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 protection354ＰＥＮＡＮＡWemRoaG33L 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 protection354ＰＥＮＡＮＡk0kq3dIrLr 尼

358Please respect copyright.ＰＥＮＡＮＡ2I1ou3tcFz
Section A. ETag1234 copyright protection354ＰＥＮＡＮＡhDmxzdnS5f 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 protection354ＰＥＮＡＮＡd4uK7yFqSD 尼

The server send the etag of a resource by header like this:358Please respect copyright.ＰＥＮＡＮＡBduWiJj6Mb
---------------http response with etag----------------358Please respect copyright.ＰＥＮＡＮＡ4HsnTpEJjU
HTTP/1.1 200 OK358Please respect copyright.ＰＥＮＡＮＡG9WRPpVTwA
Date: Mon, 25 Nov 2024 10:56:12 GMT358Please respect copyright.ＰＥＮＡＮＡNjGAmuie8F
accept-ranges: bytes358Please respect copyright.ＰＥＮＡＮＡD9F4pUfmfG
ETag: "534231200_1732277116"358Please respect copyright.ＰＥＮＡＮＡSulr1oPWJZ
Content-Length: 534231200358Please respect copyright.ＰＥＮＡＮＡDG2hRTG0nm
Content-Type: application/octet-stream358Please respect copyright.ＰＥＮＡＮＡUbFu3hTR64
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection354ＰＥＮＡＮＡnGysV3Vduq 尼

(http body for the resource)358Please respect copyright.ＰＥＮＡＮＡ383AcTnGXn
-----------------------------------------------------1234 copyright protection354ＰＥＮＡＮＡKmLf4Oz6DG 尼

358Please respect copyright.ＰＥＮＡＮＡRrE2wOH0E8
Section B. if-none-match1234 copyright protection354ＰＥＮＡＮＡjnkaiu9All 尼

The browser send back the etag to ask server if it has been changed by header like this:358Please respect copyright.ＰＥＮＡＮＡXfBZSoHmgf
-----http request for a cached resource------358Please respect copyright.ＰＥＮＡＮＡJxhwGjHC2t
GET /zen.iso HTTP/1.1358Please respect copyright.ＰＥＮＡＮＡQ3pys7Iuwz
if-none-match: "534231200_1732277116"358Please respect copyright.ＰＥＮＡＮＡ6PXyEvhnKt
---------------------------------------------1234 copyright protection354ＰＥＮＡＮＡn1uG8ri6qZ 尼

If the etag is unchanged, the server response like this:1234 copyright protection354ＰＥＮＡＮＡancgEidZ1k 尼

1. for get and head requests:358Please respect copyright.ＰＥＮＡＮＡVwD1r4DyEh
-----------http response for unchanged resource------358Please respect copyright.ＰＥＮＡＮＡVkkF9kHfs2
HTTP/1.1 304 Not Modified1234 copyright protection354ＰＥＮＡＮＡGrJuhl7mQ1 尼

(http body canbe empty, the client will used its cache)358Please respect copyright.ＰＥＮＡＮＡgajP4WkhMW
----------------------------------------------------1234 copyright protection354ＰＥＮＡＮＡr6COvOyhi9 尼

2. for other methods requests:358Please respect copyright.ＰＥＮＡＮＡ8CbmGcOvmf
-----------http response for unchanged resource------358Please respect copyright.ＰＥＮＡＮＡBTR9pHwT41
HTTP/1.1 412 Precondition Failed 1234 copyright protection354ＰＥＮＡＮＡXEKNjTv7pF 尼

(http body canbe empty, the client will used its cache)358Please respect copyright.ＰＥＮＡＮＡLPphpeSE32
----------------------------------------------------1234 copyright protection354ＰＥＮＡＮＡt8K0ozLWdK 尼

