An edge system is any computer (host, printer, even a toaster...) which is "connected
to" the Internet -- that is, it has access to the Internet's packet
delivery system, but doesn't itself form part of that delivery
system.
A transport service provides communications
between application processes running on edge systems. As we have
already seen, application processes communicate with each another
using application protocols such as HTTP and SMTP.
The interface between an application process and the transport
service is normally provided using the socket
mechanism.
Most application protocols require reliable data
transfer, which in the Internet is provided by the
TCP transport service/protocol. Note: some
applications do not require reliablity[1], so the unreliable UDP
transport service/protocol is also provided as an alternative, see
later.
[1] That is, it's not
a problem for some data to fail to be delivered. The DNS is an
example of such a protocol.