[802.3_DMLT] Negotiation turn-on of IET
An opinion on the principles of negotiating the turn-on of IET:
1. The negotiation for one direction is independent of the negotiation for
the other direction. Whether I can transmit to you is s separate question
from whether you can transmit to me. Any negotiation whose goal is to
flip a switch that turns the feature on or off for both transmission and
reception at one end (e.g, a "turn on the feature in this chip" switch) is
a bad idea. This is because it requires both sides doing something at the
same time, which is inherently difficult. It inherently requires a firm
commitment to action in the future, which is never easy to achieve with a
protocol.
2. A device that has the capability of receiving both non-IET frames and
IET frames is enabled for receiving both kinds of frames at all times.
"Be liberal in what you receive, and conservative in what you send" is a
good idea. It eliminates half of the need for negotiation. Eliminating
negotiation is better than doing negotiation, no matter how clever.
3. The following scenario is a good basis to start with, in my opinion:
A. As mentioned, if I have IET capability, I always enable reception of
both IET and non-IET frames.
B. After link down, I turn off IET transmission mode.
C. We define an information element for IET negotiation. This could
well be transmitted in 802.1AB LLDP. But, whether transmitted in LLDP or
by hardware negotiation, or by some new protocol, doesn't matter. Whet
matters is the information.
D. The information transmitted has three parts:
i. I have the ability to receive IET. (The fact that I'm sending
the information is sufficient; we don't need a 0/1 bit.)
ii. I have, or have not (0/1) turned on my ability to transmit IET.
iii. I have, or have not, recently received an IET frame on this link.
E. Every time I transmit a PDU, I send the above state information.
F. If I receive a PDU indicating that you have the ability to receive
IET, I ensure that my IET transmission mode is turned on. If I ever
receive an indication that you do not understand IET, I turn off IET mode
in my transmitter. (I don't need to do anything with reception mode --
it's always on.)
G. If I have sent you a lot of IET frames, and I have received several
PDUs from you that indicate that you have not received any IET frames,
then something is terribly wrong (e.g. There is a silent non-IET device
between us), and I turn off IET (and bit ii of the PDU).
Purely for diagnostic purposes, we may add a fourth information element:
Iv. I have not received anything from you indicating that you are
able to receive IET.
-- Norm