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Short version

  • IPv4 download from server: fast
  • IPv4 upload to server: fast
  • IPv6 download from server: fast
  • IPv6 upload to server: slow

enter image description here

Full version

I am copying files from a GBe desktop to a GBe server. The copy proceeds at a horrendously slow:

  • 23 kB/s
  • i.e. 0.2 Mbps
  • 1% of GBe

Slow moving progress

But if I copy a file to a virtual machine hosted inside that same GBe server, copy proceeds as normal (512 Mbps = 50% of GBe):

Normal moving progress

I thought to myself, "What can Windows Server 2012 possibly being doing, that the virtual Windows Server 2003 is not?"

What about...

  • It cannot be a cabling issue, as both transfers go across the same cables, through the same switch.
  • It cannot be a drive speed issue, as the targets of both file copies end up on the same drive (a Storage Space parity drive).
  • It cannot be drive write speed issue, as I can copy files locally around on the destination drive, at full speed (111 MB/s = 888 Mbps).
  • It is not network speed, as I can download from the server at 994 Mbps.

As I was pinging both servers, I realized I was communicating:

  • old server over IPv4
  • new server over IPv6

Which then led me to my final, frustrating, test:

  • Download from the server over IPv6
  • and upload back to the same server over IPv4

And it's fast:

 Desktop                \\corsair (IPv6 identity)
+----------+           +--------------------------------+
|          | 994Mbps   |                                |
|          |<──────────|                                |  
|          |           |                                |
|          | 0.2Mbps   |                                |
|          |──────────>|                                |
|          |           |                                |
|        ┌─|<──────────|                                |
|        │ |           ┇                               ┇ 
|        │ |           |\\ (IPv4 Identity) |
|        │ | 504Mbps   |                                |
|        └─|──────────>|                                |
|          |           |                                |
+----------+           +--------------------------------+

What is Windows Server 2012 doing that is causing it to fail so horrendeously on receiving IPv6 packets?

Note: I would have tried to test uploading over IPv6 to the virtual machine inside, but Windows XP/Server 2003 did not support File Sharing over IPv6 (cannot resolve the NetBIOS name).


  • Desktop: Windows 7 64-bit
    IPv6 MTU: 1472
    IPv4 MTU: 1472

  • Server: Windows Server 2012
    IPv6 MTU: 1472
    IPv4 MTU: 1472

Bonus: the MTU setting on client and server 1472:

>netsh interface ipv6 show subinterfaces

   MTU  MediaSenseState   Bytes In  Bytes Out  Interface
------  ---------------  ---------  ---------  -------------
  1472                1  11739088006  581016212  Local Area Connection

The Questions

  • Why is uploading IPv6 slow?
  • Why is downloading IPv6 not slow?
  • Why is IPv4 not slow?
  • What setting must be adjusted to fix it?


It's a litter over a year later, and the problem remains unsolved. Today i switched my desktop back to backup to the network server. It's a few hours later and the backup is still going. I remembered that Windows Server 2012 is slow when trying to send it IPv6 packets:

enter image description here

share|improve this question
The 1472 MTU isn't normal for a native IPv6 connection. Are you tunneling IPv6 directly to this machine? – Michael Hampton Aug 28 '13 at 0:35
@MichaelHampton These two machines are on the same LAN. But there is a router, that routes IPv6 packets to the Internet.**That** router is connected to the IPv6 Internet using a tunnel. – Ian Boyd Aug 28 '13 at 1:04
What's the MTU on the other machine? And of your tunnel? – Michael Hampton Aug 28 '13 at 1:06
@MichaelHampton Both client and server (IPv6) MTU is 1472. (Question updated) – Ian Boyd Aug 28 '13 at 1:14
Looks like we've found the problem. – Michael Hampton Aug 28 '13 at 1:22

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