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Chapter 21XCP: eXplicit Congestion control ProtocolXCP is a feedback-based congestion control system that uses direct, explicit, router feedback to avoid congestion in thenetwork. It is designed for both scalability and generality. It was developed by Dina Katabi, starting from a suggestion byMark Handley (refer to [?] for detailed descriptions). The ns code for XCP which was originally developed by Dina Katabiwas modified, extended and integrated into ns-2.28 at USC/ISI. It still continues to evolve as of today. If you are interested inlooking at Dina’s original source code please go to her website at http://www.ana.lcs.mit.edu/dina/XCP/21.1 What is XCP?XCP is a congestion control protocol that can be applied to any transport protocol. It performs especially well in very highdelay-bandwidth product networks. Typically in large bandwidth-delay product networks, efficiency of TCP goes down inthe event of multiple of packet losses and becomes unstable irrespective of queueing schemes used. However in similarenvironments, XCP, using a control theory based feedback model, achieves much more efficiency, stability and fairness bysending feedback from the network to the sender for setting the data flow into the network.XCP’s scalability results from the fact that it requires no per-flow state in the router to calculate the feedback. Most routerassistedcongestion control systems maintain per-flow information used to allocate the resources. XCP keeps very littleinformation in the router, and this information is chosen to minimize both the amount of router state and the per-packetoperations needed to update that state.For generality, XCP divides the resource allocation function between two controllers: a congestion controller that ensuresthat flows use all available capacity, and a fairness controller that ensures that flows are allocated the capacity fairly. Mostcongestion control systems fail to make this division, much less to implement as two conceptually distinct systems. Thisdivision allows a clear exposition and implementation of two basic resource allocation functions in XCP. XCP sources sendadditional information about their current round-trip times and router-assigned throughput in each packet. XCP routers insertfeedback into the packets that is interpreted by the sources.Although XCP may be implemented for any transport protocol, however as an initial test it has been implemented in TCP.The next section gives details of the way XCP is implemented in ns.201
21.2 Implementation of XCP in NSIn ns, the XCP implementation can be found under ~ns/xcp directory. The protocol needs to be deployed in the TCP endpoints (source and receiver) as well within the intermediate nodes which is mostly the router and may sometimes be a linklayerswitch as well. The end-point part of XCP code may be found under xcp-end-sys.cc,h and the router portion of the codemay be found under xcp.cc,h and xcpq.cc,h.21.2.1 Endpoints in XCPThe end points consist of TCP source and sink agents using XCP as their congestion control mechanism. The intermediatenode or router writes feedback in each packet header as the delta_throughput value, about the data capacity that may be incrementedif feedback is positive and should be decreased if negative. When this packet reaches the receiver this delta_throughputvalue is returned to the sender in a reverse_feedback field of a congestion header in the returning packet, which is an ACKpacket in case of TCP.The sender upon receiving this reverse_feedback value adjusts its sending rate by increasing or decreasing its congestionwindow size as the case maybe.The packet header that is used by XCP is implemented as a structure called hdr_xcp in ns and is defined as follows:double x_; // idealized inter-packet timedouble rtt_;enum {XCP_DISABLED = 0,XCP_ENABLED,XCP_ACK,} xcp_enabled_; // to indicate that the flow is XCP enabledint xcpId_; // Sender’s ID (debugging only)double cwnd_; // The current window (debugging only)double reverse_feedback_;// --- Initialized by source and Updated by Routerdouble delta_throughput_;unsigned int controlling_hop_;// router ID (debugging only)The xcp receiver is responsible for copying the delta_throughput value into the reverse_feedback field of the ack packets. Insome cases where delayed ack is used, the receiver calculates the sum of the delta_throughput values in arriving packets forcopying into the reverse_feedback field of the outgoing ack packet.The controlling_hop_ field that carries the address of the router who has last updated the feedback is used for debuggingpurposes only.In case of a packet loss in the network, TCP’s Van Jacobson congestion control should most likely override XCP. Howeverin nsthis happens a little differently. With receiving of duplicate acks indicating packet loss, the cwnd gets halved and fastretransmit and fast recovery algorithms come into play. However xcp routers continue to send feedback to the source based onwhich the source tries to open its cwnd. So it seems to be a mish-mash of VJCC and XCP algorithms. However for most casesthis issue doesnot arise as XCP router very rarely experiences a pkt drop as the queue is continuously regulated and drainedby XCP. Understanding the correct behaviour of XCP in face of pkt loss is an area of current study and shall be implementedin the future.202
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18 Radio Propagation Models 17718.1
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30 Multicast Routing 25130.1 Multic
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38.2.5 An example . . . . . . . . .
