Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756704AbZFYHiR (ORCPT ); Thu, 25 Jun 2009 03:38:17 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752828AbZFYHiF (ORCPT ); Thu, 25 Jun 2009 03:38:05 -0400 Received: from woodchuck.wormnet.eu ([77.75.105.223]:60494 "EHLO woodchuck.wormnet.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752576AbZFYHiF (ORCPT ); Thu, 25 Jun 2009 03:38:05 -0400 Date: Thu, 25 Jun 2009 08:38:05 +0100 From: Alexander Clouter To: david@lang.hm Cc: linux-kernel@vger.kernel.org, Herbert Xu Subject: Re: When does Linux drop UDP packets? Message-ID: <20090625073805.GB2014@woodchuck> References: <20090625050052.GA30594@gondor.apana.org.au> <20090625061348.GA31407@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090625061348.GA31407@gondor.apana.org.au> Organization: diGriz X-URL: http://www.digriz.org.uk/ X-JabberID: alex@digriz.org.uk User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1972 Lines: 46 Hi, * Herbert Xu [2009-06-25 14:13:48+0800]: > > On Wed, Jun 24, 2009 at 10:37:14PM -0700, david@lang.hm wrote: > > > > please explain more? how do they differ? > > An IGMP-snooping switch will only forward multicast traffic to > ports that have subscribed to that group. Broadcast traffic > on the otherhand is forwarded to all ports. > [directing over to David...] The more intelligent switches actually peek at L3/L4 (even if it's just a L2 switch, like a Cisco 2950) to decode IGMP packets as they come out from the hosts. The switch uses that to build up an Ethernet multicast routing table, in addition to it's usual unicast based one. If you bought a managed switch in the past five years (ours from 8 years ago that we have just binned also supported it) it should support IGMP snooping. Any one buying switches now needs to make sure they support IPv6 and MLD snooping...bearing in mind you are going to be lumbered with those switches for at least five years. In addition and mentioned before, the NIC in hardware will also filter any multicast traffic that is un-needed if you are using el-cheapo switches; crime and punishment. Broadcast traffic *always* spans the whole VLAN and *always* gets passed up to the OS to make decisions on whether to process the packet or not. Now, when you roll back in the 'automaticness' of service discovery and other such things on the network...multicast is a very powerful tool. Just because it can be many<->many or one<->many does not mean it cannot be just as efficiently used to bootstrap one<->one communications. Cheers -- Alexander Clouter .sigmonster says: Don't take life seriously, you'll never get out alive. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/