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

[Erik Nordmark <erik.nordmark@sun.com>]

Bernard Aboba wrote:

> 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?

If we are trying to solve the DNA issue, and ignore that hosts might 
have multiple NICs, then we can come up with what we need operationally.
My take is that this should be sufficient:
They layer 2 probably has some internal set of states at which it is 
known that no IP packets can be passed. Thus the loss probability is 
100% due to the state at the driver/NIC/AP/whatever.

For DNAs purposes it is useful to get a 'link up' notification from L2 
when L2 moves out of the state(s) where no IP packets are passed.
Likewise, if DNA needs a 'link down' notification, it is probably most 
useful for DNA if that notification arrives when L2 moves from a state 
where IP packets might be passed, to a state where they are guaranteed 
to not pass. We do need to ensure that at least a 'link up' notification 
is generated when the L2 connectivity changes, even if that change is 
near instantaneous (quickly unplugging the Ethernet cable and plugging 
one back in again; associating with an 802.11 AP even if the old AP was 
believed to still be reachable).

With the above definitions we'll minimize what DNA would consider 
spurious notifications just because of signal strength, yet never miss 
the case when the layer 3 link is different.

That is the simple case.
Enter multihoming (multi-NICcing if you'd like)...

When a host has multiple NICs there might be any number of policy 
related ways the host wants to choose. This could be based on a 
combination of signal strength, "flakiness history", cost, and it might 
be different for different applications running on the same host.
I think that is out of scope for DNA, but it probably makes sense to 
explicitly mention it in the context of the L2 event notifications, 
saying that the host implementation can benefit from finding out about 
significant changes in signal strength, observed L2 retransmissions, 
etc., as some of the inputs to the hosts policy choices.

    Erik