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X Other 40347 Educational use of NS
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# so, we lied. now, we define the t
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Chapter 2Undocumented FacilitiesNs
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Part IInterface to the Interpreter1
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• if you can do what you want by
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• tcl.result(const char* s)Pass t
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By convention in ns, the class Agen
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$object set bwvar 1500kb$object set
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For a C++ variable to be traceable,
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3.5 Class TclClassThis compiled cla
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class Packet {......static int hdrl
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The actual arguments passed by the
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class TclClass (Section 3.5) define
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Chapter 4The Class SimulatorThe ove
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4.2.2 the heap schedulerThe heap sc
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4.4 Commands at a glanceSynopsis:ns
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$ns_ dumpqCommand for dumping event
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NODEPortClassifierAgentAgentAddrCla
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The Node instance variable, entry_,
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The default values for all the abov
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The classify() method is pure virtu
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};The class imposes no direct seman
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flow-specific queuing disciplines a
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5.5 Routing Module and Classifier O
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Module NameRtModule/BaseRtModule/Mc
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$node neighborsThis returns the lis
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Linkhead_enqT_queue_ deqT_ link_ tt
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6.2 ConnectorsConnectors, unlink cl
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$ns_ link-lossmodel This function
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Chapter 7Queue Management and Packe
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}}}void Queue::resume(){Packet* p =
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7.3 Different types of Queue object
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maxidle_ is the maximum amount of t
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dst_ The destination address of pac
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7.5.2 ConfigurationRunning a JoBS s
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mean_pkt_size_ Used to set the expe
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Demarker objects$q trace-file This
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The recv() method overrides the bas
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9.2 ImplementationThe procedures an
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Average sending rateTSW window leng
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The following command adds an entry
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$qCE configQ 0 1 10 20 0.10Note tha
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Chapter 10AgentsAgents represent en
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CtrMcast/EncapCtrMcast/DecapMessage
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}bind("windowOption_", &wnd_option_
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hdr_flags* nf = (hdr_flags*)npkt->a
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public:ECHO_Timer(ECHO_Agent *a) :
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10.6.4 Using the agent through OTcl
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State Variables are:dupacks_ Number
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$agent attach-tbf Attaches a token
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(void)Scheduler::instance().schedul
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* outstanding, cancel it.*/void Tcp
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Chapter 12Packet Headers and Format
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method is now obsolete; its usage i
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Packethdrsize_next_bits()points to
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the system’s memory allocator. In
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}incr hdrlen_ $incrreturn $baseFrom
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Chapter 13Error ModelThis chapter d
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SimpleLink::errormodule argsSimulat
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This is a simple example of how to
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Higher LayersNode 1Node 2. . .Node
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};void recv(Packet* p, Handler*);vi
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and the frame type. It then passes
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$ll_ set delay_ $delay$ll_ set band
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14.10 Commands at a glanceThe follo
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15.2.1 Default Hierarchical Setting
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Chapter 16Mobile Networking in nsTh
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entry_addrdemuxIP addressdefaulttar
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16.1.2 Creating Node movementsThe m
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Page 171 and 172: Chapter 17Satellite Networking in n
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Page 175 and 176: • Position/Sat/Geo A geostationar
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Page 181 and 182: This will add an error model to the
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Page 187 and 188: $satnode add-isl This method c
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Page 193 and 194: Chapter 19Energy Model in nsEnergy
Page 195 and 196: Chapter 20Directed DiffusionThe dir
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Page 215 and 216: Part IIISupport214
Page 217 and 218: endenddocument pargvcPrint out argc
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Page 221 and 222: Chapter 25Mathematical SupportThe s
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Page 225 and 226: }$sizeRNG next-substream# print the
Page 227 and 228: protected:RNG* rng_;};Classes deriv
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Page 231 and 232: Chapter 26Trace and Monitoring Supp
Page 233 and 234: The monitor-queue function is const
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Page 245 and 246: Chapter 27Test Suite SupportThe ns
