Re: [tcpPrague] [tsvwg] ecn-l4s-id: Proposed Changed to Normative Classic ECN detection Text

[Jonathan Morton <[e-mail]>]

> On 1 Nov, 2020, at 3:07 am, Christian Huitema <[e-mail]> wrote:
> 
> I am reading the L4S ECN-AQM proposal with an eye on responding to it in an implementation of QUIC, and I have a couple of questions regarding use of ECN marking with QUIC. 
> 
> The document does not mention QUIC, yet QUIC is already used in a large fraction of Internet traffic. QUIC does specify support for ECN, and QUIC acknowledgements may carry counts of each category of ECN marks received from the peer -- three counters for ECT(0), ECT(1) and CE. In theory, QUIC implementations could take advantage of L4S -- in fact, at least one implementation supports DC-TCP like CC already. Is there interest in specifying L4S for QUIC?
> 
> My next question regards the interaction of the proposed L4S ECN-AQM with CC algorithms like BBR that attempt to discover the bottleneck packet rate for the connection, and use pacing to send packets at that rate. I observed that BBR is never mentioned in the draft, yet BBR is used in a sizeable part of Internet traffic. Do we have data on how a non-L4S aware implementation of BBR interacts with the proposed L4S AQM?
> 
> My last question regards potential use of ECT(1) marking. Most current implementations set ECT(0), but setting ECT(1) instead is trivial. This should elicit an L4S compatible response in L4S-AQM, and the BBR implementation might be modified to use the signals as part of the bottleneck bandwidth tracking. But there is a small issue there. With BBR, QUIC packets are supposedly paced at just under the bottleneck rate, except during "probe" periods in which they probe for 1 RTT at a slightly higher rate. The L4S AGM might degenerate in a form of ON-OFF control -- no feedback at all most of the time, then a bunch of CE marks if the probe rate exceeds the bottleneck bandwidth. As anyone experimented with that?

In my view, the biggest question you should be asking is how QUIC will distinguish between CE marks applied by an L4S AQM on the path, and those applied by an RFC-3168 AQM on the path.  The latter will treat ECT(1) marked packets the same as ECT(0), and expects the same multiplicative-decrease congestion response, but L4S expects a qualitatively different linear response.

You should probably not do any substantial work on integrating L4S into QUIC until you have a good answer to that question.  Alternative approaches to high-fidelity congestion control exist which resolve that particular conundrum easily.  In particular, the ECN field feedback mechanism in QUIC can also support SCE.

 - Jonathan Morton