Subject: RE: uni-based-mcast and malloc-ipv6-guide

From: "Dave Thaler" <[email-masked]>
Date: Mon, 17 Sep 2001 12:34:06 -0700

> -----Original Message-----
> From: Thomas Narten [mailto:[email-masked]]
> Sent: Monday, September 17, 2001 11:17 AM
> 
> > > The last paragraph in section 3 is about lifetimes.
> > > I don't understand what the intended effect is of the statement
> > > since I don't know what the lifetime is of a multicast address.
> 
> > See RFC 2908.
> 
> 2908 seems to say mcast addresses have a start and end lifetime. That
> makes sense. What I think is potentially significantly different with
> the approach described in *this* document is that the lifetimes are
> much shorter. I.e., it is conceivable that unicast addresses will be
> advertised with prefixes of several days or a week in common
> cases. That doesn't mean the addresses will expire then, just that
> short unicast lifetimes are being advertised, just in case, it becomes
> necessary to deprecate the address relatively quickly. However, if
> multicast addresses derive their lifetimes from the same lifetime,
> there may be a mismatch in expectations.

How so?  One can renew multicast addresses.
(Conceivably an implementation could support the ability to
automatically renew multicast addresses as long as the unicast
address stays valid.)

If the multicast address is being advertised with say SAP,
each SAP advertisements can contain the extended lifetime,
just like RAs do for unicast prefixes.
 
> Seems to me that it might very well cause problems for multicast
> applications that request an address with a start and end time that
> don't match the advertised lifetimes of unicast addresses.

If so, the same problems exist with any SSM channel that an app uses.
 
> I wonder if this point should to be discussed in the document
> explicitely. I.e., seems like this is a potential difference between
> IPv4 multicast and IPv6 multicast. We should be making such
> differences very clear to minimize confusion, if nothing else.

I don't think there's any difference between IPv4 and IPv6 multicast,
can you elaborate?  (By the way, you should see a uni-based-mcast
draft for IPv4 sometime before next IETF.)

> Will the tying of mcast adddresses to unicast lifetimes put additional
> pressure on using longer unicast lifetimes?  Is this a good thing?

I don't know.

> What are typical start and end times that folks use in IPv4 today? 

Anywhere from 1-hour duration, to maybe 6 months or so.

> Do they match expectations for unicast lifetimes in IPv6?

One option might be to make it a SHOULD NOT (for advertised multicast
lifetime to exceed known unicast lifetime) with the note
that after the unicast lifetime, there is no guarantee that packets
will reach their destination.

-Dave
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---

From: Thomas Narten <[email-masked]>
Date: Mon, 17 Sep 2001 16:51:44 -0400

> How so?  One can renew multicast addresses.  (Conceivably an
> implementation could support the ability to automatically renew
> multicast addresses as long as the unicast address stays valid.)

Suppose I want to get a multicast address so I can advertise a session
(e.g., on a web page). I do this two weeks in advance of the event. I
want the address to last one day only. The key point is that I will
want to use the address staring two weeks from now.

However, because unicast lifetimes are only a week, that's too far
into the future to actually get the address (yet), so the address
can't be obtained yet. This would seem like it might be a problem.  Is
it? (I don't know how this scenario fits with the way multicast is
actually used in IPv4, so this really is a question.)

Thomas

---

From: Marshall Eubanks <[email-masked]>
Date: Mon, 17 Sep 2001 18:39:41 -0400

> actually used in IPv4, so this really is a question.)

In IPv4 I would say, use a GLOP address for this. (We'll rent you one if you don't have any.)

                                 Regards
                                 Marshall Eubanks

T.M. Eubanks
Multicast Technologies, Inc
10301 Democracy Lane, Suite 410
Fairfax, Virginia 22030
Phone : 703-293-9624       Fax     : 703-293-9609
e-mail : [email-masked]
http://www.on-the-i.com

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