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Asymmetric Routing Asymmetric routi
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class RouteLogic This class defines
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— The procedure init-all{} is a g
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Global Actions Once the detailed ac
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This dumps next hop information in
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$ns mrtproto ST;# specify shared tr
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Dense Mode The Dense Mode protocol
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and unique label (id). Thus, “inc
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add-iif{ifid link},add-oif{link if}
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CtrMcastComp is the centralised mul
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$ns_ mrtproto This command specifi
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Chapter 31Network DynamicsThis chap
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v 0.8123 link-up 3 5v 0.8123 link-u
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The first argument is the time at w
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$ns_ rtmodel This command defines
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$ns set-address-format hierarchical
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32.4 Hierarchical Routing with Sess
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Chapter 33UDP Agents33.1 UDP Agents
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Chapter 34TCP AgentsThis section de
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set ns [new Simulator]set node1 [$n
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34.2.1 The Base TCP SinkThe base TC
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Agent/TCP/FullTcp set dupseg_fix_ t
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34.5 Tracing TCP DynamicsThe behavi
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Chapter 35SCTP AgentsThis chapter d
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Figure 35.1: Example of a Multihome
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Note: the actual value of these tra
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1.526624 1 4 sctp 1500 -------D 0 1
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$ns at 5.0 "finish"$ns run35.5.2 Mu
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Chapter 36Agent/SRMThis chapter des
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36.1.2 Other Configuration Paramete
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3.6274 n 0 m r 1 type repair servi
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3.6029 n 3 m r 2 Q NTIMER at 3.730
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36.4 Loss Detection—The Class SRM
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same packet. The repair objet does
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}hdr_asrm* seh = (hdr_asrm*) p->acc
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set grp [Node allocaddr]$srm set ds
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#set up the multicast routingDM set
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37.3 Architecture of the PLM Protoc
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We add in void PLMLossMonitor::recv
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Part VIApplication330
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Traffic generatorsApplication/Traff
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• recv(int nbytes)—Announces th
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1. EXPOO_Traffic—generates traffi
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set src [new Agent/UDP]set sink [ne
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Application FTP FTP objects produce
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send_data(ADU)Application(HttpApp,
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39.1.4 Transmitting user data over
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and teardown of connections. Only O
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TclObjectPagePoolPagePool/CompMathP
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PagePool/ProxyTrace takes these two
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}int id_; // object IDWebTrafSessio
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An Http/Server object waits for inc
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set tmp [new RandomVariable/Exponen
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Object Type Event Type Explaination
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Chapter 40Worm ModelIn this chapter
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$w local-p 0.5Following are some co
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(nsnode) clientcloudPackM ime(nsnod
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HTTP responsesclients servers Delay
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41.5 Commands at a GlanceThe follow
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• HTTP response size (bytes)• s
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Chapter 42Session-level Packet Dist
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42.1.2 Inserting a Loss ModuleWhen
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Delay and Loss Modules Each receive
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Chapter 43Asim: approximate analyti
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set n(1) [$ns node]set link(0:1) [$
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Part VIIIEmulation382
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When using the emulation mode, a sp
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set intf [$pf1 open readonly]puts "
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puts "install nets into taps..."$a0
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Chapter 45Nam45.1 IntroductionNam i
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• Button 6 (Chevron logo) - Close
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Second, when dealing with randomly
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dst_portaddr,seqno,flags,sname);A n
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• up• down• right• left•
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46.1.7 Agent TracingAgent trace eve
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v -t 1.0 -e node_tclscript 2 "Echo
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If nam ever gets to the end of an e
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l :link-t time-s source id-d des
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E :D :P :a :session enqueue-t time
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v :V :N :W :g :A :c :q :execute tcl
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$ns duplex-link-op orient right$ns
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Chapter 47Educational use of NS and
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Bibliography[1] C. Alaettinoğlu, A
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