本文描述了HTTP协议使用Etag对内容进行缓存以便减少不必要的网络传输以提高运行效率的机制。后面用英文写，但我的英文不太好，如有错误之处，希望大家帮我纠正，我的联系方式在后面。1234 copyright protection366ＰＥＮＡＮＡFap4PhJD2n 尼

This article describes the HTTP procotol mechanism of using Etag to cache resources and resume file download in order to reduce unnecessory network transfer so the performance can increase.1234 copyright protection366ＰＥＮＡＮＡ2uBqjcP8Sw 尼

370Please respect copyright.ＰＥＮＡＮＡq95tLXdCXf
Section A. ETag1234 copyright protection366ＰＥＮＡＮＡMnftpAGvSy 尼

Etag is an identity of a resource, for example, we can use "10000_20241125150500" as the identity of a file where 10000 is the file size and 20241125150500 is the last modifed time of the file. We tell client browsers the etag of a file, then the browser cache the file and its etag, when next time the file is requested again, the browser can first just send the etag of the file to server, and if the server find the etag remain unchanged, then the browser can use the cache, no need to get the file from server once again.1234 copyright protection366ＰＥＮＡＮＡhtW3yNXJPZ 尼

The server send the etag of a resource by header like this:370Please respect copyright.ＰＥＮＡＮＡe6bYyIyZxC
---------------http response with etag----------------370Please respect copyright.ＰＥＮＡＮＡPzsOTislxi
HTTP/1.1 200 OK370Please respect copyright.ＰＥＮＡＮＡLGUYNkijGE
Date: Mon, 25 Nov 2024 10:56:12 GMT370Please respect copyright.ＰＥＮＡＮＡ3QTzzrM40l
accept-ranges: bytes370Please respect copyright.ＰＥＮＡＮＡc0ePnSXTf7
ETag: "534231200_1732277116"370Please respect copyright.ＰＥＮＡＮＡKWGRBzuMGz
Content-Length: 534231200370Please respect copyright.ＰＥＮＡＮＡT4IYymBZGe
Content-Type: application/octet-stream370Please respect copyright.ＰＥＮＡＮＡg1Z09AVrzq
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection366ＰＥＮＡＮＡdc7fdCEzm0 尼

(http body for the resource)370Please respect copyright.ＰＥＮＡＮＡcftQpogiOu
-----------------------------------------------------1234 copyright protection366ＰＥＮＡＮＡ2aFP7aUYeh 尼

370Please respect copyright.ＰＥＮＡＮＡyBNQXxE4uz
Section B. if-none-match1234 copyright protection366ＰＥＮＡＮＡ8veW5mhYgu 尼

The browser send back the etag to ask server if it has been changed by header like this:370Please respect copyright.ＰＥＮＡＮＡx0qrmTUVu2
-----http request for a cached resource------370Please respect copyright.ＰＥＮＡＮＡbVVFjKFHDH
GET /zen.iso HTTP/1.1370Please respect copyright.ＰＥＮＡＮＡ6JUAyuMEiC
if-none-match: "534231200_1732277116"370Please respect copyright.ＰＥＮＡＮＡyziifjKDb6
---------------------------------------------1234 copyright protection366ＰＥＮＡＮＡtFPUOZDrjk 尼

If the etag is unchanged, the server response like this:1234 copyright protection366ＰＥＮＡＮＡUBTnHRWPOK 尼

1. for get and head requests:370Please respect copyright.ＰＥＮＡＮＡYPTOCU22zt
-----------http response for unchanged resource------370Please respect copyright.ＰＥＮＡＮＡP3XhiFjAff
HTTP/1.1 304 Not Modified1234 copyright protection366ＰＥＮＡＮＡmy0u4tnzW8 尼

(http body canbe empty, the client will used its cache)370Please respect copyright.ＰＥＮＡＮＡcGOAnR8IPN
----------------------------------------------------1234 copyright protection366ＰＥＮＡＮＡiPunHGGIdj 尼

2. for other methods requests:370Please respect copyright.ＰＥＮＡＮＡM2pZueZkn0
-----------http response for unchanged resource------370Please respect copyright.ＰＥＮＡＮＡWPecswzaJ9
HTTP/1.1 412 Precondition Failed 1234 copyright protection366ＰＥＮＡＮＡlBiVzXBquJ 尼

(http body canbe empty, the client will used its cache)370Please respect copyright.ＰＥＮＡＮＡbpN5x95paD
----------------------------------------------------1234 copyright protection366ＰＥＮＡＮＡUrlTUY0Rxn 尼

