From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757643Ab0KOPmg (ORCPT ); Mon, 15 Nov 2010 10:42:36 -0500 Received: from mx1.redhat.com ([209.132.183.28]:35595 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756186Ab0KOPmf (ORCPT ); Mon, 15 Nov 2010 10:42:35 -0500 Subject: [PATCH net-next-2.6] clarify documentation for net.ipv4.igmp_max_memberships From: Jeremy Eder Reply-To: jeder@redhat.com To: netdev@vger.kernel.org Cc: rdunlap@xenotime.net, davem@davemloft.net, opurdila@ixiacom.com, apetlund@simula.no, William.Allen.Simpson@gmail.com, ian.campbell@citrix.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Jiri Pirko Content-Type: text/plain; charset="UTF-8" Organization: Red Hat, Inc. Date: Mon, 15 Nov 2010 10:41:31 -0500 Message-ID: <1289835691.8257.1274.camel@jerms-wks.usersys.redhat.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This patch helps clarify documentation for net.ipv4.igmp_max_memberships by providing a formula for calculating the maximum number of multicast groups that can be subscribed to, plus defining the theoretical limit. Signed-off-by: Jiri Pirko Signed-off-by: Jeremy Eder diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt index fe95105..ae55227 100644 --- a/Documentation/networking/ip-sysctl.txt +++ b/Documentation/networking/ip-sysctl.txt @@ -707,10 +707,28 @@ igmp_max_memberships - INTEGER Change the maximum number of multicast groups we can subscribe to. Default: 20 -conf/interface/* changes special settings per interface (where "interface" is - the name of your network interface) -conf/all/* is special, changes the settings for all interfaces + Theoretical maximum value is bounded by having to send a membership + report in a single datagram (i.e. the report can't span multiple + datagrams, or risk confusing the switch and leaving groups you don't + intend to). + The number of supported groups 'M' is bounded by the number of group + report entries you can fit into a single datagram of 65535 bytes. + + M = 65536-sizeof (ip header)/(sizeof(Group record)) + + Group records are variable length, with a minimum of 12 bytes. + So net.ipv4.igmp_max_memberships should not be set higher than: + + (65536-24) / 12 = 5459 + + The value 5459 assumes no IP header options, so in practice + this number may be lower. + + conf/interface/* changes special settings per interface (where + "interface" is the name of your network interface) + + conf/all/* is special, changes the settings for all interfaces log_martians - BOOLEAN Log packets with impossible addresses to kernel log.