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

[Alper Yegin <alper.yegin@samsung.com>]

Hi Bernard,

>
http://www.ietf.org/internet-drafts/draft-ietf-dna-link-information-01.t
xt

We have the definitions as:

   Node's establishment of a link-layer connection with an attachment
   point that signifies the availability of IP service (i.e., being able
   to send and receive IP packets) between the two is considered a link
   up event.


   Link down event signifies the discontinuation of the IP service
   between the node and the attachment point.  When the link-layer
   connection is physically or logically torn down and it can no longer
   carry IP packets, this is considered to be a link down event.


> 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?  

Yes. This event should be associated with a state transition within the
L2. In the new state, the L2 should be willing to accept IP packets from
L3, and look for an opportunity to transmit them. (as opposed to
returning an immediate error message stating the request cannot be
fulfilled -- interface is down, IP service not available, etc.)

> 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?

Yes and yes, as we stated in the dna-link-info I-D.


> 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?

I don't think so.


> 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.

Yes, we covered those in the same I-D as well.

> 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?

Eventually a continued "low quality" may lead to the L2 deciding a state
change that leads to a "link down", but this is a L2 design and
implementation decision. 

> 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.

I think this is a L2 implementation issue. At what point the L2 decides
that it has lost its connection to an access network. 

Alper