Re: [dna] DNA and DHCPv6

Ralph Droms <rdroms@cisco.com> Fri, 20 November 2009 17:14 UTC

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From: Ralph Droms <rdroms@cisco.com>
To: Bernard Aboba <bernard_aboba@hotmail.com>
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Cc: DNA <dna@eng.monash.edu.au>, dna@ietf.org, draft-ietf-dna-simple@tools.ietf.org, IESG IESG <iesg@ietf.org>, dhc WG <dhcwg@ietf.org>
Subject: Re: [dna] DNA and DHCPv6
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So, would this be an accurate description of Simple DNA:

* Send NS for each candidate link to the default router for that link
* Initiate RS/RA exchange as specified in RFC 4861
* Initiate DHCPv6 exchange as specified in RFC 3315

* If an NA is received, used cached info for corresponding link from  
SDAT
* Process any received RAs as specified in RFC 4861
* Use info from DHCPv6 exchange as specified in RFC 3315
* Info from RA and/or DHCPv6 overrides any reused cached info based on  
NA

- Ralph


On Nov 20, 2009, at 11:39 AM 11/20/09, Bernard Aboba wrote:

>> I believe that many of Ralph's issues below are the result of poor
>> wording in the document.
>>
>> The draft specifically says that it does not change the DHCPv6
>> protocol, state machine or timers, so that any situation in which
>> there is an implied DHCPv6 change represents an error.
>
> "It makes sense to me that DHCPv6 would not be changed by this
> document.  DHCPv6 already has a mechanism through which a host can
> determine, in one DHCPv6 message exchange, if it has reattached to its
> most recently attached link.  If the host has moved to a new link, an
> additional 4 message exchange (possibly optimized to 2 messages) is
> necessary to get configuration information for the new link."
>
> [BA] Right.  That mechanism should be used as is, with no changes.
>
> "In addition to the text that may imply skipping an initial delay when
> sending an initial DHCPv6 message, the third paragraph of section 4.6
> of this document changes the DHCPv6 protocol by specifying that a host
> always send a Solicit message first, possibly including addresses from
> previously visited links as hints.  I think the text about this step
> is a little fuzzy about which addresses should be sent and how they
> should be sent."
>
> [BA] The contents of DHCPv6 packets shouldn't be dependent on any  
> aspect of
> DNA.
> For example, DNA shouldn't influence whether a Solicit message is  
> sent first
> nor
> what addresses are used as hints.  Whatever any implementation already
> does should be left alone.  After all, DNA is just an optimization.
>
> "I could imagine an argument for a change to DHCPv6 that requires the
> host to send a Confirm message for each of its currently valid
> addresses and a Solicit message, all in a burst, to probe all possible
> links in parallel, rather than just checking for reattachment to the
> most recently attached link.  However, it would be a significant
> change to DHCPv6 that has the potential for significant load on the
> DHCPv6 service."
>
> [BA] I don't see a need to change DHCPv6, particularly if this would
> create potential problems.
>
> "Another link between Simple DNA and DHCPv6 comes to mind at this
> point.  Here is the second paragraph of section 4.7:
>
>    On reception of a Router Advertisement or advertising DHCPv6  
> message
>    (a REPLY or ADVERTISE) which contains prefixes that intersect with
>    those previously advertised by a known router, the host utilizes  
> the
>    configuration associated with the detected network.
>
> What, exactly, does "utilize the configuration" mean, in relation to
> DHCPv6?  Does the host use any DHCPv6-assigned addresses without
> sending a Confirm message to the DHCPv6 service?  If the host has not
> reattached to its most recently attached link, presumably the host
> will have received a NAK from the DHCPv6 service from its initial
> Confirm (which checked just the most recently attached link).
> Depending on the timing, the host may have already initiated a DHCPv6
> message exchange for the current link."
>
> [BA] I have no idea what this paragraph means (or even why it is  
> necessary).
>
> Shouldn't a host just do what it would normally do in response to  
> these
> messages?  After all, DNA is only really about use of neighbor  
> discovery,
> not RS/RA or DHCPv6.
>
> "Also, this paragraph specifies "prefixes that intersect with those
> previously advertised by a known router".  Suppose the set of prefixes
> intersects but is not identical to those previously advertised?  I
> assume the host uses the newly received configuration.  But, by now
> the host has all the current configuration information from its local
> router, so what's the point?"
>
> [BA] As far as I can see, the only case worth additional discussion is
> the one in which there are prefixes that don't intersect.
>
> "As I mentioned above, I don't think the behavior you describe here is
> currently described in the document."
>
> [BA] Right.  My take is that this (and other DHCPv6 changes you  
> mention)
> are unnecessary and should be removed.
>