Error Message: LPT<n>: Windows Cannot Write to This File

• Microsoft Windows for Workgroups version 3.11
• Microsoft TCP/IP for Windows for Workgroups version 3.11
• Microsoft Windows NT Workstation version 3.5
• Microsoft Windows NT Server version 3.5

This article contains information about using Windows for Workgroups version 3.11 with third-party products or configurations that have not been tested and are not supported by Microsoft.

If the steps in this article do not function properly, use a supported configuration or contact the manufacturer of your third-party product for more information.

SYMPTOMS

When you try to print to a shared printer from a Windows-based program such as Microsoft Word for Windows or Microsoft Excel, you receive the following error message:

   LPT<n>: Windows cannot write to this file. The disk may be full.
   Delete any unwanted files and try again.

CAUSE

This problem can occur when you print from a computer running Windows for Workgroups 3.11 using Microsoft TCP/IP-32 3.11a to a shared printer on a Windows NT Server or Workstation 3.5 computer using Microsoft TCP/IP, and the print job goes through a "black hole" router before reaching the printer.

A black hole router does not return an Internet Control Message Protocol (ICMP) "destination unreachable" message when it needs to fragment a packet that has the Don't Fragment bit enabled.

RESOLUTION

To correct this problem, follow these steps:

1. Add the following parameter to the Windows NT registry:

         ENABLEPMTUBHDETECT=1
   

   WARNING: Using the Windows NT Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.

2. Use any text editor (such as Notepad) to edit the SYSTEM.INI file on the Windows for Workgroups computer. Add the following line to the [MSTCP] section of the SYSTEM.INI file:

         PMTUBLACKHOLEDETECT=1
   

MORE INFORMATION

The PMTUBLACKHOLEDETECT setting is the Microsoft TCP/IP-32 equivalent to the EnablePMTUBHDetect setting. Setting the EnablePMTUBHDetect setting to 1 (true) causes TCP to try to detect black hole routers while performing MTU discovery. A black hole router does not return an ICMP "destination unreachable" message when it needs to fragment a TCP packet with the Don't Fragment bit set. TCP depends on receiving these message to perform MTU discovery. With this setting enabled, TCP tries to send segments without the Don't Fragment bit set if several retransmissions of a segment go unacknowledged. If the segment is acknowledged as a result, the MSS is decreased and the Don't Fragment bit is set in future packets on the connection. Enabling black hole router detection increases the maximum number of retransmissions performed for a given segment.

REFERENCES

For additional information about adding registry values in Windows NT, see the online Help file for Registry Editor, or see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q120642
   TITLE     : TCP/IP & NBT Configuration Parameters for Windows NT 3.5