netdev - Re: TX time stamping

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Date:	Mon, 30 Mar 2009 20:09:40 +0200
From:	Oliver Hartkopp <oliver@...tkopp.net>
To:	Patrick Ohly <patrick.ohly@...el.com>
CC:	Herbert Xu <herbert@...dor.apana.org.au>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
Subject: Re: TX time stamping

Patrick Ohly wrote:
> On Thu, 2009-03-26 at 14:48 +0000, Herbert Xu wrote:
>> On Wed, Mar 25, 2009 at 08:08:44AM +0100, Patrick Ohly wrote:
>>> I suggest to make it so that the sender gets the packet back once per
>>> interface, with different time stamps and information about the
>>> interface.
>> That could be awkward.  What if the second or subsequent instance
>> of the packet is held up indefinitely?
> 
> The sender must be prepared for packet loss anyway (for example, during
> stress tests I have seen that packets were dropped between socket layer
> and device driver).
> 
> Getting a TX time stamp only for some, but not all expected interfaces,
> or getting some results much later isn't that different.
> 
> After a certain timeout the sender must assume that the packet was lost
> and resend. An unexpected response for a packet that was supposed to be
> lost must be ignored.
> 

Hello Patrick,

i wonder if using the IP stack for PTP with the possibility to send TX-stamped
 PDUs on various interfaces is the best solution.

I'm not aware of all the routing, packet scheduling, etc. stuff that much -
but does it probably make sense to use AF_PACKET for PTP, where you can
specify the interface and build a PTP IP PDU directly? I assume this does not
make that big difference to the ptpd in userspace.

Any ideas? Or am i completely wrong here?

Regards,
Oliver
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