Re: [DNA] Question to DT about TSLLAO and Negotiation-freeDeterministic Fast RA

[Greg Daley <greg.daley@eng.monash.edu.au>]

Hi Erik,

Erik Nordmark wrote:
> Greg Daley wrote:
> 
>> Dear DT,
> 
> 
> Did you intend to send this on the DT list or the WG list?

I intended it to the WG mailing list.

Since the document is out there (DNA list), I guess it's OK to
discuss components on-list.

I wasn't sure if my email to DNA-DT would bounce anyway.

>> Was there a reason that Tentative Source Link-Layer Address
>> options were recommended for acting as the sequence
>> ordering mechanism in unnegotiated detfastra?
>>
>> I would have thought that the inbound Router Solicitation's
>> source address would work just as well to xor against the hash
>> (probably in reverse or reverse byte order, to be less
>> predictable) to get the ordering.
> 
> 
> Since you can't assume that your link-local address is unique should you 
> have moved, you need to run the optimistic DAD procedure.
> While doing so, the RS must have the unspecified source address.
> Hence all RSes from different hosts will all look identical, except for 
> a TSLLAO.


Optimistic DAD doesn't require unspecified source addresses on RSs.

It requires that (non-tentative) SLLAOs aren't included in RSs while
optimistic.

(section 2.2 of draft-ietf-ipv6-optimistic-dad-05:

"...
       * Never using an Optimistic Address as the source address of a Router
         Solicitation with a SLLAO.  Another address, or the unspecified
         address, may be used, or the RS may be sent without a SLLAO.

         An address collision with a router may cause neighboring
         router's IsRouter flags for that address to be cleared.
         However, routers do not appear to use the IsRouter flag for
         anything, and the NA sent in response to the collision will
         reassert the IsRouter flag.
..."
)

section 3.2:
"...
    * (modifies 6.3.7)  A node MUST NOT send a Router Solicitation with a
         SLLAO from an Optimistic Address.  Router Solicitations SHOULD
         be sent from a non-Optimistic or the Unspecified Address,
         however they MAY be sent from an Optimistic Address as long as
         the SLLAO is not included.
..."
)

There was previously some concern over the use of RS, since it clears
the IsRouter flag in the case of colliding with a router (V.low
probability). It was found that the DAD defence from the collided
address would reset it less than 1 second later, though.

In most cases, I guess that the host will have to choose between sending
RS with Unspecified Source or Optimistic Address.  There's no protocol
impediment to using the Optimistic Address though.

I'd guess that really we'll see people using the Optimistic Address
in most cases, in order to allow for a unicast response (at L3).
If that's the case, source addresses are sufficient.

Relying on TSLLAO excludes legacy devices which are more likely to use
unspecified source addresses anyway.   In the case that an RS arrives
with an address and no TSLLAO (or even with an SLLAO one), it may be 
valuable to send an RS fast anyway.

Just an idea.

Greg