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

From: NJEDJOU Eric RD-RESA-REN <[email-masked]>
Date: Thu, 02 Jun 2005 00:45:48 +0200

I have a little problem with the most recent defintion (Bernard's) of Link-Down suggested. The statement "A Link Down event is only generated once the link layer considers the link unusable" looks (to me at least) a bit contradictory to the first sentence of the definition in the sense that the link layer can consider the link unusable even without any change having occured in the link state machine: PDP context activated in GPRS/UMTS but very high BER  
Said differently, the definition implies that the event cause can be "state changed" in the link layer state machine as well "state did not changed" (as the link can become unusable within the same state) which is a problem.

Hope my concern is understood.

Insn't 
"An event provided by the link layer that signifies a state change associated with the interface no longer being capable of communicating IP packets. transient periods of high frame loss are not sufficient." enough for Link down definition?

Please advice

Eric Njedjou

-----Message d'origine-----
De : [email-masked] [mailto:[email-masked]] De la part de Bernard Aboba
Envoyé : samedi 28 mai 2005 22:58
À : [email-masked]
Objet : Re: [DNA] Definition of "Link Up" and "Link Down" events?

Erik Nordmark wisely observed:

> I think it would be good if the definitions would clarify that even if 
> the link layer on the host sees 100% packet loss for a short while, it 
> doesn't necessarily imply that a link down event should be generated.
> (It presumably shouldn't be generated until the link layer abandons 
> the
> L2 association/connection/whatever.)

How about this?

Link Down
An event provided by the link layer that signifies a state change associated with the interface no longer being capable of communicating IP packets. A Link Down event is only generated once the link layer considers the link unusable; transient periods of high frame loss are not sufficient.

Link Up
An event provided by the link layer that signifies a state change associated with the interface becoming capable of communicating IP packets.

---

From: James Kempf <[email-masked]>
Date: Wed, 01 Jun 2005 20:20:36 -0700

Eric,

I have a little problem with the most recent defintion (Bernard's) of
Link-Down suggested. The statement "A Link Down event is only generated once
the link layer considers the link unusable" looks (to me at least) a bit
contradictory to the first sentence of the definition in the sense that the
link layer can consider the link unusable even without any change having
occured in the link state machine: PDP context activated in GPRS/UMTS but
very high BER
Said differently, the definition implies that the event cause can be "state
changed" in the link layer state machine as well "state did not changed" (as
the link can become unusable within the same state) which is a problem.

Insn't
"An event provided by the link layer that signifies a state change
associated with the interface no longer being capable of communicating IP
packets. transient periods of high frame loss are not sufficient." enough
for Link down definition?

Please advice

jak>> I guess I don't understand the problem. The definition leaves it up to
the link layer to decide what "unusable" means. Whether or not that involves
a state machine change or some other consideration is pretty much up to the
link layer to decide.

            jak

---

From: Bernard Aboba <[email-masked]>
Date: Thu, 02 Jun 2005 08:16:22 -0700 (PDT)

> for Link down definition?

The above is ok from my point of view.

> a state machine change or some other consideration is pretty much up to the

I think Erik's point was that we don't want a "Link Down" to be sent just
because a few frames were lost.  While that might initiate a search for a
new point of attachment, the problem might be highly transient.  Where
there is multi-link interference, for example, we see the S/N ratio drop
20-30 dB within a very short distance.