3.1.5.2.3 HTTP Error 503

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The server returns an HTTP error 503 when more users than are allowed by the server's request queue limit have sent requests to a single server or when the actions of the client have triggered throttling.

The error returned by the server resembles the following.

 OPTIONS /Microsoft-Server-ActiveSync
 Content-Type: application/vnd.ms-sync.wbxml
 MS-ASProtocolVersion: 14.0
  
 HTTP/1.1 503 Service Unavailable
 Connection: close
 Date: Mon, 02 Mar 2009 23:51:51 GMT
 Server: Microsoft-IIS/7.0
 X-Powered-By: ASP.NET
 Content-Type: text/html

If a Retry-After header ([RFC2616]) is present in the response, the client SHOULD<6> retry the request after waiting the number of seconds indicated by the Retry-After header. Any such value represents an estimate of when the server is expected to be able to process the request.

If a Retry-Header is not present in the response, the client can retry the request after waiting a few seconds. The time to wait between continuous requests that result in HTTP error 503 responses can be increased exponentially to a predetermined maximum.

For more details about ASP.NET performance monitoring properties, see [MSDN-APM].