Re: [dhcwg] Prefix-length of an assigned address, in a DHCPv6 message

[Alexandru Petrescu <[e-mail]>]

Le 05/09/2014 16:24, Ralph Droms a écrit :
>
> 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.

But in practice if I say "ifconfig add eth0 1::1" it will result in 
"1::1/128".  That '128' is put in 'prefix length' fields.

There is a difference between the address being a 128bit value and a 
prefix length of that value.

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

Also for on-link determination.

> 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?

The snippet is from DHCPv6 client code, not from kernel code.  It is 
that line which makes subsequently that the address assigned on the 
interface has a prefix-length 64.

For the kernel part, I would go as far as to say that this highly-used 
kernel code is not in line with the standards:
linux/.../if_inet6.h:
> struct inet6_ifaddr {
> 43	struct in6_addr	addr;
> 44	__u32	prefix_len;

This is the structure which defines addresses on interfaces.  The very 
fact that the prefix-len is an integer (and not a pointer, which could 
be nil) means that every address assigned on an interface must have a 
prefix length.

This concept is very different than saying that an address may have no 
prefix length.

Alex

>
>>
>> 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
>>>
>>>
>>
>>
>
>
>