ns2_agents.ppt

7/27/2019 ns2_agents.ppt

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NS2 Agents

Leo Bhebhe


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Contents

Introduction

Application Composition

Attaching Transport Agents to Nodes

UDP Agent

TCP Agents SCTP Agents

Agent SRM


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Introduction

In real-world systems, applications typically access network servicesthrough an applications programming interface (API).

The most popular of these APIs is known as sockets.

In ns, we mimic the behavior of the sockets API through a set of well-defined API functions.

These functions are then mapped to the appropriate internal agentfunctions (e.g., a call to send(numBytes) causes TCP to increment its sendbuffer by a corresponding number of bytes)

Applications sit on top of transport agents in ns.

Running Running, e.g. an TCP simulation requires creating and configuringthe agent, attaching an application-level data source (a traffic generator),and starting the agent and the traffic generator.

There are two basic types of applications: Traffic generators and Simulated applications.


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Example of Application Composition

Traffic generators Simulated applications

Agent/UDP Agent/TCP/FullTcp

Application/

Traffic/

Exponential

Application/FTP

The diagram illustrates how agents and applications arehooked together and communicate with one another viathe API

API API


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Attaching Transport Agents to Nodes

set src [new Agent/TCP/FullTcp]

set sink [new Agent/TCP/FullTcp]$ns_ attach-agent$node_(s1) $src

$ns_ attach-agent$node_(k1) $sink

$ns_ connect $src $sink

set udp0 [new Agent/UDP]$ns attach-agent$n0 $udp0

set cbr0 [new Application/Traffic/CBR]

$cbr0 attach-agent$udp0

$udp0 set packetSize_ 536 ;

#(max=1000)set null0 [new Agent/Null]

$ns attach-agent$n1 $null0

$ns connect $udp0 $null0


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UDP Agent

A UDP agent accepts data in variable size chunks from an application,and segments the data if needed.

UDP packets also contain a monotonically increasing sequence numberand an RTP timestamp.

The default maximum segment size (MSS) for UDP agents is 1000 byte:

Agent/UDP set packetSize_ 1000 ;# max segment size

set ns [new Simulator]

set n0 [$ns node]

set n1 [$ns node]

$ns duplex-link $n0 $n1 5Mb

2ms DropTail

set udp0 [new Agent/UDP]$ns attach-agent $n0 $udp0set cbr0 [new Application/Traffic/CBR]$cbr0 attach-agent $udp0$udp0 set packetSize_ 536 ;# set MSS to 536bytesset null0 [new Agent/Null]$ns attach-agent $n1 $null0$ns connect $udp0 $null0$ns at 1.0 "$cbr0 start"

Create src, dest. nodes & a link Create agents & starting traffic generator


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TCP Agents (1/2)

The TCP agent does not generate any application data on its own

Instead, the simulation user can connect any trafc generation module to theTCP agent to generate data.

Two applications are commonly used for TCP: FTP and Telnet.


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TCP Agents (2/2)

The one-way TCP sending agents currently supported are:

Agent/TCP - a tahoe TCP sender Agent/TCP/Reno - a Reno TCP sender Agent/TCP/Newreno - Reno with a modication Agent/TCP/Sack1 - TCP with selective repeat (follows RFC2018) Agent/TCP/Vegas - TCP Vegas Agent/TCP/Fack - Reno TCP with .forward acknowledgment.

The one-way TCP receiving agents currently supported are:

Agent/TCPSink - TCP sink with one ACK per packet Agent/TCPSink/DelAck - TCP sink with congurable delay per ACK Agent/TCPSink/Sack1 - selective ACK sink (follows RFC2018) Agent/TCPSink/Sack1/DelAck - Sack1 with DelAck

The two-way experimental sender currently supports only a Reno form ofTCP: Agent/TCP/FullTcp Note:Still under development


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One Way TCP Senders (1/3)

What can they do

Attempt to capture the the essence of TCP congestion and error controlbehaviours

Segment number and ACK computations entirely in the packet units

What they cant do

Cant mimic the real-world TCP implementations

They do not have a dynamic window advertisement

Theres no SYN/FIN connection establishement/teardown

No data is ever transferred (e.g. no checksums or urgent data)


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The Base TCP Sender (Tahoe TCP)

Performs congestion control Round-trip-time estimation

RTO Timeout Selection

Creating the Agent

set ns [new Simulator] ;#preamble initialization

set node1 [$ns node] ;# agent to reside on this node

set node2 [$ns node] ;# agent to reside on this node

set tcp1 [$ns create-connection TCP $node1 TCPSink$node2 42]

$tcp set window_ 50 ;# configure the TCP agent set ftp1 [new Application/FTP]

$ftp1 attach-agent $tcp1

$ns at 0.0 "$ftp start


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Reno TCP:The Reno TCP agent is very similar to the Tahoe TCP agent,except it also includes fast recovery, where the current congestion window

is .inflated. by the number of duplicate ACKs the TCP sender has receivedbefore receiving a new ACK.

The Reno TCP agent does not return to slow-start during a fast retransmit.Rather, it reduces sets the congestion window to half the current windowand resets ssthresh_ to match this value.

Newreno TCPThis agent is based on the Reno TCP agent, but which

modifies the action taken when receiving new ACKS. In order to exit fast recovery, the sender must receive an ACK for the

highest sequence number sent. Thus, new partial ACKs. (those whichrepresent new ACKs but do not represent an ACK for all outstanding data)do not deflate the window (and possibly lead to a stall, characteristic ofReno).

Vegas TCPThis agent implements Vegas TCP. It was contributed by TedKuo.

