Re: Comments on draft-ietf-ipoib-link-multicast-04.txt
Vivek Kashyap <[email protected]>
| Newsgroups | gmane.ietf.ipoib |
|---|---|
| Message-ID | <[email protected]> |
<snip..>
> > 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.
That is a valid idea. The other alternative is to list the
parameters that may differ from the ones used by the
broadcast-GID.
The parameters for an IB multicast group are:
Q_Key, MLID, MTU, TClass, P_Key, Rate, SL, PacketLifeTime,
FlowLabel, Hoplimit, scope.
We certainly need the P_Key and MTU to be the same as the
broadcast MGID. Similarly all multicast mappings in a subnet
must have the same 'scope'.
The other parameters can be different if it is ensured that they
are all supported by all the members of the IP subnet. There is
no well defined method of ensuring this - none is being defined
by us. On the other had, by definition all members support the
parameters supported by the broadcast-GID.
Therefore I agree with you... Jerry, I think we should change the
draft to state the following about the parameters:
1. unicast communication use the parameters used by
broadcast-GID
2. multicast GIDs created should use the same parameters as
the broadcast-GID.
Vivek
--
Vivek Kashyap
Linux Technology Center, IBM
[email protected]
[email protected]
Ph: 503 578 3422 T/L: 775 3422
----- Forwarded by Vivek Kashyap/Beaverton/IBM on 05/07/03 08:28 -----
[email protected]
nux.ibm.com To: Roy Brabson/Raleigh/IBM@IBMUS
Sent by: cc: "H.K. Jerry Chu" <[email protected]>, <[email protected]>
[email protected] Subject: Re: [Ipoverib] Comments on draft-ietf-ipoib-link-multicast-04.txt
rg
05/06/03 15:43
On Tue, 6 May 2003, Roy Brabson wrote:
> > >- 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.
Roy,
Yes, I agree that a common method must be described for interoperability.
Though, in this case, even with the flexibility
interoperability is maintained - the onus being on the router :
The packet may be sent to the routers on broadcast-GID or the
all-router GID. The router itself will listen on both -
broadcast-GID since it is required by IPoIB and all-routers as a
result of other specs (IPv6 & IGMP). Therefore the packet will be
received by the router irrespective of the MGID used as long as
the router accepts any multicast packets on these MGIDs. I have
described the need for this in my previous reply.
We could do one of three things when the relevant MGID does not
exist:
1. The host must always forward on broadcast MGID
and the router must always accept IP multicast packets on the
broadcast GID.
2. The host must always forward on all-router MGID and the router
must always accept IP multicast packets on the all-router
MGID
3. The host must always forward on broadcast GID or all-router
MGID. The router must accept IP multicast packets on both the
broadcast MGID and all-router MGID.
We have chosen case 3 . Is there a strong feeling that one of the
other cases is preferable?
It appears that the method chosen(case 3) needs to be
explicitly stated in the draft.
>
> > > 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.
That is a valid idea. The other alternative is to list the
parameters that may differ from the ones used by the
broadcast-GID.
The parameters for an IB multicast group are:
Q_Key, MLID, MTU, TClass, P_Key, Rate, SL, PacketLifeTime,
FlowLabel, Hoplimit, scope.
We certainly need the P_Key and MTU to be the same as the
broadcast MGID. Similarly all multicast mappings in a subnet
must have the same 'scope'.
The other parameters can be different if it is ensured that they
are all supported by all the members of the IP subnet. There is
no well defined method of ensuring this - none is being defined
by us. On the other had, by definition all members support the
parameters supported by the broadcast-GID.
Therefore I agree with you... Jerry, I think we should change the
draft to state the following about the parameters:
1. unicast communication use the parameters used by
broadcast-GID
2. multicast GIDs created should use the same parameters as
the broadcast-GID.
Vivek
>
> Roy
> _______________________________________________
> IPoverIB mailing list
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> https://www1.ietf.org/mailman/listinfo/ipoverib
>
>
__
Vivek Kashyap
Linux Technology Center, IBM
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