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Configuration Manager PXE TFTP Window Size Bug

Published by Popovici Ioan · Jul 14, 2023 · 3 mins read
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Quick Summary

Configuring Distribution Points to speed up PXE boot, can be achieved by tweaking the RamdiskTFTPBlockSize__ and _RamdiskTFTPWindowSize registry keys.

If your Configuration Manager version is below Configuration Manager 1603, you can’t use RamdiskTFTPWindowSize without a small hack, which is covered in-depth below.

Prerequisites

  • Configuration Manager Instance
  • PXE Enabled Distribution Point
  • Hex Workshop

TFTP Block And Window Size

  • Block Size: The block size is the size of the data packets that the server sends to the client that is downloading the file. A larger block size allows the server to send fewer packets, so there are fewer round-trip delays between the server and the client. However, a large block size leads to fragmented packets, which most PXE client implementations don’t support.

  • Window Size: FTP requires an acknowledgment (ACK) packet for each block of data that is sent. The server doesn’t send the next block in the sequence until it receives the ACK packet for the previous block. TFTP windowing enables you to define how many data blocks it takes to fill a window. The server sends the data blocks back-to-back until the window is filled, and then the client sends an ACK packet. If you increase this window size, it reduces the number of round-trip delays between the client and server, and it decreases the overall required time to download a boot image.

Notes

A large TFTP Block Size will cause IP Fragmentation, which will not lead to a desirable outcome.

ConfigMgr Version >= 1603

This version already supports RamdiskTFTPBlockSize and RamdiskTFTPWindowSize registry keys, and they can be tweaked to speed up PXE boot.

Modify the RamDisk TFTP Window Size

  • Add the following registry key on PXE-enabled distribution points:
Location: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SMS\DP
Name: RamDiskTFTPWindowSize
Type: REG_DWORD
Value: (customized window size) The default value is 1 (one data block fills the window).
  • Restart the Windows Deployment Services service.

Modify the RamDisk TFTP Block Size

  • Add the following registry key on PXE-enabled distribution points:
Location: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SMS\DP
Name: RamDiskTFTPBlockSize
Type: REG_DWORD
Value: (customized block size) The default value is 4096.
  • Restart the Windows Deployment Services service.

ConfigMgr Version < 1603

A bug in the smspxe.dll library prevents the parsing of the RamdiskTFTPWindowSize registry key, and using just the RamdiskTFTPBlockSize to speed up PXE boot can be problematic because of IP Fragmentation.

To fix this, patching of the smspxe.dll library is required, so it uses the RamDiskTFTPBlockSize as the RamDiskTFTPWindowSize value.

Patching smspxe.dll

  • Back-up smspxe.dll, located in the \BIN\I386\, \BIN\X64\ or \BIN\ folder of your currently PXE-enabled Distribution Point.
  • Stop the Windows Deployment Services service to remove the lock from the library file.
  • Open smspxe.dll with Hex Workshop.
  • Select Type as Hex Values.
  • Select in Options the Find All Instances and search for 07000035 as value.
    If you find more than one instance search for BA07000035.
  • Change the 07000035 or BA07000035 value to 08000035 or BA08000035 respectively.
  • Save the changes smspxe.dll.
article card image hex-workshop-patch.png

Modify the RamDisk TFTP block size

  • Add the following registry key on PXE-enabled distribution points:
Location: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SMS\DP
Name: RamDiskTFTPBlockSize
Type: REG_DWORD
Value: (customized block size) The default value is 4096.
  • Restart the Windows Deployment Services service.
Notes

Setting the RamdiskTFTPBlockSize instead of the RamDiskTFTPWindowSize key in the registry is intentional because the patched smspxe.dll will look for this key when setting the TFTP Window Size.

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Published by · Jul 14, 2023 configmgr · 1 mins read

Configuration Manager detailed, filterable Port Documentation

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