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

[JinHyeock Choi <jinchoe@gmail.com>]

Erik

> > 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 took the latter approach. (that may be the road less traveled. :-))  
Allow me to explain my reasoning too. 

In the previous CPL draft, we wrote that 

   Thus we think the prefix-based approach has a stronger assumption
   here than the Link Identifier-based approach, because the latter can
   operate reliably without any link-layer event notifications [14].

though the current version omits this.

So I tried to build 'linkid prefix' scheme as independent of Link UP
event notification as possible. I avoided to rely on Link UP. 
 
For example, with Link UP notification, Linkid scheme may work 
to detect link change from non-supporting link to supporting one 
as below. 

Assume a host is at a non-supporting link. Then it doesn't have 
a linkid prefix. When it moves to a supporting link, it will see a 
new linkid. 

The host may assume a link change upon receiving this new linkid.
But the current draft mandates that the host doesn't make any 
decision becuase there is the other possibility that the host is at 
the same link but a router on the link becomes a DNA router and
starts advertising the linkid. 

I contemplated the idea to distinguish the above two cases with 
Link UP indication but gave it up because I thought the simplicity
and less assumption is better. 

> 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.
 
We intended Linkid scheme not to depend on Link UP and the 
scheme is supposed to work rubustly even without any Link UP 
notificaiton. We can't deny the possibility of confusion (even Titanic
sank in its maiden voyage :-)) but can't find the example yet. 

Thanks in advance for your kind consideration. 

Best Regards

JinHyeock