Re: [LOOPS] Measuring forward latency

[Carsten Bormann <[e-mail]>]

On Jun 18, 2019, at 14:52, Michael Welzl <[e-mail]> wrote:
> 
> I thought the spin bit is meant for a router somewhere along the path to understand the end-to-end RTT of a flow.
> We, on the other hand, need a LOOPS ingress node to understand the RTT (or, better, OWD) to the LOOPS egress node. So this is (LOOPS-)end-to-(LOOPS)-end.

We need a number of measurements, and an implementation could also make use of additional measurements.  If a measurement for the end-to-end RTT is available for a flow, it can be used to properly dose congestion event signaling for non-ECT flows in the way described at the end of Section 2.4 of gen-info [1].  It may also be useful to derive an upper limit for the timing of retransmissions (and the latency variance these will introduce).

But I would argue that getting that end-to-end RTT measurement is not a function of LOOPS — it is a function that implementations could provide to improve the LOOPS function of congestion signaling.  (Besides, that measurement is per-flow, not a per-tunnel wholesale measurement.)

As Yizhou listed [2], we need a number of different per-tunnel measurements, and we need them to be available for different functions of the LOOPS protocol.  Specifically:

— per-tunnel RTT, to time retransmissions
— per-tunnel one-way latency, to detect congestion and sort congestion from non-congestion losses
— per-tunnel loss rate (and other statistical properties of the loss), for parameterizing FEC

These differ in how they can be achieved, where the data is produced and where it is needed, and that should guide the design.  E.g., moving the timestamps into the forward information makes latency variation immediately available at the egress, which is also the place where the congestion signaling to non-ECT transports will need to happen.

Grüße, Carsten


[1]: https://tools.ietf.org/html/draft-welzl-loops-gen-info-00#section-2.4
[2]: https://mailarchive.ietf.org/arch/msg/loops/P3LdmvVvMeeYUzv-6r9oB91pkvQ