From the specs:
l) If desiredTotalRequestTime is unspecified, the request may be retried for a duration, which is dependent on the SRM, in case transient error occurs.
m) If input parameter desiredTotalRequestTime is 0 (zero), each file request must be tried at least once. Negative value must be invalid.
which is in contradiction with what stated in the Wiki (Assumptions):
The value of desiredTotalRequestTime MUST be zero or positive. If desiredTotalRequestTime is 0 it indicates the site defined default time.
Not clear what to do when it's zero.
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ElisaLanciotti - 23 Feb 2009