358Please respect copyright.ＰＥＮＡＮＡjXesm2oEEC
If the etag changes, the server response 200 OK with the new file content like this:358Please respect copyright.ＰＥＮＡＮＡcLcF73LNRM
---------------http response with etag----------------358Please respect copyright.ＰＥＮＡＮＡKaybpeXLZP
HTTP/1.1 200 OK358Please respect copyright.ＰＥＮＡＮＡe7fc9z7CnM
Date: Mon, 25 Nov 2024 10:56:12 GMT358Please respect copyright.ＰＥＮＡＮＡ9pMXKGlrXY
accept-ranges: bytes358Please respect copyright.ＰＥＮＡＮＡuUeJkJYgqZ
ETag: "534231200_1732277116"358Please respect copyright.ＰＥＮＡＮＡXXcsUuu6r5
Content-Length: 534231200358Please respect copyright.ＰＥＮＡＮＡHCpLulBgTz
Content-Type: application/octet-stream358Please respect copyright.ＰＥＮＡＮＡbmEavlhI0S
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection354ＰＥＮＡＮＡE9OdeWD91z 尼

(http body for the resource)358Please respect copyright.ＰＥＮＡＮＡyVkgtmY4tU
-----------------------------------------------------1234 copyright protection354ＰＥＮＡＮＡ4WDm9omrar 尼

358Please respect copyright.ＰＥＮＡＮＡvpgVNB10zF
Section C. if-match1234 copyright protection354ＰＥＮＡＮＡOwlLSfVozL 尼

When resuming a download in the browser download manager, the browser sends a range request with the if-match header like this:358Please respect copyright.ＰＥＮＡＮＡUY7x30JPkC
------resume download request-------358Please respect copyright.ＰＥＮＡＮＡekweNcIEhW
GET /zen.iso HTTP/1.1358Please respect copyright.ＰＥＮＡＮＡTDTw77kMDF
Range: bytes=287718144-358Please respect copyright.ＰＥＮＡＮＡHfkcKwEClA
If-Match: "534231200_1732277116"358Please respect copyright.ＰＥＮＡＮＡV4z2fyZIpN
-----------------------------------358Please respect copyright.ＰＥＮＡＮＡXnTOJGvUdm
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 protection354ＰＥＮＡＮＡDZNtMTiDms 尼

The server response for unchanged etag is like this:358Please respect copyright.ＰＥＮＡＮＡHgzrbc3opU
-------------range response-------------------358Please respect copyright.ＰＥＮＡＮＡgIHanZjL6H
HTTP/1.1 206 Partial Content358Please respect copyright.ＰＥＮＡＮＡcY5wwXNk62
accept-ranges: bytes358Please respect copyright.ＰＥＮＡＮＡ8Iy3n23mNO
content-range: bytes 287718144-534231199/534231200358Please respect copyright.ＰＥＮＡＮＡwlRQnxPCGm
ETag: "534231200_1732277116"358Please respect copyright.ＰＥＮＡＮＡ2zW8l087kG
Content-Length: 246513056358Please respect copyright.ＰＥＮＡＮＡ3THvuP2KVn
Content-Type: application/octet-stream358Please respect copyright.ＰＥＮＡＮＡCGq9UyCZMc
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection354ＰＥＮＡＮＡMZM2qWXolc 尼

(http body is the remain file content)358Please respect copyright.ＰＥＮＡＮＡ8HPpChla9T
-------------------------------------------1234 copyright protection354ＰＥＮＡＮＡlZGr2WsrzK 尼

If the etag changes, response 412 like this:358Please respect copyright.ＰＥＮＡＮＡOn2RbW30lR
-----------http response for changed resource------358Please respect copyright.ＰＥＮＡＮＡxxVnWIWB6l
HTTP/1.1 412 Precondition Failed 1234 copyright protection354ＰＥＮＡＮＡFqHVNQAayo 尼

(http body canbe empty, the client will stop downloading the resumed file)358Please respect copyright.ＰＥＮＡＮＡorCl50eQkZ
---------------------------------------------------1234 copyright protection354ＰＥＮＡＮＡ1IUctpFRw0 尼

358Please respect copyright.ＰＥＮＡＮＡThpox5eUPf
Section D. if-range1234 copyright protection354ＰＥＮＡＮＡDFlOkQ4mt9 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 protection354ＰＥＮＡＮＡNpyWte4Iu6 尼

