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 protection367ＰＥＮＡＮＡAt8Ur0AwF7 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 protection367ＰＥＮＡＮＡ6CgVIt85hn 尼

371Please respect copyright.ＰＥＮＡＮＡWhDMBPHoGL
Section A. ETag1234 copyright protection367ＰＥＮＡＮＡKiY64yo2bt 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 protection367ＰＥＮＡＮＡWsuqFp90SP 尼

The server send the etag of a resource by header like this:371Please respect copyright.ＰＥＮＡＮＡQSIykI1JWQ
---------------http response with etag----------------371Please respect copyright.ＰＥＮＡＮＡGRfQCNIdFp
HTTP/1.1 200 OK371Please respect copyright.ＰＥＮＡＮＡSOwEfmjmLH
Date: Mon, 25 Nov 2024 10:56:12 GMT371Please respect copyright.ＰＥＮＡＮＡ8Eaxhsb4nT
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡzEjTIyFMRA
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡQ5e5K7TVK7
Content-Length: 534231200371Please respect copyright.ＰＥＮＡＮＡZywabOCPFp
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡocnNZNTlIH
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡIbdigRzER2 尼

(http body for the resource)371Please respect copyright.ＰＥＮＡＮＡVlfDAk52Qy
-----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡBMTDekPEQt 尼

371Please respect copyright.ＰＥＮＡＮＡjMrDnqT0e8
Section B. if-none-match1234 copyright protection367ＰＥＮＡＮＡv1l7XKtt8d 尼

The browser send back the etag to ask server if it has been changed by header like this:371Please respect copyright.ＰＥＮＡＮＡF3kyXtKnD6
-----http request for a cached resource------371Please respect copyright.ＰＥＮＡＮＡnTv9iaC9Xv
GET /zen.iso HTTP/1.1371Please respect copyright.ＰＥＮＡＮＡfPU4kjHnbR
if-none-match: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡty85NLjrkC
---------------------------------------------1234 copyright protection367ＰＥＮＡＮＡPqmJhquA7Z 尼

If the etag is unchanged, the server response like this:1234 copyright protection367ＰＥＮＡＮＡluS4zCDCBv 尼

1. for get and head requests:371Please respect copyright.ＰＥＮＡＮＡIN7xzay86C
-----------http response for unchanged resource------371Please respect copyright.ＰＥＮＡＮＡUIopI0Fnxg
HTTP/1.1 304 Not Modified1234 copyright protection367ＰＥＮＡＮＡrlOIl3fyor 尼

(http body canbe empty, the client will used its cache)371Please respect copyright.ＰＥＮＡＮＡG2fdjzYjeT
----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡmxL1Z58xBA 尼

2. for other methods requests:371Please respect copyright.ＰＥＮＡＮＡm2ZpypEyOU
-----------http response for unchanged resource------371Please respect copyright.ＰＥＮＡＮＡWmiiiqrkzy
HTTP/1.1 412 Precondition Failed 1234 copyright protection367ＰＥＮＡＮＡUW0agnbYgG 尼

(http body canbe empty, the client will used its cache)371Please respect copyright.ＰＥＮＡＮＡg5ZxRhI97E
----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡoiXc1PixUF 尼

371Please respect copyright.ＰＥＮＡＮＡT2YQKfEUg3
If the etag changes, the server response 200 OK with the new file content like this:371Please respect copyright.ＰＥＮＡＮＡuY1JfIv2gz
---------------http response with etag----------------371Please respect copyright.ＰＥＮＡＮＡUx9oCjlskR
HTTP/1.1 200 OK371Please respect copyright.ＰＥＮＡＮＡJZcRYj0ijf
Date: Mon, 25 Nov 2024 10:56:12 GMT371Please respect copyright.ＰＥＮＡＮＡoi3ZS3BytA
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡ861gmR0NCR
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡFp4Jv5pAwW
Content-Length: 534231200371Please respect copyright.ＰＥＮＡＮＡtlLyM6nzeN
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡfFdBp39DgL
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡRPvC2RFtti 尼

(http body for the resource)371Please respect copyright.ＰＥＮＡＮＡ1MxAPZTuki
-----------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡp2miEMqfLY 尼

371Please respect copyright.ＰＥＮＡＮＡzmnQOpXDsz
Section C. if-match1234 copyright protection367ＰＥＮＡＮＡWA5DZacGd0 尼

When resuming a download in the browser download manager, the browser sends a range request with the if-match header like this:371Please respect copyright.ＰＥＮＡＮＡxG0georCli
------resume download request-------371Please respect copyright.ＰＥＮＡＮＡ0o5pPvGbHv
GET /zen.iso HTTP/1.1371Please respect copyright.ＰＥＮＡＮＡAOZkvW0ug3
Range: bytes=287718144-371Please respect copyright.ＰＥＮＡＮＡrWEKyouITr
If-Match: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡhAVcslPpdC
-----------------------------------371Please respect copyright.ＰＥＮＡＮＡAKLjgqN1jj
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 protection367ＰＥＮＡＮＡIP2hwo6uIc 尼

