The active approach must violate the TCP specification in order to execute
the trick. The drawback to the active technique is that it is detectable to the
fingerprinted device. Furthermore, the entire approach only applies to TCP
tra?±c and can be evaded by spoofing the TCP timestamp field or setting it to
an arbitrary value. Sensor networks utilize optimized, modified, lightweight
protocols for data delivery. Pursuing a transport specific approach such as
426
Chapter 18 Unauthorized Sensor Node Identification
Kohno??™s, requires developing a di?®erent technique for each transport delivery
protocol.
3 Organization of a Radio Interface
An RI is responsible for carrying out the physical transmission of a packet over
the air waves. To do so, the 802.11 specification requires the implementation of
two layers: the physical layer (PHY) and the medium access control (MAC)
layer. To support this implementation the RI is organized into hardware,
firmware, and software (driver software and utility software). The functions
of 802.11 PHY are entirely implemented in hardware. The firmware is a microprogram
semi-permanently embedded into ROM to control the hardware.
It works to communicate between the hardware and driver software. Driver
software accepts generic I/O commands from the operating system (OS) of the
host and then converts them into instructions the interface can understand.
The utility software is used to configure parameters to change the overall behavior
of the hardware and software.
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