a0d929ca_WSRP_v2_0_Specification__Statement_text
[#] CS072 [Section] 5.2.1.1 [Statement text] 
If the cacheControl field is empty, the Portlet has provided no guidance and the Consumer MAY apply whatever cache policy it chooses. For a Portlet to indicate the markup is not cacheable, it will need to return a cacheControl structure with a value of zero in the expires field.
WSRP v2.0 Specification
Statement text