A question about RFC7232#2.2.2

Michael Lee <michael.lee@zerustech.com> Sat, 03 December 2016 23:47 UTC

Return-Path: <ietf-http-wg-request+bounce-httpbisa-archive-bis2juki=lists.ie@listhub.w3.org>
X-Original-To: ietfarch-httpbisa-archive-bis2Juki@ietfa.amsl.com
Delivered-To: ietfarch-httpbisa-archive-bis2Juki@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 53AC7129408 for <ietfarch-httpbisa-archive-bis2Juki@ietfa.amsl.com>; Sat, 3 Dec 2016 15:47:03 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -9.797
X-Spam-Level:
X-Spam-Status: No, score=-9.797 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, HEADER_FROM_DIFFERENT_DOMAINS=0.001, RCVD_IN_DNSWL_HI=-5, RP_MATCHES_RCVD=-2.896, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (2048-bit key) header.d=zerustech-com.20150623.gappssmtp.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 DqJAyHbWlwp8 for <ietfarch-httpbisa-archive-bis2Juki@ietfa.amsl.com>; Sat, 3 Dec 2016 15:47:01 -0800 (PST)
Received: from frink.w3.org (frink.w3.org [128.30.52.56]) (using TLSv1.2 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 192AD129441 for <httpbisa-archive-bis2Juki@lists.ietf.org>; Sat, 3 Dec 2016 15:46:59 -0800 (PST)
Received: from lists by frink.w3.org with local (Exim 4.80) (envelope-from <ietf-http-wg-request@listhub.w3.org>) id 1cDJxy-0008Rk-Nq for ietf-http-wg-dist@listhub.w3.org; Sat, 03 Dec 2016 23:43:38 +0000
Resent-Date: Sat, 03 Dec 2016 23:43:38 +0000
Resent-Message-Id: <E1cDJxy-0008Rk-Nq@frink.w3.org>
Received: from mimas.w3.org ([128.30.52.79]) by frink.w3.org with esmtps (TLS1.2:RSA_AES_128_CBC_SHA1:128) (Exim 4.80) (envelope-from <michael.lee@zerustech.com>) id 1cDJxm-0008Qi-BA for ietf-http-wg@listhub.w3.org; Sat, 03 Dec 2016 23:43:26 +0000
Received: from mail-pg0-f47.google.com ([74.125.83.47]) by mimas.w3.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.84_2) (envelope-from <michael.lee@zerustech.com>) id 1cDJxW-0002O2-Fm for ietf-http-wg@w3.org; Sat, 03 Dec 2016 23:43:16 +0000
Received: by mail-pg0-f47.google.com with SMTP id p66so121672140pga.2 for <ietf-http-wg@w3.org>; Sat, 03 Dec 2016 15:42:44 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zerustech-com.20150623.gappssmtp.com; s=20150623; h=to:from:subject:message-id:date:user-agent:mime-version :content-transfer-encoding; bh=6QqASv5oJoJb7giqu2GUy/la4alIBa9e8wfxd6FGqnI=; b=ZgopsszPAnhEO7EP0zp9lo274JsYOMd/ieMspqerVTNonHAyQf7C5HCtY5mEEiKOQa s6wMzLIpWJw2bxR4Zujhn6pueJRL+HihcTx1kYyzehtIcQX/vdMwmtqpye4JCTYWgqad Ofg5l1K8+twUP2SG1wwh7h2bFSEI/afznI2Fj3cXmV6jgVe3dwi7dJXEfCX5dqe+q57A Hz8jwRFByUlFZFYpLMFFRazZHBwDupUODNPlPf9ylFi2e3DP4TnU6WfTqHGHRlUsCbFu 1UOXEBXDCvX1iY8tZRTHCQHmXCboEhjJBUqX9LG4zMhWUloiKNX8vzopOPVF05agqLVC pnjg==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:to:from:subject:message-id:date:user-agent :mime-version:content-transfer-encoding; bh=6QqASv5oJoJb7giqu2GUy/la4alIBa9e8wfxd6FGqnI=; b=fjnog1y+kt1bZTOKWs4LiYhYvIY+D6wwhBrGzW/ELU3Gm25x9QVHW728mZPXJuYglM Tp09nzdekqiojhgG0g5ZWcXFoc+52cK0zHsu4s7P8JxPHchU1tHqb77lE/JAbpNi8ATV 8ZpRtekANosvVxhyAk5fap4fk32HVa3mm9qHKXPzAPRqpdpPUjA87NrjhXsD2U0U+Zsu WieXml8m4u9HDcK+R8XpNM88SWy3HLoma4lcC8AVPgG21IMfnHZrsvNfv1wr6oQPxdZ6 d1MiB2UUeb+l49V1yXUBKi7cGq7n491nJvj0xr0u5mzvxFWubX0SMF6gv6p6M5RRSqYY c7Ow==
