NS2 Agents

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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 services through 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 agent functions (e.g., a
  call to send(numBytes) causes TCP to increment
  its send buffer 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 configuring the 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
Simulated applications
Traffic generators
Application/ Traffic/ Exponential
Application/FTP
API
API
Agent/UDP
Agent/TCP/FullTcp
The diagram illustrates how agents and
applications are hooked together and communicate
with one another via the 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 (max1000)
• 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 number and an RTP timestamp.
• The default maximum segment size (MSS) for UDP
  agents is 1000 byte
• Agent/UDP set packetSize_ 1000 max segment
  size

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

• set udp0 new Agent/UDP
• ns attach-agent n0 udp0
• set cbr0 new Application/Traffic/CBR
• cbr0 attach-agent udp0
• udp0 set packetSize_ 536 set MSS to 536 bytes
• set null0 new Agent/Null
• ns attach-agent n1 null0
• ns connect udp0 null0
• ns at 1.0 "cbr0 start"

• set ns new Simulator
• set n0 ns node
• set n1 ns node
• ns duplex-link n0 n1 5Mb 2ms DropTail

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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 the TCP 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 of TCP
• 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 control behaviours
• 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/teard
  own
• No data is ever transferred (e.g. no checksums
  or urgent data)

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One Way TCP Senders (2/3)

• 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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One Way TCP Senders (3/3)

• 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 received before 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 window
  and resets ssthresh_ to match this value.
• Newreno TCP This 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 which
  represent new ACKs but do not represent an ACK
  for all outstanding data) do not deflate the
  window (and possibly lead to a stall,
  characteristic of Reno).
• Vegas TCP This agent implements Vegas TCP. It
  was contributed by Ted Kuo.
• Sack TCP This agent implements selective repeat,
  based on selective ACKs provided by the receiver.
  It follows the ACK scheme developed with Matt
  Mathis and Jamshid Mahdavi.
• Fack TCP This agent implements forward ACK TCP,
  a modication of Sack TCP

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One Way TCP Receiver (Sinks)

• The base TCP sink object (Agent/TCPSink ) is
  responsible for returning ACKs to a peer TCP
  source object.
• It generates one ACK per packet received. The
  size of the ACKs may be configured.
• The creation and configuration of the TCP sink
  object is generally 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 are exchanged)
• 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 in a
  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 has been ported
  into ns from Kathy Nicholes/Van Jacobson's group.
  It is called
• The basic difference between BayFullTcp and
  FullTcp (the two-way tcp version already present
  in ns) are as follows
• BayTcp supports a client-server application
  model while FullTcp makes no 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 not the case for
  baytcp.
• Both implementations have different set of API's
  .
• There might be other finer differences between
  the two as well. One of our future 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 the University of Delaware.
• The SCTP agents are all two-way agents, which
  means they are symmetric in the sense that they
  represent both a sender and receiver.
• However, bi-directional data has not yet been
  implemented. Each instance of 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
• Supports features of RFC2960
• Does not yet support
• Normal Establishment of an Association
  (rudimentary handshake)
• 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 cwnd reductions 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

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THANK YOU!