The emulation facility can be subdivided into
In opaque mode, the simulator
treats network data as uninterpreted packets.
In particular, real-world protocol fields
are not directly manipulated by the simulator.
In opaque mode, live data packets may be dropped, delayed, re-ordered, or
duplicated, but because no protocol processing is performed,
protocol-specific traffic manipulation scenarios (e.g. ``drop the TCP segment
containing a retransmission of sequence number 23045'') may not be performed.
In protocol mode, the simulator is able to interpret and/or generate
live network traffic containing arbitrary field assignments.
To date (Mar 1998), only Opaque Mode is currently implemented.
- opaque mode - live data treated as opaque data packets
- protocol mode - live data may be interpreted/generated by simulator
The interface between the simulator and live network is provided by
a collection of objects including tap agents and network objects.
Tap agents embed live network data into simulated packets and
Network objects are installed in tap agents and provide an entrypoint
for the sending and receipt of live data.
Both objects are described in the following sections.
When using the emulation mode, a special version of the system
scheduler is used: the RealTime scheduler.
This scheduler uses the same underlying structure as the
standard calendar-queue based scheduler, but ties the execution of
events to real-time.
It is described below.