From the HTTPD specs at ftp://ftp.isi.edu/in-notes/rfc2616.txt

The server has fulfilled the partial GET request for the resource. The request MUST have included a Range header field indicating the desired range, and MAY have included an If-Range header field to make the request conditional.

The response MUST include the following header fields:

• Either a Content-Range header field indicating the range included with this response, or a multipart/byteranges Content-Type including Content-Range fields for each part. If a Content-Length header field is present in the response, its value MUST match the actual number of OCTETs transmitted in the message-body.
• Date
• ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request
• Expires, Cache-Control, and/or Vary, if the field-value might differ from that sent in any previous response for the same variant