[openib-general] IPv6oIB neighbour discover broken when MCGs overflow

[Hal Rosenstock]

On Fri, 2007-02-16 at 12:00, Roland Dreier wrote:
>  > We have a customer issue regarding IPv6oIB. In the subnet, there are
>  > limited number of MCGs supported. So when there are multiple IPv6 addresses
>  > are assigned to one interface, each IPv6 address will have one unique
>  > solicited-node address (depends on their groupID). Then in a large subnet,
>  > we will have tons of MCGs. If IPv6 solicited node addresses exceed the
>  > number of MDGs in this subnet, then IPv6 neighbour discovery will be
>  > broken, this won't happen in Ethernet since sendonly doesn't require sender
>  > to be joined any MCG.
> 
>  > I have done an initial patch to addresss MCG overflow problem and redirect
>  > the solicited-node address to all hosts node address, thus IPv6 neighbour
>  > discovery will work no matter how many IPv6 addresses in this subnet. This
>  > patch is only triggered with IPv6 enabled and MGC overflows, so there is
>  > almost no performance penalty.
> 
> I really don't like this approach, since it can break things in very
> subtle ways (eg suppose one node fails to join its solicited node
> group, but then a later node wants to talk to it and succeeds in
> joining the solicited node group as a send-only member -- since the
> first node is not a member then it will never see the ND messages).
> 
> I much prefer to fix the SM not to impose too-low limits on the number
> of MCGs.  Supporting O(# nodes) MCGs is really not a very onerous
> requirement on the SM.

Is this a MFT size issue or SM issue or both ?

-- Hal

>  - R.
> 
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