Tipos de llamadas ingresantes

A new Trust Value (TV) is a float value in the range [0, 1]. 0 denotes the lowest value of trustworthiness and 1 refers to the highest value. The new Trust Value (TV) is generated using three values: Existing Trust Value (ETV), Bad Transaction Factor (BTF), and Error Factor (EF). Each factor is multiplied by a weight that reflects the importance of the factor in the system and shows to what extent the factor affects the final Trust Value (TV). Each weight is a percentage that shows how much the factor will affect the general equation and the new Trust Value (TV).

The weights are Existing Trust Value Weight (ETVW), Bad Transaction Factor Weight (BTFW), and Error Factor Weight (EFW). All ETVW, BTFW, and EFW percentages can be set in line with the organisation’s policies. For example, if the organisation does not consider the Error Factor important then EFW can be 1% but if the organisation considers the error rate to be important then EFW it could be 10%.

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Both Bad Transaction Factor Weight (BTFW) and Error Factor Weight (EFW) range between 1% and 10%. The Existing Trust Value Weight (ETVW) range is between 80% and 98%. Range values are selected to keep the Trust Value (TV) within suitable bounds. The maximum for both bad transaction and error weights is 10% and not higher because the aim of the system is to adjust user privilege according to behaviour and not to block user access completely. For example, if this weight much higher say as 50%, then the Trust Value (TV) would drop suddenly and dramatically and may cause other access problems. The Existing Trust Value Weight (ETVW) is regarded as the basic value to calculate the new TV. The new TV is derived from the existing one and this explains why its weight should be high. In the practice the range between 80% and 98% was used. This was arrived at by series of experiments.

The trust calculator completes the calculation process for the new Trust Value in a number of steps:

 Find the number of bad transactions and the number of errors recorded in the XLog file.

 Assign values to both the Bad Transaction Factor and the Error Factor depending on the trust policy file.

 Assign weights to each factor of ETVW, BTFW, and EFW depending on the trust policies.

 Calculate the new Trust Value according to the equations that are recorded in the trust policy file.

 Send the new Trust Value to the access decision maker to update the users’ privileges.

The value of Bad Transaction Factor (BTF) depends on the Bad Transaction Number (BTNum) found in the XLog file. After the BTNum has been counted, the range for the BTF is selected. There are five ranges:

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negligible, low, moderate, high, and extreme. These ranges are defined according to the organisation’s policy for classifying bad transactions.

For the purposes of these experiments in Trust Based Access Control for XML databases, the ranges are defined as follows. The negligible range is used when there are no bad transactions in the XLog file and the BTF will be 0. The low range is selected when the BTNum is between 1 and 5 bad transactions. Then BTF will be equal to 0.25. If the number of bad transactions is between 6 and 10, the moderate range is selected and the BTF will be 0.50. The high range reflects a BTNum between 11 and 15 and the BTF will be 0.75. The BTF reaches 1 when the BTNum fits into the extreme range that is defined to be larger than 15. Table 7.1 shows these ranges and the equivalent BTF.

Table ‎7.1 The equivalent range for the Bad Transaction Number (BTNum)

Like the Bad Transaction Factor (BTF), the Error Factor (EF) is defined according to the Error Number (ENum) counted in the XLog file which leads to a range. The EF ranges are also negligible, low, moderate, high, and extreme. Each one reflects how many errors are detected in the XLog file. These ranges can be defined according to the organisation’s policy for classifying errors and are shown in Table 7.2.

Range Name Bad Transaction Number (BTNum)

Bad Transaction Factor (BTF)

Negligible 0 0

Low 0< BTNum< = 5 0.25

Moderate 5< BTNum< =10 0.50

High 10< BTNum < =15 0.75

Extreme 15< BTNum 1

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Table ‎7.2 The equivalent range for the Error Number (ENum)

Range Name Error Number (ENum) Error Factor (EF)

Negligible 0 0

Low 0< ENum< = 5 0.25

Moderate 5< ENum< =10 0.50

High 10< ENum < =15 0.75

Extreme 15< ENum 1

After the trust calculator finds the BTF and the EF, the Trust Value (TV) is calculated. The TV increases when there are no bad transactions or errors but drops markedly when the Bad Transaction Factor (BTF), Error Factor (EF), or both increase. There are four different equations to calculate the Trust Value and each one applies to specific cases. Trust Value equations are:

{

( ) ( ) ( ) ( ) ( ) ( )

Equation 1 is used to calculate Trust Value (TV) when there are no errors or bad transactions and Trust Value increases slightly. In this specific case, the three weights must sum to 1. This equation could be simplified, since it is used when the error and bad transaction factors are zero. It can be shown in the following simple form:

( ). If there are errors or bad transactions, (2) or (3) is used to calculate the TV. In general, if there are bad transactions or errors the TV will decrease. As a consequence, the Bad Transaction Factor (BTF) is subtracted from the Existing Trust Value (ETV) in (2). The same principle applies to (3) when

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there are only errors without bad transactions; the Error Factor (EF) is subtracted from Existing Trust Value (ETV). Equation 4 is used when there are both bad transactions and errors. It subtracts both the Bad Transaction Factor (BTF) and the Error Factor (EF) from Existing Trust Value (ETV) to find the new Trust Value (TV).

Table 7.3 illustrates some examples of the calculation of a new Trust Value for some general cases when the Existing Trust Value (ETV) = 0.5.

More examples and calculation case studies on the Trust Value will be mentioned in the experimental part of the trust module in Chapter 10. The next Section describes the trust policy file.

Table ‎7.3 The calculation of Trust Value (TV)