Sessions

When a network includes more than one host processor, and therefore more than one VTAM® (or VTAM in one or more host processors and other SNA access methods in one or more other host processors), a session called an SSCP-SSCP session must be established between the SSCP in one VTAM and any other SSCP with which the first SSCP communicates.

Within the portion of the network controlled by each SSCP, different types of sessions are established in stages. The SSCP must first establish an SSCP-PU session with each PU that is active in the configuration. Then, for each active LU associated with a PU, the SSCP must establish an SSCP-LU session. And finally, when a pair of LUs indicates that they want to communicate with each other, the SSCP must establish an LU-LU session between them.

In a particular session between two LUs, one LU adheres to a set of SNA-defined primary protocols and is known as the primary logical unit (PLU) for that session. The other LU adheres to a set of secondary protocols and is known as the secondary logical unit (SLU) for that session. More than one session can exist between two LUs. Multiple concurrent sessions between the same two LUs are referred to as parallel sessions. Not all LUs have parallel session capability.

A VTAM application program can act as a PLU, as an SLU, or as both at the same time. It can be a PLU in some sessions while it is also functioning as an SLU in others. In a set of parallel sessions involving an application program and another LU, the application program can operate as the PLU for some sessions, and the SLU for the remainder. The LUs associated with cluster controllers are called device-type LUs in this book; they can act only as SLUs.

A CP-CP session is actually a pair of sessions between control points in adjacent nodes in an APPN network. This pair of sessions is an LU 6.2 contention-winner session and an LU 6.2 contention-loser session that together provide a CP-CP session. LU 6.2 protocols are used to communicate network control information and session control information.

A network node establishes a CP-CP session with each adjacent network node and with each served end node. Each end node is required to have a CP-CP session with a network node, which acts as its current network node server. Connections to adjacent LEN nodes do not support CP-CP sessions. After connection has been established between two attached nodes, a CP-CP session can be established. Identification information is exchanged between the nodes. A CP-CP session is then automatically started between the control points, if the nodes indicate that they want the CP-CP session and if no CP-CP session already exists between them. Each node informs the other about the capabilities of its control point and about the links connected to it. If both nodes are network nodes, they exchange updated information about the nodes and links (and their characteristics) between network nodes within the network.

The sections that follow describe the steps required to establish some of the different types of sessions. The numbers beneath the headings correspond to the numbers in Figure 1. The example used is a device-type LU communicating with an application program LU.

Figure 1. Example of establishing an LU-LU session