[Detnet] The scalabilities of the queuing solutions in ADN

[정진우_교수_휴먼지능정보공학전공 <[e-mail]>]

Dear Toerless, and members of DetNet WG,

Thanks for pointing out the scalability issue on the queuing solutions,
at yesterday's interim meeting.
I would like to add some comments, which I couldn't make due to limited
time.
We have to always be deterministic, you know :)

The ATS IR has to maintain individual flow states, and it is the limiting
factor on its scalability.
However, the IR has only one queue per input port,
therefore I think it is feasible to be implemented in core routers.
The only real-time operation here is to look up the flow table and to
overwrite once per packet,
no matter how many the flows are active. (This was also stated by the
authors of ATS.)
But I understand if there are millions of flows, then simply looking up can
take time.

The FAIR (Flow aggregate - IR) framework can be considered as a generalized
ATS.
Therefore it has the same scalability as the ATS.
However, in the FAIR framework, the IRs can be placed only where they are
necessary.
Moreover, the IR can be implemented independently of queuing and forwarding
functions,
for example in a separate device, in a more flexible way.

The PFAR (Per port FA regulator) has only one queue per input port,
and maintains a flow aggregate (FA) state (per input port),
instead of individual flow states. It is therefore more scalable.
As flows become active and inactive, the FA state parameters
have to be modified, but the modification process is not a real-time
operation
and can be executed in bulk.

The C-SCORE (Work-conserving stateless core)
enjoys the best flow protection and the best scalability.
Usually the classical fair queuing algorithms (like Virtual Clock) require
per-flow queue,
but the C-SCORE can be implemented, as it is suggested in the
draft-joung-detnet-asynch-detnet-framework-01, with per-input port queues.
The scheduler only has to decide which HoQ packet has the smallest finish
time.

The drawback of C-SCORE is that packets are required to carry metadata.
And that metadata has to be updated in real-time as the packet is being
transmitted,
just like the flow state in ATS has to be updated.

The CQF and its variations seem to be scalable in packet level,
but I think the difficulty is in the schedulability.
I just wonder how the millions of dynamic flows can be scheduled to fit in
the slots,
in real operating environments.
Larger slots would make the slot scheduling easier, but introduce more
latency and jitter.

Best regards,
Jinoo