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Message-ID: <1289835691.8257.1274.camel@jerms-wks.usersys.redhat.com> Date: Mon, 15 Nov 2010 10:41:31 -0500 From: Jeremy Eder <jeder@...hat.com> To: netdev@...r.kernel.org Cc: rdunlap@...otime.net, davem@...emloft.net, opurdila@...acom.com, apetlund@...ula.no, William.Allen.Simpson@...il.com, ian.campbell@...rix.com, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, Jiri Pirko <jpirko@...hat.com> Subject: [PATCH net-next-2.6] clarify documentation for net.ipv4.igmp_max_memberships 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 <jpirko@...hat.com> Signed-off-by: Jeremy Eder <jeder@...hat.com> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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