Re: [v6ops] DHCPv6/SLAAC Make Hosts Confusing-//RE: new draft: draft-liu-bonica-v6ops-dhcpv6-slaac-problem

A single address does not have a mask. 
DHCP gives out addresses, not prefixes that can be used for onlink determination. 

Cheers,
Ole

> On 23 Oct 2013, at 11:11, Mark ZZZ Smith <markzzzsmith@yahoo.com.au> wrote:
> 
> Hi,
> 
> 
> ----- Original Message -----
>> From: Mikael Abrahamsson <swmike@swm.pp.se>
>> To: Mark ZZZ Smith <markzzzsmith@yahoo.com.au>
>> Cc: Liubing (Leo) <leo.liubing@huawei.com>; "v6ops@ietf.org" <v6ops@ietf.org>; "draft-liu-bonica-v6ops-dhcpv6-slaac-problem@tools.ietf.org" <draft-liu-bonica-v6ops-dhcpv6-slaac-problem@tools.ietf.org>
>> Sent: Wednesday, 23 October 2013 2:40 PM
>> Subject: Re: [v6ops] DHCPv6/SLAAC Make Hosts Confusing-//RE: new draft: draft-liu-bonica-v6ops-dhcpv6-slaac-problem
>> 
>>> On Tue, 22 Oct 2013, Mark ZZZ Smith wrote:
>>> 
>>>   Is this saying that the prefix length of the address on host is /128?
>> 
>> A single address is always a /128. This /128 can be part of an on-link 
>> other network, or it isn't.
> 
> My question was more specificaly about you using "/128", rather than "an address" or "a single address", because I wondered if the DHCPv6 server supplies a /128 prefix length and the host configures a /128 prefix length on the address.
> 
> As you say below, address prefix length doesn't indicate on-link or off-link presence. So I'd expect a DHCPv6 server to hand out single IPv6 addresses with a /64 prefix length, as I think that would be more consistent and more expected when the subnet's prefix length is /64.
> 
> For somebody with a IPv4 experience, who wasn't aware an IPv6 prefix length doesn't indicate on-link presence, I think the use of a /128 prefix length in this scenario would imply that prefix length does indicate on-link presence. Somewhat pedantic perhaps, however I think anything that may give false indications of IPv6's behaviour, when it is different to IPv4's, is better to avoid.
> 
> 
> Regards,
> Mark.
> 
>>>   If that is the case, as the prefix length in doesn't indicate on-link
>> or 
>>>   off-link status (as RA PIOs do), is there any specific reason for the 
>>>   prefix length the DHCPv6 server hands out to be /128 instead of the 
>>>   subnet's prefix length of /64? 
>> 
>> The DHCPv6 server always hands out /128. This /128 can be within a subnet 
>> advertised in RA, or it can be outside of it. The host doesn't care. When 
>> a subnet is being advertised in RA you get a network route pointing to the 
>> interface without an IPv6 address as next-hop.
>> 
>> So it's perfectly achievable today (and it works) to have the following:
>> 
>> Host gets 2001:db8:fff::1/128 from dhcp
>> host gets on-link 2001:db8:1::/64 from RA and creates a route towards the 
>> interface for this, but doesn't do SLAAC because A=0.
>> Host gets default route from router and installs it.
>> 
>> Now, the *router* needs to understand that 2001:db8:fff::1/128 is on-link, 
>> but no other hosts on the network does not.
>> 
>> So what networks are being announced in RA is completely decoupled from 
>> addresses handed out by DHCPv6 IA_NA. It's perfectly valid to hand out a 
>> /128 and then have no on-link prefixes at all, or have some with A=0.
>> 
>> 
>> -- 
>> Mikael Abrahamsson    email: swmike@swm.pp.se
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