370Please respect copyright.ＰＥＮＡＮＡMToRkbBfVs
If the etag changes, the server response 200 OK with the new file content like this:370Please respect copyright.ＰＥＮＡＮＡiJiX5nAoji
---------------http response with etag----------------370Please respect copyright.ＰＥＮＡＮＡ5TQKULPxYr
HTTP/1.1 200 OK370Please respect copyright.ＰＥＮＡＮＡsxqal6xKCc
Date: Mon, 25 Nov 2024 10:56:12 GMT370Please respect copyright.ＰＥＮＡＮＡfDeaBk0LcC
accept-ranges: bytes370Please respect copyright.ＰＥＮＡＮＡQtHs628HIu
ETag: "534231200_1732277116"370Please respect copyright.ＰＥＮＡＮＡuJAq2RAhHR
Content-Length: 534231200370Please respect copyright.ＰＥＮＡＮＡJtUmc8kHpE
Content-Type: application/octet-stream370Please respect copyright.ＰＥＮＡＮＡNkA29BwJF1
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection366ＰＥＮＡＮＡZLdAqcn8Yk 尼

(http body for the resource)370Please respect copyright.ＰＥＮＡＮＡ1Q0W5xpKDr
-----------------------------------------------------1234 copyright protection366ＰＥＮＡＮＡ12w8xmfKlf 尼

370Please respect copyright.ＰＥＮＡＮＡkHLBZJIU9X
Section C. if-match1234 copyright protection366ＰＥＮＡＮＡ2WTAjOVT1d 尼

When resuming a download in the browser download manager, the browser sends a range request with the if-match header like this:370Please respect copyright.ＰＥＮＡＮＡCe74NSL1lI
------resume download request-------370Please respect copyright.ＰＥＮＡＮＡzJmuljPSJ8
GET /zen.iso HTTP/1.1370Please respect copyright.ＰＥＮＡＮＡ84bZR8FJGJ
Range: bytes=287718144-370Please respect copyright.ＰＥＮＡＮＡOcw4h0Cy6N
If-Match: "534231200_1732277116"370Please respect copyright.ＰＥＮＡＮＡqPNX1Uud9Y
-----------------------------------370Please respect copyright.ＰＥＮＡＮＡueQc66mhNe
It tells the server to check the etag in If-Match, if unchanged, resume to get the remain content; otherwise, return 412 Precondition Failed.1234 copyright protection366ＰＥＮＡＮＡoU6rO4llUs 尼

The server response for unchanged etag is like this:370Please respect copyright.ＰＥＮＡＮＡbpFkT9teOB
-------------range response-------------------370Please respect copyright.ＰＥＮＡＮＡhVHGPbZvfp
HTTP/1.1 206 Partial Content370Please respect copyright.ＰＥＮＡＮＡPvC3213DSV
accept-ranges: bytes370Please respect copyright.ＰＥＮＡＮＡhi3srKuXHw
content-range: bytes 287718144-534231199/534231200370Please respect copyright.ＰＥＮＡＮＡq7hhcyUeQA
ETag: "534231200_1732277116"370Please respect copyright.ＰＥＮＡＮＡE99qvEBC8X
Content-Length: 246513056370Please respect copyright.ＰＥＮＡＮＡ78MkJGBwj6
Content-Type: application/octet-stream370Please respect copyright.ＰＥＮＡＮＡQsoytbBzWY
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection366ＰＥＮＡＮＡpfdKjdNN1x 尼

(http body is the remain file content)370Please respect copyright.ＰＥＮＡＮＡFZnDTdTw9D
-------------------------------------------1234 copyright protection366ＰＥＮＡＮＡQGRRBujoeK 尼

If the etag changes, response 412 like this:370Please respect copyright.ＰＥＮＡＮＡav4cNmetcu
-----------http response for changed resource------370Please respect copyright.ＰＥＮＡＮＡLOyaKbZUO6
HTTP/1.1 412 Precondition Failed 1234 copyright protection366ＰＥＮＡＮＡnwcNtS33x3 尼

(http body canbe empty, the client will stop downloading the resumed file)370Please respect copyright.ＰＥＮＡＮＡsxxlgHIzHA
---------------------------------------------------1234 copyright protection366ＰＥＮＡＮＡli6RbWcjmF 尼

370Please respect copyright.ＰＥＮＡＮＡHF9vwNUTpB
Section D. if-range1234 copyright protection366ＰＥＮＡＮＡQmgrzQYJ0a 尼

If-Range is similar with If-Match, it's used by chrome when resuming a download in the download manager, whereas firefox use If-Match. They are the same except that when the etag changes, if-range response the resource content from beginning, so the resumed download will start downloading from beginning, while if-match just response the 412 error to stop the resumed download. I think chrome do a better job.1234 copyright protection366ＰＥＮＡＮＡnXtndMzLuc 尼

