Re: [DNA] Definition of "Link Up" and "Link Down" events?

[James Kempf <kempf@docomolabs-usa.com>]

> > But is this really of concern to IP? The IP layer just wants to know
whether
> > or not the link layer thinks there's a high probability of getting a
packet
> > across. The details of how that's done are indeed part of the state
machine
> > in the driver, but are they of any concern to the IP stack?
>
> "High probability" is different from "the link layer is in a state where
> IP packet transmission is not possible".  The former depends on link
> quality, whereas the latter depends on the link layer state machine.  As a
> result, the former is likely to be highly transient, while the latter is
> likely to be less so.
>

Ok,  so is ~(IP packet transmission not possible) == (IP packet transmission
possible)? From your previous emails, I have the impression that you think
this is an oversimplification, is that right?

> > >Sure, but I'd suggest that we should be clear that this additional info
is
> > >not a determinant of "Link Up" and "Link Down" events.
> >
> > Hmm, what about the case where 802.1x authentication must complete prior
to
> > "Link Up"? That has nothing whatsoever to do with the quality of the
link
> > from a frame transmission standpoint?
>
> Since EAP supports retransmission, and on 802.11 802.1X frames are sent to
> a unicast address, it is possible for 802.1X authentication to complete
> even in situations where there is high frame loss.  So it is possible for
> an 802.11i link to reach the "Link Up" state even though link quality is
> poor.
>

From the IP stack's standpoint, "Link Up" on a link where 802.1x is required
means: "802.1x is complete and the controlled port is now open for traffic".
Even if the link is of stellar quality, the IP stack can't get any packets
out prior to that. If the state is: "802.1x is complete and has failed, the
controlled port is closed" means the link is still down. This particular
state transition has nothing to do with the link quality, from the IP
stack's viewpoint, even though the transition of the state machine regarding
 whether frames can successfully get across the link does affect whether or
not 802.1x completes.

If 802.1x is not required on the link, then the transition of interest to
the IP stack is controlled by the link quality alone.

So my point was, link quality is not the only factor that influences whether
the driver can report "Link Up" to the IP stack. Sorry for being not clear.

            jak