[RE] On worst-case latency for Ethernet networks and alternative shaping concept
Gents,
on our previous call somebody (I believe
it was Geoff) have expressed a desire to have an exact worst-case latency
formula. Since I couldn't find anything readily available on the topic,
I've took up on an exercise in algebra and formal logic to come up with
the formula. Please see the link to the document with formula and the proof.
http://public.wpanther.fast-mail.org/WorstCaseLatency.pdf
I also sensed the general agreement
that it would be beneficial to come up with alternatives to "shaping"
as it is proposed at the moment.
While studying the subject it would
appear that such solution is possible to get. In particular I would suggest
a synchronized network access, which can also be called "orchestra"-concept.
In essence idea is that all nodes use
precisely synchronized wall clock to orchestrate a predictable network
access, thus making is possible to reduce latencies to the levels well
below suggested in the attached paper. Very similar to orchestra which
has to tune for the same frequency and tempo in order to play a piece,
end-nodes and routers "tune" onto the same time/schedule before
than can play a melody... well, provide predictable propagation delay in
our case.
When reserving a path end node would
look at the topology and "occupation map" along the communication
route to determine latency it can be expecting. This "occupation map"
would essentially consist of time-slots and number of streams passing through
this output port at this slot. Each router/output port has such map and
client can access it. Using this information client can determine its own
transmission schedule producing the least latency (most empty slots) and
decide whether this is satisfactory for its purposes.
Essentially this is a fusion between
a conventional isochronous access arbitration and conventional best-effort
absence of arbitration with compromises made to achieve a simple router
design. Obviously number of details need to be worked out, such as interaction
between streams occupying the same time slot on given the switch/output
port and between neighboring slots.
Pros:
1) No changes are need in traffic routing
algorithms. All burden of arbitration is distributed to end-points.
2) Deterministic latency guarantees
are possible.
Cons:
1) Limited scalability. Here we need
to acknowledge that scalability for ResE is not a prime target. We have
to sacrifice something to get a cheap router.
2) Relatively precise time synchronization
(probably within 10s of microseconds) has to occur before latency-bound
traffic is possible.
This is another compromise that has to be made. I think
that it may be acceptable to expect real-time traffic service hick-ups
when backbone configuration changes (i.e. clock-master is disconnected).
Even local buses such as IEE1394 require bus reset under certain topology
changes.
3) Assumes that routers have high degree
of determinism in routing traffic (SW routers will not do).
In addition, we could use lower 802.1
priority for traffic which is not overly sensitive to the latency, i.e.
traffic which is OK with worst-case figures. I'd imagine that stored video
playback and even life TV would be good candidates.
Feedback on both paper and "orchestra"
concept would be greatly appreciated!
Max Azarov
SMSC