Subject: Re: r8169: fix driver drop incoming packets >= 1515 if MTU is set between 1515 and 1536

From: Raimonds Cicans <[email-masked]>
Date: Wed, 11 Nov 2009 22:05:35 +0200

Ben Hutchings:
> On Wed, 2009-11-11 at 20:20 +0200, Raimonds Cicans wrote:
>> Driver drop incoming packets >= 1515(1) if MTU is set between 1515(1) and 1536.
>>
>> 1) exact number depends on some factors:
>> - VLAN tagged or not
>> - patch "r8169: Fix card drop incoming VLAN tagged MTU byte large jumbo frames"
>>   applied or not
> [...]
> 
> MTU is a limit on transmission; it is not required to limit received
> frames.

???

In r8169 driver MTU is used to calculate receive buffer size.
Receive buffer size is used to configure hardware incoming packet filter.

For jumbo frames:
Receive buffer size = Max frame size = MTU + 14 (ethernet header) + 4
(vlan header) + 4 (ethernet checksum) = MTU + 22

Bug:
driver for all MTU up to 1536 use receive buffer size 1536

As you can see from formula, this mean all IP packets > 1536 - 22
(for vlan tagged, 1536 - 18 for not tagged) are dropped by hardware
filter.

Example:

host_good>  ifconfig eth0 mtu 1536
host_r8169> ifconfig eth0 mtu 1536
host_good>  ping host_r8169
Ok
host_good>  ping -s 1500 host_r8169
Fail
host_good>  ifconfig eth0 mtu 7000
host_r8169> ifconfig eth0 mtu 7000
host_good>  ping -s 1500 host_r8169
Ok

Raimonds Cicans