Unable to Transfer Files Across DEC 250 and DEC 500 Routers

• Microsoft Windows NT Workstation version 3.5
• Microsoft Windows NT Server version 3.5
• Microsoft Windows NT Server version 4.0
• Microsoft Windows NT Workstation version 4.0

SYMPTOMS

When you attempt to copy large files across Digital Equipment Corporation (DEC) 250 and DEC 500 routers using the TCP/IP protocol, the file transfer fails.

CAUSE

DEC 250 and DEC 500 are IP/DECNet Routers. For wide area networks (WAN) with DECNet as the primary protocol, the MTU size may be set to 512 bytes. These routers may not send ICMP Destination Unreachable messages with Fragmentation Needed in its code field if the datagram requires fragmentation. Instead the frame is dropped. When the Windows NT EnablePMTUDiscovery parameter is turned on (set by default), the "Don't Fragment" bit is set in the IP datagram indicating to the router that it should not fragment this packet. At this time the router should respond with a destination unreachable ICMP message.

RESOLUTION

To correct this problem, you must modify the Registry.

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

1. Run Registry Editor (REGEDT32.EXE).

2. From the HKEY_LOCAL_MACHINE subtree, go to the following key:

         \SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
   

3. From the Edit menu choose Add Value.

4. Add the following:

         Value Name:   EnablePMTUDiscovery
         Data Type:    REG_DWORD
         Data:         0
         Radix:        Hex
   
   

5. Quit Registry Editor.

6. Shutdown and restart Windows NT.