netdev - Re: 802.3x pause frames

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Date:	Tue, 11 Nov 2014 12:29:39 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Julia Niewiejska <julia.niewiejska@...e.fraunhofer.de>
Cc:	netdev@...r.kernel.org
Subject: Re: 802.3x pause frames

On Thu, 2014-11-06 at 13:59 +0100, Julia Niewiejska wrote:
> Hello,
> 
> I indend to do some experiments with 802.3x pause frames, but I have yet 
> to find a setup that works. I used a tool that generates pause frames 
> [1] in two setups. First one consists of two virtual machines on a 
> VMWare ESXi 5.5 server connected with each other through a virtual 
> switch. In second setup two physical machines (Desktop PCs) were 
> connected directly by an ethernet cable. The VMs are running Debian 
> Wheezy 32bit while Debian Testing (Jessie) 64bit is installed on both 
> desktop PCs. Below you will find some information on the ethernet 
> adapters and drivers used in the VM [2], the desktop PC with the more up 
> to date hardware [3] and the older machine [4]. I used the following 
> command to activate flow control:
> 
> ethtool -A <eth> autoneg off rx on tx on
> 
> First of all I noticed some discrepancies between the output of 
> mii-tools or ethtool -a and the attempt to change the flow control 
> settings with the command above. Only the Realtek adapter actually 
> output that it didn't support the operation, the other adapters accepted 
> the settings without any error message.
> 
> In both setups pause frames were generated on one machine while a ping 
> was sent simultaneously, as suggested in [1]. While the VM connection 
> was set to 1 Gbps the whole time, I also tested a 10 Mbps setting on the 
> physical connection. Even though the pause frames were always visible in 
> tcpdump at the receiver, I didn't notice any influence whatsoever in the 
> ping results.
[...]

MACs that implement flow control will not pass pause frames to the host;
therefore these MACs do not implement flow control.

Ben.

-- 
Ben Hutchings
Experience is directly proportional to the value of equipment destroyed.
                                                         - Carolyn Scheppner

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