Re: [dhcwg] Prefix-length of an assigned address, in a DHCPv6 message (was: draft-dhcwg-dhc-rfc3315bis-02.txt)

[Ralph Droms <[e-mail]>]

On Sep 5, 2014, at 6:59 AM 9/5/14, Alexandru Petrescu <[e-mail]> wrote:

> Le 07/08/2014 18:24, 神明達哉 a écrit :
>> At Thu, 07 Aug 2014 17:07:33 +0200,
>> Alexandru Petrescu <[e-mail]> wrote:
>> 
>>> That aside, I wonder why programmers have set that 64 in the kernel data
>>> absent a prefix length field in DHCP/RA, if not because of the fact that
>>> specifications are lacking.  Were it for DHCPv6 to have a prefix length
>>> field for assigned address A, then programmer would be clearer of what
>>> to put on that field, I think.
>> 
>> I guess you're basically re-raising a long standing discussion of
>> whether to provide an on-link prefix via DHCPv6.
>> 
>> My understanding is that we have never reached a consensus on this at
>> IETF (consider, for example, how
>> http://tools.ietf.org/html/draft-droms-dhc-dhcpv6-default-router-00
>> was discussed and we still don't have any published outcome), so a
>> simple rehash of it won't bring us anywhere.  At the very least it's
>> safer to discuss it outside the scope of rfc3315bis (because otherwise
>> we'll never be able to publish the bis).
> 
> Jinmei, thanks for the pointer.
> 
> But I wonder whether there is any draft that talks about how DHCPv6 sends a prefix-length of the assigned IPv6 address, to the Client?  (not Prefix Delegation, but the equivalent of the Prefix Length field of a PIO of a Router Advertisement)?

According to current published IPv6 standards, there is no prefix length associated with an IPv6 address.  An IPv6 address assigned to an interface in a host is a 128-bit value.  There is no implication that any leftmost bits of an IPv6 address assigned to an interface correspond to any prefixes assigned to a link.

An IPv6 host is made aware of prefix(es) on a link, which it then uses for forwarding decisions, through ND PIOs.

DHCPv4 includes a subnet mask with the assigned address as a way for a host to infer a prefix assigned to the associated link.

Can you describe how the prefix length associated with an assigned address is used in the kernel code from which you extracted the snippet below?

> 
> Alex
> 
>>        } else {
>>            /* Current practice is that all subnets are /64's, but
>>             * some suspect this may not be permanent.
>>             */
>>            client_envadd(client, prefix, "ip6_prefixlen",
>>                      "%d", 64);
>                              ^ make this variable.
> 
>> 
>> --
>> JINMEI, Tatuya
>> 
>> 
> 
>