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Range requests

HTTP range requests allow to send only a portion of an HTTP message from a server to a client. Partial requests are useful for large media or downloading files with pause and resume functions, for example.

Checking if a server supports partial requests

If the Accept-Ranges is present in HTTP responses (and its value isn't "none"), the server supports range requests. You can check this by issuing a HEAD request and cURL, for example.

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 unit 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 for partial requests. Some sites also explicitly send "none" as a value, indicating no support. In some apps, download managers disable their pause buttons in that case.

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

HTTP/1.1 200 OK
...
Accept-Ranges: none

Requesting a specific range from a server

If the server supports range requests, you can issue such a request by using the Range header. It indicates the part(s) of a document that the server should return.

Single part ranges

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 206 Partial Content status:

HTTP/1.1 206 Partial Content
Content-Range: bytes 0-1023/146515
Content-Length: 1024
...
(binary content)

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.

Multipart ranges

The 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 206 Partial Content status and a Content-Type: multipart/byteranges; boundary=3d6b6a416f9b5 header, indicating that a multipart byterange follows. Each part contains its own Content-Type and 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--

Conditional range requests

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: if the condition is fulfilled, the range request will be issued and the server sends back a 206 Partial Content answer with the appropriate body. If the condition is not fulfilled, the full resource is sent back, with a 200 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

Partial request responses

There are three relevant statuses, when working with range requests:

In case of a successful range request, the 206 Partial Content status is sent back from a server.
In case of a range request that is out of bounds (range values overlap the extent of the resource), the server responds with a 416 Requested Range Not Satisfiable status.
In case of no support of range requests, the 200 OK status is sent back from a server.

Comparison to chunked `Transfer-Encoding`

The Transfer-Encoding header allows chunked encoding, which 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 servers 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.