In most cases this happens silently -- all
that's observed externally is that the packet never arrives.
Two other disasters can befall delivery: a sequence of packets may
not arrive in the same order in which they were
sent, and a packet can even become duplicated
during delivery -- that is, the same packet is received twice.
Big Idea #4
these are basic design characteristics of IP -- Internet packet
delivery is said to be unreliable.
However, note one important fact -- in this context, unreliable
doesn't mean "no good", or "poor quality". It simply says that the
delivery system may fail to deliver a packet correctly. In
fact, most packets do get delivered correctly. This is because the
second design concept for the Internet is best
effort -- under normal operation, it works. Sections of
the network should only exhibit unreliability under abnormally
heavy loads.
A final design concept, that of connectionless IP
packet delivery, will be discussed later.