当OOS系统服务繁忙，暂时不可使用时，OOS会向客户端返回503响应码和Retry-After响应头。示例如下：
HTTP/1.1 503 Service Unavailable
x-ctyun-request-id: abdcf4945d14406a-30383a4b4e4948542d6b6e696854
Retry-After: 90
Date: Tue, 6 May 2014 08:02:58 GMT
Content-Length: 133
Content-Type: text/xml
Server: CTYUN
<?xml version='1.0' encoding='UTF-8'?>
<Error>
<Code>SlowDown</Code>
<Message>Please reduce your request rate.</Message>
</Error>
