[ippm] Which bits to use for explicit performance measurements?

[Cociglio Mauro <[e-mail]>]

Hi Martin and All.

I would like to answer your question from yesterday IPPM session in a more detailed way.
If I understood well your question is:
"Which bits, at the end, we suggest to use among all those proposed in the draft for explicit performance measures?"
(https://datatracker.ietf.org/doc/html/draft-mdt-ippm-explicit-flow-measurements-02)

Taking for granted that the usable space consists of 3 bits I would say to use 1 bit for RTT and 2 bits for Packet Loss.
Starting from the results of Aachen University ANRW paper (https://arxiv.org/pdf/2106.13710.pdf) we can say that, for Packet Loss, 1 bit is not enough:
. T-bit is not a so accurate measurement.
. Q-bit is not an end-to-end measure.
. L-bit does not allow the localization of the impairment.
So the best proposition, in our opinion, is to use 2 bits.
The 1st is definitely the Q-bit, coupled either with L-bit or with R-bit.
The L-bit is very accurate and works well even with short sessions, but it is a signal of the losses recorded by the protocol, not an independent measure.
If you want a protocol independent measurement you need to use R-bit, which even with some limitations in the case of short sessions (it needs to wait for the completion of a Q period on one side of the connection before reflecting it on the other) has proven to work quite well.

We now come to RTT measurement, considering in particular the QUIC protocol, where there is already a standardized bit, even if not mandatory, on this measure.
The Spin bit, albeit with some limitations in case of impairments (in particular out-of-sequence) and the fact that it "mixes" the application delay with the network delay, has proved to work quite well. The problem is that, up to now, it has not been implemented and in the QUIC traffic on the network it is not present.
It is a question of understanding the reasons for this fact.
If the issue of privacy is the main problem, the proposal of the D-bit, in the version with Hidden RTT, could be considered for the next version of QUIC protocol.

BR.

Mauro

_____________________
Mauro Cociglio
TIM - CI.TIS.IPT
Via G. Reiss Romoli, 274
10148 - Torino (Italy)
Tel.: [phone]
Mobile: [phone]
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