NS(1) General Commands Manual NS(1)
NAME
ns - network simulator (version 2)
SYNOPSIS
ns [ file [ arg arg ... ] ]
DESCRIPTION
ns is an event-driven network simulator. An extensible simulation engine is implemented in C++ that uses MIT's Object Tool Command Lan-
guage, OTcl (an object oriented version of Tcl) as the command and configuration interface. A previous version of the simulator i.e. ns
version 1 used the Tool Command Language, Tcl as the configuration language. The current version still supports simulation scripts written
in Tcl meant for the ns version 1 simulator.
This manual page documents some of the interfaces for ns. For much more complete documentation, please see "ns Notes and Documentation"
[13], available in the distribution and on the web.
The simulator is invoked via the ns interpreter, an extension of the vanilla otclsh command shell. A simulation is defined by a OTcl
script. The scripts use the Simulator Class as the principal interface to the simulation engine. Using the methods defined in this class,
a network topology is defined, traffic sources and sinks are configured, the simulation is invoked, and the statistics are collected. By
building upon a fully functional language, arbitrary actions can be programmed into the configuration.
The first step in the simulation is to acquire an instance of the Simulator class. Instances of objects in classes are created and
destroyed in ns using the new and delete methods. For example, an instance of the Simulator object is created by the following command:
e.g. set ns [new Simulator]
A network topology is realized using three primitive building blocks: nodes, links, and agents. The Simulator class has methods to create/
configure each of these building blocks. Nodes are created with the node Simulator method that automatically assigns an unique address to
each node. Links are created between nodes to form a network topology with the simplex-link and duplex-link methods that set up unidirec-
tional and bidirectional links respectively. Agents are the objects that actively drive the simulation. Agents can be thought of as the
processes and/or transport entities that run on nodes that may be end hosts or routers. Traffic sources and sinks, dynamic routing modules
and the various protocol modules are all examples of agents. Agents are created by instantiating objects in the subclass of class Agent
i.e., Agent/type where type specifies the nature of the agent. For example, a TCP agent is created using the command:
set tcp [new Agent/TCP]
Once the agents are created, they are attached to nodes with the attach-agent Simulator method. Each agent is automatically assigned a
port number unique across all agents on a given node (analogous to a tcp or udp port). Some types of agents may have sources attached to
them while others may generate their own data. For example, you can attach ``ftp'' and ``telnet'' sources to ``tcp'' agents but ``constant
bit-rate'' agents generate their own data. Applications are attached to agents using the attach-app method.
Each object has some configuration parameters associated with it that can be modified. Configuration parameters are instance variables of
the object. These parameters are initialized during startup to default values that can simply be read from the instance variables of the
object. For example, $tcp set window_ returns the default window size for the tcp object. The default values for that object can be
explicitly overridden by simple assignment either before a simulation begins, or dynamically, while the simulation is in progress. For
example the window-size for a particular TCP session can be changed in the following manner.
$tcp set window_ 25
The default values for the configuration parameters of all the class objects subsequently created can also be changed by simple assignment.
For example, we can say
Agent/TCP set window_ 30
to make all future tcp agent creations default to a window size of 30.
Events are scheduled in ns using the at Simulator method that allows OTcl procedures to be invoked at arbitrary points in simulation time.
These OTcl callbacks provide a flexible simulation mechanism -- they can be used to start or stop sources, dump statistics, instantiate
link failures, reconfigure the network topology etc. The simulation is started via the run method and continues until there are no more
events to be processed. At this time, the original invocation of the run command returns and the Tcl script can exit or invoke another
simulation run after possible reconfiguration. Alternatively, the simulation can be prematurely halted by invoking the stop command or by
exiting the script with Tcl's standard exit command.
Packets are forwarded along the shortest path route from a source to a destination, where the distance metric is the sum of costs of the
links traversed from the source to the destination. The cost of a link is 1 by default; the distance metric is simply the hop count in
this case. The cost of a link can be changed with the cost Simulator method. A static topology model is used as the default in ns in
which the states of nodes/links do not change during the course of a simulation. Network Dynamics could be specified using methods
described in NETWORK DYNAMICS METHODS section. Also static unicast routing is the default in which the routes are pre-computed over the
entire topology once prior to starting the simulation. Methods to enable and configure dynamic unicast and multicast routing are described
in the UNICAST ROUTING METHODS and MULTICAST ROUTING METHODS sections respectively.
NS COMMANDS
This section describes the basic commands to create the building blocks of the simulation (i.e. the node, link and agent objects) and to
run the simulation.
The first step in running a simulation as stated before is to acquire an instance of the Simulator class that has methods to configure and
run the simulation. Throughout this section the object variable name $ns is used to imply a Simulator object.
$ns node
Create a new node object and return a handle to it.
$ns all-nodes-list
Returns a list of all the node objects defined in the simulation.
$ns simplex-link node1 node2 bw delay type
Create a new unidirectional link between node1 and node2 with bandwidth bw in bits per second and link propagation delay delay in
seconds. node1 and node2 must have already been created with the node method. bw and delay default to 1.5 Mbits/sec and 100 ms
respectively. The defaults can be changed by modifying the relevant configuration parameters of the DelayLink Object (see DELAYLINK
OBJECTS section). node1 and node2 must have already been created with the node method. The queuing discipline of the link is spec-
ified by type, which may be DropTail, FQ, SFQ, DRR, RED, CBQ, or CBQ/WRR. A DropTail link is a simple FIFO queue which drops the
last packet in the queue when the queue overflows. A FQ link is for Fair Queuing (for details see [?]). A SFQ link is for Stochas-
tic Fair Queuing (for details see [?]). A DRR link is for deficit round robin scheduling (for details see [9]). A RED link is a
random-early drop queue (for details see [2]). A CBQ link is for class-based queuing using a packet-by-packet round-robin scheduler
(for details see [3]). A CBQ/WRR link is for class-based queuing with a weighted round robin scheduler. If multicast routing is
used links with interface labels are required. Such links are created by setting Simulator NumberInterfaces_ variable to 1. All
the subsequently created links will have interface labels. To disable creation of interfaces simply reset NumberInterfaces_ to 0
(this is the default).
$ns duplex-link node1 node2 bw delay type
Create a new bidirectional link between node1 and node2 with bandwidth bw in bits per second and link propagation delay delay in
seconds. node1 and node2 must have already been created with the node method. bw and delay default to 1.5 Mbits/sec and 100 ms
respectively. The defaults can be changed by modifying the relevant configuration parameters of the DelayLink Object (see DELAYLINK
OBJECTS section). The queuing discipline of the link is specified by type, which may be DropTail, FQ SFQ, DRR, RED, CBQ, or
CBQ/WRR. A DropTail link is a simple FIFO queue which drops the last packet in the queue when the queue overflows. A FQ link is
for Fair Queuing (for details see [?]). A SFQ link is for Stochastic Fair Queuing (for details see [?]). A DRR link is for deficit
round robin scheduling (for details see [9]). A RED link is a random-early drop queue (for details see [2]). A CBQ link is for
class-based queuing using a packet-by-packet round-robin scheduler (for details see [3]). A CBQ/WRR link is for class-based queuing
with a weighted round robin scheduler. If multicast routing is used links with interface labels are required. Such links are cre-
ated by setting Simulator NumberInterfaces_ variable to 1. All the subsequently created links will have interface labels. To dis-
able creation of interfaces simply reset NumberInterfaces_ to 0 (this is the default).
$ns link node1 node2
Returns a reference to the link connecting nodes node1 and node2. This is useful for setting link configuration parameters and to
invoke tracing methods (see LINK OBJECTS section).
$ns queue-limit node1 node2 queue-limit
Set the maximum number of packets that can be queued on the link in the direction from node1 to node2 to queue-limit. The link
between node1 and node2 should have already been created.
$ns delay node1 node2 time-interval
Set the latency of the link in the direction from node1 to node2 to time-interval seconds. The link between node1 and node2 should
have already been created.
$ns cost node1 node2 cost-val
Assign the cost cost-val to the link between nodes node1 and node2. The costs assigned to links are used in unicast route computa-
tions. All the links default to a cost of 1.
$ns multi-link node-list bw delay type
Connects the nodes specified in node-list by a mesh of duplex links (to simulate a broadcast LAN) with bandwidth bw in bits per sec-
ond and link propagation delay delay in seconds. node-list is a list of node object handles that have already been created with the
node method. bw and delay default to 1.5 Mbits/sec and 100 ms respectively. The defaults can be changed by modifying the relevant
configuration parameters of the DelayLink Object (see DELAYLINK OBJECTS section). The queuing discipline of the link is specified
by type, which may be DropTail, FQ SFQ, DRR, RED, CBQ, or CBQ/WRR. A DropTail link is a simple FIFO queue which drops the last
packet in the queue when the queue overflows. A FQ link is for Fair Queuing (for details see [?]). A SFQ link is for Stochastic
Fair Queuing (for details see [?]). A DRR link is for deficit round robin scheduling (for details see [9]). A RED link is a ran-
dom-early drop queue (for details see [2]). A CBQ link is for class-based queuing using a packet-by-packet round-robin scheduler
(for details see [3]). A CBQ/WRR link is for class-based queuing with a weighted round robin scheduler.
