Presentation 6:
Workflow of TCP in
NS3
Presented By :
1705033
1705034
1705037
1705048
TcpSocketBase
TcpSocketBase
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A base class for
implementation of a
stream socket using TCP.
This class contains the
essential components of
TCP.
Does Congestion Control
together with
TcpSocketState Class.
TcpSocketBase
Member Functions Related to
Congestion Control
SetCongestionControlAlgorithm()
●
Install a congestion
control algorithm on
this socket.
TcpL4Protocol::CreateSocket()
SetInitialCwnd()
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Set the initial
Congestion Window.
SetInitialSSThresh()
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Set the initial Slow
Start Threshold.
Other Setter Functions
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SetMinRTO()
SetRtt()
SetRcvBufSize()
SetSegSize()
SetConnTimeout() etc etc
Most of the Setter functions have getter and update functions too!!
TcpSocketState
TcpSocketState
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Data Structure that
records Congestion State
of a connection.
Has lots of public
attributes that keeps the
information of a
congestion state.
TcpSocketState
Public Attributes
Public Attributes
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m_cWnd: Congestion Window Size
m_initialCWnd: Initial Congestion Window size
m_initialSSThresh : Initial Slow Start Threshold Value
m_lastRtt : Last Rtt Sample Collected
m_minRtt: Minimum Rtt Observed Throughout the Connection
m_segmentSize: Segment Size
m_ssThresh: Slow Start Threshold
etc etc….
If we want to get any value of a congestion control related parameter, most of the
time we will get them through these attributes of an object of TcpSocketState
Class
Code Examples
Set value to the attributes:
Constructor of tcpSocketBase()
Keep these values up-to-date:
Constructor of tcpSocketBase()
TcpSocketState
Member Functions
GetCwndInSegments()
●
Get Congestion
Window in Segments
rather than bytes.
TcpCubic: GetSsThresh()
GetSsThreshInSegments()
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Get Slow Start Threshold in Segments rather than bytes.
Similiar to GetCwndInSegments()
TcpCongestionOps
TcpCongestionOps
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The congestion control is split from the
main socket code, and it is a pluggable
component.
It is an abstract class
Variables are maintained in
the TcpSocketState class
Subclasses of
TcpCongestionOps operate
over an instance of that
class.
TcpCongestionOps
Member Functions
IncreaseWindow()
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Congestion avoidance algorithm implementation.
New segments have been ACKed, and the congestion control duty is to update the
window.
This function is allowed to change directly cWnd and/or ssThresh.
IncreaseWindow()
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Reimplemented in TcpNewReno :
ns-3.35/src/internet/model/tcp-congestion-ops.cc : line 214
IncreaseWindow()
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Reimplemented in TcpVegas :
./ns-3.35/src/internet/model/tcp-vegas.cc:156:
PktsAcked()
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The function is called every time an ACK is received and contains timing
information.
It is optional (congestion controls need not implement it)
The default implementation does nothing.
PktsAcked()
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Reimplemented in TcpVegas :
ns-3.35/src/internet/model/tcp-vegas.cc :100
Usage of IncreaseWindow and PktsAcked
ns-3.35/src/internet/test/tcp-vegas-test.cc : 118
CongControl()
●
Called when packets are delivered to update cwnd and pacing rate.
Reimplemented in TcpBbr :
./ns-3.35/src/internet/model/tcp-bbr.cc:689:
CongestionStateSet()
●
Trigger events/calculations specific to a congestion state.
CongestionStateSet()
Reimplemented in TcpVegas :
./ns-3.35/src/internet/model/tcp-vegas.cc:141:
CongestionStateSet()
Reimplemented in TcpBbr :
…}
./ns-3.35/src/internet/model/tcp-bbr.cc:701:
CongestionStateSet()
Usage :
./ns-3.35/src/internet/test/tcp-bbr-test.cc:77:
CwndEvent()
● Trigger events/calculations on occurrence of congestion window event.
CwndEvent()
Reimplemented in TcpBbr :
…
…
./ns-3.35/src/internet/model/tcp-bbr.cc:744:
TcpRecoveryOps
TcpRecoveryOps
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Inspired by the TcpCongestionOps class
The fast recovery is split from the main socket code, and it
is a pluggable component.
Subclasses of TcpRecoveryOps modifies TcpSocketState
variables
TcpRecoveryOps
Member Functions
EnterRecovery()
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Called when the first loss is guessed
Performs variable initialization at the start of recovery.
The function is called when the TcpSocketState is changed to
CA_RECOVERY.
DoRecovery()
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Called each time a duplicate ACK is received
Performs recovery based on the recovery algorithm.
The function is called on arrival of every ack when
TcpSocketState is set to CA_RECOVERY.
It performs the necessary cwnd changes as per the recovery
algorithm.
ExitRecovery()
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Called when the sequence transmitted when the socket
entered the Recovery phase is ACKed, therefore ending phase
Performs cwnd adjustments at the end of recovery.
The function is called when the TcpSocketState is changed
from CA_RECOVERY.
