Programming WCF Services, 2nd Edition

Transport-Level Sessions

In traditional programming, an object is indirectly associated with a client by virtue of the call stack. That is, each object is tied to a particular client. But in WCF, since the client sends a message to the service and never invokes the instance directly, such an association is not possible. The analogous concept in WCF is the transport session, which ensures that all messages coming from a particular client are sent to the same transport channel on the host. It is as if the client and the channel maintain a logical session at the transport level (hence the name). As in traditional programming, with a transport session the calls (or rather, the messages) are strictly processed in the order in which they were received. The transport session is unrelated to any application-level session the client may or may not have with the instance itself. Note that using a transport session is optional and is largely an aspect of the binding configuration, so the client and the service may or may not have a transport session. The transport session is one of the key fundamental concepts of WCF, affecting reliability, instance management, error management, synchronization, transactions, and security.

A transport session relies on WCF's ability to identify the client and correlate all its messages to a particular channel. Thus, there has to be something in the transport or in the message that identifies the client.

Transport Session and Binding

Both the TCP and the IPC bindings ...