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Some IPv6LL operational experience




I realize that as an employee of a company that sells a product and 
tries to implement standards the IETF blesses to solve problems, my 
voice doesn't really count, but I wanted to toss in my two cents.

We have been using IPv6LL addresses with some success. The next release 
of Mac OS X implements something similar to LLMNR that we call mDNS 
with support for IPv6. For the time being, the only IPv6 addresses we, 
or most of our customers have, are IPv6LL addresses. This combination 
of mDNS and IPv6LL addresses works very well. IPv6LL addresses are 
important to us because a multi-homed host may not have an IP address 
on every interface. IPv4LL addresses don't work on more than one 
interface because they lack a scope id. The scope id in IPv6LL 
addresses makes them much more compelling.

IPv6LL addresses let us enable networking in situations that wouldn't 
otherwise work. For example, we've implemented IP and IPv6 over 
Firewire. In most cases, it is unlikely that an IPv4 address will be 
assigned to the IP over Firewire link. This is usually used in the 
scenario where two computers are hooked directly together. We can't 
assign an IPv4LL address to the Firewire interface because it causes a 
number of problem on a multi-homed host. The IPv6LL address is 
configured automatically and is always there. Very few people have the 
skill required to successfully type an IPv6 address, which is why the 
mDNS component is so important. In addition to allowing users the 
ability to type a name instead of an address, the question of scope id 
goes away. The name is resolved using mDNS. The scope id is derived 
from the interface the response came back from. The application is none 
the wiser because it gets a full sockaddr_in6 from the call to resolve 
the name.

Applications that perform referrals may fail, but I'm not aware of any 
of these that are currently shipping and support IPv6. IPv6 is a new 
beast, we don't have to be as concerned about applications making 
stupid assumptions. If we explain that IPv6 link local addresses work 
this way and here's a list of limitations, that's good enough. The 
advantages of IPv6 link-local addresses far outweigh the disadvantages.

IPv6LL is a major selling point. IPv6LL is a sneaky way to get everyone 
exposed to IPv6 and to encourage developers to start supporting IPv6. 
Sure, connectivity off of the local link for those of us in the US is 
only for a few elite, but IPv6 on the local link can solve real world 
problems today and work for everyone.

-josh

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