The SCP subsystem is an abstract implementation of SCP, a protocol for federated byzantine agreement, intended to drive a distributed system built around the "replicated state machine" formalism. SCP is defined without reference to any particular interpretation of the concepts of "slot" or "value", nor any particular network communication system or replicated state machine.
This separation from the rest of the system is intended to make the implementation of SCP easier to model, compare to the paper describing the protocol, audit for correctness, and extract for reuse in different programs at a later date.
The SCPDriver class should be subclassed by any module wishing to
implement consensus using the SCP protocol, implementing the necessary abstract
methods for handling SCP-generated events, and calling methods from the central
SCP base-class methods to receive incoming messages.
The messages making up the protocol are defined in XDR,
in the file Stellar-SCP.x, however, library users are
most likely to want to modify Stellar-types.x,
which contains all the base types used through the implementation
(such as the hash type, or the NodeID type, used to represent a node's identity).
The stellar-core program has a single subclass of SCPDriver called
Herder, which gives a specific interpretation to "slot" and
"value", and connects SCP up with a specific broadcast communication medium
(Overlay) and specific replicated state machine
(LedgerManager).
For details of the protocol itself, see the paper on SCP.