$ns multi-link-of-interfaces node-list bw delay type
Connects the nodes specified in node-list by a mesh of duplex links with interfaces (to simulate a broadcast LAN) with bandwidth bw
in bits per second and link propagation delay delay in seconds. node-list is a list of node object handles that have already been
created with the node method. bw and delay default to 1.5 Mbits/sec and 100 ms respectively. The defaults can be changed by modi-
fying the relevant configuration parameters of the DelayLink Object (see DELAYLINK OBJECTS section). The queuing discipline of the
link is specified by type, which may be DropTail, FQ SFQ, DRR, RED, CBQ, or CBQ/WRR. A DropTail link is a simple FIFO queue which
drops the last packet in the queue when the queue overflows. A FQ link is for Fair Queuing (for details see [?]). A SFQ link is
for Stochastic Fair Queuing (for details see [?]). A DRR link is for deficit round robin scheduling (for details see [9]). A RED
link is a random-early drop queue (for details see [2]). A CBQ link is for class-based queuing using a packet-by-packet round-robin
scheduler (for details see [3]). A CBQ/WRR link is for class-based queuing with a weighted round robin scheduler.
new Agent/type
Create an Agent of type type which may be:
Null - Traffic Sink
LossMonitor - Traffic Sink that monitors loss parameters
TCP - BSD Tahoe TCP
TCP/FullTcp - Full Reno TCP with two-way connections [11]
TCP/Reno - BSD Reno TCP
TCP/Newreno - a modified version of BSD Reno TCP
TCP/Vegas - Vegas TCP (from U. Arizonia via USC)
TCP/Sack1 - BSD Reno TCP with selective ACKs
TCP/Fack - BSD Reno TCP with forward ACKs
TCPSink - standard TCP sink
TCPSink/DelAck - TCP sink that generates delayed ACKs
TCPSink/Sack1 - TCP sink that generates selective ACKs
TCPSink/Sack1/DelAck - delayed-ack TCP sink with selective ACKs
UDP - UDP Transport
RTP - RTP agent
Session/RTP -
RTCP - RTCP agent
IVS/Source -
IVS/Receiver -
SRM -
The methods, configuration parameters and the relevant state variables associated with these objects are discussed in detail in
later sections. Note that some agents e.g. TCP or SRM do not generate their own data. Such agents need sources attached to them to
generate data (see attach-source and attach-traffic methods in AGENT OBJECTS section).
$ns attach-agent node agent
Attach the agent object agent to node. The agent and node objects should have already been created.
$ns detach-agent node agent
Detach the agent object agent from node.
$ns connect src dst
Establish a two-way connection between the agent src and the agent dst. Returns the handle to src agent. A helper method has been
defined to facilitate creating and attaching an agent to each of two nodes and establishing a two-way connection between them. (see
BUILTINS section).
$ns use-scheduler type
Use an event scheduler of type type in the simulations. type is one of List, Heap, Calendar, RealTime. The List scheduler is the
default. A Heap scheduler uses a heap for event queueing. A Calendar scheduler uses a calendar queue to keep track of events.
RealTime scheduler is used in emulation mode when the simulator interacts with an external agent.
$ns at time procedure
Evaluate procedure at simulation time time. The procedure could be a globally accessible function (proc) or an object method (inst-
proc). This command can be used to start and stop sources, dynamically reconfigure the simulator, dump statistics at specified
intervals, etc. Returns an event id.
$ns cancel eid
Remove the event specified by the event id eid from the event queue.
$ns now
Return the current simulation time.
$ns gen-map
Walks through the simulation topology and lists all the objects that have been created and the way they are hooked up to each other.
This is useful to debug simulation scripts.
ns-version
Return a string identifying the version of ns currently running. This method is executed in the global context by the interpreter.
ns-random [ seed ]
If seed is not present, return a pseudo-random integer between 0 and 2^31-1. Otherwise, seed the pseudo-random number generator
with seed and return the seed used. If seed is 0, choose an initial seed heuristically (which varies on successive invocations).
This method is executed in the global context by the interpreter.
Ns has other facilities for random number generation; please see documentation for details [13].
OBJECT HIERARCHY
A brief description of the object hierarchy in ns is presented in this section. This description is not intended to be complete. It has
been provided to depict how the methods and configuration parameters associated with the various objects are inherited. For more complete
information see "ns notes & documentation" and the automatically generated class library information on the ns web page.
Objects are associated with configuration parameters that can be dynamically set and queried, and state variables that can be queried (usu-
ally modified only when the state variables need to be reset for another simulation run).
Configuration parameters represent simulation parameters that are usually fixed during the entire simulation (like a link bandwidth), but
can be changed dynamically if desired. State variables represent values that are specific to a given object and that object's implementa-
tion.
The following diagram depicts a portion the object hierarchy:
Simulator
MultiSim
Node
Link
SimpleLink
CBQLink
DummyLink
DelayLink
Queue
DropTail
FQ
SFQ
DRR
RED
CBQ
CBQ/WRR
QueueMonitor
ED
Flowmon
Flow
rtObject
RouteLogic
Agent
rtProto
Static
Session
DV
Direct
Null
LossMonitor
TCP
FullTcp
Reno
Newreno
Sack1
Fack
TCPSink
DelAck
Sack1
DelAck
UDP
RTP
RTCP
IVS
Source
Receiver
SRM
Session
RTP [how is this diff from Agent/CBR/RTP]
Appplication
FTP
Telnet
Traffic
Expoo
Pareto
CBR
Trace
Integrator
Samples
For a complete, automatically generated, object hierarchy, see the link "class hierarchy" (which points to http://www-
sop.inria.fr/rodeo/personnel/Antoine.Clerget/ns/) on the ns web pages. (Thanks to Antoine Clerget for maintaining this!)
For example, any method that is supported by a TCP agent is also supported by a Reno or a Sack1 agent. Default configuration parameters
are also inherited. For example, $tcp set window_ 20 where $tcp is a TCP agent defines the default TCP window size for both TCP and Reno
objects.
OBJECT METHODS
The following sections document the methods, configuration parameters and state variables associated with the various objects as well as
those to enable Network dynamics, Unicast routing, Multicast routing and Trace and Monitoring support. The object class is specified
implicitly by the object variable name in the description. For example, $tcp implies the tcp object class and all of its child classes.
NODE OBJECTS
[NOTE: This section has not been verified to be up-to-date with the release.]
$node id
Returns the node id.
$node neighbors
Returns a list of the neighbour node objects.
$node attach agent
Attach an agent of type agent to this node.
$node detach agent
Detach an agent of type agent from this node.
$node agent port
Return a handle to the agent attached to port port on this node. Returns an empty string if the port is not in use.
$node reset
Reset all agents attached to this node. This would re-initialize the state variables associated with the various agents at this
node.
$node rtObject?
Returns a handle to rtObject if there exists an instance of the object at that node. Only nodes that take part in a dynamic unicast
routing protocol will have this object (see UNICAST ROUTING METHODS and RTOBJECT OBJECTS section).
$node join-group agent group
Add the agent specified by the object handle agent to the multicast host group identified by the address group. This causes the
group membership protocol to arrange for the appropriate multicast traffic to reach this agent. Multicast group address should be
in the range 0x8000 - 0xFFFF.
$node allocaddr
Returns multicast group address in ascending order on each invocation starting from 0x8000 and ending at 0xFFFF.
$node shape shape
Set the shape of the node to "shape". When called before the simulator starts to run, it changes the default shape of the node in
the nam trace file. The default shape of a node is """circle"""
$node color color
Set the color of the node to color. It can be called anytime to change the current color of the node in nam trace file, if there is
one.
$node get-attribute name
Get the specified attribute name of the node. Currently a Node object has two attributes: COLOR and SHAPE. Note: these letters must
be capital.
$node add-mark name color shape
Add a mark (in nam trace file) with color and shape around the node. The shape can be """circle""", """hexagon""" and """square"""
(case sensitive). The added mark will be identified by name.
$node delete-mark name
Delete the mark with name in the given node.
There are no state variables or configuration parameters specific to the node class.
LINK OBJECTS
[NOTE: This section has not been verified to be up-to-date with the release.]
$link trace-dynamics ns fileID
Trace the dynamics of this link and write the output to fileID filehandle. ns is an instance of the Simulator or MultiSim object
that was created to invoke the simulation (see TRACE AND MONITORING METHODS section for the output trace format).
$link trace-callback ns cmd
Trace all packets on the link with the callback cmd. Cmd is invoked for each trace event (enqueue, dequeue, drop) with the text
that would be logged as parameters. (See the description of the log file for this information.) A demo of trace callbacks is in
the program tcl/ex/callback_demo.tcl in the distribution.
$link color color
Set the color of the Link object. It can be called anytime to change the current color of the link in nam trace file, if there is
one.
$link get-attribute name
Get the specified attribute name of the Link. Currently a Link object has three attributes: COLOR, ORIENTATION, and QUEUE_POS.
Currently the following two functions should not be directly called. Use $ns duplex-link-op instead. Refer to the corresponding section in
this man page.
