Re: CUD and FBD

[Jari Arkko <jari.arkko@piuha.net>]

marcelo bagnulo braun wrote:

> So, in FBD each node has a clear view about a failure in their 
> incoming path, while in CUD, each node can detect a failure but it is 
> not certain of which direction is affected.
>
> This difference may not be relevant, depending on which are the 
> following steps.
> In FBD, the node that has detected the outage needs to inform the 
> other end about the failure, in a reliable manner. This may result in 
> starting the full path exploration procedure as described in the 
> draft, so that the other end starts the procedure itself.
> In CUD, the behaviour is likely to be the same, since the node that 
> has detected the outage does not knows which direction has failed, so 
> both nodes need to perform the full path exploration procedure.
> So, i think that the result in both mechanisms is similar.

Agreed.

> - Overhead
>
> In CUD, each time there is an idle period, 2 packets are generated, 
> one in each direction.
> In FBD, each time there is an idle period, i packet is generated 

Not sure I like either one of these options. Preferrably, the probing 
frequency
would depend on whether you have something to send or not. For instance, if
we don't have any payload packets to send, the frequency = n and if we 
do have
packets to send, it would m, m << n. If you combine this with the ULP 
feedback
and packet reception in CUD, we have a mechanism that normally does not need
extra probes during traffic and which generates a very small amount of 
traffic
when its idle.

> - ULP interaction
>
> As defined in the draft, CUD requires ULP feedback in order to greatly 
> reduce overhead. However, as i mentioned above, i think that a 
> workaround can be used to deal with this issue.

I think we can assume that an optimized Shim6 implementation gets ULP 
feedback.
NUD needs it anyway, so its not a big assumption.

> IMHO, both mechanisms can benefit from negative ULP feedback in a 
> similar way, since CUD wouldn't need to wait for Tu and FBD wouldn't 
> have to wait for n*t. Probably, the benefit is much greater in FDB.

Yes.

> So, as far as i can see, both mechanisms behave quite similarly. 
> Probably, the main difference is w.r.t. the overhead, where FBD is 
> superior.

The mechanisms are very close indeed, particularly if you consider that both
benefit from some ULP feedback. What you wrote later:

> In this extended CUD scheme there are three elements in the failure 
> detection:
> - ULP feedback
> - incoming traffic
> - explicit reachability tests 

seems to make a lot of sense to me. Does the no-exponential-backoff property
hold for this too (except for the 3rd item)?

--Jari