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

[Alexandru Petrescu <[e-mail]>]

Le 05/09/2014 18:42, Ralph Droms a écrit :
>
>
>> On Sep 5, 2014, at 8:07 AM, Alexandru Petrescu
>> <[e-mail]> wrote:
>>
>> 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.
>
> I seem to recall having a similar discussion several years ago, in
> which I argued that the ifconfig syntax and semantics are wrong and
> that ifconfig should omit the prefix length entirely.  The consensus
> was that it was too late to fix ifconfig.
>
> I'm hoping someone else might also recall that conversation and
> confirm or correct my memory...

WEll, your discussion was maybe at a time when ifconfig required a 
prefix-len parameter.  It no longer does.  But ifconfig introduces a 
'128' when the user does not specify that parameter.  BEcause that 
prefix-length is mandatory in the structure.

>>> 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.
>
> Right (I sort of included on-link determination under forwarding)...
>
>>
>>> 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.
>
> Ok, but there is still the question of why the API in the code
> snippet includes a prefix length and...
>>
>> 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.
>
> ... how is the prefix_len component of the struct used in the
> kernel?

Good question, let's see.

Alex

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