$link orient ori
Set the orientation of the link to ori. When called before the simulator starts to run, it changes the default orientation of the
link in nam trace file, if there is one. If orientation is unspecified for any link(s), nam will use automatic layout. The default
orientation of a Link object is unspecified.
$link queuePos pos
Set the queue position of the link to pos. When called before the simulator starts to run, it changes the default queue placement of
the simplex link in nam trace file, if there is one. pos specifies the angle between the horizontal line and the line along which
queued packets will be displayed.
SIMPLELINK OBJECTS
[NOTE: This section has not been verified to be up-to-date with the release.]
$link cost cost-val
Make cost-val the cost of this link.
$link cost?
Return the cost of this link.
Any configuration parameters or state variables?
DELAYLINK OBJECTS
[NOTE: This section has not been verified to be up-to-date with the release.] The DelayLink Objects determine the amount of time required
for a packet to traverse a link. This is defined to be size/bw + delay where size is the packet size, bw is the link bandwidth and delay
is the link propagation delay. There are no methods or state variables associated with this object.
Configuration Parameters
bandwidth_
Link bandwidth in bits per second.
delay_ Link propagation delay in seconds.
There are no state variables associated with this object.
NETWORK DYNAMICS METHODS
This section describes methods to make the links and nodes in the topology go up and down according to various distributions. A dynamic
routing protocol should generally be used whenever a simulation is to be done with network dynamics. Note that a static topology model is
the default in ns.
$ns rtmodel model model-params node1 [node2]
Make the link between node1 and node2 change between up and down states according to the model model. In case only node1 is speci-
fied all the links incident on the node would be brought up and down according to the specified model. model-params contains the
parameters required for the relevant model and is to be specified as a list i.e. the parameters are to be enclosed in curly brack-
ets. model can be one of Deterministic, Exponential, Manual, Trace. Returns a handle to a model object corresponding to the speci-
fied model.
In the Deterministic model model-params is [start-time] up-interval down-interval [finish-time]. Starting from start-time the link
is made up for up-interval and down for down-interval till finish-time is reached. The default values for start-time, up-interval,
down-interval are 0.5s, 2.0s, 1.0s respectively. finish-time defaults to the end of the simulation. The start-time defaults to
0.5s in order to let the routing protocol computation quiesce.
If the Exponential model is used model-params is of the form up-interval down-interval where the link up-time is an exponential dis-
tribution around the mean up-interval and the link down-time is an exponential distribution around the mean down-interval. Default
values for up-interval and down-interval are 10s and 1s respectively.
If the Manual distribution is used model-params is at op where at specifies the time at which the operation op should occur. op is
one of up, down. The Manual distribution could be specified alternately using the rtmodel-at method described later in the section.
If Trace is specified as the model the link/node dynamics is read from a Tracefile. The model-params argument would in this case be
the file-handle of the Tracefile that has the dynamics information. The tracefile format is identical to the trace output generated
by the trace-dynamics link method (see TRACE AND MONITORING METHODS SECTION).
$ns rtmodel-delete model-handle
Delete the instance of the route model specified by model-handle.
$ns rtmodel-at at op node1 [node2]
Used to specify the up and down times of the link between nodes node1 and node2. If only node1 is given all the links incident on
node1 will be brought up and down. at is the time at which the operation op that can be either up or down is to be performed on the
specified link(s).
QUEUE OBJECTS
A queue object is a general class of object capable of holding and possibly marking or discarding packets as they travel through the simu-
lated topology.
Configuration Parameters
limit_ The queue size in packets.
blocked_
Set to false by default, this is true if the queue is blocked (unable to send a packet to its downstream neighbor).
unblock_on_resume_
Set to true by default, indicates a queue should unblock itself at the time the last packet packet sent has been transmitted
(but not necessarily received).
DROP-TAIL OBJECTS
Drop-tail objects are a subclass of Queue objects that implement simple FIFO queue. There are no methods that are specific to drop-tail
objects. The only configuration parameter is drop-front_, which when set to true causes the queue to behave as a drop-from-front queueing
discipline. This variable is set to false by default.
FQ OBJECTS
FQ objects are a subclass of Queue objects that implement Fair queuing. There are no methods that are specific to FQ objects.
Configuration Parameters
secsPerByte_
There are no state variables associated with this object.
SFQ OBJECTS
SFQ objects are a subclass of Queue objects that implement Stochastic Fair queuing. There are no methods that are specific to SFQ objects.
Configuration Parameters
maxqueue_
buckets_
There are no state variables associated with this object.
DRR OBJECTS
DRR objects are a subclass of Queue objects that implement deficit round robin scheduling. These objects implement deficit round robin
scheduling amongst different flows ( A particular flow is one which has packets with the same node and port id OR packets which have the
same node id alone). Also unlike other multi-queue objects, this queue object implements a single shared buffer space for its different
flows.
Configuration Parameters
buckets_
Indicates the total number of buckets to be used for hashing each of the flows.
blimit_
Indicates the shared buffer size in bytes.
quantum_
Indicates (in bytes) how much each flow can send during its turn.
mask_ mask_, when set to 1, means that a particular flow consists of packets having the same node id (and possibly different port
ids), otherwise a flow consists of packets having the same node and port ids.
RED OBJECTS
RED objects are a subclass of Queue objects that implement random early-detection gateways. The object can be configured to either drop or
``mark'' packets. There are no methods that are specific to RED objects.
Configuration Parameters
bytes_ Set to "true" to enable ``byte-mode'' RED, where the size of arriving packets affect the likelihood of marking (dropping)
packets.
queue-in-bytes_
Set to "true" to measure the average queue size in bytes rather than packets. Enabling this option also causes thresh_ and
maxthresh_ to be automatically scaled by mean_pktsize_ (see below).
thresh_
The minimum threshold for the average queue size in packets.
maxthresh_
The maximum threshold for the average queue size in packets.
mean_pktsize_
A rough estimate of the average packet size in bytes. Used in updating the calculated average queue size after an idle
period.
q_weight_
The queue weight, used in the exponential-weighted moving average for calculating the average queue size.
wait_ Set to true to maintain an interval between dropped packets.
linterm_
As the average queue size varies between "thresh_" and "maxthresh_", the packet dropping probability varies between 0 and
"1/linterm".
setbit_
Set to "true" to mark packets by setting the congestion indication bit in packet headers rather than drop packets.
drop-tail_
Set to true to use drop-tail rather than random-drop or drop-from-front when the queue overflows or the average queue size
exceeds "maxthresh_". This is the default behavior. For a further explanation of these variables, see [2].
drop-rand_
Set to true to use random-drop rather than drop-tail or drop-from-front when the queue overflows or the average queue size
exceeds "maxthresh_".
drop-front_
Set to true to use drop-from-front rather than drop-tail or random drop when the queue overflows or the average queue size
exceeds "maxthresh_".
ns1-compat_
Set to true to avoid resetting the count since the last packet drop, after a forced packet is dropped. This gives compati-
bility with previous behavior of RED. The default is set to false.
entle_ Set to true to increase the packet drop rate slowly from max_p to 1 as the average queue size ranges from maxthresh to twice
maxthresh. The default is set to false, and max_p increases abruptly from max_p to 1 when the average queue size exceeds
maxthresh.
State Variables
None of the state variables of the RED implementation are accessible.
CBQ OBJECTS
CBQ objects are a subclass of Queue objects that implement class-based queueing.
$cbq insert $class
Insert traffic class class into the link-sharing structure associated with link object cbq.
$cbq bind $cbqclass $id1 [$id2]
Cause packets containing flow id $id1 (or those in the range $id1 to $id2 inclusive) to be associated with the traffic class $cbq-
class.
$cbq algorithm $alg
Select the CBQ internal algorithm. $alg may be set to one of: "ancestor-only", "top-level", or "formal".
CBQ
/WRR OBJECTS
CBQ/WRR objects are a subclass of CBQ objects that implement weighted round-robin scheduling among classes of the same priority level. In
contrast, CBQ objects implement packet-by-packet round-robin scheduling among classes of the same priority level.
Configuration Parameters
maxpkt_
The maximum size of a packet in bytes. This is used only by CBQ/WRR objects in computing maximum bandwidth allocations for
the weighted round-robin scheduler.
CBQCLASS OBJECTS
CBQClass objects implement the traffic classes associated with CBQ objects.
$cbqclass setparams parent okborrow allot maxidle prio level extradelay
Sets several of the configuration parameters for the CBQ traffic class (see below).
$cbqclass parent [$cbqcl|none]
specify the parent of this class in the link-sharing tree. The parent may be specified as ``none'' to indicate this class is a
root.
$cbqclass newallot $a
Change the link allocation of this class to the specified amount (in range 0.0 to 1.0). Note that only the specified class is
affected.
$cbqclass install-queue $q
Install a Queue object into the compound CBQ or CBQ/WRR link structure. When a CBQ object is initially created, it includes no
internal queue (only a packet classifier and scheduler).
