Re: [v6ops] draft-ietf-v6ops-host-addr-availability discussion - address out of the delegated prefix, on the egress - no DAD

Alexandre Petrescu <alexandre.petrescu@gmail.com> Tue, 03 November 2015 09:24 UTC

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To: "Templin, Fred L" <Fred.L.Templin@boeing.com>, "v6ops@ietf.org" <v6ops@ietf.org>
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From: Alexandre Petrescu <alexandre.petrescu@gmail.com>
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Subject: Re: [v6ops] draft-ietf-v6ops-host-addr-availability discussion - address out of the delegated prefix, on the egress - no DAD
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Le 03/11/2015 16:13, Templin, Fred L a écrit :
> Hi Alex,
>
>> -----Original Message----- From: v6ops
>> [mailto:v6ops-bounces@ietf.org] On Behalf Of Alexandre Petrescu
>> Sent: Monday, November 02, 2015 6:56 PM To: v6ops@ietf.org Subject:
>> Re: [v6ops] draft-ietf-v6ops-host-addr-availability discussion -
>> address out of the delegated prefix, on the egress - no DAD
>>
>>
>>
>> Le 03/11/2015 10:31, Templin, Fred L a écrit :
>>> Bumping up one level - is it clear to everyone that it is OK to
>>> assign addresses taken from a DHCPv6 delegated prefix to the
>>> interface over which the prefix was received? And, that DAD is
>>> not required for those addresses?
>>
>> This indeed new enough at least to me.
>>
>> I agree that if the prefix is delegated for Host with DHCP-PD then
>> it has a tighter bind to that Host, tighter than a prefix
>> _advertised_ to it with an RA.
>>
>> In that sense, certainly yes the Host may self-form and assign an
>> address on its interface over which the application DHCP-PD
>> received it earlier.
>>
>> And, since the prefix is administratively unique, it would make
>> little sense for the Host to DAD that address on that interface.
>>
>> Moreover, it would bring some advantages for privacy.  Privacy
>> addresses as we know them make only the IID variable, while still
>> keeping a trackable prefix (the advertised prefix).  With this way
>> of prefix delegation, the Host may decide more ways to obfuscate
>> its identity: use sometimes the allocated prefix, other times the
>> advertised prefix, in some hard-to-detect sequence.
>>
>> But, if a Host forms an address out of the delegated prefix and
>> wants to talk to its Gateway on that interface, maybe it wants to
>> send an RA to that Gateway so the Gateway forms an address out of
>> the delegated prefix too.  At that point DAD would be needed.
>
> "Host sends an RA to the Gateway" doesn't make any sense that I am
> aware of.

I should have said "to the link on which the Gateway is present".

On a shared link, where each such Host is delegated a prefix (no prefix 
advertised by the RA from the gateway), these Hosts will want to reach 
each other directly w/o being ICMP Redirected by the Gateway.  Which 
address should such a Host use to reach its neighbor.

Alex

>
> Thanks - Fred
>
>>
>> Alex
>>
>>>
>>> Thanks - Fred
>>>
>>> *From:*v6ops [mailto:v6ops-bounces@ietf.org] *On Behalf Of
>>> *Templin, Fred L *Sent:* Monday, November 02, 2015 5:24 PM *To:*
>>> Lorenzo Colitti *Cc:* v6ops@ietf.org *Subject:* Re: [v6ops]
>>> draft-ietf-v6ops-host-addr-availability discussion
>>>
>>> Hi Lorenzo,
>>>
>>> Responses below in "green":
>>>
>>> *From:*Lorenzo Colitti [mailto:lorenzo@google.com] *Sent:*
>>> Monday, November 02, 2015 5:04 PM *To:* Templin, Fred L *Cc:*
>>> Fred Baker (fred); v6ops@ietf.org <mailto:v6ops@ietf.org>
>>> *Subject:* Re: [v6ops] draft-ietf-v6ops-host-addr-availability
>>> discussion
>>>
>>> On Tue, Nov 3, 2015 at 8:59 AM, Templin, Fred L
>>> <Fred.L.Templin@boeing.com <mailto:Fred.L.Templin@boeing.com>>
>>> wrote:
>>>
>>> I have one text addition suggestion and one question. On P. 7,
>>> in Table 1, suggest adding a new final row as follows:
>>>
>>> requires DAD               Yes                  Yes No N/A
>>>
>>> Meaning that multi-addresses configured by SLAAC or DHCPv6
>>> IA_NA/IA_TA must use DAD to check for duplicates on the link
>>> they were obtained. In a multi-addressing environment where
>>> millions of addresses are required, this could amount to a
>>> substantial amount of DAD multicast traffic. On the other hand,
>>> DAD is not needed for DHCPv6 PD because the network has
>>> unambiguously delegated the prefix for the node's exclusive use.
>>>
>>> I don't think "Requires DAD: No" is correct. Even if the device
>>> gets a /64 prefix entirely for its own use, it still needs to do
>>> DAD with any other devices on that /64 (e.g., tethered devices,
>>> VMs, etc.).
>>>
>>> I'm not opposed to adding a line to the table, though I don't
>>> think it provides much value - if we put our mind to it, I'm sure
>>> we could come up with lots of things we could add to the table
>>> that aren't there at the moment. My main concern is that if we
>>> add something to the table it needs to be correct.
>>>
>>> What I mean is "Requires DAD on the interface over which the
>>> prefix was received",
>>>
>>> but that was too long to fit in the table. Let's call the
>>> interface "A". If the node gets
>>>
>>> SLAAC addresses or DHCP IA_NA/IA_TA addresses over interface
>>> "A", then it needs
>>>
>>> to do DAD on interface "A" for each such address. If the node
>>> gets a DHCPv6 PD
>>>
>>> over interface "A", however, it does not need to do DAD over
>>> interface "A" at all.
>>>
>>> If the node assigns the delegated prefix to interface "B", then
>>> you are right that
>>>
>>> that DAD will be required among all tethered devices, VMs, etc.
>>> on interface "B".
>>>
>>> But, there will still be no need for DAD on interface "A". Does
>>> that clarify?
>>>
>>> I have a question also on table 1. Under ""Unlimited" endpoints",
>>> why does it say "no" for DHCPv6 PD? I think it should say "yes"
>>> instead, since a prefix obtained by DHCPv6 PD can be used to
>>> configure an unlimited number of addresses on the link over which
>>> the prefix was received.
>>>
>>> The table is written from the perspective of the network
>>> assigning addresses to devices that connect to it. Therefore, it
>>> says "no" because if you use DHCPv6 PD you can't assign address
>>> space to an unlimited number of endpoints - you are limited to
>>> however many /64s you have available.
>>>
>>> If you use IA_NA or SLAAC, any network with a /64 subnet has, at
>>> least in theory, an "unlimited" number of addresses to assign to
>>> clients. Of course, that's only true in theory. In practice,
>>> there's going to be a limit due to scaling reasons.
>>>
>>> I don't understand this. True that SLAAC and DHCPv6 IA_NA/IA_TA
>>> can be used
>>>
>>> to assign an unlimited number of addresses to interface "A". But,
>>> so can DHCPv6
>>>
>>> PD. When the node receives the delegated prefix (e.g., a /64), it
>>> can assign as
>>>
>>> many unique IPv6 addresses as it likes to interface "A". And
>>> again, it need not
>>>
>>> do DAD for any of them.
>>>
>>>
>>>
>>> _______________________________________________ v6ops mailing
>>> list v6ops@ietf.org https://www.ietf.org/mailman/listinfo/v6ops
>>>
>>
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