Declaration
./ns-3.35/build/ns3/tcp-recovery-ops.h:115:
Implementation
./ns-3.35/src/internet/model/tcp-recovery-ops.cc:108:
Usage
./ns-3.35/src/internet/test/tcp-classic-recovery-test.cc:80:
Using CongestionOps and RecoveryOps in Socket
./ns-3.35/src/internet/model/tcp-socket-base.h:1327:
Using CongestionOps and RecoveryOps in Socket
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The fork method from both CongestionOps and RecoveryOps class is used to
copy the congestion control algorithm and recovery algorithm across socket.
…
}
./ns-3.35/src/internet/model/tcp-socket-base.cc:386:
Tcp Vegas
TCP VEGAS
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TCP Vegas is a TCP congestion avoidance
algorithm that emphasizes packet delay, rather
than packet loss, as a signal to help determine
the rate at which to send packets.
TCP Vegas is a pure delay-based congestion
control algorithm
TCP Vegas Inheritance Diagram
Tcp Vegas
Set Attributes for Major Phases
AddAttribute
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AddAttribute() method associates a given string with a strongly typed value in the class
./ns-3.35/src/internet/model/tcp-vegas.cc:37
AddAttribute
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The key point is that now the value of the variables like m_alpha and its default
value are accessible in the attribute namespace, which is based on string such as
"Alpha"
./ns-3.35/src/internet/test/tcp-vegas-test.cc:158
Constructor
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Attribute initialization in the
constructor of tcp-vegas.cc
./ns-3.35/src/internet/model/tcp-vegas.cc:60
Diff Calculation
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Vegas continuously samples the RTT and computes the actual throughput a
connection achieves and compares it with the expected throughput.
./ns-3.35/src/internet/test/tcp-vegas-test.cc:146
Diff Calculation
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The difference between these two sending rates reflects the amount of extra
packets being queued at the bottleneck.
./ns-3.35/src/internet/test/tcp-vegas-test.cc:154
Alpha
● Alpha is the lower bound of
packets in the network
./ns-3.35/src/internet/model/tcp-vegas.cc:44
Beta
● Beta is the upper bound of
packets in the network
./ns-3.35/src/internet/model/tcp-vegas.cc:48
Gamma
● Gamma is the limit on increase
./ns-3.35/src/internet/model/tcp-vegas.cc:52
Tcp Vegas
Slow Start Phase
Slow Start Phase
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Vegas calls TcpNewReno::SlowStart when congestion window is less than slow start
threshold and diff is less than gamma.
./ns-3.35/src/internet/model/tcp-vegas.cc:233
TCP NewReno Slow Start Phase
./ns-3.35/src/internet/model/tcp-congestion-ops.cc:165
Tcp Vegas
Congestion Avoidance
Phase
Congestion Avoidance Phase
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Vegas starts congestion avoidance phase when congestion window is less than
slow start threshold but diff is greater than gamma.
./ns-3.35/src/internet/model/tcp-vegas.cc:163
Congestion Avoidance Phase
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Vegas linearly decreases
congestion window if diff
is greater than beta.
./ns-3.35/src/internet/test/tcp-vegas-test.cc:179
Congestion Avoidance Phase
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Vegas linearly increases
congestion window if diff
is greater than beta.
./ns-3.35/src/internet/test/tcp-vegas-test.cc:184
Congestion Avoidance Phase
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Vegas does nothing if diff
is in between the
predefined threshold
value
./ns-3.35/src/internet/test/tcp-vegas-test.cc:184
Implementation
./ns-3.35/src/internet/test/tcp-vegas.cc:155
Fast
Retransmission and
Recovery
Fast Retransmission And Recovery (FRR)
● Without FRR, the TCP uses a timer that requires a retransmission timeout if a
packet is lost.
● No new or duplicate packets can be sent during the timeout period.
● With FRR, if a receiver receives a data segment that is out of order, it immediately
sends a duplicate acknowledgement to the sender.
● If the sender receives three duplicate acknowledgements, it assumes that the
data segment is lost and immediately retransmits the lost segment.
Fast Retransmission and Recovery In NS3
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Implemented inside tcp-socket-base.cc in processAck() function.
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It first checks if :
○ The incoming ACK is a duplicate acknowledgment
○ And the TCP is not currently in loss recovery
./ns-3.35/src/internet/model/tcp-socket-base.cc:1922:
Then it calls DupAck() functidfdsf
Inside DupAck()
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Here it checks if ‘DupAcks >= DupThresh’
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DupThresh is set to 3 by default
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If true, then it calls ‘EnterRecovery()’
./ns-3.35/src/internet/model/tcp-socket-base.cc:1721:
Inside EnterRecovery()
● Recovery point is set to the highest seq number.
● Recovery state is activated.
./ns-3.35/src/internet/model/tcp-socket-base.cc:1626:
Inside EnterRecovery()
● ssThresh is reduced.
● DoRetransmit() is called.
./ns-3.35/src/internet/model/tcp-socket-base.cc:1635:
Inside
DoRetransmit()
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Finds the first lost segment
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Updates the trace
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Retransmit the segment by calling SendDataPacket()
./ns-3.35/src/internet/model/tcp-socket-base.cc:3859:
./ns-3.35/src/internet/model/tcp-socket-base.cc:3873:
Thank You