A Comparative Study on Self-tolerant Strategies for Hardware Immune Systems - Artificial Immune Systems - page 205

detector set updating strategy is a key component in hardware immune systems. A good
detector set updating strategy should have the following characteristics.
(1) It should maintain a low false positive ratio when the new self individual is
collected. “False positive” means recognizing a self individual as a non-self.
(2) It should maintain a low false negative ratio after the detector set is updated.
“False negative” means recognizing a non-self individual as a self.
(3) The false negative ratio will not clearly increase even if the self set is
unavailable.
(4) It has reasonable time and space complexities, and can be easily implemented in
hardware immune systems.
In [9], during the detection process, if a detector is activated by a state transition
without a co-stimulation signal, it will be deleted from the detector set, and the failure
probability of detecting an invalid state transition (the false negative ratio) will be
clearly increased. In the ASTA-CED algorithm proposed in [8], if a detector is deleted,
a new detector generated randomly will replace the deleted one. But in some cases a
new detector can not be generated easily. Moreover, the self set must be kept by the HIS
because the newly generated candidate detector should be filtered by the self set to
make sure it will not match a known self individual. This is a very time consuming
operation, and is not practical for some hardware immune systems.
Two novel detector set updating strategies are introduced in this paper, and
compared with the strategies in [8] and [9]. The experimental results show that the
detector sets being updated by these two novel strategies are less affected by the
growing self set, and have a better coverage on the non-self space.
3 Detector Set Updating Strategies
Before introducing the two novel detector set updating strategies in this section, some
symbols used in this paper are defined as following.
r : Matching length threshold. Note that r-continuous-bits matching rule [11] is
adopted in this paper.
st : State transition. Here a state transition means a 0/1 string to be monitored [8].
l : The string length of a detector.
m : Maximum continuous matching length between two strings in the corresponding
positions.
S : Set of self strings.
R 0 : Immature detector set.
R : Mature detector set.
N R0 : The number of immature detectors.
N R : The number of mature detectors.
N S : The size of the self set.
The overall flowchart of the detector set detecting and updating process is described
in Fig. 1. And the process is described as following [8].