APPC DLL Has A Hardcoded Limit of 4 KB GDS Data Size

ID: Q196606

The information in this article applies to:

Microsoft SNA Server, versions 2.11, 2.11 SP1, 2.11 SP2, 3.0, 3.0 SP1, 3.0 SP2, 3.0 SP3, 4.0, 4.0 SP1

SUMMARY

The IBM host will reject General Data Stream (GDS) data that is larger than the 4 KB increments hardcoded in the Appc.dll file.

MORE INFORMATION

The application writes the data message and indicates the length of the data, but the APPC .dll file formats the message into LU 6.2 data format. The data flow should be similar to the following:

The CPIC application calls CMSEND, passes a pointer to the data, and sets the send length to 8872.

The CPIC .dll file calls the MC_SEND_DATA function, passes a pointer to the data, and sets the send length to 8872.

The APPC .dll file formats the RU with the Logical Length size and GDS header information (to indicate LU 6.2 application data) and includes the application data (following IBM LU 6.2 data formatting specifications):
9000 12FF <first 4 KB of data>
9000 <next 4 KB of data>
02B0 <last 688 bytes of data>

The Logical Length fields (9000, 9000, 02B0) and GDS ID (12FF) account for 8 additional bytes, making the full data sent equal to 8880 bytes.

The data is split into the RU size, and segmented within the maximum frame size (265 bytes on this Synchronous Data Link Control (SDLC) connection).

The host stack does not recognize the continuation bit, therefore, any data larger than 4 KB that is sent, is rejected.

The APPC .dll file is hardcoded to send application GDS data sizes of 4 KB.

REFERENCES

IBM SNA Format and Protocol Reference Manual: Architectural Logic for LU Type 6.2 (IBM #SC30-3269), Chapter 2: GDS Variables.

Additional query words:

Keywords :
Version : WINDOWS:2.11,2.11 SP1,2.11 SP2,3.0,3.0 SP1,3.0 SP2,3.0 SP3,4.0,4.0 SP1
Platform : WINDOWS
Issue type : kbinfo