The IP network-layer protocol provides a:
~  Reliable datagram service
~+  Best-effort datagram service
~  Reliable stream service
~  Best-effort stream service
~~
The IP network-layer protocol delivers packets to:
~+  Hosts
~  Applications
~  Routers
~~
The IP transport-layer protocols (UDP, TCP) deliver packets to:
~  Hosts
~+  Applications
~  Routers
~~
The TCP protocol is a:
~  Reliable datagram service
~  Best-effort datagram service
~+  Reliable stream service
~  Best-effort stream service
~~
One way of making the client vs server distinction is by the behavior 
of programs during the initial establishment of communication between them.
In this terminology
~   Clients listen passively and are contacted by servers
~+   Servers listen passively and contacted by clients
~   Either clients or servers can actively establish communication