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

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

Bernard,

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?
Given the diversity of link technologies, I think that is really all one can
say from the IP layer. 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. In addition, there are likely to be other policy matters - how
the radio transport is used (like, for example, actual number of ARQs v.s.
the maximum number, which is typically in the radio link standard), whether
reauthorization is required on handover, etc. - that can differ even between
two different terminals utilizing the same transport, or even two of the
same terminals talking to two different wireless service providers.

That said, I'm not saying that defining exactly what constitutes acceptable
link quality for a particular radio transport isn't an interesting and
important problem. 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.

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?

            jak




----- Original Message ----- 
From: "Bernard Aboba" <aboba@internaut.com>
To: "Brett Pentland" <brett.pentland@eng.monash.edu.au>
Cc: <dna@eng.monash.edu.au>
Sent: Monday, May 23, 2005 11:33 PM
Subject: [DNA] Definition of "Link Up" and "Link Down" events?


> 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.
>