47.1 Introduction

The emulation facility can be subdivided into two modes:

1. opaque mode - live data treated as opaque data packets
2. protocol mode - live data may be interpreted/generated by simulator

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.

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 vice-versa. 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.