Re: Comments on draft-ietf-ipoib-link-multicast-04.txt

> >- Section 9.0 indicates that "The packet SHOULD be forwarded to locally 

> >  connected routers.  [snip] The specific mechanism for a sender to 
> >  forward packets to routers are left to implementations".  At a 
> >  minimum, I think the exact implementation needs to be specified.
> 
> Why? (The draft strives to avoid dictating implementation details that
> do not affect interoperability.)

For interoperatibilty.  The requirement is the host and router must agree 
on using a common mechanism for exchanging the IP multicast packet. 
Implementations are free to choose a mechanism as long as all other 
implementations with which they communicate also choose to implement the 
same mechanism.  I don't think this is an area where we can or should punt 
- a minimal, common approach needs to be mandated such that all IPoIB 
nodes implement that mechanism so that any two IPoIB nodes will 
interoperate.

> >  The 
> >  text suggests using all-router multicast group or the broadcast 
group. 
> >  If a router is required to create the all-router multicast group (see 

> >  the previous comment), then wouldn't it be sufficient to check for 
the 
> >  existence of the all-router multicast group and, if it exists, send 
to 
> >  that?  If it doesn't exist, then there can't be any multicast routers 

> >  on the link, and since there aren't any local listeners there are no 
> >  on-link listeners there isn't any need to send the packet.
> 
> Don't the code sinppet following 9.B match pretty much what you 
described
> above?

Yes, it does, and if the text matched the code snippet then I wouldn't 
have an issue.  But the text which supports the code snippet doesn't.  The 
text basically says you could send to the all-router multicast group OR 
the broadcast group.  And either should work, as long as the receiving 
IPoIB node MUST accept the IP multicast packets regardless of the IB group 
over which they are received (I'm not sure I've seen this explicitly 
stated in the draft, but I may have missed it).  But do we really need to 
have multiple mechanisms to accomplish the same result?  Wouldn't we be 
better off just choosing one and going with that?  And how are we going to 
find multiple interoperable implementations for each option when the draft 
puts no limits on the approaches which can be taken?

> >
> >  And shouldn't the IPoIB sender subscribe to the IB multicast group 
> >  creation events using a wildcard MGID so that it can 
> >  "SendOnlyNonMember" join the group if a listener appears on the local 

> >  IB subnet, or "SendOnlyNonMember" join the all-routers group if a 
> >  router appears on-link and creates the all-routers multicast group? 
> >  And doesn't it need to subscribe to the IB multicast group deletion 
> >  events so that it knows when to switch back to sending to the 
> >  all-router multicast group and/or stop sending?
> 
> Aren't you saying the same thing (more or less) as what is described in
> the section following the code snippet?

You are right - I must have missed the text.  But you need to change the 
"should" to a "MUST" in that paragraph.
 
> >
> >  But I'm wondering why a node needs to do this at all.  Wouldn't it 
> >  make more sense for the IPoIB node instead create the multicast group 

> >  and join the group so that IPoIB multicast routers can join the 
group? 
> >  Yes, the IPoIB sender will need create the group as a "FullMember", 
> >  but this seems to provide a simpler mapping of IP multicast to IB 
> >  multicast.
> 
> While what you suggested may simplify the procedure, it essentially
> removes the difference between a multicast sender and a multicast 
receiver,
> which is wrong. In the IP multicast model a multicast sender doesn't 
need
> to be a member of a multicast group to send. The draft has tried to
> keep the same distinction.

However, you are mapping the IP multicast model to the IB multicast model, 
and the two are not the same.  The reality is a sender must join an IB 
multicast group in order to send to it (albeit as a "SendOnlyNonMember"), 
and this step does not exist in the IP model.  I imagine that the vast 
majority of the code/complexity of an IPoIB implementation is going to be 
dealing with IB multicasting.  I was hoping there was some way to reduce 
both at an IPoIB host.  Maybe it is just wishful thinking.

By the way, after thinking about this a little more, I'm not sure that 
creating a group as I suggested makes sense in all cases.  For instance, 
it is only useful for an IP multicast group which has a scope greater than 
link-local.  For link-local scope, only nodes directly attached to the 
IPoIB link are able to receive the packet.  Creating the group for the 
purpose of forwarding to an IPoIB multicast router is simply wasteful, as 
the IPoIB multicast router will not be able to forward the packet to 
another IP link.

A similar optimization can also be taken with the current approach 
described in this draft - for an IP multicast group with link-local scope, 
if the IB MGID for that IP multicast group does not exist then an IPoIB 
node really doesn't need to send the multicast packet to the all-router 
MGID or broadcast GID.

> >- Section 9.0 also indicates that "A multicast sender must join the 
> >  target multicast group as a "SendOnlyNonMember" before outgoing 
> >  messages from it can be successfully routed." This text should be 
> >  clarified to indicate this is only true if the node has not already 
> >  joined the group as either a "FullMember" or "SendOnlyNonMember". The 

> >  If-Then-Else logic below correctly shows this behavior, but this 
> >  paragraph does not reflect that behavior.
> 
> Although your comment is correct, I'd argue this kind of details can
> be found in the IBA spec. It has nothing to do with IPoIB in particular,
> So the question becomes how much IBA info we need to repeat in an IPoIB 
RFC.
> Since the code sippet already covers this correctly, if you still 
believe
> this needs to be clarified i can certain add more text to it.

My feeling is enough to describe HOW an IPoIB node will USE the IBA 
services, and this is one of those cases.  I would prefer to see the text 
clarified if you do another revision of the draft, but given the code 
snippet describes the correct behavior I wouldn't hold this draft up just 
to clarify this paragraph.

> >- Section 9.0 indicates that "For the rest of attributes, it is 
> >  recommended the same values from the all-node multicast/broadcast 
> >  group be used." Should this instead say "For the rest of the 
> >  attributes, the same values from the all-node multicast/broadcast 
> >  group SHOULD be used." Regardless of the wording, this text seems to 
> >  contradict the Section 7.0 which states a node must "use the rest of 
> >  the link attributes associated with the group for all future 
> >  communication on the link.  If Section 7.0 is correct, should this be 

> >  a MUST instead of a SHOULD?  And, if not, under what circumstances 
> >  would a node choose to not use the same values?
> 
> 7.0 describes the attributes used by a node when conducting unicast
> communication. 9.0 describes the attributes used to create a multicast
> group, i.e., for multicast communication. So the question is:
> Must path attributes used for unicast match with those for multicast?
> 
> The flexibility allowed in the multicast case in the current draft is
> deliberate. I don't know in reality if this is useful, or even good to
> have. (Comments are welcome.) My design principle has been to not to
> make something a requirement unless it is needed for interoperability.

Flexibility is often times good, but so is describing a minimal set of 
functions necessary for interoperability.  If a node selects link 
attributes when creating an MGID which does not match the broadcast GID's 
attributes, then it is possible that not all nodes which are members of 
the IPoIB link will be able to support those attributes.  If this happens, 
then the two nodes will not be able to communicate using the IP multicast 
group mapped to the MGID.  All of a sudden, an IPoIB node can't send an 
ARP request or Neighbor Solicitation to another node.

Perhaps it would be better to state that a node which creates a multicast 
group MUST use the same values for the attributes as the all-node 
multicast/broadcast group, but a node joining a multicast group must not 
verify the values match.  If the node joining the group can't support the 
values for the multicast group, then it should log the error.  That way, a 
future RFC could describe why/when/how a node would choose to use 
different values for the link attributes but still be able to interoperate 
with IPoIB nodes.

Roy