Subject: Re: Draft minutes for MALLOC @ IETF 45

From: Bill Fenner <[email-masked]>
Date: Fri, 23 Jul 1999 14:50:20 -0700

>One open issue is how best to map IPv6 multicast addresses onto
>Ethernet multicast addresses. RFC 2373 to use only the low 32 bits of
>the group ID. It would probably be better to include more bits.

I think that the open issue is actually how this scheme interacts with
the IPv6->ethernet group address mapping, which uses the low 32 bits
of the group address.  There aren't more bits available in the ethernet
group address space, unless we want to redefine the mapping, and there
are a max of 14 more bits available even if IPv6 multicast claims the
entire "local use" ethernet multicast address space.

The concern is more that RFC2373 says that you only use the low 32
bits of the IPv6 multicast address, in order to reduce the number of
group address collisions (especially for layer-2 devices doing GMRP or
mapped IGMP snooping).  If we assign /80's as this proposal suggests,
one obvious tendency is to start assigning group addresses starting at
.1, meaning that there will be lots of groups using the ethernet group
address 33:33:00:00:00:01.

One of the suggestions was to hash (e.g. md5) the assigned range and pick
32 bits from the hash and start assigning with that address.  This moves
towards address allocator policy, though, and it's kind of hard to say
exactly how much the group can specify.

  Bill

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From: Dino Farinacci <[email-masked]>
Date: Fri, 23 Jul 1999 16:53:50 -0700 (PDT)

>> entire "local use" ethernet multicast address space.

    I think we should seriously consider redefining the mapping. We get endless
    worry requests from customers about the 32-to-1 mapping for IPv4. And
    it creates Havoc for switches when a video group maps to the OSPF or
    PIM group addresses.

Dino

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From: Steve Hanna <[email-masked]>
Date: Mon, 26 Jul 1999 08:57:04 -0400

This is certainly an interesting and worthwhile topic (how to map IPv6
multicast addresses onto Ethernet group addresses). 

I don't think we discussed this in any depth at the MALLOC meeting in
Oslo. Maybe it was discussed more at MBONED. I don't know. But for the
purposes of the MALLOC minutes, I suggest the following (revised) text:

  One open issue is how best to map IPv6 multicast addresses onto
  Ethernet group addresses. RFC 2373 uses only the low 32 bits of
  the group ID. There may be other schemes that would result in
  fewer collisions.

Please let me know if you have concerns with this.

As for my personal opinion on this issue, hashing seems like the
simplest and most effective solution. MD5 would be fine, but I don't
think we need a cryptographic hash. CRC should be OK, too.

-Steve

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From: Bill Fenner <[email-masked]>
Date: Mon, 26 Jul 1999 07:15:05 -0700

>  the group ID. There may be other schemes that would result in

I would have said that the open issue was how this allocation scheme
interacts with the existing mapping scheme; I'm guessing there will
be a fair amount of resistance to changing the mapping due to existing
implementations.

  Bill

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From: Steve Hanna <[email-masked]>
Date: Mon, 26 Jul 1999 11:01:04 -0400

> be a fair amount of resistance to changing the mapping due to existing

Good point. So you're saying that we should try to ensure that multicast
addresses are allocated and used in such a way that collisions will be
unlikely with the RFC 2373 mapping?

I guess I missed that point originally. Thanks for pointing out the
problem.

Here's some revised text:

  One open issue is how this allocation scheme may interact with
  the technique used for mapping IPv6 multicast addresses to
  Ethernet group addresses. RFC 2464 uses only the low 32 bits of
  the multicast address. We should try to avoid collisions by
  attempting to ensure that the IPv6 multicast addresses in use map
  evenly over the Ethernet group address space.

Note that I changed RFC 2373 to RFC 2464, since it is the latter which
specifies the mapping from IPv6 multicast addresses to Ethernet group
addresses.

More comments?

Thanks,

Steve

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From: Brian Haberman <[email-masked]>
Date: Mon, 26 Jul 1999 12:19:21 -0400

> specifies the mapping from IPv6 multicast addresses to Ethernet group

Steve,
     That sounds good to me.  There are a few other points that I
want to make :

     1.  There was concern voiced by a few IPv6 people as well as
         Tom Narten about interaction between this new scheme and
         existing implementations of RFC 2464.

     2.  I agree that CRC should work as well.  MD5 was just mentioned
         as one possibility.

Brian