Configuration Parameters
okborrow_
is a boolean indicating the class is permitted to borrow bandwidth from its parent.
allot_ is the maximum fraction of link bandwidth allocated to the class expressed as a real number between 0.0 and 1.0.
maxidle_
is the maximum amount of time a class may be required to have its packets queued before they are permitted to be forwarded
priority_
is the class' priority level with respect to other classes. This value may range from 0 to 10, and more than one class may
exist at the same priority. Priority 0 is the highest priority.
level_ is the level of this class in the link-sharing tree. Leaf nodes in the tree are considered to be at level 1; their parents
are at level 2, etc.
extradelay_
increase the delay experienced by a delayed class by the specified number of seconds.
QUEUEMONITOR Objects
QueueMonitor Objects are used to monitor a set of packet and byte arrival, departure and drop counters. It also includes support for
aggregate statistics such as average queue size, etc. [see TRACE AND MONITORING METHODS].
$queuemonitor reset
reset all the cumulative counters described below (arrivals, departures, and drops) to zero. Also, reset the integrators and delay
sampler, if defined.
$queuemonitor set-delay-samples delaySamp_
Set up the Samples object delaySamp_ to record statistics about queue delays. delaySamp_ is a handle to a Samples object i.e the
Samples object should have already been created.
$queuemonitor get-bytes-integrator
Returns an Integrator object that can be used to find the integral of the queue size in bytes. (see Integrator Objects section).
$queuemonitor get-pkts-integrator
Returns an Integrator object that can be used to find the integral of the queue size in packets. (see Integrator Objects section).
$queuemonitor get-delay-samples
Returns a Samples object delaySamp_ to record statistics about queue delays (see Samples Objects section).
There are no configuration parameters specific to this object.
State Variables
size_ Instantaneous queue size in bytes.
pkts_ Instantaneous queue size in packets.
parrivals_
Running total of packets that have arrived.
barrivals_
Running total of bytes contained in packets that have arrived.
pdepartures_
Running total of packets that have departed (not dropped).
bdepartures_
Running total of bytes contained in packets that have departed (not dropped).
pdrops_
Total number of packets dropped.
bdrops_
Total number of bytes dropped.
bytesInt_
Integrator object that computes the integral of the queue size in bytes. The sum_ variable of this object has the running
sum (integral) of the queue size in bytes.
pktsInt_
Integrator object that computes the integral of the queue size in packets. The sum_ variable of this object has the running
sum (integral) of the queue size in packets.
QUEUEMONITOR
/ED Objects
This derived object is capable of differentiating regular packet drops from early drops. Some queues distinguish regular drops (e.g. drops
due to buffer exhaustion) from other drops (e.g. random drops in RED queues). Under some circumstances, it is useful to distinguish these
two types of drops.
State Variables
epdrops_
The number of packets that have been dropped ``early''.
ebdrops_
The number of bytes comprising packets that have been dropped ``early''
Note: because this class is a subclass of QueueMonitor, objects of this type also have fields such as pdrops_ and bdrops_. These fields
describe the total number of dropped packets and bytes, including both early and non-early drops.
QUEUEMONITOR
/ED/FLOWMON Objects
These objects may be used in the place of a conventional QueueMonitor object when wishing to collect per-flow counts and statistics in
addition to the aggregate counts and statistics provided by the basic QueueMonitor.
$fmon classifier [$cl]
insert (read) the specified classifier into (from) the flow monitor object. This is used to map incoming packets to which flows
they are associated with.
$fmon dump
Dump the current per-flow counters and statistics to the I/O channel specified in a previous attach operation.
$fmon flows
Return a character string containing the names of all flow objects known by this flow monitor. Each of these objects are of type
QueueMonitor/ED/Flow.
$fmon attach $chan
Attach a tcl I/O channel to the flow monitor. Flow statistics are written to the channel when the dump operation is executed.
Configuration Parameters
enable_in_
Set to true by default, indicates that per-flow arrival state should be kept by the flow monitor. If set to false, only the
aggregate arrival information is kept.
enable_out_
Set to true by default, indicates that per-flow departure state should be kept by the flow monitor. If set to false, only
the aggregate departure information is kept.
enable_drop_
Set to true by default, indicates that per-flow drop state should be kept by the flow monitor. If set to false, only the
aggregate drop information is kept.
enable_edrop_
Set to true by default, indicates that per-flow early drop state should be kept by the flow monitor. If set to false, only
the aggregate early drop information is kept.
QUEUEMONITOR
/ED/FLOW Objects
These objects contain per-flow counts and statistics managed by a QUEUEMONITOR/ED/FLOWMON object. They are generally created in an OTcl
callback procedure when a flow monitor is given a packet it cannot map on to a known flow. Note that the flow monitor's classifier is
responsible for mapping packets to flows in some arbitrary way. Thus, depending on the type of classifier used, not all of the state vari-
ables may be relevant (e.g. one may classify packets based only on flow id, in which case the source and destination addresses may not be
significant).
State Variables
src_ The source address of packets belonging to this flow.
dst_ The destination address of packets belonging to this flow.
flowid_
The flow id of packets belonging to this flow.
UNICAST ROUTING METHODS
A dynamic unicast routing protocol can be specified to run on a subset of nodes in the topology. Note that a dynamic routing protocol
should be generally used whenever a simulation is done with network dynamics.
$ns rtproto proto node-list
Specifies the dynamic unicast routing protocol proto to be run on the nodes specified by node-list. Currently proto can be one of
Static, Session, DV. Static routing is the default. Session implies that the unicast routes over the entire topology are instanta-
neously recomputed whenever a link goes up or down. DV implies that a simple distance vector routing protocol is to be simulated.
node-list defaults to all the nodes in the topology.
$ns compute-routes
Compute routes between all the nodes in the topology. This can be used if static routing is done and the routes have to be recom-
puted as the state of a link has changed. Note that Session routing (see rtproto method above) will recompute routes automatically
whenever the state of any link in the topology changes.
$ns get-routelogic
Returns an handle to a RouteLogic object that has methods for route table lookup etc.
ROUTELOGIC OBJECTS
$routelogic lookup srcid destid
Returns the id of the node that is the next hop from the node with id srcid to the node with id destid.
$routelogic dump nodeid
Dump the routing tables of all nodes whose id is less than nodeid. Node ids are typically assigned to nodes in ascending fashion
starting from 0 by their order of creation.
RTOBJECT OBJECTS
Every node that takes part in a dynamic unicast routing protocol will have an instance of rtObject (see NODE OBJECTS section for the method
to get an handle to this object at a particular node). Note that nodes will not have an instance of this object if Session routing is done
as a detailed routing protocol is not being simulated in this case.
$rtobject dump-routes fileID
Dump the routing table to the output channel specified by fileID. fileID must be a file handle returned by the Tcl open command and
it must have been opened for writing.
$rtobject rtProto? proto
Returns a handle to the routing protocol agent specified by proto if it exists at that node. Returns an empty string otherwise.
$rtobject nextHop? destID
Returns the id of the node that is the next hop to the destination specified by the node id, destID.
$rtobject rtpref? destID
$rtobject metric? destID
MULTICAST ROUTING METHODS
Multicast routing is enabled by setting Simulator EnableMcast_ variable to 1 at the beginning of the simulation. Note that this variable
must be set before any node, link or agent objects are created in the simulation. Also links must have been created with interface labels
(see simplex-link and duplex-link methods in NS COMMANDS section).
$ns mrtproto proto node-list
Specifies the multicast routing protocol proto to be run on the nodes specified by node-list. Currently proto can be one of CtrM-
cast, DM, detailedDM, dynamicDM, pimDM. node-list defaults to all the nodes in the topology. Returns an handle to a protocol-spe-
cific object that has methods, configuration parameters specific to that protocol. Note that currently CtrMcastComp object is
returned if CtrMcast is used but a null string is returned if DM, detailedDM, dynamicDM or pimDM are used.
If proto is 'CtrMcast' a Rendezvous Point (RP) rooted shared tree is built for a multicast group. The actual sending of prune, join
messages etc. to set up state at the nodes is not simulated. A centralized computation agent is used to compute the fowarding
trees and set up multicast forwarding state, (*,G) at the relevant nodes as new receivers join a group. Data packets from the
senders to a group are unicast to the RP. Methods are provided in the CtrMcastComp object (see CTRMCASTCOMP OBJECTS section) that
is returned by mrtproto to switch to source-specific trees, choose some nodes as candidate RPs etc. When a node/link on a multicast
distribution tree goes down, the tree is instanteously recomputed.
If proto is 'DM' DVMRP-like dense mode is simulated. Parent-child lists are used to reduce the number of links over which the data
packets are broadcast. Prune messages are sent by nodes to remove branches from the multicast forwarding tree that do not lead to
any group members. The prune timeout value is 0.5s by default (see DM OBJECTS section to change the default). This does not adapt
to network changes. There is also currently no support for proper functioning in topologies with LANs.
If proto is 'detailedDM' a dense mode protocol based on Protocol Independent Multicast - Dense Mode (PIM-DM) is simulated. This is
currently the most complete version of the dense mode protocol in the simulator and is recommended for use over the other dense mode
protocols. It adapts to network dynamics and functions correctly in topologies with LANs (where LANs are created using the multi-
link-of-interfaces method - see NS COMMANDS). In case there are multiple potential forwarders for a LAN, the node with the highest
id is chosen as the forwarder (this is done through the Assert mechanism). The default values for the prune timeout, interface
deletion timeout (used for LANs) and graft retransmission timeout are 0.5s, 0.1s and 0.05s respectively. (see Prune/Iface/Timer,
Deletion/Iface/Timer and GraftRtx/Timer objects respectively to change the default values and for more information about the
timers).
