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

[Shirley Ma]

Hello, Roland,

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.

According to IPoIB RFCs, it only covers MCG beyond this subnet,  For MCG
not in this subnet, direct the packet to all routes or broadcast. ( for
IPv6 should be all hosts address here), but not cover MCG overflow in this
subnet. (Currently it's not implemented in openFabric.)

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.

The patch seems working, although it is still under validation and test. I
would like to hear your opinion regarding how to address this problem,
whether I am in the right direction ...

Thanks
Shirley Ma
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