How CloudFront Processes Partial Requests for an Object (Range GETs)
For a large object, an end user's browser or client might make multiple
requests and use the
Range request header to download the object in smaller
units. These requests for ranges of bytes, sometimes known as
Range GET requests,
improve the efficiency of partial downloads and the recovery from partially failed transfers.
When CloudFront receives a
Range GET request, it checks the cache in the edge
location that received the request. If the cache in that edge location already contains the
entire object or the requested portion of the object, CloudFront immediately serves the requested
range from the cache.
If the cache doesn't contain the requested range, CloudFront forwards the request to the origin.
(To optimize performance, CloudFront may request a larger range than the client requested in the
Range GET.) What happens next depends on whether the origin supports
Range GET requests:
If the origin supports
Range GETrequests: It returns the requested range. CloudFront serves the requested range and also caches it for future requests. (Amazon S3 supports
Range GETrequests, as do some HTTP servers, for example, Apache and IIS. For information about whether your HTTP server does, see the documentation for your HTTP server.)
If the origin doesn't support
Range GETrequests: It returns the entire object. CloudFront serves the entire object and also caches the entire object for future requests. After CloudFront caches the entire object in an edge cache, it responds to
Range GETrequests by serving the requested range.
In either case, CloudFront begins to serve the requested range or object to the end user as soon as the first byte arrives from the origin.
If the viewer makes a
Range GET request and the origin returns
Transfer-Encoding: chunked, CloudFront returns the entire object to the viewer
instead of the requested range.
CloudFront generally follows the RFC specification for the
Range header. However,
Range headers don't adhere to the following requirements, CloudFront will
return HTTP status code 200 with the full object instead of status code 206 with the specified
The ranges must be listed in ascending order. For example,
300-400,100-200is not valid.
The ranges must not overlap. For example,
100-200,150-250is not valid.
All of the ranges specifications must be valid. For example, you can't specify a negative value as part of a range.
For more information about the
Range request header, see "Section
14.35 Range" in Hypertext Transfer Protocol -- HTTP/1.1 at http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35.