Sack TCPThis agent implements selective repeat, based on selectiveACKs provided by the receiver. It follows the ACK scheme developed withMatt Mathis and Jamshid Mahdavi.

Fack TCPThis agent implements forward ACK TCP, a modication of


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One Way TCP Receiver (Sinks)

The base TCP sink object (Agent/TCPSink ) is responsible forreturning ACKs to a peer TCP source object.

It generates one ACK per packet received. The size of the ACKsmay be configured.

The creation and configuration of the TCP sink object isgenerally performed automatically when creating a connection.

Configuration parameters

Base TCP Sink

Agent/TCPSink set packetSize_ 40

Delayed-ACK TCP Sink

Agent/TCPSink/Delack set interval_ 100ms

SACK TCP Sink

Agent/TCPSink set maxSacksBlocks_ 3


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Two Way TCP Senders (1/2)

The Agent/TCP/FullTcp

Connections may be establised and town down (SYN/FIN packets areexchanged)

Bidirectional data transfer is supported

Sequence numbers are in bytes rather than packets

Creating the Agent set src [new Agent/TCP/FullTcp] ;# create agent

set sink [new Agent/TCP/FullTcp] ;# create agent $ns_ attach-agent $node_(s1) $src ;# bind src to node $ns_ attach-agent $node_(k1) $sink ;# bind sink to node $src set fid_ 0 ;# set flow ID field $sink set fid_ 0 ;# set flow ID field $ns_ connect $src $sink ;# active connection src to sink # set up TCP-level connections $sink listen ;# will figure out who its peer is

$src set window_ 100;

The creation of the FullTcp agent is similar to the other agents, but the sink is placed ina listening state by the listen method.


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Two Way TCP Senders (2/2)

The Agent/TCP/BayFullTcp: Different implementation of two-way TCP hasbeen ported into ns from Kathy Nicholes/Van Jacobson's group. It iscalled

The basic difference between BayFullTcp and FullTcp (the two-way tcpversion already present in ns) are as follows:

BayTcp supports a client-server application model while FullTcp makesno assumption about its application layer.

The tcp-application interface is different for both;

FullTcp supports partial ack (BayTcp doesn't).

FullTcp supports different flavors of tcp (tahoe, reno etc) which is notthe case for baytcp.

Both implementations have different set of API's .

There might be other finer differences between the two as well. One of ourfuture plans is to redefine the APIs to allow fulltcp to use baytcp's client-server model.


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SCTP Agents

SCTP agents were developed for ns by the Protocol Engineering Lab at theUniversity of Delaware.

The SCTP agents are all two-way agents, which means they are symmetricin the sense that they represent both a sender and receiver.

However, bi-directional data has not yet been implemented. Each instanceof an SCTP agent is either a sender or receiver

The SCTP agents currently supported are: Agent/SCTP - RFC2960 Agent/SCTP/Newreno - includes Fast Recovery Agent/SCTP/HbAfterRto - experimental extension (HEARTBEAT after RTO) Agent/SCTP/MultipleFastRtx - experimental extension (UD PEL's Multiple Fast

Retransmit algorithm) Agent/SCTP/Timestamp - experimental extension (TIMESTAMP chunk)

The SCTP agents are implemented in les matching the regular expression~ns/sctp/sctp*.{cc, h}.


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SCTP Agents

The base SCTP Agent

Supportsfeatures of RFC2960

Does not yet support

Normal Establishment of an Association (rudimentaryhandshake)

Transmission of DATA Chunks

Acknowledgment on Reception of DATA Chunks

Management Retransmission Timer Multihomed SCTP Endpoints

Stream Identier and Stream Sequence Number

Ordered and Unordered Delivery

Report Gaps in Received DATA TSNs

SCTP Slow-Start and Congestion Avoidance

Endpoint Failure Detection

Path Failure Detection

Path Heartbeat (without upper layer control)


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SCTP Agents

The base SCTP Agent

Configuration

Agent/SCTP set pathMaxRetrans_ 5 ;# Changes the class variable

$sctp set pathMaxRetrans_ 5 ;# Changes pathMaxRetrans_ for the $sctp object only


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SCTP Agents Extensions

Newreno SCTP Similar to the base SCTP agent,

Introduces Fast Recovery mechanism to avoid multiple cwndreductions in a single round-trip time. Restricts the cwnd from being increased during Fast Recovery

HbAfterRto SCTP Better RTT estimatimation A heartbeat is sent immediately to the destination on which a

timeout occurred.

MultipleFastRtx SCTP Attempts to minimize the number of timeouts which occur Algorithm allows the same TSN to be Fast Retransmitted

several times if needed without waiting for thr RTO

Timestamp SCTP Introduces timestamps into each packet, thus allowing a sender

to disambiguate original transmissions from retransmissions. Retransmissions on the alternate path can be used to update

the RTT estimate and keep the RTO value more accurate. With timestamps, the sender has more samples for updating

the RTT estimate of alternate destination(s)


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Agent/SRM

SRM agent performs the following functions

Packet handling, Loss recovery, and Session message activity.

Creating the Agent

set ns [new Simulator] ;#preamble initialization $ns enableMcast set node [$ns node] ;# agent to reside on this node set group [$ns allocaddr] ;# multicast group for this agent

set srm [new Agent/SRM] $srm set dst_ $group ;# configure the SRM agent $ns attach-agent $node $srm

$srm set fid_ 1 ;# optional configuration $srm log [open srmStats.tr w] ;# log statistics in this file $srm trace [open srmEvents.tr w] ;# trace events for this agent

The key steps in configuring a virgin SRM agent are to assign its multicast group,and attach it to a node.


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THANK YOU!

Documents

ns2_agents.ppt