If proto is 'dynamicDM' DVMRP-like dense mode protocol that adapts to network changes is simulated. 'Poison-reverse' information
(i.e. the information that a particular neighbouring node uses this node to reach a particular network) is read from the routing
tables of neighbouring nodes in order to adapt to network dynamics (DVMRP runs its own unicast routing protocol that exchanges this
information). The current implementation does not support proper functioning in topologies with LANs. The prune timeout value is
0.5s by default (see DM OBJECTS section to change the default).
If proto is 'pimDM' Protocol Independent Multicast - Dense mode is simulated. In this case the data packets are broadcast over all
the outgoing links except the incoming link. Prune messages are sent by nodes to remove the branches of the multicast forwarding
tree that do not lead to any group members. The current implementation does not adapt to network dynamics and does not support
proper functioning in topologies with LANs. The prune timeout value is 0.5s by default (see DM OBJECTS section to change the
default).
CTRMCASTCOMP OBJECTS
A handle to the CtrMcastComp object is returned when the protocol is specified as 'CtrMcast' in mrtproto.
$ctrmcastcomp switch-treetype group-addr
Switch from the Rendezvous Point rooted shared tree to source-specific trees for the group specified by group-addr. Note that this
method cannot be used to switch from source-specific trees back to a shared tree for a multicast group.
$ctrmcastcomp set_c_rp node-list
Make all the nodes specified in node-list as candidate RPs and change the state of all the other nodes to not be candidate RPs.
Note that all nodes are candidate RPs by default. Currently the node with the highest node id serves as the RP for all multicast
groups. This method should be invoked before any source starts sending packets to the group or any receiver joins the group.
$ctrmcastcomp get_rp node group
Returns the RP for the group as seen by the node node for the multicast group with address group-addr. Note that different nodes
may see different RPs for the group if the network is partitioned as the nodes might be in different partitions.
DM OBJECTS
DM Objects implement DVMRP style densemode multicast where parent-child lists are used to reduce the number of links over which initial
data packets are broadcast. There are no methods or state variables specific to this object.
Configuration parameters
PruneTimeout
Timeout value for the prune state at nodes.
PRUNE
/IFACE/TIMER OBJECTS
The Prune/Iface/Timer objects are used to implement the prune timer for detailedDM. There are no methods or state variables specific to
this object.
Configuration parameters
timeout
Timeout value for the prune state at nodes.
DELETION
/IFACE/TIMER OBJECTS
The Deletion/Iface/Timer objects are used to implement the interface deletion timer that are required for correct functioning at nodes that
are part of LANs. If a node has a LAN as its incoming interface for packets from a certain source and it does not have any downstream mem-
bers it sends out a prune message onto the LAN. Any node that has the LAN as its incoming interface for the same source and has downstream
members on hearing the prune message sent on the LAN. will send a join message onto the LAN. When the node that is acting as the for-
warder for the LAN hears the prune message from the LAN, it does not immediately prune off the LAN as its outgoing interface. Instead it
starts an interface deletion timer for the outgoing interface. The forwarder will remove the LAN as its outgoing interface only if it does
not receive any join messages from the LAN before its deletion timer expires. There are no methods or state variables specific to this
object.
Configuration parameters
timeout
Timeout value for the interface deletion timer.
GRAFTRTX
/TIMER OBJECTS
The GraftRtx/Timer objects are used to implement the graft retransmission timer at nodes. This is to ensure the reliability of grafts sent
upstream by a node.
Configuration parameters
timeout
Timeout value for the graft retransmission timer.
AGENT OBJECTS
[NOTE: This section has not been verified to be up-to-date with the release.]
$agent port
Return the transport-level port of the agent. Ports are used to identify agents within a node.
$agent dst-addr
Return the address of the destination node this agent is connected to.
$agent dst-port
Return the port at the destination node that this agent is connected to.
$agent attach-source type
Install a data source of type type in this agent. type is one of FTP or bursty[???]. See the corresponding object methods for
information on configuration parameters. Returns a handle to the source object.
$agent attach-traffic traffic-object
Attach traffic-object to this agent traffic-object is an instance of Traffic/Expoo, Traffic/Pareto or Traffic/Trace. Traffic/Expoo
generates traffic based on an Exponential On/Off distribution. Traffic/Pareto generates traffic based on a Pareto On/Off distribu-
tion. Traffic/Trace generates traffic from a trace file. The relevant configuration parameters for each of the above objects can
be found in the TRAFFIC METHODS section.
$agent connect addr port
Connect this agent to the agent identified by the address addr and port port. This causes packets transmitted from this agent to
contain the address and port indicated, so that such packets are routed to the intended agent. The two agents must be compatible
(e.g., a tcp-source/tcp-sink pair as opposed a cbr/tcp-sink pair). Otherwise, the results of the simulation are unpredictable.
Configuration Parameters
dst_ Address of destination that the agent is connected to. Currently 32 bits with the higher 24 bits the destination node ID and
the lower 8 bits being the port number.
There are no state variables specific to the generic agent class.
NULL OBJECTS
[NOTE: This section has not been verified to be up-to-date with the release.] Null objects are a subclass of agent objects that implement
a traffic sink. They inherit all of the generic agent object functionality. There are no methods, configuration parameters or state vari-
ables specific to this object.
LOSSMONITOR OBJECTS
[NOTE: This section has not been verified to be up-to-date with the release.] LossMonitor objects are a subclass of agent objects that
implement a traffic sink which also maintains some statistics about the received data e.g., number of bytes received, number of packets
lost etc. They inherit all of the generic agent object functionality.
$lossmonitor clear
Resets the expected sequence number to -1.
Configuration Parameters
There are no configuration parameters specific to this object.
State Variables
nlost_ Number of packets lost.
npkts_ Number of packets received.
bytes_ Number of bytes received.
lastPktTime_
Time at which the last packet was received.
expected_
The expected sequence number of the next packet.
TCP OBJECTS
TCP objects are a subclass of agent objects that implement the BSD Tahoe TCP transport protocol as described in [7]. They inherit all of
the generic agent functionality.
To trace TCP parameters, mark each parameter with ``$tcp trace window_'' and then send the output to a trace file with ``$tcp attach [open
trace.tr w]''.
Tcp segments can be sent with the advance and advanaceby commands. When all data is sent, the done method will be invoked (which can be
overridden in OTcl).
$tcp advance n
Send up to the nth packets.
$tcp advanceby n
Send n more packets.
$tcp done
Functional called when all packets (specified by advance/advanceby/maxpkts_) have been sent. Can be overriden on a per-object
basis.
Configuration Parameters
window_
The upper bound on the advertised window for the TCP connection (in packets).
maxcwnd_
The upper bound on the congestion window for the TCP connection. Set to zero to ignore. (This is the default.) Measured in
packets.
windowInit_
The initial size of the congestion window on slow-start. (in packets).
wnd_init_option_
The algorithm used for determining the initial size of the congestion window. Set to 1 for a static algorithm using the
value in windowInit_. Set to 2 for a dynamic algorithm using a function of packetSize_.
syn_ Set to true to model the initial SYN/ACK exchange in one-way TCP. Set to false as default.
delay_growth_
Set to true to delay the initial congestion window until after one packet has been sent and acked. Set to false as default.
windowOption_
The algorithm to use for managing the congestion window in linear phase. The standard algorithm is 1 (the default). Other
experimental algorithms are documented in the source code.
windowThresh_
Gain constant to exponential averaging filter used to compute awnd (see below). For investigations of different window-
increase algorithms.
overhead_
The range (in seconds) of a uniform random variable used to delay each output packet. The idea is to insert random delays at
the source in order to avoid phase effects, when desired [4]. This has only been implemented for the Tahoe ("tcp") version
of tcp, not for tcp-reno. This is not intended to be a realistic model of CPU processing overhead.
ecn_ Set to true to use explicit congestion notification in addition to packet drops to signal congestion. This allows a Fast
Retransmit after a quench() due to an ECN (explicit congestion notification) bit.
packetSize_
The size in bytes to use for all packets from this source.
tcpip_base_hdr_size_
The size in bytes of the base TCP/IP header.
tcpTick_
The TCP clock granularity for measuring roundtrip times. Note that it is set by default to the non-standard value of 100ms.
Measured in seconds.
bugFix_
Set to true to remove a bug when multiple fast retransmits are allowed for packets dropped in a single window of data.
maxburst_
Set to zero to ignore. Otherwise, the maximum number of packets that the source can send in response to a single incoming
ACK.
slow_start_restart_
Boolean; set to 1 to slow-start after the connection goes idle. On by default.
srtt_init_
Initial value for the smoothed roundtrip time estimate. Default is 0 seconds.
t_rttvar_
Initial value for the variance in roundtrip time. Default is 3 seconds.
rtxcur_init_
Initial value for the retransmit value. Default is 6 seconds.