---resume download request by chrome---358Please respect copyright.ＰＥＮＡＮＡr4CxNzBOOn
GET /zen.iso HTTP/1.1358Please respect copyright.ＰＥＮＡＮＡ5DiVralSmr
Range: bytes=287718144-358Please respect copyright.ＰＥＮＡＮＡLiA4cWPGnn
If-Range: "534231200_1732277116"358Please respect copyright.ＰＥＮＡＮＡeVw77Fw92h
------------------------------------358Please respect copyright.ＰＥＮＡＮＡqrFJS1iWC8
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 protection354ＰＥＮＡＮＡtLoiOlkcqy 尼

The server response for unchanged etag is like this:358Please respect copyright.ＰＥＮＡＮＡHlDgvkPDB6
-------------range response-------------------358Please respect copyright.ＰＥＮＡＮＡdDQqdQgyMT
HTTP/1.1 206 Partial Content358Please respect copyright.ＰＥＮＡＮＡleqQ1Es3Ib
accept-ranges: bytes358Please respect copyright.ＰＥＮＡＮＡb8dVwfkG76
content-range: bytes 287718144-534231199/534231200358Please respect copyright.ＰＥＮＡＮＡPC0zfdSq50
ETag: "534231200_1732277116"358Please respect copyright.ＰＥＮＡＮＡMcW4crrQUB
Content-Length: 246513056358Please respect copyright.ＰＥＮＡＮＡ9zoK6iqfvs
Content-Type: application/octet-stream358Please respect copyright.ＰＥＮＡＮＡqJDD9XLpCD
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection354ＰＥＮＡＮＡhHLwKecbFa 尼

(http body is the remain file content)358Please respect copyright.ＰＥＮＡＮＡFKVi0h3RyO
-------------------------------------------1234 copyright protection354ＰＥＮＡＮＡXm437oD24S 尼

If the etag changes, response 200 from beginning like this:358Please respect copyright.ＰＥＮＡＮＡUtZzUYGenK
-----------http response from the beginning------358Please respect copyright.ＰＥＮＡＮＡfGVejh3SEt
HTTP/1.1 200 OK358Please respect copyright.ＰＥＮＡＮＡJE6JQTgslV
Date: Mon, 25 Nov 2024 10:56:12 GMT358Please respect copyright.ＰＥＮＡＮＡUYsXyByIP1
accept-ranges: bytes358Please respect copyright.ＰＥＮＡＮＡW3mpkIgtHj
ETag: "534231200_1732277116"358Please respect copyright.ＰＥＮＡＮＡi7b3iHqjue
Content-Length: 534231200358Please respect copyright.ＰＥＮＡＮＡMOhoEVCelT
Content-Type: application/octet-stream358Please respect copyright.ＰＥＮＡＮＡzkMXiZ3yD2
Content-Disposition: attachment; filename="zen.iso"1234 copyright protection354ＰＥＮＡＮＡT2H1Ny0nOv 尼

(http body for the resource)358Please respect copyright.ＰＥＮＡＮＡrMgWASfDU3
---------------------------------------------------1234 copyright protection354ＰＥＮＡＮＡXy5PoxxcHx 尼

358Please respect copyright.ＰＥＮＡＮＡ8G9zKVFPZn
Section X. Thanks1234 copyright protection354ＰＥＮＡＮＡDWphGpukAf 尼

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match358Please respect copyright.ＰＥＮＡＮＡ5XGdp4CeSt
https://blog.csdn.net/catoop/article/details/134174653358Please respect copyright.ＰＥＮＡＮＡpEk0hF1rOP
https://blog.csdn.net/phker/article/details/507226191234 copyright protection354ＰＥＮＡＮＡgno6D0c8RC 尼

Section Y. Contacts Me1234 copyright protection354ＰＥＮＡＮＡu2X8PKky4p 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]358Please respect copyright.ＰＥＮＡＮＡxlMOQadFfJ
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 protection354ＰＥＮＡＮＡ9Sgxt6XXCW 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 protection354ＰＥＮＡＮＡ4br9HBZTvc 尼

216.73.216.69