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

[Jim Busse <jimbusse@pacbell.net>]

Historically, a lot of IETF documents use "link up" as triggers for other
events, without defining the meaning of "link up" state.  Scary.

Sun defines the "link up" state on an ethernet device as the result of the
link meeting a basic set of tunable parameters.  These statistics determine
when the Sun OS sets the "link up" state.  Sun docs reprinted here without
permission....

"When auto-negotiation is enabled, the adv_*_cap statistics show which
capabilities are advertised to the link partner. When auto-negotiation is
disabled in forced mode, the statistics precisely show how a link should
function and that it must be matched on the link partner to achieve a valid
link up.

"The lp_cap_* parameters exist as kernel statistics for an Ethernet device.
The statistics are the advertised capabilities provided by the link partner
on completion of auto-negotiation. If the capabilities match the
capabilities provided in the local advertisement, the link can proceed to a
link up state. If no match is found, the link remains down. In two other
instances, lp_cap_* values might all be zero: when a cable is not present,
when forced mode is enabled."

Microsoft and Apple may have similar definitions, but I couldn't find any in
my MS references or on-line.  I don't know how to look at Apple stuff...
:-(

There are a couple of instances of Link Up definition in other IETF docs,
but the definitions are pretty loose:

From draft-pentland-dna-protocol-00.txt

DNAv6 assumes that the host's wireless link interface software and
   hardware is capable of delivering a 'link up' event notification when
   layer 2 on the host is configured and sufficiently stable for IP
   traffic. "

Link down is not defined in that document.

From draft-jung-mobileip-fastho-chain-00.txt
"two simple triggers of  Layer 2 are required. One is the Link Up trigger.
MN or new AR
     receive the Link Up trigger when the MN arrives on the link with  the
new AR. "

I'm not sure if the definition of "link up" and "link down" states are
within the scope of the DNA charter.  I think the OS determines whether the
link is stable enough to perform the duties required by the OS, and is
therefore an OS construct, not a DNA construct.

Best regards

Jim Busse

----- 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.