X-Gm-Message-State: AKaTC03tYqhz1E54JiCX3HpY7UjbqYgWvBQ7wG8KvhAfjRTv90pKwqzQa42nl9ND7HH3lg==
X-Received: by 10.84.210.167 with SMTP id a36mr112046701pli.125.1480808557449; Sat, 03 Dec 2016 15:42:37 -0800 (PST)
Received: from 10.0.0.10 (ec2-52-78-85-40.ap-northeast-2.compute.amazonaws.com. [52.78.85.40]) by smtp.gmail.com with ESMTPSA id h7sm17248480pgn.13.2016.12.03.15.42.35 for <ietf-http-wg@w3.org> (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 03 Dec 2016 15:42:36 -0800 (PST)
To: ietf-http-wg@w3.org
From: Michael Lee <michael.lee@zerustech.com>
Message-ID: <58435873.2000408@zerustech.com>
Date: Sun, 4 Dec 2016 07:42:43 +0800
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:38.0) Gecko/20100101 Thunderbird/38.7.2
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Received-SPF: none client-ip=74.125.83.47; envelope-from=michael.lee@zerustech.com; helo=mail-pg0-f47.google.com
X-W3C-Hub-Spam-Status: No, score=-2.8
X-W3C-Hub-Spam-Report: BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, FSL_HELO_BARE_IP_2=1.118, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, W3C_AA=-1, W3C_WL=-1
X-W3C-Scan-Sig: mimas.w3.org 1cDJxW-0002O2-Fm 769ed69ccc1d669bd198735dc9329741
X-Original-To: ietf-http-wg@w3.org
Subject: A question about RFC7232#2.2.2
Archived-At: <http://www.w3.org/mid/58435873.2000408@zerustech.com>
Resent-From: ietf-http-wg@w3.org
X-Mailing-List: <ietf-http-wg@w3.org> archive/latest/33102
X-Loop: ietf-http-wg@w3.org
Resent-Sender: ietf-http-wg-request@w3.org
Precedence: list
List-Id: <ietf-http-wg.w3.org>
List-Help: <http://www.w3.org/Mail/>
List-Post: <mailto:ietf-http-wg@w3.org>
List-Unsubscribe: <mailto:ietf-http-wg-request@w3.org?subject=unsubscribe>

I am currently working on a tutorial of HTTP/1.1 caching, but I got 
stuck with the following statement in RFC7232 section 2.2.2:

" This method relies on the fact that if two different responses were 
sent by the origin server during the same second, but both had the same 
Last-Modified time, then at least one of those responses would have a 
Date <http://httpwg.org/specs/rfc7231.html#header.date> value equal to 
its Last-Modified time. The arbitrary 60-second limit guards against the 
possibility that the Date and Last-Modified values are generated from 
different clocks or at somewhat different times during the preparation 
of the response. An implementation /MAY/ use a value larger than 60 
seconds, if it is believed that 60 seconds is too short."

I don't understand why under the circumstance above, at least one of 
those responses would have a Date value equal to its Last-Modified time.

And what's the point of ensuring a 60 seconds gap between the 
Last-Modified time and Date?

-- 
Michael Lee / Managing Director / ZerusTech Ltd

Tel: +86 (21) 6107 3305

Mobile: +86 186 021 03818

Skype: zerustech

Email: michael.lee@zerustech.com

www.zerustech.com

Suite 9208
Building No. 9, 4361 HuTai Road
Shanghai
P.R.China
201906