T_SRTT_BITS
Exponent of weight for updating the smoothed round-trip time t_srtt_. Default is 3, for a weight of 1/2^T_SRTT_BITS or 1/8.
T_RTTVAR_BITS
Exponent of weight for updating variance in round-trip time, t_rttvar_. Default is 2, for a weight of 1/2^T_RTTVAR_BITS or
1/4.
rttvar_exp_
Exponent of multiple of the mean deviation in calculating the current retransmit value t_rtxcur_. Default is 2, for a multi-
ple of 2^rttvar_exp_ or 4.
Defined Constants
MWS The Maximum Window Size in packets for a TCP connection. MWS determines the size of an array in tcp-sink.cc. The default
for MWS is 1024 packets. For Tahoe TCP, the "window" parameter, representing the receiver's advertised window, should be
less than MWS-1. For Reno TCP, the "window" parameter should be less than (MWS-1)/2.
State Variables
dupacks_
Number of duplicate acks seen since any new data was acknowledged.
seqno_ Highest sequence number for data from data source to TCP.
t_seqno_
Current transmit sequence number.
ack_ Highest acknowledgment seen from receiver.
cwnd_ Current value of the congestion window (in packets).
awnd_ Current value of a low-pass filtered version of the congestion window. For investigations of different window-increase algo-
rithms.
ssthresh_
Current value of the slow-start threshold (in packets).
rtt_ Round-trip time estimate. In seconds (expressed in multiples of tcpTick_).
srtt_ Smoothed round-trip time estimate. In seconds (in multiples of tcpTick_/8).
rttvar_
Round-trip time mean deviation estimate.
t_rtxcur_
Current retransmit value. In seconds.
backoff_
Round-trip time exponential backoff constant.
TCP
/RENO OBJECTS
TCP/Reno objects are a subclass of TCP objects that implement the Reno TCP transport protocol as described in [7]. There are no methods,
configuration parameters or state variables specific to this object.
TCP
/NEWRENO OBJECTS
TCP/Newreno objects are a subclass of TCP objects that implement a modified version of the BSD Reno TCP transport protocol.
There are no methods or state variables specific to this object.
Configuration Parameters
newreno_changes_
Set to zero for the default NewReno described in [7]. Set to 1 for additional NewReno algorithms as suggested in [10]; this
includes the estimation of the ssthresh parameter during slow-start.
TCP
/VEGAS OBJECTS
This section of the man page has not yet been written.
TCP
/SACK1 OBJECTS
TCP/Sack1 objects are a subclass of TCP objects that implement the BSD Reno TCP transport protocol with Selective Acknowledgement Exten-
sions as described in [7].
They inherit all of the TCP object functionality. There are no methods, configuration parameters or state variables specific to this
object.
TCP
/FACK OBJECTS
TCP/Fack objects are a subclass of TCP objects that implement the BSD Reno TCP transport protocol with Forward Acknowledgement congestion
control.
They inherit all of the TCP object functionality. There are no methods or state variables specific to this object.
Configuration Parameters
ss-div4
Overdamping algorithm. Divides ssthresh by 4 (instead of 2) if congestion is detected within 1/2 RTT of slow-start.
(1=Enable, 0=Disable)
rampdown
Rampdown data smoothing algorithm. Slowly reduces congestion window rather than instantly halving it. (1=Enable, 0=Disable)
TCP
/FULLTCP OBJECTS
This section has not yet been added to the man page. The implementation and the configuration parameters are described in [11].
TCPSINK OBJECTS
TCPSink objects are a subclass of agent objects that implement a receiver for TCP packets. The simulator only implements "one-way" TCP
connections, where the TCP source sends data packets and the TCP sink sends ACK packets. TCPSink objects inherit all of the generic agent
functionality. There are no methods or state variables specific to the TCPSink object.
Configuration Parameters
packetSize_
The size in bytes to use for all acknowledgment packets.
maxSackBlocks_
The maximum number of blocks of data that can be acknowledged in a SACK option. For a receiver that is also using the time
stamp option [RFC 1323], the SACK option specified in RFC 2018 has room to include three SACK blocks. This is only used by
the TCPSink/Sack1 subclass. This value may not be increased within any particular TCPSink object after that object has been
allocated. (Once a TCPSink object has been allocated, the value of this parameter may be decreased but not increased).
TCPSINK
/DELACK OBJECTS
DelAck objects are a subclass of TCPSink that implement a delayed-ACK receiver for TCP packets. They inherit all of the TCPSink object
functionality. There are no methods or state variables specific to the DelAck object.
Configuration Parameters
interval_
The amount of time to delay before generating an acknowledgment for a single packet. If another packet arrives before this
time expires, generate an acknowledgment immediately.
TCPSINK
/SACK1 OBJECTS
TCPSink/Sack1 objects are a subclass of TCPSink that implement a SACK receiver for TCP packets. They inherit all of the TCPSink object
functionality. There are no methods, configuration parameters or state variables specific to this object.
TCPSINK
/SACK1/DELACK OBJECTS
TCPSink/Sack1/DelAck objects are a subclass of TCPSink/Sack1 that implement a delayed-SACK receiver for TCP packets. They inherit all of
the TCPSink/Sack1 object functionality. There are no methods or state variables specific to this object.
Configuration Parameters
interval_
The amount of time to delay before generating an acknowledgment for a single packet. If another packet arrives before this
time expires, generate an acknowledgment immediately.
SRM OBJECTS
SRM objects are a subclass of agent objects that implement the SRM reliable multicast transport protocol. They inherit all of the generic
agent functionalities.
$srm traffic-source source
Attach a traffic source, e.g., Application/Traffic/CBR, to the SRM agent.
$srm start
Join the multicast group, start the SRM agent and its attached traffic source.
$srm delete
Stop the SRM agent, delete all its status and detach the traffic source.
$srm trace trace-file
Write the traces generated by the SRM agent to trace-file. The traces includes timer settings, request and repair sending and
receipts, etc. Two related files that are not built into ns are tcl/mcast/srm-debug.tcl that permits more detailed tracing of the
delay computation functions, and tcl/mcast/srm-nam.tcl that separately marks srm control messages from data. The latter is useful
to enhance nam visualisation.
$srm log log-file
Write the recovery statistics during each request or repair to log-file. The statistics include start time, duration, message id,
total number of duplicate requests and repairs.
$srm distance? node
Return the distance estimate to node in this SRM agent.
$srm distances? node
Returns a list of <group member, distance> tuples of the distances to all group members that this node is aware of. The group mem-
ber is identified as the address of the remote agent. The first tuple is this agent's token. The list can be directly loaded into
a Tcl array.
Configuration Parameters
packetSize_
The data packet size in bytes that will be used for repair messages. The default value is 1024.
requestFunction_
The algorithm used to produce a retransmission request, e.g., setting request timers. The default value is SRM/request. Other
possible request functions are SRM/request/Adaptive, used by the Adaptive SRM code.
repairFunction_
The algorithm used to produce a repair, e.g., compute repair timers. The default value is SRM/repair. Other possible request
functions are SRM/repair/Adaptive, used by the Adaptive SRM code.
sessionFunction_
The algorithm used to generate session messages. Default is SRM/session
sessionDelay_
The basic interval of session messages. Slight random variation is added to this interval to avoid global synchronization of
session messages. User may want to adjust this variable according to their specific simulation. Measured in seconds; default
value is 1.0 seconds.
C1_, C2_
The parameters which control the request timer. Refer to [8] for detail. The default value is C1_ = C2_ = 2.0.
D1_, D2_
The parameters which control the repair timer. Refer to [8] for detail. The default value is D1_ = D2_ = 1.0.
requestBackoffLimit_
The maximum number of exponential backoffs. Default value is 5.
State Variables
stats_ An array containing multiple statistics needed by adaptive SRM agent. Including: duplicate requests and repairs in current
request/repair period, average number of duplicate requests and repairs, request and repair delay in current request/repair
period, average request and repair delay.
SRM
/Adaptive OBJECTS
SRM/Adaptive objects are a subclass of the SRM objects that implement the adaptive SRM reliable multicast transport protocol. They inherit
all of the SRM object functionalities.
State Variables Refer to the SRM paper by Sally et al ([11]) for more detail.
pdistance_
This variable is used to pass the distance estimate provided by the remote agent in a request or repair message.
D1_, D2_
The same as that in SRM agents, except that they are initialized to log10(group size) when generating the first repair.
MinC1_, MaxC1_, MinC2_, MaxC2_
The minimum/maximum values of C1_ and C2_. Default initial values are defined in [8]. These values define the dynamic range
of C1_ and C2_.
MinD1_, MaxD1_, MinD2_, MaxD2_
The minimum/maximum values of D1_ and D2_. Default initial values are defined in [8]. These values define the dynamic range
of D1_ and D2_.
AveDups
Higher bound for average duplicates.
AveDelay
Higher bound for average delay.
eps AveDups - dups determines the lower bound of the number of duplicates, when we should adjust parameters to decrease delay.
APPLICATION OBJECTS
Application objects generate data for transport agents to send.
