gtps2m35ACF/SNA Data Communications Reference

General Information

In a PU 2.1 network, the TPF system can be a LEN node or an APPN node. The TPF system can be channel-attached to devices like 37x5 NCP, 3174 APPN controller, RISC System/6000 system, and other PU 2.1 nodes.

The following information provides general information about the TPF system in a PU 2.1 environment. It also provides more specific information based on the TPF node type (LEN or APPN) and which PU 2.1 devices are connected to the TPF system.

Session Control

The considerations for sessions in a PU 2.1 environment are as follows:

Session Identifiers

The routing of message traffic between the TPF system, as a PU 2.1 node, and NCP (or other channel-attached PU 2.1 device) is based on session identifiers (SIDs), as contrasted with network addresses used for subarea routing when TPF is a PU 5 node. The SID uniquely identifies a session between a TPF LU and another LU and is associated with a link. A SID is a 17 bit number with the following structure:

Which node, TPF or NCP, assigned the SID is denoted by the ODAI. TPF always assigns an ODAI value of zero (0), and the NCP assigns an ODAI value of one (1). How a node, TPF or NCP, sets the ODAI value is determined at XID. That is, the primary side of the link uses the ODAI value zero (0), and the secondary side uses one (1). Since TPF is considered the primary side of the channel, it always uses zero (0) as its ODAI value.

SIDs are assigned by the BIND sender from a pool of available numbers. Assignment starts from the lowest numbers available number (X'0101') and proceeds to the higher numbers. When a session ends, its SID is available for re-assignment. This procedure ensures that the address space is not fragmented and limits the main storage the TPF system needs to correlate SIDs to session control blocks (RVTs or SCBs).

Extended BIND

The SNA architecture requires that a TPF system as a PU 2.1 node supports extended BINDs, which implies support of the following:

A negotiable BIND allows the PLU, SLU, and NCP boundary function (BF) to negotiate the BIND parameters to be used for the session. The SLU and its boundary function can change the BIND parameters it disagrees with. The changed parameters are then reviewed by the PLU. If the PLU agrees, then normal data traffic can begin. If the PLU disagrees, it must UNBIND the session.

For certain LU-LU session types, the TPF system uses negotiable BINDs to allow the NCP BF and VTAM to negotiate the maximum request unit (MAXRU) sizes and pacing values for the session. If a BIND receiver changes any of the BIND parameters whose negotiated value is unacceptable to the PLU, the session is unbound.

The TPF system supports negotiable and nonnegotiable BINDs that contain the following control vectors:

The TPF system will always send an extended BIND for an LU-LU session over a PU 2.1 link. The BIND response received by the TPF system may or may not be extended.

SNA Restart and ALS Discovery

During SNA restart, the TPF system verifies that ALS connections that were active at the time of the IPL are still active. In addition, the TPF system discovers new ALS connections that have become active and automatically creates new resource definitions for these ALS connections, if necessary.

The restart portion consists of identifying ALS connections that were active when TPF went down, and verifying their status by issuing a non-activation XID.

The discovery portion is identifying all ALSs that have become active since the TPF system went down. This function is only for ALSs (and PU 5 CTC links); PU 5 NCPs become active via the ZNETW activate command.

An overview of the SNA restart logic for ALSs is as follows:

  1. Scan the RVT entries to find ALS connections that were active before the IPL. For each active ALS found, issue a non-activation XID and rebuild the session index table (SIT) entry for that ALS.
  2. Scan the symbolic device address table (SDAT) in keypoint 2 (CTK2) to find SDAs that are not currently assigned to active ALS connections. For each such SDA found, issue a pre-negotiation XID to check if the ALS is ready to become active. (This is known as self discovery of an ALS.)

    Before issuing the pre-negotiation XID, a sense is issued to determine if the SDA is connected to a PU 5 NCP. If it is, a pre-negotiation XID is not issued because this is not a PU 2.1 device.

When an XID is sent by the TPF system, the information provided must include the fully qualified control point (CP) name of the TPF system. The network identifier portion of the fully qualified CP name is the value of the LENNETID parameter on the SNAKEY macro in keypoint 2 (CTK2). The name portion of the fully qualified CP name is the name of the application that was defined to the TPF system as the local CP. This is done by coding ASNA=LOCP on the MSGRTA statement that defines the application to the TPF system during the SIP process.

Because an ALS becomes active dynamically, there is no way of knowing which ALS links will be active when the TPF system comes up; therefore, it is not possible to allocate SNA CCW areas at OSTG time. Instead, the TPF system allocates a CCW area as follows: