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Commit d67ef35f authored by Jeremy Eder's avatar Jeremy Eder Committed by David S. Miller
Browse files

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: default avatarJiri Pirko <jpirko@redhat.com>
Signed-off-by: default avatarJeremy Eder <jeder@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 9920239c

Documentation/networking/ip-sysctl.txt

+21 −3
Original line number Diff line number Diff line
@@ -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.