Subject: Re: Comments on 2292bis (Adv. API)

From: Erik Nordmark <[email-masked]>
Date: Wed, 1 Sep 1999 22:22:35 -0700 (PDT)

> => multicast addresses are exactly like link-local addresses even
> they have their own scope because a multicast destination without
> the outgoing interface is incomplete.

Hmm, the IPv4 code (and our IPv6 code as well) can send multicast
packets without any interface information.
If there is no IP*_MULTICAST_IF (or sin6_scope_id, or IPV6_PKTINFO with
ifindex) then the kernel will do a *unicast* route lookup for 224.0.0.0
in the IPv4 case. This will find the "default multicast interface".

If we preserve this behavior in IPv6 we can treat the multicast scope
issues the same as the unicast scope issues.

  Erik

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From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Fri, 03 Sep 1999 21:32:28 +0900

>>>>> On Wed, 1 Sep 1999 22:22:35 -0700 (PDT), 
>>>>> Erik Nordmark <[email-masked]> said:

> issues the same as the unicast scope issues.

That's right, but my impression is that it was a workaround for those
who didn't have enough API to specify the outgoing interface of a
multicast packet. Even if we install a route for ff00::/8 to an
interface, we have to specify another interface via some API after
all. IMO, we should not rely on a `default route' in the kernel for
sending a multicast packet.

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[email-masked]
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From: Erik Nordmark <[email-masked]>
Date: Tue, 7 Sep 1999 14:07:26 -0700 (PDT)

> sending a multicast packet.

Something needs to pick the "default multicast interface".
It could be one of
1. The user (start the application with a command line option specifying the
   interface)
2. The application (without user information)
3. The kernel

It makes sense having the kernel pick one if neither the user nor
the application specifies one.
(Of course, the user or application might want to override this but that
doesn't change the question of where the default comes from).

IPv4 applications can use the kernel default today.
Why should we not provide the same ability for IPv6 applications?

   Erik

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From: Francis Dupont <[email-masked]>
Date: Sat, 11 Sep 1999 14:49:44 +0200

=> I agree. In fact I have a ff00::/8 router per interface with
multicast support then I've got another reason to remplace the old
route lookup hack (which is incorrect in most of kernels because
the route structure is in the stack and not fully initialized)
by a "default interface" notion (as suggested by the ipi6_ifindex 0
text in the I-D)...

Regards

[email-masked]

PS: I use a sysctl() for the "default interface". This sysctl() gives
the interface index to use when sin6_scope_id and others are 0.
When the "default interface" index is 0 (which should happen only
with multiple interfaces) then an error is returned (this can be
convenient for a router in some situations).
I share the same "default interface" for multicasts and link-local unicast
(because for both scope_id is an interface index). I don't know if
this is the best choice?
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From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Mon, 13 Sep 1999 12:12:32 +0900

>>>>> On Tue, 7 Sep 1999 14:07:26 -0700 (PDT), 

> Why should we not provide the same ability for IPv6 applications?

I didn't intend to get rid of `a default route for multicast'. I meant
that an application should not *RELY ON* the default route, i.e. it
should provide an explicit way to specify an interface(e.g. as a
command line option) as well as using the kernel's default route.

					JINMEI, Tatuya