HTTP

范围请求

An HTTP range request asks the server to send only a portion of an HTTP message back to a client.

Range requests are useful for download managers that let the user pause and resume the download.

判断服务器是否支持：If an HTTP response includes the Accept-Ranges header and its value is anything other than none, then the server supports range requests.

没有 Accept-Ranges 字段或者 Accept-Ranges 字段值为 none，均不支持。A download manager might disable its pause button in that case.

You can perform a manual check by issuing a HEAD request with a tool like cURL.

curl -I http://i.imgur.com/z4d4kWk.jpg

curl -I https://www.youtube.com/watch?v=EwTZ2xpQwpA

If the server supports range requests, then by including the Range header in your HTTP request, you can specify which part or parts of the document you want the server to return.

curl https://i.imgur.com/z4d4kWk.jpg -i -H "Range: bytes=0-1023"

HTTP/2 206
last-modified: Thu, 02 Feb 2017 11:15:53 GMT
etag: "18c50e1bbe972bdf9c7d9b8f6f019959"
content-type: image/jpeg
accept-ranges: bytes
content-range: bytes 0-1023/146515
content-length: 1024

The server responses with the 206 Partial Content status.

The Content-Length header now indicates the size of the requested range (and not the full size of the image).

The Content-Range response header indicates where in the full resource this partial message belongs.

如果请求分片下载，但是原资源已经被更改了怎么办呢？When resuming to request more parts of a resource, you need to guarantee that the stored resource has not been modified since the last fragment has been received. The If-Range HTTP request header makes a range request conditional. This header can be used either with a Last-Modified validator, or with an ETag, but not with both.

The ETag (or entity tag) HTTP response header is an identifier for a specific version of a resource. Etags are similar to fingerprints.

实战发现，有些网站响应头带了 Accept-Ranges，却不支持断点续传，例如这个。因此浏览器的逻辑是，首次请求时，若响应头不带 Content-Length，判断为不支持；再次请求时，请求头带 Range，若响应头没有 Content-Range，则同样表示不支持断点续传。

HTTP

范围请求​

范围请求