'Set the TTL of the rdataset to be the lesser of the set\'s current
TTL or the specified TTL.  If the set contains no rdatas, set the TTL
to the specified TTL.
@param ttl: The TTL
@type ttl: int'

def update_ttl(self, ttl):

    if (len(self) == 0):
        self.ttl = ttl
    elif (ttl < self.ttl):
        self.ttl = ttl
