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

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

JinHyeock Choi wrote:

> I use flapping in more general case as below. 
> 
> When the set of prefixes on a link changes, 
> there is the possibility of temporary lack of synchronization, i.e. 
> the nodes on the link (such as routers and hosts) 
> may have different set of prefixes (on the link) for the time being.  
> 
> For example, if the prefix set is {P1, P2} and P3 is added, one would
> ideally have all the nodes on the link have the prefix set {P1, P2, P3} 
> instantly. But due to packet loss, it is possible that one router has the
> prefix set {P1, P2} and the other router has {P1, P2, P3} for some time.

It isn't only packet loss that is the problem; in a distributed system 
the notion of "at the same time" isn't a useful concept. We've talked 
about this in the DT and I referred to the Lamport paper which I think 
is titled "time, clocks, and the ordering of events in a distributed 
system".

So we shouldn't require that DNA solve a known unsolvable problem :-)

Hence I postulated "flapping" as being the more specific case when a 
single host, instead of seeing a "clean" switch from one prefix set to 
another, sees the set of prefixes flap back and forth before settling 
down on the new set. Such flapping is unavoidable for the cases we are 
interested in (an unknown and changeable set of routers on the link, and 
a changeable set of prefixes being advertised.)

I think it is important that the DNA solution cope gracefully with such 
flapping.

> If hosts check for link change with prefix information, such a lack of 
> (temporary) synchronization may cause a false link identity detection. 

 From my understanding of CPL, completeRA, landmark, and linkID, none of 
them have this problem. So which solutions do you refer to with your "may"?


> ok. Allow me to clarify this
> 
> In Landmark/ CompleteRA, 
> when a host receives a solicited RA, 
> the host check for link change with both 
> 1) Landmark option with YES/NO bit and
> 2) prefixes in the RA. 
> 
> If the (solicited) RA carries 1) NO answer and 2) a known prefix, 
> the host would not assume a link change. 
> (So in effect, "known prefix" would overrule "NO".)
> 
> Is this right? 

I think that makes sense.

> Routers don't pay attention what other routers advertise as a linkid. 
> They generate the complete prefix set and pick the linkid prefix for 
> themselves. You recommended this approach. 

Blush :-)

    Erik