FTP APPLICATION OBJECTS
Application/FTP objects produce bulk data for a TCP object to send.
$ftp start
Causes FTP to produce packets indefinitely.
$ftp produce n
Causes the FTP object to produce n packets instantaneously.
$ftp stop
Causes the attached TCP object to stop sending data.
$ftp attach agent
Attaches an Application/FTP object to agent.
$ftp producemore count
Causes the Application/FTP object to produce count more packets.
Configuration Parameters
maxpkts
The maximum number of packets generated.
TELNET APPLICATION OBJECTS
Application/Telnet objects produce individual packets with inter-arrival times as follows. If interval_ is non-zero, then inter-arrival
times are chosen from an exponential distribution with average interval_. If interval_ is zero, then inter-arrival times are chosen using
the "tcplib" telnet distribution.
$telnet start
Causes the Application/Telnet object to start producing packets.
$telnet stop
Causes the Application/Telnet object to stop producing packets.
$telnet attach agent
Attaches a Application/Telnet object to agent.
Configuration Parameters
interval_
The average inter-arrival time in seconds for packets generated by the Application/Telnet object.
TRAFFIC OBJECTS
Traffic objects create data for a transport protocol to send. A Traffic object is created by instantiating an object of class Applica-
tion/Traffic/type where type is one of Exponential, Pareto, CBR, Trace.
EXPONENTIAL TRAFFIC OBJECTS
Application/Traffic/Exponential objects generate On/Off traffic. During "on" periods, packets are generated at a constant burst rate.
During "off" periods, no traffic is generated. Burst times and idle times are taken from exponential distributions.
Configuration Parameters
packet_size_
The packet size in bytes.
burst_time_
Burst duration in seconds.
idle_time_
Idle time in seconds.
rate_ Peak rate in bits per second.
PARETO TRAFFIC OBJECTS
Application/Traffic/Pareto objects generate On/Off traffic with burst times and idle times taken from pareto distributions.
Configuration Parameters
packet_size_
The packet size in bytes.
burst_time_
Average on time in seconds.
idle_time_
Average off time in seconds.
rate_ Peak rate in bits per second.
shape_ Pareto shape parameter.
CBR (CONSTANT BIT RATE) TRAFFIC OBJECTS
Application/Traffic/CBR objects generate packets at a constant rate. Dither can be added to the interarrival times by enabling the "ran-
dom" flag.
Configuration Parameters
rate_ Peak rate in bits per second.
packet_size_
The packet size in bytes.
random_
Flag that turns dithering on and off (default is off).
maxpkts_
Maximum number of packets to send.
TRACE TRAFFIC OBJECTS
Application/Traffic/Trace objects are used to generate traffic from a trace file.
$trace attach-tracefile tfile
Attach the Tracefile object tfile to this trace. The Tracefile object specifies the trace file from which the traffic data is to be
read (see TRACEFILE OBJECTS section). Multiple Application/Traffic/Trace objects can be attached to the same Tracefile object. A
random starting place within the Tracefile is chosen for each Application/Traffic/Trace object.
There are no configuration parameters for this object.
TRACEFILE OBJECTS
Tracefile objects are used to specify the trace file that is to be used for generating traffic (see TRAFFIC/TRACE OBJECTS section).
$tracefile is an instance of the Tracefile Object.
$tracefile filename trace-input
Set the filename from which the traffic trace data is to be read to trace-input.
There are no configuration parameters for this object. A trace file consists of any number of fixed length records. Each record consists
of 2 32 bit fields. The first indicates the interval until the next packet is generated in microseconds. The second indicates the length
of the next packet in bytes.
TRACE AND MONITORING METHODS
[NOTE: This section has not been verified to be up-to-date with the release.] Trace objects are used to generate event level capture logs
typically to an output file. Throughout this section $ns refers to a Simulator object, $agent refers to an Agent object.
$ns create-trace type fileID node1 node2 [option]
Create a Trace object of type type and attach the filehandle fileID to it to monitor the queues between nodes node1 and node2. type
can be one of Enque, Deque, Drop. Enque monitors packet arrival at a queue. Deque monitors packet departure at a queue. Drop mon-
itors packet drops at a queue. fileID must be a file handle returned by the Tcl open command and it must have been opened for writ-
ing. If option is not specified, the command will instruct the created trace object to generate ns traces. If option is """nam"""
the new object will produce nam traces. Returns a handle to the trace object.
$ns drop-trace node1 node2 trace
Remove trace object attached to the link between nodes node1 and node2 with trace as the object handle.
$ns trace-queue node1 node2 fileID
Enable Enque, Deque and Drop tracing on the link between node1 and node2.
$ns namtrace-queue node1 node2 fileID
Same function as $ns trace-queue, except it produces nam traces.
$ns trace-all fileID
Enable Enque, Deque, Drop Tracing on all the links in the topology created after this method is invoked. Also enables the tracing
of network dynamics. fileID must be a file handle returned by the Tcl open command and it must have been opened for writing.
$ns namtrace-all fileID
Same function as $ns trace-all, except it will produce all equivalent traces in nam format. In addition, calling this command before
the simulator starts to run will generate color configurations (if any) and topology information needed by nam (nodes, links,
queues). An example can be found at ns-2/tcl/ex/nam-example.tcl.
$ns namtrace-config fileID
Assign a file to store nam configuration information, e.g., node/link/agents and some Simulator-related traces such as annotations.
When you don't want to trace every object. call this function and then use $ns namtrace-queue, rtModel trace, etc., to insert traces
individually. Note that you should use the same file for individual traces and nam configuration. An example for this is available
at ns-2/tcl/ex/nam-separate-trace.tcl.
$ns monitor-queue node1 node2
Arrange for queue length of link between nodes node1 and node2 to be tracked. Returns QueueMonitor object that can be queried to
learn average queue size etc. [see QueueMonitor Objects section]
$ns flush-trace
Flush the output channels attached to all the trace objects.
$link trace-dynamics ns fileID [option]
Trace the dynamics of this link and write the output to fileID filehandle. ns is an instance of the Simulator or MultiSim object
that was created to invoke the simulation.
$ns color id name
Create a color index, which links the number id to the color name name. All colors created before the simulator starts to run will
be written to nam trace file, if there is any.
$ns trace-annotate string
Writes an annotation to ns and nam trace file, if there are any. The string should be enclosed in double quote to make it a single
argument.
trace_annotate string
Another version of $ns trace-annotate, which is a global function and doesn't require the caller to know ns.
$ns duplex-link-op $node1 $node2 $op $args
Perform a given operation $op on the given duplex link ($node1, $node2). The following two operations may be used:
orient - Specify the nam orientation of the duplex link. Values can be
left, right, up, down, their mixture combined by '-' (e.g.,
left-down), and a number specifying the angle between the
link and the horizontal line.
queuePos - Construct a queue of the simplex link ($node1,
$node2) in nam, and specify the angle between the
horizontal line and the line along which the queued packets
will be displayed.
$ns add-agent-trace agent name [fileID]
Write a nam trace line, which will create a trace agent for agent when interpreted by nam. The trace agent's name will be name. This
nam trace agent is used to show the position of agent and can be used to write nam traces of variables associated with the agent.
By default traces will be written to the file assigned by namtrace-all. fileID can be used to write traces to another file.
$agent tracevar name
Label OTcl variable name of $agent to be traced. Then whenever the variable name changes value, a nam trace line will be written to
nam trace file, if there is one. Note that name must be the same as the variable's real OTcl name.
$ns delete-agent-trace agent
Write a nam trace line, which will delete the nam trace associated with agent when interpreted by nam.
$agent add-var-trace name value [type]
Write a nam trace line, which creates a variable trace with name name and value value, when interpreted by nam. type indicates the
type of the variable, e.g., is it a list, array, or a plain variable. Currently only plain variable is supported, for which type =
'v'.
The following 2 functions should be called after the simulator starts running. This can be done using $ns at.
$agent delete-var-trace name
Write a nam trace line, which deletes the variable trace name when interpreted by nam.
$agent update-var-trace name value [type]
Write a nam trace line, which changes the value of traced variable name when interpreted by nam. Unlike $agent tracevar, the above 3
functions provide 'manual' variable tracing, in which variable tracing are done by placing $agent update-var-trace in OTcl code,
while tracevar automatically generates nam traces when the traced variable changes value.
The tracefile format is backward compatible with the output files in the ns version 1 simulator so that ns-1 post-processing scripts can
still be used. Trace records of traffic for link objects with Enque, Deque or Drop Tracing have the following form:
<code> <time> <hsrc> <hdst> <packet>
where
<code> := [hd+-r] h=hop d=drop +=enque -=deque r=receive
<time> := simulation time in seconds
<hsrc> := first node address of hop/queuing link
<hdst> := second node address of hop/queuing link
<packet> := <type> <size> <flags> <flowID> <src.sport> <dst.dport> <seq> <pktID>
<type> := tcp|telnet|cbr|ack etc.
<size> := packet size in bytes
<flags> := [CP] C=congestion, P=priority
<flowID> := flow identifier field as defined for IPv6
<src.sport> := transport address (src=node,sport=agent)
<dst.sport> := transport address (dst=node,dport=agent)
<seq> := packet sequence number
<pktID> := unique identifer for every new packet
Only those agents interested in providing sequencing will generate sequence numbers and hence this field may not be useful for pack-
ets generated by some agents.
