Background: Know: NMS Recognize:
Q: How can NMS access all layers?
A1: Implementation-dependent “hooks” into layers
e.g. the hardware/software that implements the protocols of a layer also provides data structures/interfaces through which the implementation can be controlled. These may take the form of standardised MIBs, or may be particular to the implementation (e.g. Microsoft Windows Registry controls of network protocol parameters)
As an example of a hook, see the source code for uIP - a small implementation of a TCP/IP stack in C. e.g. the uip.h file declares a "uip_stats" structure which exposes the statistics collected by the implementation of IP to other software (e.g. the operating system) which can access those stats through that structure.