Subject: Re: node-local mc & the API

From: Brian Haberman <[email-masked]>
Date: Wed, 01 Sep 1999 08:56:38 -0400

Richard Draves wrote:
> 
> How do node-local multicast addresses interact with the IPV6_MULTICAST_LOOP
> option? If the option is set to disallow loopback, I assume that means
> transmissions will get dropped in the bit bucket?

Rich,
     I was under the impression that node-local multicast does not
really
follow the same loopback conventions as other multicast addresses do. 
The
reason is that the packet addressed is not meant to leave the box so if
anything in the node is "joined" to the group, it will get the packet. 
I
suppose that you could look at it as when an app joins to that
node-local
address, the loopback state is automatically enabled.

Brian
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From: Francis Dupont <[email-masked]>
Date: Wed, 01 Sep 1999 20:03:51 +0200

In your previous mail you wrote:

   Does the API require/allow any special restrictions on node-local multicast
   addresses. For example, can an implementation require that node-local
   multicast addresses only be assigned to a loopback interface?
   
=> I don't understand what you means by "assigned".
Do you suggest a JOIN can be done only on a loopback interface if
the multicast address is a node-local one?

=> fortunately the default is to loopback packets then for common
applications this doesn't matter.
 The whole node-local is a bit under-specified but this doesn't make
portable applications difficult to write. Obviously it is more a
(kernel) implementor issue than a general one.

Regards

[email-masked]

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From: Erik Nordmark <[email-masked]>
Date: Wed, 1 Sep 1999 21:34:32 -0700 (PDT)

> multicast addresses only be assigned to a loopback interface?

I think "assigned" is a poor term here.
Presumably you are asking both about joining multicast groups
as well as sending to multicast groups, since interfaces can
be optionally specified for both cases.

I haven't seen anything in the API that restricts the behavior in any way.
And the implications of adding such limitations might be far reaching.
Why would node-local be special?
One could also envision requirements on greater than link-local
scope such as there must be at least one default router on the interface!

I think we don't want to head down that path and leave it unspecified.
Or we could clarify (when we open up the basic API again) that 
node-local scope can be used on any interface.

> transmissions will get dropped in the bit bucket?

Your assumption matches my assumption.

  Erik