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

[Bernard Aboba <aboba@internaut.com>]

> As a practical matter, I think the definition of Link Up for DNA is: is it
> now possible to push an IP packet across the link? And the definition of
> link down is: is it now not possible to push an IP packet across the link?

I would suggest that there is an important distinction between "the link
layer state is now such that IP transport is permitted" versus "at this
moment, the link quality is such that communication is likely to be
successful."  The former is typically well defined within a link layer,
whereas the latter is a matter of judgement.  My belief is that "Link Up"
only reflects the former but not the latter consideration.

> The details of what constitutes acceptable link
> quality to rule that an IP packet can be pushed across the link are likely
> to be different for UWB than for Super 3G or 802.16 from a radio technical
> standpoint.

Yes, but that will eventually be reflected in a state transition within
the link layer state machine.

> Actually, it is quite vital and I would expect that any
> radio link specification would have that kind of information available for
> the packet data service.

Yes, but that may be considered separately from "Link Up" and "Link Down"
indications.  For example, a link may have made a transition to a state in
which IP packet transmission is permitted, yet extensive frame loss may be
observed.  Such a link may provide information on the frame loss to upper
layers which may affect routing metrics, but it does not send a "Link
Down" indication until the loss is sufficiently persistent to cause the
link layer state machine to transition.

> If the WG can arrive at a concensus definition of Link Up and Link Down for
> DNA, do you think it would be worthwhile writing it down, possibly in the
> solution framework document?

There was a terminology document at one point, so that might be the best
place to include it.  However, it also seems necessary to include that
definition in other places which use the term, such as the DNAv4 document.