From nobody Fri Oct 11 16:14:11 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 4XQBX832rcz5Z5Hp for ; Fri, 11 Oct 2024 16:14:16 +0000 (UTC) (envelope-from michael.tuexen@lurchi.franken.de) Received: from drew.franken.de (mail-n.franken.de [193.175.24.27]) (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 4XQBX80Z2jz51H4; Fri, 11 Oct 2024 16:14:16 +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:854f:2eb9:db9d:26f8]) (Authenticated sender: lurchi) by mail-n.franken.de (Postfix) with ESMTPSA id 2D091721E283A; Fri, 11 Oct 2024 18:14:12 +0200 (CEST) Content-Type: text/plain; charset=utf-8 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 \(3818.100.11.1.3\)) Subject: Re: How does the TCP measurement period work? From: Michael Tuexen In-Reply-To: Date: Fri, 11 Oct 2024 18:14:11 +0200 Cc: FreeBSD Net Content-Transfer-Encoding: quoted-printable Message-Id: <5F7D477B-A491-4851-890B-993B493D26B8@lurchi.franken.de> References: <7FB6F5E5-C6C0-4C18-B5FD-AEB18F0D5967@lurchi.franken.de> To: Alan Somers X-Mailer: Apple Mail (2.3818.100.11.1.3) 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:193.174.0.0/15, country:DE] X-Rspamd-Queue-Id: 4XQBX80Z2jz51H4 X-Spamd-Bar: ---- > On 11. Oct 2024, at 14:55, Alan Somers wrote: >=20 > On Fri, Oct 11, 2024 at 1:05=E2=80=AFAM Michael Tuexen > wrote: >>=20 >>> On 11. Oct 2024, at 01:07, Alan Somers wrote: >>>=20 >>> Can somebody please explain to me how the TCP measurement period >>> works? When does h_ertt decide to take a new measurement? >>>=20 >>> 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? >=20 > I was using the regular freebsd TCP stack with cc_chd. The production I'm not sure how well debugged chd is... > system was running FreeBSD 14.1, but my attempts to recreate the > situation using dummynet used 15.0-CURRENT. Please report, if you are able to reproduce it. In that case some BBLogs might be useful. Best regards Michael