4447f991_P_Guide___HTTP_General_Headers__HTTP_Cache_Control_Directives_HTTP_Message_Type
[Cache-Control Directive] must-revalidate [Description] Instructs a cache to revalidate its cache entry for the given response with the original server after it becomes stale. This is used to prevent problems with certain types of transactions that can occur if stale cache entries are sent to a client (perhaps as a result of the client using the max-stale directive.) [HTTP Message Type] 
Response
R e q u e s t o r R e s p o n s e
R e s p o n s e
R e q u e s t
The TCP/IP Guide - HTTP General Headers
HTTP Message Type