The ns Manual (formerly ns Notes and Documentation)1 - NM Lab at ...

Other useful configuration parameters are to assign a separate flow id to traffic originating from this agent, to open a log filefor statistics, and a trace file for trace data 1 .The file tcl/mcast/srm-nam.tcl contains definitions that overload the agent’s send methods; this separates controltraffic originating from the agent by type. Each type is allocated a separate flowID. The traffic is separated into sessionmessages (flowid = 40), requests (flowid = 41), and repair messages (flowid = 42). The base flowid can be changed by settingglobal variable ctrlFid to one less than the desired flowid before sourcing srm-nam.tcl. To do this, the simulation scriptmust source srm-nam.tcl before creating any SRM agents. This is useful for analysis of traffic traces, or for visualizationin nam.Application Data Handling The agent does not generate any application data on its own; instead, the simulation user canconnect any traffic generation module to any SRM agent to generate data. The following code demonstrates how a trafficgeneration agent can be attached to an SRM agent:set packetSize 210set exp0 [new Application/Traffic/Exponential]$exp0 set packetSize_ $packetSize$exp0 set burst_time_ 500ms$exp0 set idle_time_ 500ms$exp0 set rate_ 100k;# configure traffic generator$exp0 attach-agent $srm0$srm0 set packetSize_ $packetSize$srm0 set tg_ $exp0$srm0 set app_fid_ 0;# attach application to SRM agent;# to generate repair packets of appropriate size;# pointer to traffic generator object;# fid value for packets generated by traffic generatorThe user can attach any traffic generator to an SRM agent. The SRM agent will add the SRM headers, set the destinationaddress to the multicast group, and deliver the packet to its target. The SRM header contains the type of the message, theidentity of the sender, the sequence number of the message, and (for control messages), the round for which this message isbeing sent. Each data unit in SRM is identified as 〈sender’s id, message sequence number〉.The SRM agent does not generate its own data; it does not also keep track of the data sent, except to record the sequencenumbers of messages received in the event that it has to do error recovery. Since the agent has no actual record of past data,it needs to know what packet size to use for each repair message. Hence, the instance variable packetSize_ specifies thesize of repair messages generated by the agent.Starting the Agent and Traffic GeneratorThe agent and the traffic generator must be started separately.$srm start$exp0 startAlternatively, the traffic generator can be started from the SRM Agent:$srm0 start-sourceAt start, the agent joins the multicast group, and starts generating session messages. The start-source triggers thetraffic generator to start sending data.1 Note that the trace data can also be used to gather certain kinds of trace data. We will illustrate this later.309