Subject: RE: [magma] Re: RC3810: MLD Hop Limit is 1 / random reply time / exclude

From: "Manfredi, Albert E" <[email-masked]>
Date: Mon, 27 Sep 2004 12:35:42 -0400

Jeroen Massar wrote:

> Brian Haberman wrote:
> > The hop limit of 1 is to keep MLD consistent with IGMP.
> 
> You mean IGMPv3, thus actually IPv4?

MLDv2 is actually IGMPv3 modified to the IPv6 way of doing things. Just as MLDv1 was basically IGMPv2.

I think that the advantages of source filtering are somewhat less critical with IPv6. I think one of the primary reasons to add source filtering in IGMPv3 was an effort to conserve on Class D addresses used for multicasting in IPv4. It's another scheme to extend the useful life of IPv4.

> But wouldn't it be better to use
> 255 here to make it consistent with the "we don't want remote hosts to
> set a TTL to 5, let the packet hop 4 routers and tada do ND 
> there' idea?

The idea with IGMP and MLD is for local hosts to inform only the last hop router(s), which must obviously be multicast routers, as to the group membership of each of their directly attached segments. The routers then relay this information up the multicast tree using multicast routing protocols, such as PIM. That's another whole subject area.

So the TTL of 1 makes sense. You don't want to clutter up multiple IP subnets with information that is meant only for the local last-hop multicast router(s). It's a scaling issue. In any event, multicast routers won't forward IGMP packets, so the TTL value is not that critical perhaps.

Bert

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