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

[Bernard Aboba <aboba@internaut.com>]

Recently, in a liaison letter from IEEE 802.21, some questions were raised
about the use of the terms "Link Up" and "Link Down" within
draft-iab-link-indications-01.txt.

In revising the document, I went about in search of definitions of these
terms.  However, I could not find a definition in the following documents:

http://www.ietf.org/internet-drafts/draft-ietf-dhc-dna-ipv4-11.txt
http://www.ietf.org/internet-drafts/draft-ietf-dna-link-information-01.txt
http://www.ietf.org/internet-drafts/draft-ietf-dna-goals-04.txt
http://www.ietf.org/internet-drafts/draft-ietf-dna-soln-frame-00.txt
http://www.watersprings.org/pub/id/draft-yamamoto-dna-term-00.txt

This struck me as more than a little odd, particularly when I found myself
asking basic questions about the meaning of these terms.

For example, in wireless communications link quality can be highly
transient;  when multi-path interference is present, the S/N ratio can dip
20-30 dB within the space of a few feet.

In a wired network, the "up" link state is one exhibiting very low
frame loss, whereas the "down" state is one exhibiting 100 percent loss.
It is therefore clear when a link transitions from "down" to "up"; a
"Link Up" event is sent upon this transition.  Similarly, it is clear
when a link transitions from "up" to "down" and a "Link Down" event
is sent upon this transition.

However, on a wireless network, intermediate frame loss rates are often
observed, so that links may be neither "up" nor "down" according to the
idealized definition.  In this case, when are "Link Up" and "Link Down"
events sent?

Does the term "Link Up" refer to an event that only occurs when a link
has entered a link layer protocol state associated with the
initial ability to handle IP traffic?  For example, within PPP does a
"Link Up" event only occur after the completion of IPCP negotiation?
Or in 802.11i, only after completion of the 4-way handshake?

Or can a "Link Up" event occur when a STA wanders out of range of an
access point, retransmits packets multiple times without a response,
scans, and then returns within range of the same AP, all without
changing state within the link layer state machine?  In this scenario,
is a "Link Down" event also sent?

Similarly, does "Link Down" refer only to an event that occurs when a link
has entered a link layer protocol state that is not associated with the
ability to handle IP traffic?  For example, in PPP a "Link Down" event
is presumably sent on receipt of an LCP-Terminate; in 802.11, a
Disassociate or Deauthenticate frame might trigger a "Link Down" event.

But can a "Link Down" event also be sent due to a transient decline in
link quality?  For example, can this event be sent when a host wanders
out of range and retransmits a frame until it gives up?  Or is a more
substantial outage required, affecting several distinct frames?  In
other words, is a "Link Down" event, unlike a "Link Up" event, affected
by link quality?

I ask these questions because it seems to me that the precise definition
of these events may determine whether they are likely to be spurious, as
well as whether they arrive in a timely way.

For example, I have observed 802.11 NICs that will retransmit a lost frame
for >30 seconds after an AP is turned off before scanning for another
point of attachment.  I have observed other NICs that will begin to scan
after two retransmissions.