pub const RETRY_AFTER: HeaderName;
The Retry-After response HTTP header indicates how long the user agent should wait before making a follow-up request. There are two main cases this header is used:
When sent with a 503 (Service Unavailable) response, it indicates how long the service is expected to be unavailable.
When sent with a redirect response, such as 301 (Moved Permanently), it indicates the minimum time that the user agent is asked to wait before issuing the redirected request.