HTTP range requests
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 clients like media players that support random access, data tools that know they need only part of a large file, and download managers that let the user pause and resume the download.
If an HTTP response includes the
Accept-Ranges header and its value is anything other than "
none", then the server supports range requests. You can perform a manual check by issuing a
HEAD request with a tool like cURL.
curl -I http://i.imgur.com/z4d4kWk.jpg HTTP/1.1 200 OK ... Accept-Ranges: bytes Content-Length: 146515
In this response,
Accept-Ranges: bytes indicates that bytes can be used as units to define a range. Here the
Content-Length header is also useful as it indicates the full size of the image to retrieve.
If sites omit the
Accept-Ranges header, they likely don't support partial requests. Some sites include the header but give it the explicit value "
none" to indicate they lack support:
curl -I https://www.youtube.com/watch?v=EwTZ2xpQwpA HTTP/1.1 200 OK ... Accept-Ranges: none
A download manager might disable its pause button in that case.
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.
We can request a single range from a resource. Again, we can test a request by using cURL. The "
-H" option will append a header line to the request, which in this case is the
Range header requesting the first 1024 bytes.
curl http://i.imgur.com/z4d4kWk.jpg -i -H "Range: bytes=0-1023"
The issued request looks like this:
GET /z4d4kWk.jpg HTTP/1.1 Host: i.imgur.com Range: bytes=0-1023
The server responses with the
Partial Content status:
HTTP/1.1 206 Partial Content Content-Range: bytes 0-1023/146515 Content-Length: 1024 ... (binary content)
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.
Range header also allows you to get multiple ranges at once in a multipart document. The ranges are separated by a comma.
curl http://www.example.com -i -H "Range: bytes=0-50, 100-150"
The server responses with the
Partial Content status and a
: multipart/byteranges; boundary=3d6b6a416f9b5 header, indicating that a multipart byterange follows. Each part contains its own
Content-Range fields and the required boundary parameter specifies the boundary string used to separate each body-part.
HTTP/1.1 206 Partial Content Content-Type: multipart/byteranges; boundary=3d6b6a416f9b5 Content-Length: 282 --3d6b6a416f9b5 Content-Type: text/html Content-Range: bytes 0-50/1270 <!doctype html> <html> <head> <title>Example Do --3d6b6a416f9b5 Content-Type: text/html Content-Range: bytes 100-150/1270 eta http-equiv="Content-type" content="text/html; c --3d6b6a416f9b5--
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.
If-Range HTTP request header makes a range request conditional: if the condition is fulfilled, the range request will be issued and the server sends back a
Partial Content answer with the appropriate body. If the condition is not fulfilled, the full resource is sent back, with a
OK status. This header can be used either with a
Last-Modified validator, or with an
ETag, but not with both.
If-Range: Wed, 21 Oct 2015 07:28:00 GMT
There are three relevant statuses, when working with range requests:
- A successful range request elicits a
Partial Contentstatus from the server.
- A range request that is out of bounds will result in a
Requested Range Not Satisfiablestatus, meaning that none of the range values overlap the extent of the resource. For example, the first-byte-pos of every range might be greater than the resource length.
- If range requests are not supported, an
OKstatus is sent back and the entire response body is transmitted.
Transfer-Encoding header allows chunked encoding, which is useful when larger amounts of data are sent to the client and the total size of the response is not known until the request has been fully processed. The server sends data to the client straight away without buffering the response or determining the exact length, which leads to improved latency. Range requests and chunking are compatible and can be used with or without each other.