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>
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@ -707,10 +707,28 @@ igmp_max_memberships - INTEGER
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Change the maximum number of multicast groups we can subscribe to.
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Default: 20
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conf/interface/* changes special settings per interface (where "interface" is
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the name of your network interface)
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conf/all/* is special, changes the settings for all interfaces
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Theoretical maximum value is bounded by having to send a membership
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report in a single datagram (i.e. the report can't span multiple
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datagrams, or risk confusing the switch and leaving groups you don't
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intend to).
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The number of supported groups 'M' is bounded by the number of group
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report entries you can fit into a single datagram of 65535 bytes.
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M = 65536-sizeof (ip header)/(sizeof(Group record))
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Group records are variable length, with a minimum of 12 bytes.
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So net.ipv4.igmp_max_memberships should not be set higher than:
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(65536-24) / 12 = 5459
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The value 5459 assumes no IP header options, so in practice
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this number may be lower.
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conf/interface/* changes special settings per interface (where
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"interface" is the name of your network interface)
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conf/all/* is special, changes the settings for all interfaces
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log_martians - BOOLEAN
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Log packets with impossible addresses to kernel log.
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