Re: [v6ops] new draft: draft-yc-v6ops-solicited-ra-unicast

Erik Nordmark <nordmark@acm.org> Thu, 23 July 2015 16:26 UTC

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To: Mark Smith <markzzzsmith@gmail.com>, Erik Nordmark <nordmark@acm.org>
References: <201507071147.t67Bl13m009348@irp-lnx1.cisco.com> <EF21B630-5D0A-415A-A93F-9058900CC80C@cisco.com> <CAO42Z2zAqMXhBZ2wa=q0wtHGhMpMWU9TSjfFyd2quiki9w0oSw@mail.gmail.com> <85CADAA2-8DF2-4A6B-812B-7A77081936F5@cisco.com> <CAO42Z2w3fOxGJHasKqYZRfGZ2u=7FnZBm+jgLtgDvfZ7HYW=iw@mail.gmail.com> <CAO42Z2z+DwOin23HQTysrZ9dNP924+LQ-vOExmJc_xZUEB4yCQ@mail.gmail.com> <228248C6-94FE-4C9C-A875-F732EFDC6601@cisco.com> <CAAedzxqapiWuy4Gk5t3zEe3XmaLyRc3nc5=aA1ED0tzfeXckbA@mail.gmail.com> <CAKD1Yr3Hn9qJTaM0v3+hr7NfQbLc=mOWYGwrTK-XXxKp5v+dpg@mail.gmail.com> <CAAedzxpdFsCy2Y7U0gFmQeHEvJjNj-243g_ffoJsVUeRz5RpZw@mail.gmail.com> <CAKD1Yr1uR+HyBTB=Yhy5hGs1Z6Wv=HT3wwFgLYDosDJ7a78-PA@mail.gmail.com> <D1D2A832.1B7D8F%sgundave@cisco.com> <CAAedzxoX1dD3MQO5YCS6+u1esThW0sVv=JMmivJXZ92FKZ0sZg@mail.gmail.com> <CAO42Z2w8D+G7ONS5uXDP2kgf4da2JgiHHubEMt3TVumfFbkWOA@mail.gmail.com> <55B0177F.8050703@acm.org> <CAO42Z2yTrjLudtCnbR_ziTGAKYkwtCguF+yEB4e78jEVUUT-Cg@mail.gmail.com>
From: Erik Nordmark <nordmark@acm.org>
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Date: Thu, 23 Jul 2015 18:26:14 +0200
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Cc: v6ops list <v6ops@ietf.org>, "draft-yc-v6ops-solicited-ra-unicast@tools.ietf.org" <draft-yc-v6ops-solicited-ra-unicast@tools.ietf.org>
Subject: Re: [v6ops] new draft: draft-yc-v6ops-solicited-ra-unicast
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On 7/23/15 2:37 AM, Mark Smith wrote:
> Hi Erik,
>
> On 23 July 2015 at 08:21, Erik Nordmark <nordmark@acm.org>; wrote:
>> On 7/21/15 1:25 PM, Mark Smith wrote:
>>>
>>> I thought that could be an option, however I encountered RSes without
>>> the Source Link Layer Option, which means if RFC6085 is to be used,
>>> the link-layer header has to be available to get the source link-layer
>>> address from.
>>>
>>> After I wondered about the efficiency benefit of using multicasts for
>>> solicited RAs, Fred asked me to do some testing/investigation. I wrote
>>> up what I found here, the few unusual RS cases I saw the following,
>>> which I put down some thoughts about handling via e.g., RFC6085.
>>>
>>> o  RSes with a :: source address
>>>
>>> o  RSes with a link-local source addresses, but no Source Link-Layer
>>> Address Option
>>
>> Yes, both of those are important (I had forgotten about the second one).
>>
>> As you say, an implementation might be able to use the link-layer header and
>> unicast back a packet. If the source was :: then that approach would rely on
>> sending to the multicast ff02::1 while the link-layer destination is
>> unicast.
>>
>> If not, then the implementation needs to multicast the RA.
>>
> I was wondering if instead the implementation could trigger an ND/NS
> transaction for the RS source address at that point, and then unicast
> the RA when that completes.
That would only address the lack of SLLAO case; not the unspecified 
source address case.
I don't know how common these cases are in existing implementations.
The unspecified source would appear if the host is doing DAD in parallel 
with sending the RS.
I don't know when a host would omit the SLLAO in the RS.

>
> RFC4861 seems to be a bit vague about being able to do that. It says
> that if the SLLO is not present, the router still can unicast the RA.
> As RFC4861 is pre RFC6085, then performing a ND/NS first would be the
> only way to do that. The last sentence also seems to be permitting NC
> entries to be created when there is no SLLO ("or not a Source
> Link-Layer is provided" ... "Neighbor Cache entry" ... "(or is
> created)").
>
> "If there is no existing Neighbor Cache
>     entry and no Source Link-Layer Address option was present in the
>     solicitation, the router may respond with either a multicast or a
>     unicast router advertisement.  Whether or not a Source Link-Layer
>     Address option is provided, if a Neighbor Cache entry for the
>     solicitation's sender exists (or is created) the entry's IsRouter
>     flag MUST be set to FALSE."
I think the attempt at english as opposed to pseudo-code is hard to read.
AFAIK the intent was
     if NCE
         set NCE IsRouter to FALSE

Thus the NCE might have already existed, or have been created.

I think there is text earlier to say to not create an NCE for 0::0 nor 
if there is no SLLAO (unless the link-layer has no addresses).

Regards,
    Erik

>
> Performing an NS/ND to resolve the source address of the RS without a
> SLLO would seem consistent with the corresponding RS SLLO handling -
> to load the Neighbor Cache with entries for nodes that have sent RSes.
>
>
> Regards,
> Mark.
>
>>     Erik
>>
>>> https://www.ietf.org/mail-archive/web/v6ops/current/msg22464.html
>>>
>>>
>>>
>>>
>>> https://www.ietf.org/mail-archive/web/v6ops/current/msg22464.html
>>>
>>>> But I would not say it's sufficient, if only from an "explicit
>>>> clarity" standpoint.  I think explicit mention of RAs and the other
>>>> discussion is helpful for implementors not inclined to dig to great
>>>> depths or just seeking explicit confirmation.
>>>>
>>>
>>>
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