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Thread-Topic: reno cwnd growth while app limited... 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dkim=pass header.d=netapp.onmicrosoft.com header.s=selector2-netapp-onmicrosoft-com header.b=df7tICx4; dmarc=none; spf=pass (mx1.freebsd.org: domain of Richard.Scheffenegger@netapp.com designates 40.107.78.55 as permitted sender) smtp.mailfrom=Richard.Scheffenegger@netapp.com X-Spamd-Result: default: False [-2.74 / 15.00]; TO_DN_EQ_ADDR_SOME(0.00)[]; NEURAL_HAM_MEDIUM(-1.00)[-0.999,0]; R_DKIM_ALLOW(-0.20)[netapp.onmicrosoft.com:s=selector2-netapp-onmicrosoft-com]; HAS_XOIP(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; R_SPF_ALLOW(-0.20)[+ip4:40.107.0.0/16]; MIME_GOOD(-0.10)[multipart/related,multipart/alternative,text/plain]; DMARC_NA(0.00)[netapp.com]; RCVD_COUNT_THREE(0.00)[3]; TO_MATCH_ENVRCPT_SOME(0.00)[]; DKIM_TRACE(0.00)[netapp.onmicrosoft.com:+]; NEURAL_SPAM_LONG(1.00)[1.000,0]; RCPT_COUNT_SEVEN(0.00)[9]; RCVD_IN_DNSWL_NONE(0.00)[55.78.107.40.list.dnswl.org : 127.0.3.0]; IP_SCORE(-1.24)[ipnet: 40.64.0.0/10(-3.65), asn: 8075(-2.50), country: US(-0.05)]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+,1:+,2:+,3:~,4:~,5:~,6:~,7:~]; RCVD_TLS_LAST(0.00)[]; ASN(0.00)[asn:8075, ipnet:40.64.0.0/10, country:US]; ARC_ALLOW(-1.00)[i=1] Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable X-Content-Filtered-By: Mailman/MimeDel 2.1.29 X-BeenThere: freebsd-transport@freebsd.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Discussions of transport level network protocols in FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 11 Sep 2019 12:04:23 -0000 Hi, I was just looking at some graph data running two parallel dctcp flows agai= nst a cubic receiver (some internal validation) with traditional ecn feedba= ck. [cid:image002.jpg@01D568A9.CB143AC0] Now, in the beginning, a single flow can not overutilize the link capacity,= and never runs into any loss/mark... but the snd_cwnd grows unbounded (sin= ce DCTCP is using the newreno "cc_ack_received" mechanism). However, newreno_ack_received is only to grow snd_cwnd, when CCF_CWND_LIMIT= ED is set, which remains set as long as snd_cwnd < snd_wnd (the receiver si= gnaled receive-window). But is this still* the correct behavior? Say, the data flow rate is application limited (ever n milliseconds, a few = kB), and the receiver has a large window signalled - cwnd will grow until i= t matches the receivers window. If then the application chooses to no longe= r restrict itself, it would possibly burst out significantly more data than= the queuing of the path can handle... So, shouldn't there be a second condition for cwnd growth, that e.g. pipe (= flightsize) is close to cwnd (factor 0.5 during slow start, and say 0.85 du= ring congestion avoidance), to prevent sudden large bursts when a flow come= s out of being application limited? The intention here would be to restrict= the worst case burst that could be sent out (which is dealt will different= ly in other stacks), to ideally still fit into the path's queues... RFC5681 is silent on application limited flows though (but one could thing = of application limiting a flow being another form of congestion, during whi= ch cwnd shouldn't grow...) In the example above, growing cwnd up to about 500 kB and then remaining th= ere should be approximately the expected setting - based on the average of = two competing flows hovering at aroud 200-250 kB... *) I'm referring to the much higher likelihood nowadays, that the applicati= on itself pacing and transfer volume violates the design principle of TCP, = where the implicit assumption was that the sender has unlimited data to sen= d, with the timing controlled at the full disgression of TCP. Richard Scheffenegger Consulting Solution Architect NAS & Networking NetApp +43 1 3676 811 3157 Direct Phone +43 664 8866 1857 Mobile Phone Richard.Scheffenegger@netapp.com [Welcome to Data Driven] [Facebook] [Twitter] #DataDriven https://ts.la/richard49892