[DNA] Re: LinkID v.s. Landmark Prefix

[Erik Nordmark <erik.nordmark@sun.com>]

JinHyeock Choi wrote:

> If a host has 1) a linkid and 2) receives an RA with the prefix with I-bit set, 
> it can make a decision properly. So even in the above two cases, hosts
> may satisfy these two conditions to properly check for link change without 
> resorting to prefix comparison. 
> 
> But if either of two conditions is not satisfied, the host need to fall back
> on CPL. In this case, we need reliable "Link UP" indication.  

ok

>>When it receives the RA from R2, the only thing it can rely on is the 
>>link up notification (R3 and R3 do not advertise any common prefixes), 
>>since R2 and R3 could be two old routers that are on the same link and 
>>announce disjoint prefixes.
> 
> 
> It's not clear to me. What can Link UP say to host H? It can't 
> say anything definite about whether link change happens or not. 

Link UP indicates to the host that the RA from R3 and R2 could come from 
different links.
If we want something which doesn't rely on link UP, the host has to 
always treat each RA as potentially being from a different link, which 
would be problematic when
1) the routers don't have a common prefix, and
2) the routers don't support DNA
because in the absence of relying on a link UP in this case, the host 
will end up assuming that each time it receives a RA, it has moved to a 
different link.

> After H moves from LinkB to LinkA, it receives a hint for 
> a possible link change. 
> 
> To make Link UP presence the smallest, I assume H used 
> the lack of RA from R3 as a hint. H missed 3 RAs from R3
> and came to suspect a link change. 

But that would take 90 minutes by default, since the periodic RAs arrive 
with 30 minute average interval.

Or are you suggesting that the host should always send 3 RSes when it 
receives a link UP, so that it can use this to retire any old prefixes?
In any case, I think that is orthogonal to whether or not we can come up 
with a scheme which works reliably even when there are no link UP 
notifications, for mixed links as well as links with no DNA capable routers.

> Because H doesn't have a linkid prefix, it needs to use CPL. 
> So H activates CPL module and sends an RS. 
> 
> Because H relies on CPL in this case, it needs Link UP. 

So it seems like until every link in the Internet supports DNA (it isn't 
clear to me whether all routers on every link needs to support DNA, or 
whether it is sufficient to have one per link support it), then the 
hosts MUST rely on a reliable link up notification in order to always be 
able to detect movement. Is that correct?

    Erik