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

[Tom Petch <nwnetworks@dial.pipex.com>]

The terms have been in use by the IETF since at least RFC1157 (May 1990) and
while I have seen no formal definition in that or subsequent documents,
manufacturers have shown fair consensus on the meaning, about the ability to
send an IP packet.  The beauty of IP being an unreliable network is linkUp tells
you nothing about what happens to the packet after it has left the box, only
that nothing stopped it leaving:-)

Tom Petch

----- Original Message -----
From: "Bernard Aboba" <aboba@internaut.com>
To: "Brett Pentland" <brett.pentland@eng.monash.edu.au>
Cc: <dna@eng.monash.edu.au>
Sent: Tuesday, May 24, 2005 8:33 AM
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.