From nobody Fri Oct 11 07:05:08 2024 X-Original-To: freebsd-net@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 4XPyLk4tdlz5YP8c for ; Fri, 11 Oct 2024 07:05:18 +0000 (UTC) (envelope-from Michael.Tuexen@lurchi.franken.de) Received: from drew.franken.de (drew.ipv6.franken.de [IPv6:2001:638:a02:a001:20e:cff:fe4a:feaa]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client CN "*.franken.de", Issuer "Certum Domain Validation CA SHA2" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4XPyLk195qz4fmB; Fri, 11 Oct 2024 07:05:18 +0000 (UTC) (envelope-from Michael.Tuexen@lurchi.franken.de) Authentication-Results: mx1.freebsd.org; none Received: from smtpclient.apple (unknown [IPv6:2a02:8109:1140:c3d:756d:f055:ebb2:9cd9]) (Authenticated sender: lurchi) by drew.franken.de (Postfix) with ESMTPSA id 47880722C1B63; Fri, 11 Oct 2024 09:05:09 +0200 (CEST) Content-Type: text/plain; charset=us-ascii List-Id: Networking and TCP/IP with FreeBSD List-Archive: https://lists.freebsd.org/archives/freebsd-net List-Help: List-Post: List-Subscribe: List-Unsubscribe: Sender: owner-freebsd-net@FreeBSD.org Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3731.700.6.1.2\)) Subject: Re: How does the TCP measurement period work? From: Michael Tuexen In-Reply-To: Date: Fri, 11 Oct 2024 09:05:08 +0200 Cc: FreeBSD Net Content-Transfer-Encoding: 7bit Message-Id: <7FB6F5E5-C6C0-4C18-B5FD-AEB18F0D5967@lurchi.franken.de> References: To: Alan Somers X-Mailer: Apple Mail (2.3731.700.6.1.2) X-Spam-Status: No, score=-2.9 required=5.0 tests=ALL_TRUSTED,BAYES_00 autolearn=disabled version=3.4.1 X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on mail-n.franken.de X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[]; ASN(0.00)[asn:680, ipnet:2001:638::/32, country:DE] X-Rspamd-Queue-Id: 4XPyLk195qz4fmB X-Spamd-Bar: ---- > On 11. Oct 2024, at 01:07, Alan Somers wrote: > > Can somebody please explain to me how the TCP measurement period > works? When does h_ertt decide to take a new measurement? > > Motivation: > I recently saw a long-distance connection that should've been capable > of 80+ MBps suddenly drop to < 1 MBps. Subsequent analysis of the > pcap file showed that while the typical RTT was 16.5 ms, there were a > few spikes as high as 380ms that coincided with the drop in > throughput. The surprising part was that even though RTT returned to > a good value, the throughput stayed low for the entire remaining > transfer, which lasted 750s. I would've expected throughput to > recover once RTT did. My theory is that h_ertt never made a new > measurement. However, I cannot reproduce the problem using dummynet > on a local VM. With dummynet, as soon as I return the RTT to normal, > the throughput quickly recovers, as one would expect. Which TCP stack and which CC module did you use? Which version of FreeBSD? Best regards Michael > > Grateful for any insights. > -Alan >