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[Xen-users] Xen () ethernet padding

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] Xen () ethernet padding
From: Marcelo Messa <mrmessa@xxxxxxxxx>
Date: Fri, 11 Jan 2008 19:31:12 -0200
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Hi,

I made some tests about ethernet padding ( in my dummy works: an expected behavior from ethernet 10/100 on-wire, that needs at least 60 bytes of data to be able to CRC and the NIC chipset has the job of fullfill the packet if it is smaller than this- padding ) and I notice a behavior that is not expected (at least for me).

My setup is a Centos 5.1/Xen (3.0.3) in a bridge network configuration and I've got the following results:

>>> Ethernet Padding Sender  /  Receiver Matrix <<<

\RECEIVER | Dom0 vif | FullV_DomU_2 | ParaV_DomU_2 | SMP_ParaV_DomU_2
SENDER/
Dom0 vif.......................| No (OK) | Yes (OK) | No (NOK) | No (NOK)
FullV_DomU................|  No (NOK) |  Yes (OK) |  No (NOK) | No (NOK)
ParaV_DomU..............|  No (NOK) |  Yes (OK) |  No (NOK) | No (NOK)
SMP_ParaV_DomU...|  No (NOK) |  Yes (OK) |  No (NOK) | No (NOK)

The results shows that only FullVirtualization do padding, and only when receives a packet (not when send it).

You could ask "why padding if you dont have a physical transport?". If you are playing with some L2 protocol (like AoE), this behavior do impact.

From results, we conclude that is not possible, for example, to export AoE disks from Dom0 and other Paravirtualized guests (AoE blade server verifies if the packet has a minimal size - 60 bytes minimal ethernet on-wire packet). I do not known if another layer2 protocols has the same problem, but I think that is a expected feature that a virtual NIC works as a physical one..

To have a consistent behavior, everybody should do ethernet padding (my expected behavior), or no one should do this.

Make sense? I changed the AoE initiator vblade to not discard packets "not padded" (very simple change, but I still dont have an answer why the padding limit was there), but I appreciate if someone could give more 2 cents for this issue.


Cheers
Marcelo Messa
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