gtps2m3n | ACF/SNA Data Communications Reference |
TPF's reliability and serviceability support is provided to maintain operation of the network. It includes:
Before acting upon any request, TPF analyzes the request for errors. If an error is detected, TPF returns the request along with an error indicator.
When possible, TPF support for I/O handlers and NCP attempts automatic recovery from an error. If recovery is possible, processing continues. Otherwise, a record of the error is written to the realtime log tape.
TPF detects an NCP slowdown condition and assists the NCP in return to normal operation.
All TPF operator and application program-initiated requests are tested for errors. Errors detected in an operator request result in the return to the CRAS terminal of the request and an indication of the error. If an invalid condition is detected while processing a request, TPF: 1) stops processing the request, 2) informs the operator, and 3) resets the affected resources. TPF discards invalid application requests and starts a system error dump to document the error.
In addition to invalid request notices, TPF also sends discrete error messages to the system console. These messages describe unusual conditions that affect operation of the network such as permanent hardware errors, communication line failures, or negative responses to SNA commands.
When an I/O interrupt occurs, TPF (via the appropriate error recovery procedure) tries to determine the type of error and take the appropriate recovery action. When a non-recoverable error is encountered, a message is written to the system console and an error record is written to the realtime log tape.
The NCP provides recovery for the devices attached to it. When an NCP completes error-recovery processing, it forwards a Maintenance Data Record (MDR) to TPF. The NCP also maintains temporary error counters for each device. These counters are sent to TPF (as MDRs) whenever the counters overflow or the NCP is deactivated. MDRs are written to the realtime log tape.
The record of hardware errors on the realtime log tape is formatted and printed for analysis offline.
When an NCP's message buffer is filled, the NCP automatically enters slowdown mode. In slowdown mode, the NCP reduces the number of messages it accepts from both the communication lines and host processors. Further, the NCP tries to increase its output message rate. When NCP notifies TPF of a slowdown condition, TPF stops sending data to the NCP. TPF continues to accept requests from application programs, but queues those requests directed to a slowed NCP. When an NCP exits slowdown mode, the queued requests are processed and sent. TPF data collection programs record each occurrence of an NCP slowdown.
If an NCP remains in slowdown for an extended period of time, a large output queue can build up in the TPF system. The SLOWTIME parameter on the SNAKEY macro in keypoint 2 (CTK2) determines how long an NCP is allowed to be in slowdown before the TPF system breaks the connection to the NCP.
See TPF ACF/SNA Network Generation for more information about the SNAKEY macro.
TPF recognizes buffer shortage conditions on the host on the other side of the CTC connection. VTAM indicates slowdown in one of the control fields passed on a data transfer operation. TPF honors this indication and stops initiating writes until a VTAM write operation is received with the slowdown indicator reset. TPF responds to attention interrupts by issuing the channel program with read buffers, but the write count only includes the 8 bytes of control information.
To the receiver, the slowdown indicator means that the data that has been written is now rejected. In slowdown, TPF includes the total data size in the write count. VTAM only reads 8 bytes. TPF supports slowdown as a receiving system; that is, it sets the slowdown bit if a read buffer is not available.