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Is MaximumBlockSize registry key supported on Windows 2008 R2 RTM WDS server?

  • Artigo

Hi there,

Recently I dealt with a connectivity issue that occurs when deploying OS images with Windows 2008 R2 WDS server to PXE clients running in different subnets. As you may already know, we have a workaround for router MTU size incompatibilities seen when deploying OS images to remote subnets in Windows 2008:

https://support.microsoft.com/kb/975710 Operating system deployment over a network by using WDS fails in Windows Server 2008

The same router packet drop issue was present in my case. Even we configured the MaximumBlockSize reg key on the Windows 2008 R2 WDS server, the issue was still in place and the TFTP server (part of WDS server) was still sending data in chunks requested by the client rather than the size we tried to limit wit the MaximumBlockSize reg key:

Note:
10.1.1.1 is PXE client
10.2.2.2 is TFTP server

=> Client is asking for a TFTP block size of 1456 bytes and TFTP server (that’s running as a part of WDS server) is honoring that request:

10.1.1.1 10.2.2.2 TFTP Read Request, File: boot\x86\wdsnbp.com\000, Transfer type: octet\000, blksize\000=1456\000
10.2.2.2 10.1.1.1 TFTP Option Acknowledgement, blksize\000=1456\000

=> Then TFTP server begins sending the first 1456 bytes block of wdsnbp.com file:

10.2.2.2 10.1.1.1 TFTP Data Packet, Block: 1

=> Most likely because the router drops that packet, TFTP client doesn’t receive it and hence it re-sends the Read file request once more:

10.1.1.1 10.2.2.2 TFTP Read Request, File: boot\x86\wdsnbp.com\000, Transfer type: octet\000, blksize\000=1456\000

=> And TFTP server sends the same block once more and tries a number of times:

10.2.2.2 10.1.1.1 TFTP Data Packet, Block: 1
10.2.2.2 10.1.1.1 TFTP Data Packet, Block: 1
10.2.2.2 10.1.1.1 TFTP Data Packet, Block: 1
10.2.2.2 10.1.1.1 TFTP Data Packet, Block: 1
...

So PXE boot fails at the end.

The analysis showed us that the reg key didn't take effect on Windows 2008 R2 WDS server. After a source code review, I saw that the feature wasn't integrated into Windows 2008 R2 RTM code due to release timing of Windows 2008 R2 and the fix for Windows 2008. I verified with internal resources and can say that the feature will be a part of Windows 2008 R2 SP1. The SP1 public beta is being made available by the end of July:

https://blogs.technet.com/b/itproaustralia/archive/2010/06/08/windows-7-and-windows-server-2008-r2-sp1-beta-available-end-of-july.aspx  Windows 7 and Windows Server 2008 R2 SP1 Beta available end of July

Hope this helps

Thanks,
Murat

Comments

  • Anonymous
    January 01, 2003
    Thanks for sharing this Janake. This seems to be very recent. It looks like it was decided to release an hotfix before SP1 was released. Murat

  • Anonymous
    August 17, 2010
    There is a hotfix for R2 availible now, se support.microsoft.com/.../975710