LU 6.2 APPC protocol

The LU 6.2 APPC protocol driver manages an APPC (Advanced Program-to-Program Communication) with another entity using LU 6.2. The driver can be configured to await (Listen) or initiate (Call) connections.

LU 6.2 is an asynchronous peer-to-peer SNA (Systems Network Architecture) Server LU-type protocol. SNA Server is an IBM product that is a set of programs and data files providing transparent access to SNA network resources. SNA can only be configured on AIX.

Knowledge of SNA products is required for LU 6.2 configuration.

SNA consists of these main levels:

  • Physical Unit (PU).

    This level represents a distributed control element in a network that manages and controls resources directly attached to it. For example, Token Ring, TCP/IP, SDLC, and so on.

    The PU is configured at both ends, AIX and mainframe, with the type of network, network name, and other control data.

    The PU must establish communications before anything can happen over the network.

    This is also referred to as the Link Level.

  • Logical Unit (LU).

    This level sets up communications between local programs and other networks. In SNA, there are many LUs, each with a specific function, such as Terminal Emulation (LU 2), Printers (LU 3), and others.

    LU6.2 is known as APPC. Within APPC, there are two types of LUs:

    • Dependent:

      These LUs are subservient to the mainframe and can support only a single program-to-program conversation.

    • Independent:

      These LUs are controlled also by the mainframe but can support multiple program-to-program conversations over a single LU.

    A Local LU is defined on the AIX side. This must be a name that is known to the mainframe. The mainframe defines the LU (APPLID), which becomes known as the Remote LU on the AIX side. The two sides then agree upon a mode name. The mode is a table of characteristics for each side.

    When a logical session is attempted, the mainframe acquires the LU on the AIX side. A Bind message is issued and the negotiation begins. If the negotiation is successful, then the LU-to-LU session is implemented. Only then can program-to-program communication begin.

  • Conversation.

    In this level, one program is designated the Caller and another as Listener.

    • The Caller issues an Allocate request.
    • The other side is listening, and runs its program and responds positively to the Allocate. This is where program-to-program communications begin.

    LU6.2 is a half-duplex protocol. After the Caller sends, it can reverse the sense of a logical switch to permit the other side to send. Then, the sense is reversed again, and this continues.

    Nothing can happen at any level until the previous level is successful.

    The system is involved only at the Conversation level.

Communication methods

After SNA is configured properly and sessions are available, there are two major methods of communicating:

  • Low-level APPC invocations.
  • High-level Application Program Interface (API), known as Common Programming Interface for Communications (CPIC).

The AIX version uses CPIC invocations to implement the APPC communications interface. CPIC is a well-documented API within IBM, using function invocations such as Allocate, Send_Data, Receive, Deallocate, and so on.

SNA server

Application programs use an SNA Server to communicate with traditional 3270, Remote Job Entry (RJE), and peer applications within an SNA network. SNA formally defines the functional responsibilities for components of a data communications system. It also specifies how those components must interact.

SNA Server provides an application programming interface to SNA Logical Unit (LU) 0,1, 2, 3, and 6.2 protocols over a Physical Unit (PU) Type 2.1.

SNA Server requires one or more of these adapters on the system:

  • Token Ring High-Performance Network Adapter, with appropriate cables to attach to a Token Ring LAN.
  • ETHERNET High-Performance LAN Adapter, with appropriate cables to attach to an Ethernet or IEEE 802.3 LAN.
  • X.25 Interface Co-Processor/2, with appropriate cables to attach to an X.25 packet switching network.