Re: Priority implementation complexity (was: Re: Extensible Priorities and Reprioritization)

Yoav Weiss <> Mon, 15 June 2020 08:32 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 9EF003A0AD5 for <>; Mon, 15 Jun 2020 01:32:29 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -2.648
X-Spam-Status: No, score=-2.648 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, HEADER_FROM_DIFFERENT_DOMAINS=0.249, HTML_MESSAGE=0.001, MAILING_LIST_MULTI=-1, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001] autolearn=ham autolearn_force=no
Authentication-Results: (amavisd-new); dkim=pass (2048-bit key)
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id ODWTjtmc_Aet for <>; Mon, 15 Jun 2020 01:32:27 -0700 (PDT)
Received: from ( []) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by (Postfix) with ESMTPS id A4B0F3A0AD4 for <>; Mon, 15 Jun 2020 01:32:27 -0700 (PDT)
Received: from lists by with local (Exim 4.92) (envelope-from <>) id 1jkkUQ-0003E7-Hw for; Mon, 15 Jun 2020 08:29:10 +0000
Resent-Date: Mon, 15 Jun 2020 08:29:10 +0000
Resent-Message-Id: <>
Received: from ([]) by with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from <>) id 1jkkUN-0003DL-Ct for; Mon, 15 Jun 2020 08:29:07 +0000
Received: from ([2a00:1450:4864:20::22f]) by with esmtps (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from <>) id 1jkkUL-0001Zt-9A for; Mon, 15 Jun 2020 08:29:07 +0000
Received: by with SMTP id i27so18062879ljb.12 for <>; Mon, 15 Jun 2020 01:29:04 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=wFcTUBBGue+KoMqqTRzLpiQjH2C6xOM5J6vpkpxgYrU=; b=s6bQTKv9U4N+3ZhZUx8IkP4OHbjFLthEDzeWaDKn7vjsArug9SLYgT5yv5uOCHGOfI mLg+Xb8mGnxB+FxuIhhuurSmBuDBmx2z3V40LiFsX71e1dtQ2Ln5lwTYMeIuGR+XlZy0 lYcKtCB6PN9KdY9Kch/omTjDAuROX6yi1zrO4yR1VmAtju/iDPhuGkNOox5do4KIWEGW drq0/1hy7HxSm+jHDGfyE2M8DB62QCnA8cuADQa1ztl7kesVo28IDlAAyM6fDusk1MG3 T89ptpiUw9EyhN0Q7mYxwbdHSS93pIv304Smgdtsskdb8gFiOappUPkqy+tPBbqfZpi0 4Jbw==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=wFcTUBBGue+KoMqqTRzLpiQjH2C6xOM5J6vpkpxgYrU=; b=WV9v8wZfMMc12FNIUfiUQJdUHdkELotRS0QvyGmpChwCC5adGqIeATv6qioOCKmBPl S82c+Q0/BJOJ5REIRHGBnauZjXq0ffVO4sdauIe1lsAma8LNkQJ2OK2o9OkhEvw3JWzz KlLLjLxWcEz5g02na4h+fX0dzLrKGmoYY8tumrrmw59oNO2UFOh+88MoHem028FIQywP ALoDprBhpQ6q/rjRBKK6EXQo5+zVNtZA1kvF68DTtcQA0hHkmjS2JMD5w4e1ObwkFySs AXAFkQtrB8LDiVkA5TcWqdESFuYJuGF0Vwu0ntKaRagw8xTAo1WiPsuTDw70jgaQKP07 +DCw==
X-Gm-Message-State: AOAM531Pd2BUb2OW9lVKF5CaVi7NxseaDM7TZgjbxA2ERJk3YojHzoba UHxrXv7eKaun9uywpJXjpIrdgi2qxRHTJ1GLOqMlRQ==
X-Google-Smtp-Source: ABdhPJwGu72rZ3qRwd2jFf8wa+xeDsHwIWMxG4fUKFwdbyulesQ6eCGSjHrQ9WmRvgBNG+sXXEpBb6ZgkIGDhJPXS3g=
X-Received: by 2002:a2e:b0f9:: with SMTP id h25mr12583343ljl.18.1592209733076; Mon, 15 Jun 2020 01:28:53 -0700 (PDT)
MIME-Version: 1.0
References: <> <> <> <> <> <20200609144428.GC22180@lubuntu> <> <> <> <> <>
In-Reply-To: <>
From: Yoav Weiss <>
Date: Mon, 15 Jun 2020 10:28:37 +0200
Message-ID: <>
To: Stefan Eissing <>
Cc: Kazuho Oku <>, Patrick Meenan <>, Lucas Pardue <>, HTTP Working Group <>, =?UTF-8?Q?Bence_B=C3=A9ky?= <>
Content-Type: multipart/alternative; boundary="0000000000008c69bc05a81b374a"
Received-SPF: pass client-ip=2a00:1450:4864:20::22f;;
X-W3C-Hub-Spam-Status: No, score=-8.9
X-W3C-Hub-Spam-Report: BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, W3C_AA=-1, W3C_DB=-1, W3C_IRA=-1, W3C_IRR=-3, W3C_WL=-1
X-W3C-Scan-Sig: 1jkkUL-0001Zt-9A 83986885299e559df5da91239a455141
Subject: Re: Priority implementation complexity (was: Re: Extensible Priorities and Reprioritization)
Archived-At: <>
X-Mailing-List: <> archive/latest/37761
Precedence: list
List-Id: <>
List-Help: <>
List-Post: <>
List-Unsubscribe: <>

On Mon, Jun 15, 2020 at 9:55 AM Stefan Eissing <>

> > Am 11.06.2020 um 10:41 schrieb Kazuho Oku <>om>:
> >
> > That depends on how much clients would rely on reprioritization. Unlike
> H2 priorities, Extensible Priority does not have inter-stream dependencies.
> Therefore, losing *some* prioritization signals is less of an issue
> compared to H2 priorities.
> >
> > Assuming that reprioritization is used mostly for refining the initial
> priorities of a fraction of all the requests, I think there'd be benefit in
> defining reprioritization as an optional feature. Though I can see some
> might argue for not having reprioritization even as an optional feature
> unless there is proof that it would be useful.
> > We should decide if reprioritization is good or bad, based on as much
> data as we can pull, and make sure it's implemented only if we see benefits
> for it in some cases, and then make sure it's only used in those cases.
> When thinking about priority implementations, I recommend thinking about a
> H3 reverse proxy in front of a legacy H1 server. Assume limited memory,
> disk space and backend connections.
> (Re-)prioritization in H2 works well for flow control, among the streams
> that have response data to send. Priorities can play a part in server
> scheduling, but
> it's more tricky. By "scheduling" I mean that the server has to pick one
> among the opened streams for which it wants to compute a response for. This
> is often impossible to re-prioritize afterwards (e.g. suicidal for a server
> implementation).

Can you expand on why it is "suicidal"?

> If we would do H2 a second time, my idea would be to signal priorities in
> the HTTP request in a connection header and use this in the H2 frame layer
> to allocate DATA space on the downlink. Leave out changing priorities on a
> request already started. Let the client use its window sizes if it feels
> the need.
> Cheers, Stefan (lurking)