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[176.93.29.60]) by smtp.gmail.com with ESMTPSA id u14sm2101809lfl.40.2021.03.09.11.09.25 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 09 Mar 2021 11:09:25 -0800 (PST) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 11.5 \(3445.9.7\)) From: Jonathan Morton In-Reply-To: Date: Tue, 9 Mar 2021 21:09:23 +0200 Cc: Steven Blake , ECN-Sane Content-Transfer-Encoding: quoted-printable Message-Id: References: <7ff9063df5c5e05420ef47245becb77a2de80f2f.camel@petri-meat.com> <1cb500b5cff9a570f71a9b92096f5f279f4a27e7.camel@heistp.net> <0B2ABDF5-A4B0-418B-A6C3-90FE8E4F20BC@gmail.com> <7d423efd8a380e91ae5bdf2922d38ae3c5a243a8.camel@petri-meat.com> <9cbf2c365c0ad635b6f08311d35e0681aa173af7.camel@petri-meat.com> To: "Holland, Jake" X-Mailer: Apple Mail (2.3445.9.7) Subject: Re: [Ecn-sane] IETF 110 quick summary X-BeenThere: ecn-sane@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: Discussion of explicit congestion notification's impact on the Internet List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 09 Mar 2021 19:09:27 -0000 > On 9 Mar, 2021, at 8:44 pm, Holland, Jake wrote: >=20 > =E2=80=A6classic ECN traffic will not respond as quickly as L4S=E2=80=A6= I know it wasn't you making this claim, Jake, but I have to point out = that it's completely false. Classic ECN transports actually respond = *more* quickly to a CE mark than L4S transports. Let's walk through the processes. RFC-3168 TCP: A single CE mark is applied to a data segment. The receiver immediately sends an ACK with ECE set, and keeps ECE set on = all further ACKs until a CWR cancels it. The sender gets the ECE, reduces the cwnd immediately, and sends the = next data segment with CWR set to confirm it. Proportional Rate Reduction may be used to spread out the reduction in = actual in-flight data. This takes at most one RTT. =46rom the queue's perspective, one RTT (the minimum possible) elapses = before the arrival rate from the sender halves (due to PRR). After two = RTTs maximum, the in-flight data has reached a new, substantially lower = value than the original. L4S TCP (Prague): A single CE mark is applied to a data segment. The receiver updates the CE mark counter in the next ACK. The sender sees the new counter value, and feeds it into a low-pass = filter which operates on discrete time intervals. When the filter is next processed, on average a single CE mark results = in half a segment being removed from the cwnd. Half the time, this = results in no externally visible change to the data in flight. The = other half, it is a very slight response. =46rom the queue's perspective, one RTT plus (on average) half the = filter window passes before any possible response reaches it, and half = the time it's no response anyway, the other half a single segment = reduction. Meanwhile, at least one segment has been added to the cwnd = in the RTT-plus time since the mark was applied (due to Reno-style = growth). I do not see how TCP Prague's response can be described as "faster" than = that of standard TCP. - Jonathan Morton=