Select all of the following that are disadvantages of signature-based IDS. a. They require previous knowledge of the attack to generate an accurate signature.
b. They rely on determining if packet streams are statistically unusual, which is difficult.
c. They may be overwhelmed because every packet must be compared with a large collection of signatures.
d. They cannot do deep packet inspection.
e. They may generate many false alarms because even though the signature matches, it may not be the result of an attack.