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37.3 Architecture of the PLM ProtocolThe code of the PLM protocol is divided in three files: ~ns/tcl/plm/plm.tcl, which contains the PLM protocol machinerywithout any specific interface with ns; ~ns/tcl/plm/plm-ns.tcl, which contains the specific ns interface. However, we do notguarantee the strict validity of this ns interfacing; ~ns/tcl/plm/plm-topo.tcl, which contains a user interface to build simulationscenarios with PLM flows.In the following we do not discuss the various procedures per object (for instance all the instproc of the PLM class) but ratherper functionality (for instance which instproc among the various classes are involved in the instantiation of a PLM receiver).For a given functionality, we do not describe in details all the code involved, but we give the principal steps.37.3.1 Instantiation of a PLM SourceTo create a PLM source, place it at node n, and start it at t 0 , we call the PLMTopology instproc place_source n t 0 .This instproc return addr, the address required to attach a receiver to this source. place_source calls the Simulatorinstproc PLMbuild_source_set that creates as many Application/Traffic/CBR_PP instances as there are layers (in thefollowing we call an instance of the class Application/Traffic/CBR_PP a layer). Each layer corresponds to a different multicastgroup.To speed up the simulations when the PLM sources start we use the following trick: At t = 0, PLMbuild_source_setrestricts each layer to send only one packet (maxpkts_ set to 1). That allows to build the multicast trees – one multicast treeper layer – without flooding the whole network. Indeed, each layer only sends one packet to build the corresponding multicasttree.The multicast trees take at most the maximum RTT of the network to be established and must be established before t 0 ,the PLM source starting time. Therefore, t 0 must be carrefully chosen, otherwise the source sends a large number of uselesspackets. However, as we just need to start the PLM source after the multicast trees are estabished, t 0 can be largelyoverestimated. At time t 0 , we set maxpkts_ to 268435456 for all the layers.It is fundamental, in order to have persistent multicast trees, that the prune timeout is set to a large value. For instance, withDM routing:DM set PruneTimeout 1000Each layer of a same PLM source has the same flow id fid_. Consequently, each PLM source is considered as a single flowfor a Fair Queueing scheduler. The PLM code manages automatically the fid_ to prevent different sources to have the samefid_. The fid_ starts at 1 for the first source and is increased by one for each new source. Be careful to avoid other flows(for instance concurrent TCP flows) to have the same fid_ than the PLM sources. Additionally, If you consider fid_ largerthan 32, do not forget to increase the MAXFLOW in ~ns/fq.cc (MAXFLOW must be set to the highest fid_ considered in thesimulation).37.3.2 Instantiation of a PLM ReceiverAll the PLM machinery is implemented at the receiver. In this section we decribe the instantiation process of a receiver. To create,place at node n, attach to source S, and start at t 1 a PLM receiver we call the PLMTopology instproc build_receivern addr t 1 C where addr is the address returned by place_source when S was created, and C is the check value. Thereceiver created by build_receiver is an instance of the class PLM/ns, the ns interface to the PLM machinery. Atthe initialisation of the receiver, the PLM instproc init is called due to inheritance. init calls the PLM/ns instproc326