Subject: default routers? (was: Re: draft-ietf-zeroconf-reqts-08.txt)

From: Erik Guttman <[email-masked]>
Date: Fri, 13 Jul 2001 11:37:31 +0200 (MET DST)

Aidan, Thomas,

> > draft-ietf-zeroconf-reqts-08.txt
> > >    Requirements:
> > >    - MUST configure an appropriate netmask.
> > >    - MUST have unique IP addresses within an IP subnet.
> > >    - MUST have some routing information
> > >      (for the internetwork scenario).
> "Thomas Narten" <[email-masked]> wrote:
> > 
> > On the last point, would it be more correct to say, needs the address
> > of one or more default routers? Or is there more to this?
> > 
Aidan William <[email-masked]> replied:
> I think the wording takes care to avoid the term "default router".
> Routing information may be a default route, but might not be.
> There was even discussion of hosts ARPing for every address and
> having the router do proxy ARP for destinations in different subnets.

The background is that any mention of default router configuration
implies that there are routers in the network, which may not be the
case.  Consider RFC 1122, section 3.3.1.1:

            The host IP layer MUST operate correctly in a minimal
            network environment, and in particular, when there are no
            gateways.  For example, if the IP layer of a host insists on
            finding at least one gateway to initialize, the host will be
            unable to operate on a single isolated broadcast net.

This clearly indicates that there is no requirement, in the link-local
communication case, to know of any gateways.  

What we mean in the above case is that some routing information is
needed, as stated in RFC 1122, section 3.3.1.2:

            The IP layer MUST support multiple default gateways.

The ZEROCONF requirement is that routing state is available in the
IP layer - which *may or may not* include a list of default gateways.
In one plausible case, a LL-only device will use the 'arp for everything'
forwarding policy, discussed by Stuart.  In the case where DHCP configures
an interface, default gateway(s) will be configured, while still allowing
for delivery of datagrams to connected networks without the need for a 
gateway.  

We aimed at text which supported both possibilities, without requiring
us to spell them out in detail.

Erik