Re: RFC6085 update to rfc2464bis - Ethernet or WiFi?

Alexandre Petrescu <alexandre.petrescu@gmail.com> Wed, 11 January 2017 19:00 UTC

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Subject: Re: RFC6085 update to rfc2464bis - Ethernet or WiFi?
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From: Alexandre Petrescu <alexandre.petrescu@gmail.com>
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Date: Wed, 11 Jan 2017 20:00:01 +0100
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Le 11/01/2017 à 10:48, otroan@employees.org a écrit :
> Bob, Jinmei,
>
> As one of the authors of RFC6085 let me try to clarify how it would typically be used.
> Scenario: Wireless AP

This is WiFi, it is not Ethernet.  It is not the same.  An 
IPv6-over-WiFi document remains to be written.

This multicast aspect is the tip of the iceberg of differences between 
the two.

Alex

> that already knows the L2 unicast address of all stations on a link.
> Some APs try to improve on IPv6 multicast on wireless by sending the RAs as L2 unicast to each individual station.
> The AP then runs through it's list of L2 unicast addresses and sends the multicast RA with the given L2 unicast mapping.
>
> 2464bis says:
>    An IPv6 multicast packet may also be mapped to a unicast Ethernet
>    Link layer address as defined in Section 6.
>
>    An IPv6 node receiving an IPv6 packet with a multicast destination
>    address and an Ethernet link-layer unicast address must not drop the
>    packet as a result using of this form of address mapping.
>
>
> As Jinmei also says, referring to section 6 is then wrong. That implies that the 6085 address mapping uses address resolution. Which is not the case.
>
> 6085 is indeed very underspecified in stating how the mapping is done, from 6085:
>    The determination of the unicast Ethernet link-layer
>    address and the construction of the outgoing IPv6 packet are out of
>    scope for this document.
>
>
> Either do (Jinmei):
>    An IPv6 multicast packet may also be mapped to a unicast Ethernet
>    Link layer address as described in RFC6085.
>
> Or something like:
>   An IPv6 packet with a multicast destination address may also be mapped to an
>   Ethernet link-layer unicast address [RFC6085].
>   E.g. when it is clear that only one address is relevant on the link and that the
>   mapping between an IPv6 multicast destination address and an Ethernet link-layer
>   unicast address is already known.
>
> Cheers,
> Ole
>
>
>
>> On 11 Jan 2017, at 01:05, Bob Hinden <bob.hinden@gmail.com> wrote:
>>
>> Jinmei-san,
>>
>>> On Jan 10, 2017, at 11:22 AM, 神明達哉 <jinmei@wide.ad.jp> wrote:
>>>
>>> On Tue, Jan 10, 2017 at 9:26 AM <jinmei@wide.ad.jp> wrote:
>>>
>>>>>  An IPv6 multicast packet may also be mapped to a unicast Ethernet
>>>>>  Link layer address as defined in Section 6.
>>>>
>>>> I think it's more helpful to refer to RFC6085 explicitly here.
>>>> Otherwise the proposed text looks good to me.
>>>
>>> On re-reading it more closely, I wonder whether "as defined in Section
>>> 6" may not be very appropriate.  RFC6085 intentionally left the
>>> mapping open:
>>>
>>>  [...]  The determination of the unicast Ethernet link-layer
>>>  address and the construction of the outgoing IPv6 packet are out of
>>>  scope for this document.
>>>
>>> but I suspect it doesn't really intend to perform link-layer address
>>> resolution using ND (which is in my understanding what "Section 6"
>>> talks about) to determine the unicast Ethernet address.  In fact, the
>>> address resolution itself uses a multicast IPv6 address, which is
>>> derived from the target unicast IPv6 address.  So it would be a kind
>>> of circular definition.
>>>
>>> So it's probably even better to just refer to the RFC instead of
>>> Section 6:
>>>
>>>   An IPv6 multicast packet may also be mapped to a unicast Ethernet
>>>   Link layer address as noted in [RFC6085].
>>
>> I think the problem remains that RFC6085 isn’t very clear how to do that.  Pointing to RFC6085 is that it doesn’t say what to do :-(
>>
>> It would be good to hear from the authors of RFC6085.  Also, what is current practice.
>>
>>>
>>> And, for that matter, this text of Section 6 of rfc2464bis-01 now
>>> looks a bit awkward to me:
>>>
>>>  The procedure for mapping IPv6 unicast addresses into Ethernet link-
>>>  layer addresses is described in [DISC].
>>
>> That is original text from RFC2464.  I tried to avoid changing anything that wasn’t part of an update or errata.
>>
>>>
>>> On reading both Sections 6 and 7, this "the procedure for mapping"
>>> could read some static mapping whereas it should actually refer to
>>> dynamic link-layer address resolution.
>>>
>>> I suggest revising the first paragraph from:
>>>
>>>  The procedure for mapping IPv6 unicast addresses into Ethernet link-
>>>  layer addresses is described in [DISC].  The Source/Target Link-layer
>>>  Address option has the following form when the link layer is
>>>  Ethernet.
>>>
>>> to:
>>>
>>>  When the link layer is Ethernet, the Ethernet address for an IPv6
>>>  unicast address is resolved using the address resolution protocol
>>>  as defined in [DISC].  The Source/Target Link-layer Address option
>>>  used in that protocol has the following form.
>>
>> I find that an improvement (maybe without the “When” text), but I am not sure a change is needed.
>>
>> Thanks,
>> Bob
>>
>>
>>>
>>> --
>>> JINMEI, Tatuya
>>
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