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

[Erik Nordmark <[e-mail]>]

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.


> 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.

    Erik