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authorJeremy Eder <jeder@redhat.com>2010-11-15 05:41:31 +0000
committerDavid S. Miller <davem@davemloft.net>2010-11-17 11:22:06 -0800
commitd67ef35fff67845c64d806c033cc7c569ccebfff (patch)
tree4dea9054aa80cdb0b9c4a3a020aa11509f60bf72 /Documentation/networking
parent9920239c90d5f6dadfb44325abf3568a5e3fd827 (diff)
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>
Diffstat (limited to 'Documentation/networking')
-rw-r--r--Documentation/networking/ip-sysctl.txt24
1 files changed, 21 insertions, 3 deletions
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index fe95105992c5..ae5522703d16 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.