Subject: Re: [dhcwg] DHCPv6 Client address prefix

From: "David W. Hankins" <[email-masked]>
Date: Tue, 9 Sep 2008 15:38:26 -0700

On Thu, Aug 21, 2008 at 08:11:11PM -0700, Ted Lemon wrote:
> For my own part I don't really see the point of using anything but a 
> link-local address on a routerless subnet.   So whatever the spec says, I 
> think this is a bit of a moot point.
>
> Do y'all see some reason why a link-local address wouldn't do the job in 
> this situation?

Link-local addresses are not comfortable in DNS zones without playing
dirty tricks, and it might not be clear to an application at that
layer which interface the link local address is applicable to.  The
hosts are not necessarily "off net" beacuse they may have multiple
interfaces, or multiple virtual points of attachment.

RFC4193 addresses are more likely to be used in this scenario than
link-local addresses.

In addition, there's the case where the network is online and running,
but the router is offline for the time being.  In this situation, the
client becomes dependent upon the router(s) in order to be able to
talk with the rest of the (working) network, which violates the law of
least astonishment for the user.

-- 
Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
Why settle for the lesser evil?	 https://secure.isc.org/store/t-shirt/
-- 
David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins
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From: Ted Lemon <[email-masked]>
Date: Tue, 9 Sep 2008 20:10:53 -0700

On Sep 9, 2008, at 3:38 PM, David W. Hankins wrote:
> layer which interface the link local address is applicable to.

This is an easy problem for an IPv6 stack to solve - if the host isn't  
in your routing table, you do neighbor discovery to find it.   A link- 
local address should always be discoverable; if it is not, it is not  
reachable.

> The hosts are not necessarily "off net" beacuse they may have multiple

Okay, sure, but the particular network interface we are talking about  
is off-net, isn't it?

> RFC4193 addresses are more likely to be used in this scenario than

I've never seen one of these in the wild, so I'm skeptical of your  
theory.   In practice, hosts get link-local addresses, and the RFC  
you're referring to seems to be talking about something that would  
happen on a wire with a router, not on a wire without one.

> least astonishment for the user.

This seems like a somewhat contrived scenario.   How likely is it that  
a user would have time to become astonished before the router came  
back?   Wouldn't they notice other problems as well, problems that  
would lead them to try to take some action to fix the router?   Why is  
a routed IPv6 network without its router?

---

From: "David W. Hankins" <[email-masked]>
Date: Wed, 10 Sep 2008 08:46:35 -0700

On Tue, Sep 09, 2008 at 08:10:53PM -0700, Ted Lemon wrote:
> Okay, sure, but the particular network interface we are talking about is 
> off-net, isn't it?

It is only reachable by devices directly connected, yes, but I'm not
sure you're going to find traction with the idea of performing ND on
all interfaces simultaneously.  Scaling problems, particularly on
systems connected by 802.1q tagged vlanning.  The devices you want to
reach on this off-net interface you're attached to (strawman: a
private network with dedicated bandwidth for an NFS share or "iSCSI
bus" or similar), you want to reach by name and its AAAA has a need to
also inform you what interface to try, doing its job as a locator.

> I've never seen one of these in the wild, so I'm skeptical of your theory.  
>  In practice, hosts get link-local addresses, and the RFC you're referring 
> to seems to be talking about something that would happen on a wire with a 
> router, not on a wire without one.

I think that was the intent of RFC4193, but I don't think that will be
the limit of its application.  I suspect all the hosts on this kind of
wire would have link-local addresses as well, but you wouldn't put
those in DNS.

> This seems like a somewhat contrived scenario.   How likely is it that a 
> user would have time to become astonished before the router came back?   

Experience says 100%.  People suspend and resume their laptops all the
time, and any network with more than 5 people is going to have someone
stuck in networkless land with even a short outage.

> Wouldn't they notice other problems as well, problems that would lead them 
> to try to take some action to fix the router?

This question sounds pretty silly to me, which suggests that I do not
understand it.

Are you suggesting there is some scenario where the network's router
is offline (and not just disconnected upstream) wherein that is not a
problem that must not be repaired by someone?

> Why is a routed IPv6 
> network without its router?

Failures happen.  Stuff blows up.  It can take time for equipment to
get replaced.  Not everyone keeps cold spares of every single device
in their inventory (particularly not chassis or processors).

It always chooses the worst possible moment; right when some critical
deliverable for your company is due imminently, or some other business
emergency has reared.

Generally, it will be an astonishment that Jenny Businesswoman can't
check and queue email ("smtp.mydomain.com"), get to the company shared
file store, or conduct her internal web-application to perform those
job functions that don't rely on the rest of the Internet to be
reachable.