LL17/LL18 Communication between Routable and LL

LL17 (host with Routable ARPs for LL address) was accepted, but
LL18 (host with LL address ARPs for Routable) was rejected!

This places us in the uncomfortable situation where my Mac with a manual 
routable address can send packets to my LL printer, but my LL printer 
can't reply! I hope we all agree that this is nonsense. There's no point 
saying that "Hosts with configured addresses MUST ARP for v4 LL 
addresses" if the device with the v4 LL address isn't willing to ARP back.

Below is the text I proposed on 6th February this year, in response to 
Thomas Narten's comments saying, "Two nodes, both with legitimate 
addresses that should work, for some strange reason won't talk to each 
other. Seems to me like the MAY should really be a MUST."

   If the destination address is a unicast address outside the
   169.254/16 prefix, then the host SHOULD use an appropriate routable
   source address, if it has one. If the host has no appropriate
   routable source address, then it MUST ARP for the destination address
   and then send its packet, with a link-local source IP address and a
   routable destination IP address, directly to the destination on the
   same physical link. In the case of a device with only a link-local
   address, this requirement can be paraphrased as "ARP for everything".
   In many network stacks, achieving this "ARP for everything" behaviour
   may be as simple as having no primary IP router configured, having
   the primary IP router address configured to 0.0.0.0, or having the
   primary IP router address set to be the same as the host's own
   link-local IP address. In any event, the host MUST NOT send a packet
   with a link-local source address to any router for forwarding.

LL18 wasn't rejected because people argued against it; it was rejected 
because it got forgotten and no one said anything about it at all.

I hope there's no disagreement about fixing this.

Stuart Cheshire <[email protected]>
 * Wizard Without Portfolio, Apple Computer, Inc.
 * www.stuartcheshire.org