Subject: RE: [Ipoverib] IPoIB multicast forwarding

From: "Larsen, Roy K" <[email-masked]>
Date: Fri, 12 Apr 2002 13:07:21 -0700

Aditya,

The way I understand it, the sender will need to check each time a multicast
send is attempted.  When a receiver is available on the subnet, the target
multicast group will exist.  When the multicast group has been deleted (i.e.
the receiving application has exited), the check will fail and the sender
will revert back to sending to the all-routers MGID.  Unfortunately, the
sender can't assume that once a receiver was available it will always be
available, nor can it assume that the state of multicast routers will stay
consistent.  Thus the need for persistent checks with the SM before each
multicast send.

The question I have is what happens in the case where there are two
multicast receives in the enterprise.  For example in a fault tolerant or
load balancing application.  Suppose there are three subnets (A, B, and C)
bridged by two multicast router( AB, BC).  There are two multicast receivers
(one on A and one on C) listing for the same multicast packets.  Finally,
assume there are no senders on subnet B.  If either router goes down during
the operation of the multicast application, how will it know to join the
appropriate multicast group when it comes back up?  It will not receive any
redirected all-router multicasts since senders will find a multicast receive
in each subnet?  If it can know in this situation, why the need to have a
sender redirect to the all-routers MGID at any time?

Roy

-----Original Message-----
From: Aditya Dube [mailto:[email-masked]]
Sent: Friday, April 12, 2002 11:04 AM
To: [email-masked]
Subject: RE: [Ipoverib] IPoIB multicast forwarding

Vivek

As the sender will only send packets to the all-router MGID after doing
the check, how will a receiver who creates the MGroup on the same subnet
as the sender ever receive the multicast packets that it should get?

On Ethernet the sender does no check. The sender will simply send
packets to corresponding Ethernet multicast address. Whenever, the
receiver wants it can start listening for the multicast address it is
interested in.

As you state, on Ethernet the router will be in promiscuous multicast
receive mode so the router will keep get all the multicast packets that
it needs to route. 

-----Original Message-----
From: Vivek Kashyap [mailto:[email-masked]] 
Sent: Friday, April 12, 2002 12:15 AM
To: Michael Krause
Cc: Aditya Dube; [email-masked]; [email-masked];
[email-masked]
Subject: Re: [Ipoverib] IPoIB multicast forwarding

> 
> At 09:32 PM 4/11/2002 -0700, Aditya Dube wrote:
> >Hello Vivek
> >
> >I have a question. Could you clarify what will happen in the
following
> >scenario in the third scheme that was proposed below:
> >
> >Pure senders - at IB level check for the relevant IB group.
> >            If it exists then send to the IB group else send to the
> >            'all-routers' IB group.
> >
> >What happens if a Receiver creates the 'relevant IB group' after the
> >sender has made the check?
> 
> There should not be such an option in the first place: the relevant
group 
> should always exist or else nothing is sent.

        Unfortunately we do have to have such an option. There maybe
        no IP group receiver and hence IB group on the subnet. But the
        router may still need to receive the packet to forward out.

        The router could create the IB group if it knew it has to
        forward for the relevant IP group. However, the router does
        not know this for the first multicast datagram it sends for
        this group(DVMRP). So, we need to ensure that it receives the
        packet and therefore the need to have the option to the
        all-routers IB group.

        A receiver creating the group after the sender checks is no
        different than the receiver creating the group after a
        hypothetical atomic check and send of a multicast packet by a
        sender. I thought about suggesting that sender sends to the
        broadcast-GID but then decided against it - why punish
        everyone for a once in a while race condition that can go
        either way even if the sender could do atomic 'check (i.e.
        join) and send'.

        For the next packet the sender will find the group and from
        then onwards directly send on the group.

Vivek

> 
> Mike
> 
> 
> _______________________________________________
> IPoverIB mailing list
> [email-masked]
> https://www1.ietf.org/mailman/listinfo/ipoverib
> 

-- 
Vivek Kashyap
Linux Technology Center, IBM
[email-masked] 
[email-masked] 
503 578 3422 (o)

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From: Michael Krause <[email-masked]>
Date: Fri, 12 Apr 2002 13:43:34 -0700

At 01:07 PM 4/12/2002 -0700, Larsen, Roy K wrote:
>send is attempted.

This would be an unacceptable performance overhead.

>When a receiver is available on the subnet, the target

Very impractical to implement.

>sender redirect to the all-routers MGID at any time?

The IP TCAs would be responsible for sorting this out.  From an IB 
perspective, these TCAs would all participate in the IB multicast 
operations thus they would each receive a packet.  The TCA would be then 
determine whether they inject the packet or not based on the appropriate IP 
requirements.

Mike