Re: IPoIB link address

Vivek & Vandana,

My comments are embedded in the text below (with co-chair hat OFF).

>> >         1. Define a broadcast-GID defined as follows (one of - please
>> >         comment):
>> >
>> >         a) FF1x:401B::255.255.255.255
>> >         b) FF1x:001B::255.255.255.255
>> >         c) FF1x:0000::255.255.255.255
>> >         d) FF1x:401B::1
>> >
>> >     My preference is b) or c) since it removes the IP version
>> >     completely from the mapping. a) is what we have today and requires
>> >     no change and so could be used. d) is derived from the multicast
>> >     mapping and so I'd like to leave it separate.
>>
>>I would go for a) since it is already there; if we do change, I would
>>go for d, but can live with b or c.
>>
>><VK> OK. I find (a), (b) and (c) acceptable. My order of preference (c),
>>then (b) then (a). No (d). <VK>
>
>
>My order of preference is (a) then (b). No (c) or (d). The reason I do not 
>think (c) is a good idea
>is because having a signature defined is useful to mark these MGIDs as 
>IPoIB MGIDs. There is less possibility of MGID collision with non-IPoIB 
>applications
>Also, (a) makes the most sense as a broadcast group applies only to v4, so 
>why not leave things as is.

I'd suggest "191B" (for "IPIB") as the signature (FF1x:191B::255.255.255.255)
to reflect the fact that the broadcast group is for both v4 and v6.

But if people prefer any of the above it's fine with me too, as long as
we can get a closure on this sooner.

>
>> >
>> >         2. The multicast mappings stay as currently specified.
>> >
>> >         3. The broadcast-GID (one of)
>> >
>> >         a) MUST be FullMember IB_joined
>> >
>> >         b) MUST be used to derive the attributes such as the MTU, Q_Key.
>> >         The attributes MAY be obtained by FullMember IB_Join of the
>> >         broadcast-GID or by some other operation supported by IBA. The
>> >         node MUST fail all IP operations on the interface if the
>> >         Broadcast-GID ceases to exist since that will correspond to a
>> >         downed link. It is upto the implementation to track the existence
>> >         fo the Broadcast-GID in a suitable way. Note: If the
>>broadcast-GID
>> >         is FullMember IB_joined then its existence/deletion is
>> >         automatically tracked.
>>
>>I would go with option b, but without the requirement "MUST fail all IP
>>operations ..." for multiple reasons; firstly, it requires at least a
>>memory check in the send/receive path, and secondly, if other L2's
>>(ethernet)
>>aren't requring this, why should IB? (Seems like mingling an L2 issue with
>>L3,
>>the same logic that was used against seperate groups).
>>
>><VK> On further thinking tracking with a requirement to re-acquire modified
>>attributes is needed. 'must fail' is not needed though for reasons below
>>(analogous to why we want to track other multicast GIDs):
>>
>>Assume that the broadcast-GID was not FullMember joined and so ends up
>>getting deleted. There are 3 possibilities (not counting a recreate with
>>the same attributes and MLID):
>>a) Broadcast-GID is not recreated.
>>b) Broadcast-GID is recreated with different attributes but same MLID
>>c) Broadcast-GID is recreated with different/same attributes but different
>>MLID
>>
>>
>>Now, if (a) occurs - the nodes still have agreed upon MTU, Q_Key etc. and
>>can continue to communicate with peers. No new nodes can join the link
>>though. No broadcasts can occur if the MLID was removed from the switch
>>tables => no ARP. One  could let it all go on till nodes drop off(ARP
>>timeouts for example) or declare interface is down.
>>
>>Now, if (b) occurs the link's attributes have changed and communication
>>with nodes that were on the link before, and are still using the old
>>information, can occur however communication with other nodes using new
>>attributes on the link may fail. Broadcast however will probably succeed
>>with some set of nodes (dependent on how the fabric is setup for this
>>MLID).
>>
>>if (c) occurs then broadcast with nodes that were part of the link before,
>>and are still using the old information, cannot occur (don't know the new
>>MLID) though the node can still communicate with the nodes using older
>>attributes.
>>
>>Therefore, the requriement really needs to be:
>>       - track broadcast-GID
>>       - reacquire attributes if broadcast-GID is recreated
>>       - (possibly) if no broadcast-GID down the interface
>>
>>Thoughts?, especially for the case where the broadcast-GID is deleted and
>>not brought back.
>><VK>
>
>I agree. If end-nodes do not want to join the broadcast GID, they need to 
>deal with all the requirements that
>you have detailed above.

Vivek brought up a good point that the empty broadcast group risks being
deleted by SM. But unfortunately this problem already exists in the current
draft regardless of one or two broadcast groups.

In my mind the broadcast groups for IPoIB are to be treated differently by IB
fabric adminstrator/management software compared with other IB MC groups.
They are to be created as part of configuring an IPoIB link, and their
livelihood should not depend on the condition of nodes running an IP stack.
(E.g. your Ethernet VLAN doesn't disappear when you shutdown all the nodes.)
This reasoning is compatible with IBA 1.0 spec when there was "MCGroupRecord"
to hold MC groups separate from "MCGroupRecord" that tracks membership.
Unfortunately "MCGroupRecord" got removed in 1.0a spec. Now it seems that one
must join a MCMemberRecord before an IB MC group can be successfully created.

Instead of adding more complexity to the draft, can we leave this to the
implementation, e.g., the fabric software to ensure the continuous existence of
the special broadcast group as part of IPoIB link? I've checked with some people
from IBTA and this seems to be a reasonable requirement that is very easy to
meet.

We can simply add a clause requiring implementations to ensure that the special
broadcast group won't be deleted other than when the IPoIB link is torn down.
Then we don't need to require v6 node to JOIN the broadcast group. (Note that
even if we require v6 to JOIN, it doesn't solve the problem as I stated ealier.)

Jerry

>
>-Vandana
>
>
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