# File lib/mongo/auth/cr/conversation.rb, line 103
        def start(connection = nil)
          if connection && connection.features.op_msg_enabled?
            selector = Auth::GET_NONCE.merge(Protocol::Msg::DATABASE_IDENTIFIER => user.auth_source)
            cluster_time = connection.mongos? && connection.cluster_time
            selector[Operation::CLUSTER_TIME] = cluster_time if cluster_time
            Protocol::Msg.new([], {}, selector)
          else
            Protocol::Query.new(
              user.auth_source,
              Database::COMMAND,
              Auth::GET_NONCE,
              limit: -1)
          end
        end