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

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

JinHyeock Choi wrote:

> ok. I admit this is a crude one but all I can think of right now.  
> (Because it's not clear to me how a host base its decision 
> on CompleteRA, I switch to landmark one.) 
> 
> Assume a link with two routers R1 and R2 and a host H. 
> R1 advertises Prefix1 and R2 Prefix2. 
> 
> R2 stops advertising Prefix2 and starts advertising Prefix3. 
> R2 announces the prefix change by sending an RA but
> both R1 and H don't receive it. 
> 
> Then H happens to change its AP in the same link 
> and send an RS with landmark Prefix2. It also happens that 
> R2 replies first and sends an RA with NO and Prefix3. 
> H falsely assumes a link change. 

I assumed that you'd found a case when a host would fail to detect a 
link change when there was one (which is the case we must avoid); I'm 
not too worried about there being some corner cases when the host 
interprets a change in the prefix set as a link change when in fact the 
host has stayed on the same link. (This would just result in some 
additional, e.g., MIPv6 signaling. And as Tero pointed out, this more 
rapid way to react to the prefix being removed might be viewed as a 
feature.)


> The above is not clear to me yet. Allow me a question to help 
> my understanding.  
> 
> Assume a host moves to a different link but fails to receive a 
> Link Up event Notification. Then the host will not assume a link 
> change even if it receives a different unsolicted CompleteRA? 

Let me explain my reasoning.

When we worked on CPL we concluded that if the implementation can't 
reliably deliver a 'link up' event notification from the link layer to 
the DNA module, then there are cases when the host would end up treating 
the prefixes from two links as being from one link.
Thus for CPL to work reliably, the 'link up' indication in the 
implementation must be present.

Given that we have this, then we have the choice to assume this 
notification even for the DNA solution, as a way to optimize things. 
Alternatively, we can try to make the DNA solution be more robust again 
a missing 'link up' notification.
I haven't thought through all the details, but the fact that landmark 
falls back on prefix comparisons, and linkID handles routers that 
temporarily disagree on the linkID, might imply that these schemes could 
be confused when the link up is missing. If that's the case we can't 
make the solution robust against a missing link up.

    Erik