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@redhat.com>
Signed-off-by: Jeremy Eder <jeder@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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.