Subject: Re: [homenet] Prefix stability requirements in draft-arkko-homenet-prefix-assignment-00

From: Jari Arkko <[email-masked]>
Date: Sun, 30 Oct 2011 11:40:07 +0200

Erik,

> I looked briefly at the solution in the draft, but it seems tofundamentally associate the prefix(es) with router interfaces and notwith links.

The draft associates prefixes with interfaces because that is the only thing that it can do alone. But since it monitors prefix assignments by both neighboring routers and everyone else, routers are aware of the network topology _and_ network prefix assignments. A prefix assignment by your neighbor becomes your prefix assignment, too. You use the same prefix on the same network, not add another one. And next time if the two routers boot in a different order you will remember your neighbor's prefix for this network and use it. If the two routers are moved to different networks, there will be a collision, it is detected, and one of the routers selects a new prefix.

> An interesting case is a link like this
>
> 	rest of home network
> 	  |            |
> 	 R1           R2
> 	  |            |
> 	  +------------+
> 	  |            |
> 	 H1           H2
>
> If R1 is the first router, it assigns a prefix to its southboundinterface. I take it the solution makes R2 pick up that prefix for itssouthbound interface.

Yes.

> But what happens if R1 is decomissioned? Will R2 permanently assert thatprefix for the link?

Yes. See Section 4, first bullet item, and Section 5.3, item 5 and the second bullet.

(And if R1 is just moved to a different part of the network then Section 5.3 item 5 second bullet will not apply, because the prefix has been announced elsewhere.)

Jari