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Functions for Window Management The usable send window at any time is given by the function window(). It returnsthe minimum of the congestion window and the variable wnd_, which represents the receiver’s advertised window.opencwnd() - this function opens the congestion window. It is invoked when a new ACK arrives. When in slow-start,the function merely increments cwnd_ by each ACK received. When in congestion avoidance, the standard configurationincrements cwnd_ by its reciprocal. Other window growth options are supported during congestion avoidance, but they areexperimental (and not documented; contact Sally Floyd for details).closecwnd(int how) - this function reduces the congestion window. It may be invoked in several ways: when entering fastretransmit, due to a timer expiration, or due to a congestion notification (ECN bit set). Its argument how indicates how thecongestion window should be reduced. The value 0 is used for retransmission timeouts and fast retransmit in Tahoe TCP. Ittypically causes the TCP to enter slow-start and reduce ssthresh_ to half the current window. The value 1 is used by RenoTCP for implementing fast recovery (which avoids returning to slow-start). The value 2 is used for reducing the windowdue to an ECN indication. It resets the congestion window to its initial value (usually causing slow-start), but does not alterssthresh_.Functions for Processing ACKs recv() - this function is the main reception path for ACKs. Note that because only onedirection of data flow is in use, this function should only ever be invoked with a pure ACK packet (i.e. no data). The functionstores the timestamp from the ACK in ts_peer_, and checks for the presence of the ECN bit (reducing the send windowif appropriate). If the ACK is a new ACK, it calls newack(), and otherwise checks to see if it is a duplicate of the last ACKseen. If so, it enters fast retransmit by closing the window, resetting the retransmission timer, and sending a packet by callingsend_much.newack() - this function processes a “new” ACK (one that contains an ACK number higher than any seen so far). The functionsets a new retransmission timer by calling newtimer(), updates the RTT estimation by calling rtt_update, and updates thehighest and last ACK variables.Functions for Managing the Retransmission Timer These functions serve two purposes: estimating the round-trip timeand setting the actual retransmission timer. rtt_init - this function initializes srtt_ and rtt_ to zero, sets rttvar_ to3/tcp_tick_, and sets the backoff multiplier to one.rtt_timeout - this function gives the timeout value in seconds that should be used to schedule the next retransmission timer.It computes this based on the current estimates of the mean and deviation of the round-trip time. In addition, it implementsKarn’s exponential timer backoff for multiple consecutive retransmission timeouts.rtt_update - this function takes as argument the measured RTT and averages it in to the running mean and deviation estimatorsaccording to the description above. Note that t_srtt_ and t_rttvar are both stored in fixed-point (integers). They have3 and 2 bits, respectively, to the right of the binary point.reset_rtx_timer - This function is invoked during fast retransmit or during a timeout. It sets a retransmission timer by callingset_rtx_timer and if invoked by a timeout also calls rtt_backoff.rtt_backoff - this function backs off the retransmission timer (by doubling it).newtimer - this function called only when a new ACK arrives. If the sender’s left window edge is beyond the ACK, thenset_rtx_timer is called, otherwise if a retransmission timer is pending it is cancelled.295
34.5 Tracing TCP DynamicsThe behavior of TCP is often observed by constructing a sequence number-vs-time plot. Typically, a trace is performed byenabling tracing on a link over which the TCP packets will pass. Two trace methods are supported: the default one (used fortracing TCP agents), and an extension used only for FullTcP.34.6 One-Way Trace TCP Trace DynamicsTCP packets generated by one of the one-way TCP agents and destined for a TCP sink agent passing over a traced link (seesection 26) will generate a trace file lines of the form:+ 0.94176 2 3 tcp 1000 ------ 0 0.0 3.0 25 40+ 0.94276 2 3 tcp 1000 ------ 0 0.0 3.0 26 41d 0.94276 2 3 tcp 1000 ------ 0 0.0 3.0 26 41+ 0.95072 2 0 ack 40 ------ 0 3.0 0.0 14 29- 0.95072 2 0 ack 40 ------ 0 3.0 0.0 14 29- 0.95176 2 3 tcp 1000 ------ 0 0.0 3.0 21 36+ 0.95176 2 3 tcp 1000 ------ 0 0.0 3.0 27 42The exact format of this trace file is given in section 26.4. When tracing TCP, packets of type tcp or ack are relevant. Theirtype, size, sequence number (ack number for ack packets), and arrival/depart/drop time are given by field positions 5, 6, 11,and 2, respectively. The + indicates a packet arrival, d a drop, and - a departure. A number of scripts process this file toproduce graphical output or statistical summaries (see, for example, ~ns/test-suite.tcl, the finish procedure.34.7 One-Way Trace TCP Trace DynamicsTCP packets generated by FullTcp and passing over a traced link contain additional information not displayed by defaultusing the regular trace object. By enabling the flag show_tcphdr_ on the trace