2010-11-15 15:42:36

by Jeremy Eder

Subject: [PATCH net-next-2.6] 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 <[email protected]>
Signed-off-by: Jeremy Eder <[email protected]>


diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index fe95105..ae55227 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.

2010-11-17 19:21:48

by David Miller

Subject: Re: [PATCH net-next-2.6] clarify documentation for net.ipv4.igmp_max_memberships

From: Jeremy Eder <[email protected]>
Date: Mon, 15 Nov 2010 10:41:31 -0500

> 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 <[email protected]>
> Signed-off-by: Jeremy Eder <[email protected]>

Applied, thanks Jeremy.