In order to correlate all messages from a particular client to a particular instance, WCF needs to be able to identify the client. One way of doing that is to rely on a transport-level session; that is, a continuous connection at the transport level, such as the one maintained by the TCP and IPC protocols. As a result, when using the NetTcpBinding or the NetNamedPipeBinding, WCF associates that connection with the client. The situation is more complex when it comes to the connectionless nature of the HTTP protocol. Conceptually, each message over HTTP reaches the services on a new connection. Consequently, you cannot maintain a transport-level session over the BasicHttpBinding. The WS binding, on the other hand, is capable of emulating a transport-level session by including a logical session ID in the message headers that uniquely identifies the client. In fact, the WSHttpBinding will emulate a transport session whenever security or reliable messaging is enabled.
Because the service instance remains in memory throughout the session, it can maintain state in memory, and the programming model is very much like that of the classic client/server. Consequently, it also suffers from the same scalability and transaction issues as the classic client/server model. A service configured for private sessions cannot typically support more than a few dozen (or perhaps up to a hundred or two) outstanding clients due to the cost associated with each such dedicated service instance.