Subject: RE: draft-ietf-ipngwg-site-prefixes

From: Richard Draves <[email-masked]>
Date: Thu, 9 Dec 1999 17:33:00 -0800

> I can't see a case where a site would have more than one 
> prefix (or a few once
> they consume a full /48) per ISP they connect to.
> Does anybody else have a case where this would make sense?
> I'd like to understand if requiring scalability to 100's of 
> prefixes per
> site is reasonable.

What if the ISP has more than one prefix because the ISP in turn is
multi-homed to multiple bigger ISPs?

Rich
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From: Erik Nordmark <[email-masked]>
Date: Thu, 9 Dec 1999 18:07:08 -0800 (PST)

> multi-homed to multiple bigger ISPs?

I guess my question was inexact.
If the ISPs (one or more) hand a site multiple prefixes (because
there are multiple ISPs or the ISP(s) are multihomed themselves)
this is likely to result in each subnet having N subnet prefixes
when the site has N site prefixes.
(At least that is the assumption we've made for the ipv6 multihoming work -
every node gets an address from every prefix.)

Such a scenario doesn't put any additional "strain" on the site-prefixes
mechanism of advertising prefixes in the Router Advertisements,
since the site-prefix info will be included in the subnet-prefix info
i.e. the packets do not get any larger.

What I'm explicitly looking for is the case when we can't do this
piggyback.
This happens when the site has N prefixes (of some length - could be longer
than /48) but many subnets have less than N subnet prefixes.
In this case, in the site-prefixes draft mechanism, there will be N
prefix options needed to carry the site information to every host.
If N = 100 or so this will consume some bandwith since the router
advertisement size is about 48 bytes per prefix.

Is there a case where the site has e.g. 100 prefixes but the subnets only
have say 3 prefixes?

   Erik

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From: "Matt Crawford" <[email-masked]>
Date: Fri, 10 Dec 1999 08:37:37 -0600

> > prefixes per site is reasonable.
> 

site -> 4 ISPs, each ISP -> 4 upstreams gives <= 16 global prefixes,
which I think nears, but does not quite reach, the DNS truncation
threshold.  And if a "retail" ISP is connected to multiple upper-level
ISP's which belong to a single TLA, they may use the same prefix with
all of them (at the discretion of all parties, of course).  This
would reduce the prefix-multiplication at the lowest two levels.

			Matt