[Detnet] Validity of Deadline based forwarding

Jinoo Joung <jjoung@smu.ac.kr> Mon, 13 November 2023 11:21 UTC

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From: Jinoo Joung <jjoung@smu.ac.kr>
Date: Mon, 13 Nov 2023 20:20:45 +0900
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Subject: [Detnet] Validity of Deadline based forwarding
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Hello Shaofu,

I have a question for clarification on the admission criteria (or the
schedulability condition) of the Deadline based forwarding.
Let's consider only the In-time mode operation with PIFO implementation,
for simplicity.

According to your draft, the admission is decided based on Equation 1,
which is
sum{A_i(t-d_i) for all i} <= C*t,
where A_i(t) is the arrival constraint function of the flow i.
In the flow entrance node, this A_i(t) can be obtained from the flow
specification (with max burst and arrival rate).

However, A_i(t) changes as the flow traverses hops, because of the burst
How it will change is difficult to know, since it is a function of other
flows' parameters as well.
Thus, in Options 1 and 2, you suggested the shaping (or regulation)
function to be used in core nodes,
to enforce A_i(t) to become the initial arrival process,
so that we can test the admission condition (Equation 1) even in core nodes.

Unfortunately, this approach needs per-flow state maintenance, therefore
not scalable.
So, in Options 3 and 4 you suggested the "Latency compensation", as an
alternative solution to the shaping.

How can this latency compensation apply to Equation 1, or
how do you check the admission condition in Option 3 or 4?

I suspect that the latency compensation does not make Equation 1 to be
usable in core nodes.
By utilizing the delay budget obtained from the previous nodes,
the latency compensation can only make more packets "eligible".
It is also unclear how to distinguish eligible and ineligible packets, but
it is not important to me.

As you have specified in the presentation slides,
if some of the packets become ineligible with Option 3 or 4,
then these packets will be served later than the eligible packets,
and are not guaranteed latency bounds.
But it is not the packets' faults to become ineligible.

Can you still say Option 3 or 4 is a suitable solution?

Best regards,