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clarify documentation for net.ipv4.igmp_max_memberships

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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>
This commit is contained in:
Jeremy Eder 2010-11-15 05:41:31 +00:00 committed by David S. Miller
parent 9920239c90
commit d67ef35fff
1 changed files with 21 additions and 3 deletions
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24
Documentation/networking/ip-sysctl.txt
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@ -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.