[ipxe-devel] Ipxe cannot handle bigger TFTP blocksize?

Tom.LAUWERS at ext.ec.europa.eu Tom.LAUWERS at ext.ec.europa.eu
Wed Jun 8 09:15:21 UTC 2011

Dear iPXE developers,

I'm having the following problem with iPXE; I'm using the bootable ISO that I burned to a CD-ROM.

I'm booting WinPE over PXE. This works when I'm using a DHCP server on a local subnet (on which I can set the correct scope options), but due to security policies I cannot set DHCP scope options on the rest of the network. So the objective is to boot iPXE from CD-ROM or USB-key with an embedded script that simply executes:

	set next-server x.x.x.x
	imgfetch /Boot/pxeboot.n12
	boot pxeboot.n12

I tested this and it works well. It doesn't work anymore though when I increase the TFTP block size within the BCD config (so Microsoft's bootmgr.exe will request bigger chunks of data from the TFTP-server).

Our TFTP-server must be reached over a WAN (cross-country), so TFTP-throughput is very slow (inherent of the protocol, since it's UDP and every packet needs to be acknowledged). On all of our PC's on the local subnet (using DHCP scope options and the BOOT ROM of the NIC, so not iPXE) it works well when I increase tftpblocksize to 8K and download time of a standard WinPE image drops from 10 minutes to 1,5 minute; which is great. But iPXE is unable to handle this kind of TFTP-traffic.

Here's the script that I use to create the BCD config:

	set BCD-File=C:\TFTP-ROOT\Boot\BCD
	del %BCD-File%
	Bcdedit /createstore %BCD-File%
	Bcdedit /store %BCD-File% /create {ramdiskoptions}
	Bcdedit /store %BCD-File% /set {ramdiskoptions} ramdisksdidevice boot
	Bcdedit /store %BCD-File% /set {ramdiskoptions} ramdisksdipath \boot\boot.sdi
--->	Bcdedit /store %BCD-File% /set {ramdiskoptions} ramdisktftpblocksize 1432
	Bcdedit /store %BCD-File% /set {ramdiskoptions} ramdisktftpwindowsize 2
	Set guid={10000000-1000-1000-1000-100000000000}
	Bcdedit /store %BCD-File% /create %guid% /d "WinPE PXE-boot image" /application osloader
	Bcdedit /store %BCD-File% /set %guid% systemroot \Windows
	Bcdedit /store %BCD-File% /set %guid% detecthal Yes
	Bcdedit /store %BCD-File% /set %guid% winpe Yes
	Bcdedit /store %BCD-File% /set %guid% osdevice ramdisk=[boot]\Boot\winpe_x86.wim,{ramdiskoptions}
	Bcdedit /store %BCD-File% /set %guid% device ramdisk=[boot]\Boot\winpe_x86.wim,{ramdiskoptions}
	Bcdedit /store %BCD-File% /create {bootmgr} /d "Lite Touch Installation" 
	Bcdedit /store %BCD-File% /set {bootmgr} timeout 1
	Bcdedit /store %BCD-File% /set {bootmgr} displayorder %guid%

This works, but is very slow. So I replace line 7 with:

	Bcdedit /store %BCD-File% /set {ramdiskoptions} ramdisktftpblocksize 8192

Now it still works on pc's using the internal NIC's PXE-boot code (and it's much faster), but it doesn't work anymore with iPXE.

My guess is that iPXE cannot handle TFTP-responses that are spread over multiple ethernet frames. 1432 bytes works well because with overhead it's lower than the MTU (1500 bytes). But when the TFTP-server sends 2 or more UDP packets (to send a bigger block of data, requiring less acknowledgements), these somehow get lost.

Any ideas? Thanks a lot in advance.

Best regards,
Bien à Vous,

	Tom Lauwers
	DIGIT A.2-002  Desktop, Peripherals & Mobile Solutions
	Office: B-28 03/030
	Tel. (+32-2)[29]51649 - Fax. (+32-2)[29]50164
	E-mail: tom.lauwers at ext.ec.europa.eu

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