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Q196606: APPC DLL Has A Hardcoded Limit of 4 KB GDS Data Size

Article: Q196606
Product(s): Microsoft SNA Server
Version(s): 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
Operating System(s): 
Keyword(s): 
Last Modified: 13-JUN-2001

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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 
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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:

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Keywords          :  
Technology        : kbAudDeveloper kbSNAServSearch kbSNAServ300 kbSNAServ211 kbSNAServ400 kbSNAServ211SP1 kbSNAServ211SP2 kbSNAServ300SP3 kbSNAServ300SP1 kbSNAServ400SP1 kbSNAServ300SP2
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
Issue type        : kbinfo
Solution Type     : kbnofix

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