[openib-general] IPoIB broadcast MC group membership

[Hal Rosenstock]

On Mon, 2006-02-20 at 21:53, Fabian Tillier wrote:
> On 2/20/06, Roland Dreier <rdreier at cisco.com> wrote:
> >    Fabian> Second, if so, how is IPoIB supposed to interact with
> >    Fabian> subnet managers that don't pre-create an empty broadcast
> >    Fabian> group?
> >
> >    Fabian> Shouldn't IPoIB first do a GET for the broadcast group,
> >    Fabian> and use those settings if it exist, otherwise create it?
> >
> > What parameters should it use to create it?
> 
> The only paramter that can be problematic is the QKey, but it's not a
> problem for it to just make one up, as long as it's a privileged one. 
> All other parameters can be taken from the local port info.

There are other parameters aside from QKey which cannot be derived from
local port info.

> > The IETF drafts for IPoIB say that the IPv4 broadcast group must be
> > created administratively before an IPoIB interface can be brought up.
> 
> Doesn't the IB spec require that a multicast group have a member? 

Yes.

> That is, when the last member leaves the group, the multicast group is
> destroyed?  

No; destruction can be lazy (see p.914-5 o15-0.1.14). In fact, so lazy
that it technically does not have to be done.

> Further, the IETF drafts for IPoIB only recommend
> administrative creation of the broadcast group, but allows creation by
> the first member.

Yes, but if it's precreated it can't be created by the first member. The
issue with the first member creation is how do you know which first
member has the right set of parameters ? I believe most if not all have
chosen the precreated approach (for that reason).

> An IB MC join of an non-existant group should fail unless all the
> proper parameters are provided to create the group.

Correct.

> What is the behavior of SMs that pre-create the group in response to a
> GET query for the MC group parameters?  Does the query return a
> record, or does it fail with no records?

As an IBA group does not exist without members (even an admin member is
a member), I would think that they return the group info.

-- Hal

> 
> - Fab
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