title: Is it acceptable for a server to send a HTTP response before the entire request has been received?
body: <p>Consider a large HTTP request:</p>
  
  <pre><code>POST /upload HTTP/1.1
  Content-Type: multipart/form-data
  Content-Length: 1048576
  
  ...
  </code></pre>
  
  <p>The client now begins uploading a megabyte of data, which may take a while.  However, the server determines that HTTP authorization is needed, so it decides it will respond with <code>HTTP 401 Unauthorized</code>.</p>
  
  <p>MUST the server wait until it has received the entire request (IE, headers + CRLF CRLF + <code>Content-Length</code> bytes) before it can respond?</p>
  
  <p>In practical terms, will such behavior break any browsers?  Do browsers continue uploading the file anyway, or will they stop transmitting if they receive a 'premature' response?  </p>
  
  <p>More importantly, in this scenario, will they be able to successfully authenticate and begin the upload again (with credentials), or is it unreliable to cut off the upload like this?</p>