From nobody Thu Oct 10 23:07:23 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 4XPllb4JKRz5YngD for ; Thu, 10 Oct 2024 23:07:39 +0000 (UTC) (envelope-from asomers@gmail.com) Received: from mail-ed1-f51.google.com (mail-ed1-f51.google.com [209.85.208.51]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256 client-signature RSA-PSS (2048 bits) client-digest SHA256) (Client CN "smtp.gmail.com", Issuer "WR4" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4XPllZ5MPPz4qyk for ; Thu, 10 Oct 2024 23:07:38 +0000 (UTC) (envelope-from asomers@gmail.com) Authentication-Results: mx1.freebsd.org; dkim=none; spf=pass (mx1.freebsd.org: domain of asomers@gmail.com designates 209.85.208.51 as permitted sender) smtp.mailfrom=asomers@gmail.com; dmarc=fail reason="SPF not aligned (relaxed), No valid DKIM" header.from=freebsd.org (policy=none) Received: by mail-ed1-f51.google.com with SMTP id 4fb4d7f45d1cf-5c876ed9c93so1769852a12.2 for ; Thu, 10 Oct 2024 16:07:38 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1728601655; x=1729206455; h=to:subject:message-id:date:from:mime-version:x-gm-message-state :from:to:cc:subject:date:message-id:reply-to; bh=vKXxsX839a92EvHh6PoZAlE40S09UmGXjn9FxrwOm8M=; b=MSXiR3S17c/choCmARoFP8BZEEOAEhbLVNaeC9eUNNTKmH/ipYstEgaL6FeSdKVkXA 4jbG+7hAcT62jj+sERmcPma2JCfCpt8jrTh78Eykc6jw/1cWk4ImhSbUucKsnWVi889T Oodd7qG8TvOiHMqOaCugIe+Y3188VhIErqLigfmkABbFy6h/fJa7TaMugzAKiaAYUdr7 U4HyxxUE5Ua8h8wqzbTqpSxwvgy9vBv5dnUDf5H2SYO5eZCanW4q6MV0oHTlRemmJHSj XZKbRJhWN/s7Uh595GOyp/6KhgG2qtLTlnxeE68BuuyBihCDBBz1x8kVeYE2ar1JbJYJ EWQA== X-Gm-Message-State: AOJu0YzNV/9AtKlkZHTlmMjBe9LDOCNAQdNarhsAqq8FgWQ2aHhRmBLg iGSDmo/j5JLrNIqSDLsG/ybwGRUdbwtvgGkj+f1vopKgiYdW/QIuAgBmTDM5ps2y1xrWoEVBI/0 c/wPEg7I1N9OWBqL9pgiLaYS4ObBn0A== X-Google-Smtp-Source: AGHT+IHxT8GX2AgHs36GgGuScWs/mEe9PzudBIAwvO16rwTfM6eXrhXUpfH1zWm4omzeOo1wxOnjvg4v/ONAWRGJ6/k= X-Received: by 2002:a05:6402:50d3:b0:5c8:8ddc:6233 with SMTP id 4fb4d7f45d1cf-5c948cc995cmr310038a12.16.1728601655433; Thu, 10 Oct 2024 16:07:35 -0700 (PDT) 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 From: Alan Somers Date: Thu, 10 Oct 2024 17:07:23 -0600 Message-ID: Subject: How does the TCP measurement period work? To: FreeBSD Net Content-Type: text/plain; charset="UTF-8" X-Spamd-Result: default: False [-1.76 / 15.00]; SUBJECT_ENDS_QUESTION(1.00)[]; NEURAL_HAM_SHORT(-0.97)[-0.971]; NEURAL_HAM_MEDIUM(-0.96)[-0.958]; NEURAL_HAM_LONG(-0.93)[-0.932]; FORGED_SENDER(0.30)[asomers@freebsd.org,asomers@gmail.com]; R_SPF_ALLOW(-0.20)[+ip4:209.85.128.0/17]; DMARC_POLICY_SOFTFAIL(0.10)[freebsd.org : SPF not aligned (relaxed), No valid DKIM,none]; MIME_GOOD(-0.10)[text/plain]; TO_MATCH_ENVRCPT_ALL(0.00)[]; TO_DN_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; ARC_NA(0.00)[]; RCVD_TLS_LAST(0.00)[]; MIME_TRACE(0.00)[0:+]; RCPT_COUNT_ONE(0.00)[1]; MISSING_XM_UA(0.00)[]; FREEFALL_USER(0.00)[asomers]; FREEMAIL_ENVFROM(0.00)[gmail.com]; MLMMJ_DEST(0.00)[freebsd-net@freebsd.org]; PREVIOUSLY_DELIVERED(0.00)[freebsd-net@freebsd.org]; FROM_NEQ_ENVFROM(0.00)[asomers@freebsd.org,asomers@gmail.com]; RCVD_COUNT_ONE(0.00)[1]; R_DKIM_NA(0.00)[]; RWL_MAILSPIKE_POSSIBLE(0.00)[209.85.208.51:from]; TO_DOM_EQ_FROM_DOM(0.00)[]; ASN(0.00)[asn:15169, ipnet:209.85.128.0/17, country:US]; RCVD_IN_DNSWL_NONE(0.00)[209.85.208.51:from] X-Rspamd-Queue-Id: 4XPllZ5MPPz4qyk X-Spamd-Bar: - 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. Grateful for any insights. -Alan