IBM SNA Interoperability Concepts

SNA Sessions

SNA communications are based on the establishment and termination of logical sessions between NAUs. Logical sessions are communication paths that support the operation of network devices and the flow of traffic through the network.

In SNA networks, routing information is dynamically determined when a session is established and only remains fixed until the session is terminated. During sessions, all SNA packets follow the same logical path from the source NAU to the destination NAU.

SNA packet transmissions are also time sensitive. If communication in an SNA session is interrupted, even for a short time, SNA terminates the session. As such, networks that tunnel SNA protocols across standard wide area network (WAN) connections must use routers that support Data Link Switching (DLSw), Frame Relay Assembler/Disassemblers (FRADs) that support frame relay (RFC 1490), or specialized Asynchronous Transfer Mode (ATM) switches.

Table A.6 summarizes the types of sessions that are used in hierarchical SNA networks.

Table A.6 Hierarchical SNA Session Types

Hierarchical SNA Session Type	Description
SSCP to SSCP	Supports communications between SSCPs in multidomain, hierarchical networks.
SSCP to PU	Used by system administrators to communicate with network devices.
SSCP to LU	Used by logical units in hierarchical networks to obtain session services that are controlled by the SSCP, such as logon or logoff services.
LU to LU	Supports communications between end users and applications in both hierarchical and peer-to-peer networks.

Note

SNA APPNs support control point to control point (CP to CP) sessions in addition to LU to LU sessions. CP to CP sessions allow a peer-to-peer SNA network to manage all of its functions without relying on a mainframe SSCP.