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

[JinHyeock Choi <jinchoe@gmail.com>]

Dear Erik

> > 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 :-)

ok. I don't think we can remove such a temporary "Lack of Synchronization" 
entirely. RA travels with the speed of light, so to deliver RA to all
the routers
"at the same time", we may need to study "Theory of Relativity". :-)

> 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. 

ok. From now on, I will use the term "flapping" only in the sense you 
describes. 

For the more general case I presented, I will use the term 'LOS (Lack 
of Synchronization). Is it ok? 

(The term "link" gave me enough trouble already. I don't want another 
Jason problem arising from ambiguous terms.)  

> 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.

ok. I think it's also important that DNA solution cope gracefully with 
more general case of  temporary LOS (Lack of Synchronization). If 
DNA solution can deal with LOS, I think it can deal with flapping 
automatically. 

> > 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"?

I guess I presented one problem with Landmark in the other mail. :-) 

> > 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.

ok. I think the above is a non-trivial change for Link Identification 
criteria, so needs to be reflected in the Landmark/ CompleteRA draft. 
 
Best Regards

JinHyeock