The server response for unchanged etag is like this:371Please respect copyright.ＰＥＮＡＮＡc4kRyWfd1A
-------------range response-------------------371Please respect copyright.ＰＥＮＡＮＡnhD57vXq5A
HTTP/1.1 206 Partial Content371Please respect copyright.ＰＥＮＡＮＡ8dTSgvPxR6
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡAMsncdaQvs
content-range: bytes 287718144-534231199/534231200371Please respect copyright.ＰＥＮＡＮＡPPblu5xBjG
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡdzypmkLE37
Content-Length: 246513056371Please respect copyright.ＰＥＮＡＮＡoivSiF4Lxd
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡz9q2PAQvo1
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡlNroaCMF7E 尼

(http body is the remain file content)371Please respect copyright.ＰＥＮＡＮＡ7T6YH0TuiZ
-------------------------------------------1234 copyright protection367ＰＥＮＡＮＡsKSwpkdgJE 尼

If the etag changes, response 412 like this:371Please respect copyright.ＰＥＮＡＮＡ8lkkEW5Xjn
-----------http response for changed resource------371Please respect copyright.ＰＥＮＡＮＡXXvRHn0HVO
HTTP/1.1 412 Precondition Failed 1234 copyright protection367ＰＥＮＡＮＡ6Zk5lkTkJ4 尼

(http body canbe empty, the client will stop downloading the resumed file)371Please respect copyright.ＰＥＮＡＮＡ4212s2nEai
---------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡ7NakekPdB2 尼

371Please respect copyright.ＰＥＮＡＮＡeqRpidNQF4
Section D. if-range1234 copyright protection367ＰＥＮＡＮＡWiRLVic6Dp 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 protection367ＰＥＮＡＮＡbZlCEWqnKf 尼

---resume download request by chrome---371Please respect copyright.ＰＥＮＡＮＡYuy7VKBIYo
GET /zen.iso HTTP/1.1371Please respect copyright.ＰＥＮＡＮＡhp3sBpL2qG
Range: bytes=287718144-371Please respect copyright.ＰＥＮＡＮＡitAGNAdM5h
If-Range: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡ4F35BGT2zF
------------------------------------371Please respect copyright.ＰＥＮＡＮＡq2w3qYfGym
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 protection367ＰＥＮＡＮＡqsNZ2vxWoR 尼

The server response for unchanged etag is like this:371Please respect copyright.ＰＥＮＡＮＡUIaOhLfRrF
-------------range response-------------------371Please respect copyright.ＰＥＮＡＮＡouZgnEya0w
HTTP/1.1 206 Partial Content371Please respect copyright.ＰＥＮＡＮＡTSLZUIbkTL
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡt3B7vhWn0D
content-range: bytes 287718144-534231199/534231200371Please respect copyright.ＰＥＮＡＮＡ7F8GFtNkUY
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡ7sd2qamm2O
Content-Length: 246513056371Please respect copyright.ＰＥＮＡＮＡVmh8CeFfdi
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡ847BRBBzi4
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡfDbrObHcm2 尼

(http body is the remain file content)371Please respect copyright.ＰＥＮＡＮＡjHiiSmDM1d
-------------------------------------------1234 copyright protection367ＰＥＮＡＮＡVp82LfAmiq 尼

If the etag changes, response 200 from beginning like this:371Please respect copyright.ＰＥＮＡＮＡDDAA9nmVNx
-----------http response from the beginning------371Please respect copyright.ＰＥＮＡＮＡSJkPDxgrOs
HTTP/1.1 200 OK371Please respect copyright.ＰＥＮＡＮＡMS4ACoFeFR
Date: Mon, 25 Nov 2024 10:56:12 GMT371Please respect copyright.ＰＥＮＡＮＡHHb8735tgy
accept-ranges: bytes371Please respect copyright.ＰＥＮＡＮＡl40TrhcY0S
ETag: "534231200_1732277116"371Please respect copyright.ＰＥＮＡＮＡCTTkOIgCTh
Content-Length: 534231200371Please respect copyright.ＰＥＮＡＮＡpLUTCWrAzr
Content-Type: application/octet-stream371Please respect copyright.ＰＥＮＡＮＡyUAhnt9HXm
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection367ＰＥＮＡＮＡXD1HFVmMYm 尼

(http body for the resource)371Please respect copyright.ＰＥＮＡＮＡJsyXHawLKC
---------------------------------------------------1234 copyright protection367ＰＥＮＡＮＡ9SJaomCd1Z 尼

371Please respect copyright.ＰＥＮＡＮＡQQDwQI2BsZ
Section X. Thanks1234 copyright protection367ＰＥＮＡＮＡn92sxqpBg4 尼

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match371Please respect copyright.ＰＥＮＡＮＡgIBqox3G3T
https://blog.csdn.net/catoop/article/details/134174653371Please respect copyright.ＰＥＮＡＮＡYM2qfAIlHq
https://blog.csdn.net/phker/article/details/507226191234 copyright protection367ＰＥＮＡＮＡYWw9tscoy1 尼

Section Y. Contacts Me1234 copyright protection367ＰＥＮＡＮＡqwQy2LpcL1 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]371Please respect copyright.ＰＥＮＡＮＡJnm2kUlBLT
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 protection367ＰＥＮＡＮＡBO66AeLEOA 尼

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