Re: I-D ACTION:draft-ietf-shim6-reach-detect-00.txt

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

Erik Nordmark wrote:

> I see two potential reasons for not using any negative advice in the 
> NUD-based model.
>
> 1. If the probing by the shim (which is suppressible by positive 
> advice) have been designed so that a failure can be detected in N 
> seconds, then the shim would satisfy a requirement of detecting any 
> failure in, at worst, N seconds. Is it really useful to sometimes be 
> better, or is worst case the thing that matters? (I guess this is more 
> a question than a reason.)

Ok. (I wasn't so much arguing that we need to use negative advice, just
wondered why it couldn't be combined with this model.)

> 2. When you have multiple ULP "connections" using the same shim 
> context, you have some more complexity to handle when some ULP advice 
> might be negative and some positive at the same time (for the same 
> context). If you place more weight in the negative advice, then you 
> will react more strongly to random packet loss causing one connection 
> to think there is a problem, etc.
> Only using positive advice avoids this complexity, because it 
> implicitly assumes that if anybody ULP finds the locator pair working, 
> then the probes can be suppressed.

This makes sense, but I worry that the argument
"one ULP works => others should work too" may not
hold in all cases. What if one ULP is TCP:xxxx->80, which
happens to work over this crappy firewalled network that
you are using, but it doesn't let through, say, TCP:xxxx->23?
The shim could be making the wrong decision here.

--Jari