Unable to Transfer Files Across DEC 250 and DEC 500 Routers

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.

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.

To correct this problem, you must modify the Registry.

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.

The products included here are manufactured by Digital Equipment Corporation, a vendor independent of Microsoft; we make no warranty, implied or otherwise, regarding these products' performance or reliability.