From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rick Jones Subject: Re: [PATCH v3 net-next 2/2] netem: add cell concept to simulate special MAC behavior Date: Thu, 01 Dec 2011 10:25:28 -0800 Message-ID: <4ED7C698.40003@hp.com> References: <20111201082436.20f21e52@nehalam.linuxnetplumber.net> <20111201085701.02f885c5@nehalam.linuxnetplumber.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: David Laight , Hagen Paul Pfeifer , Eric Dumazet , netdev@vger.kernel.org To: Stephen Hemminger Return-path: Received: from g1t0028.austin.hp.com ([15.216.28.35]:17922 "EHLO g1t0028.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756016Ab1LASZb (ORCPT ); Thu, 1 Dec 2011 13:25:31 -0500 In-Reply-To: <20111201085701.02f885c5@nehalam.linuxnetplumber.net> Sender: netdev-owner@vger.kernel.org List-ID: On 12/01/2011 08:57 AM, Stephen Hemminger wrote: > On Thu, 1 Dec 2011 16:38:51 -0000 > "David Laight" wrote: > >> >>> One idea to do small delays at higher speed is to insert >>> dummy pad frames into the device. >>> It would mean generating garbage, but would allow for >>> highly accurate fine grain delay. >> >> Not a good idea. >> They would have to be sent to a known MAC address >> otherwise all the ethernet switches would forward them >> on all output ports. >> >> David >> >> > > Yes it would have to be a constant destination, not sure if there > is a discard value in Ethernet protocol spec. Aren't there special addresses that aren't supposed to be forwarded by (intelligent) switches? IIRC LLDP uses such things. Though the IEEE may take a dim view of using it for such a purpose, and knuth only knows what switch bugs would be uncovered that way... http://standards.ieee.org/develop/regauth/grpmac/public.html http://en.wikipedia.org/wiki/Link_Layer_Discovery_Protocol rick jones