The recent version of DAG (http://www.icann.org/en/topics/new-gtlds/rfp-clean-30may11-en.pdf) suggests the following on the availability.

In the section SPECIFICATION 10, REGISTRY PERFORMANCE SPECIFICATIONS

…

4.1. RDDS availability. Refers to the ability of all the RDDS services for the TLD, to respond to

queries from an Internet user with appropriate data from the relevant Registry System. If 51% or

more of the RDDS testing probes see any of the RDDS services as unavailable during a given

time, the RDDS will be considered unavailable

regards,
Sarmad

P.S. Find “51%” in the document and you will get to this section.  It seems not to have a page number