Re: [quicwg/base-drafts] Why min CWND of 2 instead of 1 (#3586)

Gorry Fairhurst <notifications@github.com> Wed, 22 April 2020 09:19 UTC

Return-Path: <noreply@github.com>
X-Original-To: quic-issues@ietfa.amsl.com
Delivered-To: quic-issues@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 62BC73A0C45 for <quic-issues@ietfa.amsl.com>; Wed, 22 Apr 2020 02:19:42 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.222
X-Spam-Level:
X-Spam-Status: No, score=-1.222 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, HTML_IMAGE_ONLY_24=1.618, HTML_MESSAGE=0.001, HTML_OBFUSCATE_05_10=0.26, MAILING_LIST_MULTI=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (1024-bit key) header.d=github.com
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id QOLPq4LjHPqy for <quic-issues@ietfa.amsl.com>; Wed, 22 Apr 2020 02:19:41 -0700 (PDT)
Received: from out-20.smtp.github.com (out-20.smtp.github.com [192.30.252.203]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id E5DC93A0C44 for <quic-issues@ietf.org>; Wed, 22 Apr 2020 02:19:40 -0700 (PDT)
Received: from github-lowworker-d93c4b6.va3-iad.github.net (github-lowworker-d93c4b6.va3-iad.github.net [10.48.17.47]) by smtp.github.com (Postfix) with ESMTP id 7E9C68C0448 for <quic-issues@ietf.org>; Wed, 22 Apr 2020 02:19:39 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=github.com; s=pf2014; t=1587547179; bh=i0sBUdtNLSSkdUHqdgdGiKAwVn5KF3eu9hsoQe5dh5s=; h=Date:From:Reply-To:To:Cc:In-Reply-To:References:Subject:List-ID: List-Archive:List-Post:List-Unsubscribe:From; b=ItEJ3eHDqk+aez/fF7cpLIDj+GRckcKiJh6mOZyBlrTBqnof35NCq5LpnYkSc1JmP yW6/TyeoLC1oSJ5yoh+a9TZUFKBsgL1r22WWLL5tZWhZlx3fBOjArBYdYQR32ple5+ Tp1lUZ0jRPQn6YBM5GGMDwVuaZKJCbJz8AowtuqY=
Date: Wed, 22 Apr 2020 02:19:39 -0700
From: Gorry Fairhurst <notifications@github.com>
Reply-To: quicwg/base-drafts <reply+AFTOJK6XNWVJXE4Y3MUIVB54VPWSXEVBNHHCH3TLYY@reply.github.com>
To: quicwg/base-drafts <base-drafts@noreply.github.com>
Cc: Subscribed <subscribed@noreply.github.com>
Message-ID: <quicwg/base-drafts/pull/3586/review/398001435@github.com>
In-Reply-To: <quicwg/base-drafts/pull/3586@github.com>
References: <quicwg/base-drafts/pull/3586@github.com>
Subject: Re: [quicwg/base-drafts] Why min CWND of 2 instead of 1 (#3586)
Mime-Version: 1.0
Content-Type: multipart/alternative; boundary="--==_mimepart_5ea00c2b47a7f_4a2d3fe2528cd95c6503ca"; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: list
X-GitHub-Sender: gorryfair
X-GitHub-Recipient: quic-issues
X-GitHub-Reason: subscribed
X-Auto-Response-Suppress: All
X-GitHub-Recipient-Address: quic-issues@ietf.org
Archived-At: <https://mailarchive.ietf.org/arch/msg/quic-issues/9VJ2RXfC4GFduM4lCiUBCJDYvKE>
X-BeenThere: quic-issues@ietf.org
X-Mailman-Version: 2.1.29
List-Id: Notification list for GitHub issues related to the QUIC WG <quic-issues.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/quic-issues>, <mailto:quic-issues-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/quic-issues/>
List-Post: <mailto:quic-issues@ietf.org>
List-Help: <mailto:quic-issues-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/quic-issues>, <mailto:quic-issues-request@ietf.org?subject=subscribe>
X-List-Received-Date: Wed, 22 Apr 2020 09:19:42 -0000

@gorryfair commented on this pull request.



> @@ -245,6 +245,13 @@ QUIC specifies a time-based definition to ensure one or more packets are sent
 prior to a dramatic decrease in congestion window; see
 {{persistent-congestion}}.
 
+### Increase the min congestion window to 2 packets
+
+QUIC recommends a minimum congestion window of 2 packets instead of TCP's 1.
+2 packets avoid waiting for a delayed acknowledgement and allow the PTO to
+send 2 packets instead of 1, which can be particularly important during the

I see this as a more fundamental deviation to TCP than other changes, because although it will have some performance benefit in common cases, it also changes the flow aggressiveness in cases of extreme overload. As I udnerstand, the basis of TCP's back-off is that it scales back cwnd to 1 MSS, and then should scale back in the interval between sending 1 MSS (fractional MSS). If QUIC has a minimum of 2*MSS in QUIC, does it immediately change to 2*RTT interval, otherwise this appears to be significantly more aggressive flow under congestion collapse?

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/quicwg/base-drafts/pull/3586#discussion_r412816589