Vista problem copying large files across network connections

Written by on Thursday, June 12, 2008 17:30 - 0 Comments

VISTA: Fix bug/Problem With Copying Large Files Across Network Connections

Some people, some of the time, have a problem with Windows Vista locking up when copying large files.  This can happen when copying across a network, or the Internet, and usually only happens when attempting to copy large files.  The problem is directly related to how Windows Vista accesses the network.

There is a process, first described in RFC 1323, way back in 1992, called window scaling.  What this does is adjust the acceptable size of of incoming TCP/IP packets according to connection speed, size of the file, and a few other factors.  In theory, this sounds like a really great idea.  Large file transfers across a fast connection may be sped up, saving the user valuable time.  This factor is set during the initial transfer handshake, and remains in effect throughout the transfer.  In essence, the size of the packet transfer window is negotiated by the machines involved.

Older routers, though, don’t always handle window scaling correctly.  They will pass on the scaling information, but instead of setting the packet size internally to match, set this number to zero.  Now your machine is expecting very small data packets, while the sending machine has been told, and agrees to send larger blocks.  The result is a maddeningly slow transfer, which sometimes results in packets being dropped.  When a packet is dropped, the network connection, for all intents, has been broken.  This is what causes those super slow transfers and apparent lockups when copying large files in Windows Vista, and can sometimes be noticeable in Remote Desktop Connections.

There are a series of steps that can be taken to identify and correct this, as follows:

1.  Make sure that window scaling is supported by your firewall and router.  This can be done by checking with the manufacturer, or run Microsoft’s the connectivity diagnostic suite (especially, TCP High Performance Test).
2. Check to see if a firmware update has been released that can fix the problem. Another option is to replace the device with one which does support window scaling properly.
3. If firmware updates or product replacement are not options, try this:
       Click START
       Type “CMD”, and press ctrl+shift+ENTER to run it with administrator rights.
       Type “netsh interface tcp set global autotuninglevel=restricted”  Restricted mode will often correct the problem.
4.     Click START
    Type “CMD”, and press ctrl+shift+ENTER to run it with administrator rights.
    Type “netsh interface tcp set global autotuninglevel=disabled” to disable autotuning (packet window scaling).

If you should decide to turn this feature back on again, the command is: “netsh interface tcp set global autotuninglevel=normal”, which is also run from the command line as outlined above.


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