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The procedures and functions described in this chapter can be found in ~ns/tcl/lib/ns-lib.tcl, ~ns/tcl/lib/ns-packet.tcl, and ~ns/packet.{cc, h}.
Objects in the Packet../ns-2/packets.h are the fundamental unit of exchange between objects in the simulation. The class Packet provides enough information to link a packet on to a list (, in a PacketQueue or on a free list of packets), refer to a buffer containing packet headers that are defined on a per-protocol basis, and to refer to a buffer of packet data. New protocols may define their own packet headers or may extend existing headers with additional fields.
New packet headers are introduced into the simulator by defining a C++ structure with the needed fields, defining a static class to provide OTcl linkage, and then modifying some of the simulator initialization code to assign a byte offset in each packet where the new header is to be located relative to others.
When the simulator is initialized through OTcl, a user may choose to enable only a subset of the compiled-in packet formats, resulting in a modest savings of memory during the execution of the simulation. Presently, most configured-in packet formats are enabled. The management of which packet formats are currently enabled in a simulation is handled by a special packet header manager object described below. This object supports an OTcl method used to specify which packet headers will be used in a simulation. If an object in the simulator makes use of a field in a header which has not been enabled, a run-time fatal program abort occurs.