For links that use RED gateways, there are additional trace records as follows:
<code> <time> <value>
where
<code> := [Qap] Q=queue size, a=average queue size,
p=packet dropping probability
<time> := simulation time in seconds
<value> := value
Trace records for link dynamics are of the form:
<code> <time> <state> <src> <dst>
where
<code> := [v]
<time> := simulation time in seconds
<state> := [link-up | link-down]
<src> := first node address of link
<dst> := second node address of link
INTEGRATOR Objects
Integrator Objects support the approximate computation of continuous integrals using discrete sums. The running sum(integral) is computed
as: sum_ += [lasty_ * (x - lastx_)] where (x, y) is the last element entered and (lastx_, lasty_) was the element previous to that added
to the sum. lastx_ and lasty_ are updated as new elements are added. The first sample point defaults to (0,0) that can be changed by
changing the values of (lastx_,lasty_).
$integrator newpoint x y
Add the point (x,y) to the sum. Note that it does not make sense for x to be less than lastx_.
There are no configuration parameters specific to this object.
State Variables
lastx_ x-coordinate of the last sample point.
lasty_ y-coordinate of the last sample point.
sum_ Running sum (i.e. the integral) of the sample points.
SAMPLES Objects
Samples Objects support the computation of mean and variance statistics for a given sample.
$samples mean
Returns mean of the sample.
$samples variance
Returns variance of the sample.
$samples cnt
Returns a count of the sample points considered.
$samples reset
Reset the Samples object to monitor a fresh set of samples.
There are no configuration parameters or state variables specific to this object.
BUILTINS
[NOTE: This section has not been verified to be up-to-date with the release.] Because OTcl is a full-fledged programming language, it is
easy to build high-level simulation constructs from the ns primitives. Several library routines have been built in this way, and are
embedded into the ns interpreter as methods of the Simulator class. Throughout this section $ns represents a Simulator object.
$ns create-connection srcType srcNode dstType dstNode class
Create a source agent of type srcType at node srcNode and connect it to a destination agent of type dstType at node dstNode. Also,
connect the destination agent to the source agent. The traffic class of both agents is set to class. This method returns the
source agent.
EXAMPLE
set ns [new Simulator]
#
# Create two nodes
#
set n0 [$ns node]
set n1 [$ns node]
#
# Create a trace and arrange for all the trace events of the
# links subsequently created to be dumped to "out.tr"
#
set f [open out.tr w]
$ns trace-all $f
#
# Connect the two nodes with a 1.5Mb link with a transmission
# delay of 10ms using FIFO drop-tail queuing
#
$ns duplex-link $n0 $n1 1.5Mb 10ms DropTail
#
# Set up BSD Tahoe TCP connections in opposite directions.
#
set tcp_src1 [new Agent/TCP]
set tcp_snk1 [new Agent/TCPSink]
set tcp_src2 [new Agent/TCP]
set tcp_snk2 [new Agent/TCPSink]
$ns attach-agent $n0 $tcp_src1
$ns attach-agent $n1 $tcp_snk1
$ns attach-agent $n1 $tcp_src2
$ns attach-agent $n0 $tcp_snk2
$ns connect $tcp_src1 $tcp_snk1
$ns connect $tcp_src2 $tcp_snk2
#
# Create ftp sources at the each node
#
set ftp1 [$tcp_src1 attach-source FTP]
set ftp2 [$tcp_src2 attach-source FTP]
#
# Start up the first ftp at the time 0 and
# the second ftp staggered 1 second later
#
$ns at 0.0 "$ftp1 start"
$ns at 1.0 "$ftp2 start"
#
# run the simulation for 10 simulated seconds
#
$ns at 10.0 "exit 0"
$ns run
DEBUGGING
To enable debugging when building ns from source:
% ./configure --enable-debug
% make
For more details about ns debugging please see <http://www-mash.cs.berkeley.edu/ns/ns-debugging.html>.
DIFFERENCES FROM NS-1
In general, more complex objects in ns-1 have been broken down into simpler components for greater flexibility and composability. Details
of differences between ns-1 and ns-2 can be found at <http://www-mash.cs.berkeley.edu/ns/ns.html>.
HISTORY
Work on the LBL Network Simulator began in May 1990 with modifications to S. Keshav's (keshav@research.att.com) REAL network simulator,
which he developed for his Ph.D. work at U.C. Berkeley. In Summer 1991, the simulation description language was revamped, and later, the
NEST threads model was replaced with an event driven framework and an efficient scheduler. Among other contributions, Sugih Jamin
(jamin@usc.edu) contributed the calendar-queue based scheduling code to this version of the program, which was known as tcpsim. In Decem-
ber 1994, McCanne ported tcpsim to C++ and replaced the yacc-based simulation description language with a Tcl interface, and added prelimi-
nary multicast support. Also at this time, the name changed from tcpsim to the more generic ns. Throughout, Floyd has made modifications
to the TCP code and added additional source models for her investigations into RED gateways, resource management, class-based queuing,
explicit congestion notification, and traffic phase effects. Many of the papers discussing these issues are available through URL
http://www-nrg.ee.lbl.gov/.
SEE ALSO
Tcl(1), tclsh(1), nam(1), otclsh
[1] S. Keshav, ``REAL: A Network Simulator''. UCB CS Tech Report 88/472, December 1988. See http://min-
nie.cs.adfa.oz.au/REAL/index.html for more information.
[2] Floyd, S. and Jacobson, V. Random Early Detection gateways for Congestion Avoidance. IEEE/ACM Transactions on Networking, Vol. 1,
No. 4. August 1993. pp. 397-413. Available from http://www-nrg.ee.lbl.gov/floyd/red.html.
[3] Floyd, S. Simulator Tests. July 1995. URL ftp://ftp.ee.lbl.gov/papers/simtests.ps.Z.
[4] Floyd, S., and Jacobson, V. On Traffic Phase Effects in Packet-Switched Gateways. Internetworking: Research and Experience, V.3
N.3, September 1992. pp. 115-156.
[5] Floyd, S., and Jacobson, V. Link-sharing and Resource Management Models for Packet Networks. IEEE/ACM Transactions on Networking,
Vol. 3 No. 4, August 1995. pp. 365-386.
[6] Floyd, S., Notes of Class-Based Queueing: Setting Parameters. URL ftp://ftp.ee.lbl.gov/papers/ params.ps.Z. September 1995.
[7] Fall, K., and Floyd, S. Comparisons of Tahoe, Reno, and Sack TCP. December 1995. URL ftp:// ftp.ee.lbl.gov/papers/sacks.ps.Z.
[8] David Wetherall and Christopher J. Linblad. Extending Tcl for Dynamic Object-Oriented Programming. In Proceedings of the USENIX
Tcl/Tk Workshop, Toronto, Ontario, USENIX. July, 1995. At <http://www.tns.lcs.mit.edu/publications/tcltk95.djw.html>.
[9] M. Shreedhar and G. Varghese. Efficient Fair Queueing Using Deficit Round Robin. In Proc. of SIGCOMM, pp. 231-242, 1995.
[10] Hoe, J., Improving the Start-up Behavior of a Congestion Control Scheme for TCP. in SIGCOMM 96, August 1996, pp. 270-280. URL
http://www.acm.org/sigcomm/sigcomm96/papers/hoe.html.
[11] Fall, K., Floyd, S., and Henderson, T., Ns Simulator Tests for Reno FullTCP. URL ftp://ftp.ee.lbl.gov/papers/fulltcp.ps. July
1997.
[12] Floyd, S., Jacobson, V., Liu, C.-G., McCanne, S. and Zhang, L., A Reliable Multicast Framework for Light-weight Sessions and Appli-
cation Level Framing. To appear in IEEE/ACK Transaction on Networking, November 1996. ftp://ftp.ee.lbl.gov/papers/srm1.ps.gz
[13] Fall, K., and Varadhan, K., (eds.), "Ns notes and documentation", work in progress. http://www-mash.cs.berkeley.edu/ns/nsDoc.ps.gz
Research using ns is on-going. A list of recent research contributions employing ns can be found at <http://www-mash.cs.berke-
ley.edu/ns/ns-research.html>.
Work on ns is on-going. Information about the most recent version is available at <http://www-mash.cs.berkeley.edu/ns/ns.html>.
A mailing list for ns users and announcements is also available, send mail to ns-users-request@mash.cs.berkeley.edu or ns-announce-
request@mash.cs.berkeley.edu to join. Questions should be forwarded to ns-users; ns-announce will be low-traffic announcements only.
AUTHORS
Steven McCanne (mccanne@ee.lbl.gov), University of California, Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA, and Sally
Floyd (floyd@ee.lbl.gov) Lawrence Berkeley National Laboratory, Berkeley, CA. A complete list of contributors to ns is at <http://www-
mash.cs.berkeley.edu/ns/ns-contributors.html>.
BUGS
Not all of the functionality supported in ns-1 has been ported to ns-2.
This manual page is incomplete.
25 July 1997 NS(1)