TN3270 Server Does Not Send All Data to Host (161812)

CAUSE

The problem is caused by the TN3270 Server's logic for handling SYSREQ requests. In TN3270, one way for a TN3270 client to send a SYSREQ request is to send an SNA TESTREQ key value 0x01. (Note: TESTREQ/SYSREQ are normally 0xF0. However, the original specification for TN3270 did not specify what message a TN3270 client should send when the user press the TESTREQ/SYSREQ key, so different vendors implemented it in different ways.) The TN3270 Server has logic to spot this key as the first byte of an RU, and not to send it to the host, but instead to send a SYSREQ request. However, this code should have been looking for a TESTREQ as the first byte of a chain, not the first byte of an RU. In this case, one of the subsequent RUs in the chain began with 0x01 and the TN3270 Server treated this as a TESTREQ key and ended the chain (EC) when it should not have.

NOTE: Splitting a chain into RUs is arbitrary, so there is no significance to where the split occurs.

This problem is seen when chunks of data are larger than the RU size supported for the session, and the chunks need to be split up. The chunk size (chain size) is controlled by the Structured Field Size, configurable in some TN3270 emulators. The RU size is controlled by the primary half session's bind image (byte 10).

The following is a sample trace that shows the problem:

Beginning of Chain (BC):

|000000c9.000000ca DLC    ----------------------------------------------
17:06:52.39
|000000c9.000000ca DLC    01020101->0A160080 DLC DATA
|000000c9.000000ca DLC                       DAF:01 OAF:03 ODAI:off Normal
|000000c9.000000ca DLC                       RQE FMD BC DR1 BB
|000000c9.000000ca DLC
|000000c9.000000ca DLC    ---- Header  at address 00CC4620, 4 elements ---
-
|000000c9.000000ca DLC    00050004 00032C00 01030068 0100E800
<......,....h..Y.>
|000000c9.000000ca DLC
|000000c9.000000ca DLC    ---- Element at address 011BCA3C, start 10, end
268 ----
|000000c9.000000ca DLC    02908088 ...etc

Middle Chain (MC):

|000000c9.000000ca DLC    ----------------------------------------------
17:06:52.59
|000000c9.000000ca DLC    01020101->0A160080 DLC DATA
|000000c9.000000ca DLC                       DAF:01 OAF:03 ODAI:off Normal
|000000c9.000000ca DLC                       RQE FMD MC DR1
|000000c9.000000ca DLC
|000000c9.000000ca DLC    ---- Header  at address 00CC444C, 4 elements ---
-
|000000c9.000000ca DLC    00050004 40032C00 01030069 0100E800
<....@.,....i..Y.>
|000000c9.000000ca DLC
|000000c9.000000ca DLC    ---- Element at address 011BD7EC, start 10, end
268 ----
|000000c9.000000ca DLC    009000FF ...etc

More middle chains flow...

End Chain (EC)

|000000c9.000000ca DLC    ----------------------------------------------
17:06:54.79
|000000c9.000000ca DLC    01020101->0A160080 DLC DATA
|000000c9.000000ca DLC                       DAF:01 OAF:03 ODAI:off Normal
|000000c9.000000ca DLC                       RQE FMD EC DR1 CD
|000000c9.000000ca DLC
|000000c9.000000ca DLC    ---- Header  at address 00CC46F0, 4 elements ---
-
|000000c9.000000ca DLC    00050400 00032C00 01030072 0100E800
<......,....r..Y.>
|000000c9.000000ca DLC
|000000c9.000000ca DLC    ---- Element at address 011BE7E4, start 10, end
268 ----
|000000c9.000000ca DLC    01902001 ...etc
                                ^^ - This is the TESTREQ key.
				

The SYSREQ and TESTREQ AID bytes are found in the "IBM 3270 Information Display System" reference (Part #: GA23-0061-2).