Subject: Re: A Good Schism Brightens Anyone's Day (was: A Simple Question)

From: Keith Moore <[email-masked]>
Date: Wed, 30 Apr 2003 11:30:37 -0400

> Today, dial-up concentrators usually have an address 
> range that it used to assign addresses to people that dial in. That 
> means at most a handful of routes per dial-up concentrator in the 
> interior routing protocol. If everyone has their own /48, that means a 
> route in the IGP for each customer that's online. There are no hard and 
> fast rules about how many routes you can have in an IGP, but somewhere 
> between 10k and 1M you run into trouble. 

this is an interesting point, but I think it has more to do with whether
the prefixes are statically bound to customers than the length of those
prefixes.  why would  giving customers static /64s result in fewer routes in
your IGP than giving them static /48s?   in neither case is there a direct
correspondence between the customer's address and the concentrator.

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From: "Stephen Sprunk" <[email-masked]>
Date: Wed, 30 Apr 2003 11:20:14 -0500

Thus spake "Keith Moore" <[email-masked]>
> > interior routing protocol. If everyone has their own /48, that means
> > a route in the IGP for each customer that's online. There are no
> > hard and fast rules about how many routes you can have in an
> > IGP, but somewhere between 10k and 1M you run into trouble.
>
> prefixes.  why would  giving customers static /64s result in fewer
> routes in your IGP than giving them static /48s?   in neither case is
> there a direct correspondence between the customer's address and
> the concentrator.

IMHO, dialup is a bad example because static IPs per customer are rare;
let's switch to the cable/dsl market.

Standard practice is to connect all customers in a given area (or signed up
in a given period) to a single concentrator via some sort of virtual circuit
(PPPoE, ATM, FR, etc).  This concentrator then internally bridges all of
these virtual circuits into a single subnet with a single prefix, giving you
one route for N customers.  OTOH, if you assign a prefix to each customer,
you then have between N+1 and 2N routes for N customers.  The latter might
be justified if we're truly committed to eliminating NATs, but it costs a
lot more in routes, in administration, and in address waste (assigning a /48
to what is, in nearly all cases, 1-4 hosts).

S

Stephen Sprunk         "God does not play dice."  --Albert Einstein
CCIE #3723         "God is an inveterate gambler, and He throws the
K5SSS        dice at every possible opportunity." --Stephen Hawking

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From: Michael Thomas <[email-masked]>
Date: Wed, 30 Apr 2003 09:45:06 -0700 (PDT)

> to what is, in nearly all cases, 1-4 hosts).

Huh? Why wouldn't these prefixes summarized at the
first hop router? And even the first hop router
needs to have routes to get to the last mile, so
whether you do it with an arp cache, SID binding,
or routing table seems like a rather minor
difference in kind.

	   Mike

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From: Keith Moore <[email-masked]>
Date: Wed, 30 Apr 2003 12:53:22 -0400

> > this is an interesting point, but I think it has more to do with
> > whether the prefixes are statically bound to customers than the
> > length of those prefixes.  why would  giving customers static /64s
> > result in fewer routes in your IGP than giving them static /48s?  
> > in neither case is there a direct correspondence between the
> > customer's address and the concentrator.
> 
> IMHO, dialup is a bad example because static IPs per customer are
> rare; let's switch to the cable/dsl market.

well, they might not be so rare in the IPv6 market, but whatever...

> Standard practice is to connect all customers in a given area (or
> signed up in a given period) to a single concentrator via some sort of
> virtual circuit(PPPoE, ATM, FR, etc).  This concentrator then
> internally bridges all of these virtual circuits into a single subnet
> with a single prefix, giving you one route for N customers.  OTOH, if
> you assign a prefix to each customer, you then have between N+1 and 2N
> routes for N customers.  

well, for cable at least, seems like you'd want to assign a prefix to
each concentrator and give each customer a /48 subnet out of the
concentrator that serves that location.  as I understand dsl it affords
more flexibility than that, since each customer can get a VC to a
concentrator which can essentially be anywhere, but you'd still want
to assign an appropriate-sized prefix to the concentrator and dole /48
subnets out of that.  either way you get to do route aggregation at the
concentrator.

I suspect a lot of "standard practice" for IPv4 is designed to conserve
address space; it's not clear that such practices are either desirable
or optimal for IPv6.  more generally, IPv6 is going to be driven by 
different markets, different applications, and a different set of
customer demands than IPv4.  so anytime you find yourself thinking that 
"standard practice" (meaning v4 practice) axiomatically applies to IPv6,
it might be worth re-examining the assumptions behind that practice.

Keith