Re: [Efficientnd-dt] "DAD issues" draft

Andrew Yourtchenko <ayourtch@cisco.com> Mon, 27 October 2014 13:26 UTC

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Date: Mon, 27 Oct 2014 14:26:22 +0100
From: Andrew Yourtchenko <ayourtch@cisco.com>
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To: Erik Nordmark <nordmark@sonic.net>
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Subject: Re: [Efficientnd-dt] "DAD issues" draft
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Erik,

On Thu, 23 Oct 2014, Erik Nordmark wrote:

> On 10/15/14 1:07 PM, Andrew Yourtchenko wrote:
>> 3.  Interaction with Spanning Tree Protocol
>>
>>     When a port on an STP-enabled switch comes up, it goes through three
>>     phases of Listening then Learning then Forwarding.  The default is to
>>     keep it for 15 seconds in Listening and 15 seconds in Learning
>>     states.  During this time no user traffic is forwarded by the switch
>>     from and to this port.  Therefore, if a DAD process happens during
>>     this period it is guaranteed to not detect any duplicates.  This
>>     results in DAD being ineffective for link-local and otherwise pre
>>     configured addresses.
>
> STP is one instance of a somewhat predictable outage.
> If a link uses some modems (e.g., DSL and cable modems whose up/down status 
> is not visible to the IP stack) we could also have outages that last for tens 
> of seconds making DAD be ineffective. Thus it might make sense to add a 
> sentence about this being a more general issue than only STP.

Thanks - reworded to generalize.

>
>
>> 7.  Partition-join tolerance
>>
>>     [RFC4862] explicitly mentions this problem: "Note that the method for
>>     detecting duplicates is not completely reliable, and it is possible
>>     that duplicate addresses will still exist (e.g., if the link was
>>     partitioned while Duplicate Address Detection was performed)."
> It might make sense to add a reference to ACD (RFC 5227) as an example of a 
> way to detect duplicates in IPv4/ARP - at the cost of broadcast ARP 
> responses.

Added a reference. we can discuss the better wording later.

Thanks a lot for the comments!

--a

>
>   Erik
>