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

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

> IP packet transmission "possible" is an indication that the link
> layer will accept an IP packet for transmission.  Whether it actually
> arrives depends on the link quality.
>

I agree that an indication from the link that it is "up" and ready to accept
IP traffic is no guarantee that the packet will be transmitted. The IP layer
also needs to exercise some control if packet loss becomes unacceptably
high, perhaps by switching to another interface, or triggering a handover on
the interface where packet loss is high if the driver allows IP level
handover triggering.

> Where we have perhaps over-simplified is in assuming that links are either
> "up" or "down" in terms of their frame loss (e.g. either low loss or high
> loss in both directions).  In some cases (e.g. outdoor mesh networks) the
> data shows that links can be in intermediate states as well as exhibiting
> considerable asymmetry.
>
> However, those over-simplifications won't affect DNA as long as our
definition
> of "Link Up" is not dependent on frame loss.  However, it is not
> clear to me that other "link indications" are as robust.  For
> example, are indications such as "Link Going Down" dependent on frame
> loss, are they indicative of a decision to bring the link down?
>
>
>

I interpret "Link Going Down" to mean that the mobile node and base station
have begun some procedure to initiate handover. In cellular protocols, there
is extensive negotiation between the mobile node and base station prior to a
handover. 802.16e also has this characteristic. An indication of "Link Going
Down" on such a protocol would mean that negotiation has begun for handover
and, at some short time in the future (perhaps indeterminate), packets will
no longer be accepted for transmission. For 802.11, "Link Going Down" is
meaningless because handover is completely controlled by the mobile node,
and the link continues to accept packets until the AP scan interrupts packet
transmission (and even there, I've seen some NICs that interrupt for AP scan
then send a few packets out prior to the reassociation).

            jak