---resume download request by chrome---370Please respect copyright.ＰＥＮＡＮＡk8WdHGMDUE
GET /zen.iso HTTP/1.1370Please respect copyright.ＰＥＮＡＮＡAo1Wg6OIo7
Range: bytes=287718144-370Please respect copyright.ＰＥＮＡＮＡdyGs7AlJvy
If-Range: "534231200_1732277116"370Please respect copyright.ＰＥＮＡＮＡYoupYEjCCT
------------------------------------370Please respect copyright.ＰＥＮＡＮＡAAV1N6nDKp
It tells the server to check the etag in If-Range, if unchanged, resume to get the remain content; otherwise, return 200 OK and the file content from the beginning.1234 copyright protection366ＰＥＮＡＮＡb5Qlw6ZyeM 尼

The server response for unchanged etag is like this:370Please respect copyright.ＰＥＮＡＮＡO73xhccsiR
-------------range response-------------------370Please respect copyright.ＰＥＮＡＮＡS2xC0TWyrV
HTTP/1.1 206 Partial Content370Please respect copyright.ＰＥＮＡＮＡxYkYjfH1gF
accept-ranges: bytes370Please respect copyright.ＰＥＮＡＮＡ6AmNwcKzXy
content-range: bytes 287718144-534231199/534231200370Please respect copyright.ＰＥＮＡＮＡ2ZxxYIQvmX
ETag: "534231200_1732277116"370Please respect copyright.ＰＥＮＡＮＡburCh6ryaC
Content-Length: 246513056370Please respect copyright.ＰＥＮＡＮＡJVlDA8CMVs
Content-Type: application/octet-stream370Please respect copyright.ＰＥＮＡＮＡkw9OnCfijv
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection366ＰＥＮＡＮＡKsXk8EB15H 尼

(http body is the remain file content)370Please respect copyright.ＰＥＮＡＮＡa8jp44QiFK
-------------------------------------------1234 copyright protection366ＰＥＮＡＮＡSjgU766U0T 尼

If the etag changes, response 200 from beginning like this:370Please respect copyright.ＰＥＮＡＮＡWSfkeuw74C
-----------http response from the beginning------370Please respect copyright.ＰＥＮＡＮＡGNRKsC9A9W
HTTP/1.1 200 OK370Please respect copyright.ＰＥＮＡＮＡpNMMMgICQm
Date: Mon, 25 Nov 2024 10:56:12 GMT370Please respect copyright.ＰＥＮＡＮＡBXpl1gRRMo
accept-ranges: bytes370Please respect copyright.ＰＥＮＡＮＡmmE4CleSyI
ETag: "534231200_1732277116"370Please respect copyright.ＰＥＮＡＮＡQRcJkQqckC
Content-Length: 534231200370Please respect copyright.ＰＥＮＡＮＡuLHjZsYqP0
Content-Type: application/octet-stream370Please respect copyright.ＰＥＮＡＮＡ6NyKuV8JI9
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection366ＰＥＮＡＮＡf8KsC42Kze 尼

(http body for the resource)370Please respect copyright.ＰＥＮＡＮＡKdAYLqLmKC
---------------------------------------------------1234 copyright protection366ＰＥＮＡＮＡB3JjocfGe6 尼

370Please respect copyright.ＰＥＮＡＮＡf4KRfEbQW9
Section X. Thanks1234 copyright protection366ＰＥＮＡＮＡ5yygdbWBWv 尼

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match370Please respect copyright.ＰＥＮＡＮＡBQcnyQSRJL
https://blog.csdn.net/catoop/article/details/134174653370Please respect copyright.ＰＥＮＡＮＡppwNK88h2q
https://blog.csdn.net/phker/article/details/507226191234 copyright protection366ＰＥＮＡＮＡyMCWKR5TSf 尼

Section Y. Contacts Me1234 copyright protection366ＰＥＮＡＮＡlxNStVXXDk 尼

If you found any errors or have any suggestions for this article, please let me know, my wechat: si_jinmin, my email: [email protected]370Please respect copyright.ＰＥＮＡＮＡ3LPIiz4pfw
如果您发现本文有任何错误，或者对本文有好的建议，欢迎与我联系探讨，我的微信: si_jinmin, 我的email: [email protected]1234 copyright protection366ＰＥＮＡＮＡbyrqYKS7nS 尼

如果您對C/C++ Programming, Linux, HTTP Protocol, Website Development, Vue, Git, VsCode感興趣，邀請您加入「Linux/C/C++ Website Development」微信群，請加我的微信（si_jinmin）以便拉您进群。1234 copyright protection366ＰＥＮＡＮＡ9AGj7vZisa 尼

216.73.216.253