/bugzilla3/ Bug 1140 – Xen and Ethernet Padding
Bug 1140 - Xen and Ethernet Padding
: Xen and Ethernet Padding
Status: NEW
Product: Xen
: 3.0.3
: x86-64 All
: P2 normal
Assigned To: Xen Bug List
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Reported: 2008-01-14 16:23 CST by Marcelo Messa
Modified: 2008-01-14 16:23 CST (History)
1 user (show)

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Description Marcelo Messa 2008-01-14 16:23:49 CST

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

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

      \RECEIVER  | Dom0_vif |FullV_DomU_2|ParaV_DomU_2|SMP_ParaV_DomU_2
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)

       My setup is a Centos 5.1 (2.6.18-53.1.4.el5xen) / Xen (3.0.3) in a
bridge network configuration. 

       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.

       A direct related problem: it's not possible to export AoE disks from
Dom0 and other Paravirtualized guests (AoE blade server verifies if the packet
has a minimal size - 60 bytes) to any local VM. 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.

Best Regards
      Marcelo Messa