Because SNA is a connection-oriented protocol, before two entities in the network can communicate, a connection has to be set up between them. End users (application programs and individuals) gain access to an SNA network through logical units and exchange information over LU-LU sessions. Once network resources are active, LU-LU sessions can be initiated.

LU-LU sessions can be initiated in several ways:

• Either of the participating logical units can initiate an LU-LU session.
• A network operator can initiate an LU-LU session.
• A system definition can specify that an LU-LU session be initiated automatically when certain resources become active.

Typically, one of the participating logical units initiates an LU-LU session. The two logical units that communicate with each other over a session are called session partners.

Here's how it works:

In SNA, every LU can initiate a session with a partner LU. The LU sends a session request to its control point. The control point locates the partner LU, either within the domain of the control point, or in another domain, or in another network. In an SNA subarea, after the partner LU is located by the SSCP, the virtual route is chosen and a BIND message (RU) is sent by the LU and the session is started. The virtual route is mapped to the explicit route.

LU-LU session initiation generally begins when the session manager in an LU submits a session-initiation request to the appropriate control point. In a subarea network, it can be either the system services control point controlling the LU's domain or, in the case of a type 2.1 peripheral node, the LU's local control point (CP). A session-initiation request specifies the requested session partner's network name and a mode name. The mode name identifies which set of session parameters that the requesting logical unit chooses for the session. In a subarea network, the mode name is associated with the parameters through a mode table created during system definition.

Using the specified set of session parameters, the control point builds a BIND image. The control point transmits the BIND image in a control initiate request (CINIT request) to the primary logical unit. The primary logical unit (PLU) is the LU responsible for activating the session. The PLU activates the session by sending a bind session request (BIND request, also called a session-activation request) to the secondary logical unit (SLU). The SLU then returns a BIND response to the PLU. A response unit flows between the session partners and the session is started.

The session parameters set up by a BIND include the maximum message size (RU size) that a session partner can send or receive, the protocol used between the session partners, the window size (pacing in SNA terminology), and more.

Figure 1 illustrates a session initiation using SSCP-dependent protocols.

Figure 1. Session initiation in SNA subarea network

By comparison, TCP is a based on a client-server relationship. The client initiates the connection with the server. The TCP server cannot initiate connections with the client; instead, it listens on a specific port for connection requests. The client must know the IP address of the host where the server exists.