object (see section refsec:traceformat), 3additional header fields are written to the trace file: ack number, tcp-specific flags, and header length.34.8 Commands at a glanceThe following is a list of commands used to setup/manipulate TCP flows for simulations:set tcp0 [new Agent/TCP]This creates an instance of a TCP agent. There are several flavors of TCP-sender and TCP-receiver (or sink) agent currentlyimplemented in ns. TCP-senders currently available are: Agent/TCP, Agent/TCP/Reno, Agent/TCP/Newreno,Agent/TCP/Sack1, Agent/TCP/Vegas, Agent/TCP/Fack.TCP-receivers currently available are: Agent/TCPSink, Agent/TCPSink/DelAck, Agent/TCPSink/Sack1,Agent/TCPSink/Sack1/DelAck.There is also a two-way implementation of tcp called Agent/TCP/FullTcp. For details on the different TCP flavors see earliersections of this chapter.Configuration parameters for TCP flows maybe set as follows:296
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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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set netif $netif_($t)set mac $mac_(
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the hardware address of a packet’
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destined to itself to the port dmux
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switch(ch->ptype()) {case PT_MAC:br
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16.1.7 Revised format for wireless
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Packet info at "Application level"
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The main problem facing the wired-c
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target_target_encapsulatorreg_agent
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why: This avoids header clashes bet
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This command is used to create a Go
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Chapter 17Satellite Networking in n
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in counter-rotating planes (where t
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• Position/Sat/Geo A geostationar
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$ns add-isl $ltype $node1 $node2 $b
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$satrouteobject_ compute_routeswher
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This will add an error model to the
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lh_firstnameobj‘‘name’’ is
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from routing agentto Node->entrySat
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$satnode add-isl This method c
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set prop [new Propagation/FreeSpace
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18.3.2 Using shadowing modelBefore
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Chapter 19Energy Model in nsEnergy
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Chapter 20Directed DiffusionThe dir
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However if we have a large and dens
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yr application in the ns context. T
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value of opt(pre-stop) is usually t
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21.2 Implementation of XCP in NSIn
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the persistent queue length in the
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Thruput2 0.054024 60000residue_pos_
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Chapter 22DelayBox: Per-Flow Delay
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$ns attach-agent $n_sink $sink# mak
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Chapter 23Changes made to the IEEE
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Part IIISupport214
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endenddocument pargvcPrint out argc
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24.4.2 Memory Conservation TipsSome
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Chapter 25Mathematical SupportThe s
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set run [lindex $argv 0]}if {$run <
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}$sizeRNG next-substream# print the
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protected:RNG* rng_;};Classes deriv
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ns-random is implemented in ~ns/mis
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Chapter 26Trace and Monitoring Supp
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The monitor-queue function is const
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also used to accumulate packet drop
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}name,th->size(),flags,iph->flowid(
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PT_TORA,PT_DSR,PT_AODV,// insert ne
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The QueueMonitor class is not deriv
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$ns_ trace-all This is the command
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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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