Re: [PATCH net-next 1/8] net: ipv6: mld: fix v1/v2 switchback timeout to rfc3810, 9.12.

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

On Tue, Sep 03, 2013 at 09:59:31AM +0200, Daniel Borkmann wrote:
> i) RFC3810, 9.2. Query Interval [QI] says:
> 
>    The Query Interval variable denotes the interval between General
>    Queries sent by the Querier. Default value: 125 seconds. [...]
> 
> ii) RFC3810, 9.3. Query Response Interval [QRI] says:
> 
>   The Maximum Response Delay used to calculate the Maximum Response
>   Code inserted into the periodic General Queries. Default value:
>   10000 (10 seconds) [...] The number of seconds represented by the
>   [Query Response Interval] must be less than the [Query Interval].
> 
> iii) RFC3810, 9.12. Older Version Querier Present Timeout [OVQPT] says:
> 
>   The Older Version Querier Present Timeout is the time-out for
>   transitioning a host back to MLDv2 Host Compatibility Mode. When an
>   MLDv1 query is received, MLDv2 hosts set their Older Version Querier
>   Present Timer to [Older Version Querier Present Timeout].
> 
>   This value MUST be ([Robustness Variable] times (the [Query Interval]
>   in the last Query received)) plus ([Query Response Interval]).
> 
> Hence, on *default* the timeout results in:
> 
>   [RV] = 2, [QI] = 125sec, [QRI] = 10sec
>   [OVQPT] = [RV] * [QI] + [QRI] = 260sec
> 
> Having that said, we currently calculate [OVQPT] (here given as 'switchback'
> variable) as ...
> 
>   switchback = (idev->mc_qrv + 1) * max_delay
> 
> RFC3810, 9.12. says "the [Query Interval] in the last Query received". In
> section "9.14. Configuring timers", it is said:
> 
>   This section is meant to provide advice to network administrators on
>   how to tune these settings to their network. Ambitious router
>   implementations might tune these settings dynamically based upon
>   changing characteristics of the network. [...]
> 
> iv) RFC38010, 9.14.2. Query Interval:
> 
>   The overall level of periodic MLD traffic is inversely proportional
>   to the Query Interval. A longer Query Interval results in a lower
>   overall level of MLD traffic. The value of the Query Interval MUST
>   be equal to or greater than the Maximum Response Delay used to
>   calculate the Maximum Response Code inserted in General Query
>   messages.
> 
> I assume that was why switchback is calculated as is (3 * max_delay), although
> this setting seems to be meant for routers only to configure their [QI]
> interval for non-default intervals. So usage here like this is clearly wrong.
> 
> Concluding, the current behaviour in IPv6's multicast code is not conform
> to the RFC as switch back is calculated wrongly. That is, it has a too small
> value, so MLDv2 hosts switch back again to MLDv2 way too early, i.e. ~30secs
> instead of ~260secs on default.
> 
> Hence, introduce necessary helper functions and fix this up properly as it
> should be.
> 
> Introduced in 06da92283 ("[IPV6]: Add MLDv2 support."). Credits to Hannes
> Frederic Sowa who also had a hand in this as well. Also thanks to Hangbin Liu
> who did initial testing.
> 
> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
> Cc: David Stevens <dlstevens@us.ibm.com>
> Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>

Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>