Example to verify TCP FACK performance based on Mathis & Mahdavi paper. More...

#include "ns3/applications-module.h"
#include "ns3/core-module.h"
#include "ns3/flow-monitor-module.h"
#include "ns3/internet-module.h"
#include "ns3/network-module.h"
#include "ns3/point-to-point-module.h"
#include "ns3/traffic-control-module.h"

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Functions
static void	BytesInFlightTracer (Ptr< OutputStreamWrapper > stream, uint32_t oldval, uint32_t newval)
	Trace BytesInFlight in segments.
void	CheckQueueSize (Ptr< QueueDisc > qd)
	Check and trace the queue size.
static void	CwndTracer (Ptr< OutputStreamWrapper > stream, uint32_t oldval, uint32_t newval)
	Trace congestion window.
static void	FackAwndTracer (Ptr< OutputStreamWrapper > stream, uint32_t oldval, uint32_t newval)
	Trace FACK's inflight(AWND) in segments.
void	TraceBytesInFlight (uint32_t nodeId, uint32_t socketId)
	Bind the BytesInFlight trace source.
void	TraceCwnd (uint32_t nodeId, uint32_t socketId)
	Bind the Congestion Window trace source.
void	TraceFackAwnd (uint32_t nodeId, uint32_t socketId)
	Bind the FackAwnd trace source.
static void	TracePacketDrop (Ptr< OutputStreamWrapper > stream, Ptr< const QueueDiscItem > item)
	Trace packets dropped at Queue.
static void	TraceThroughput (Ptr< FlowMonitor > monitor)
	Calculate and trace throughput.

Variables
std::string	g_dir
	Output directory.
std::ofstream	g_individualLostSegments
	Lost segments output stream.
uint32_t	g_prev = 0
	Previous throughput byte count.
Time	g_prevTime
	Previous throughput measurement time.
std::ofstream	g_queueSize
	Queue size output stream.
uint32_t	g_segmentSize = 1448
	Segment size.
std::ofstream	g_throughput
	Throughput output stream.

Detailed Description

Example to verify TCP FACK performance based on Mathis & Mahdavi paper.

The network topology used in this example is based on the Fig. 1 described in Mathis, M., & Mahdavi, J. (1996, August). Forward acknowledgement: Refining TCP congestion control.

           10 Mbps           1.5 Mbps          10 Mbps
Sender -------------- R1 -------------- R2 -------------- Receiver
             2ms               5ms               33ms

This program runs by default for 100 seconds and creates a new directory called 'fack-results' in the ns-3 root directory. The program creates one sub-directory called 'pcap' in 'fack-results' directory (if pcap generation is enabled) and six .dat files.

'pcap' sub-directory contains six PCAP files:
- fack-0-0.pcap for the interface on Sender
- fack-1-0.pcap for the interface on Receiver
- fack-2-0.pcap for the first interface on R1
- fack-2-1.pcap for the second interface on R1
- fack-3-0.pcap for the first interface on R2
- fack-3-1.pcap for the second interface on R2
cwnd.dat file contains congestion window trace for the sender node
throughput.dat file contains sender side throughput trace (throughput is in Mbit/s)
queueSize.dat file contains queue length trace from the bottleneck link
lostSegments.dat file contains packet lost trace at queue
bytesInFlight.dat contains trace of inflight data(SND.NXT - SND.UNA).
fackAwnd.dat contains trace of the FACK-specific inflight estimate.

This example allows the user to quantitatively verify two key mechanisms described in the Mathis & Mahdavi paper:

The preservation of the TCP Self-Clock during recovery (Section 4.2).
The "Overdamping" mechanism to drain queues after overshoot (Section 4.4).

To observe these insights, you must run the simulation twice with different configurations:

Step 1: Run with FACK enabled (Default)
./ns3 run "fack-example --fack=true"

ns3
Every class exported by the ns3 library is enclosed in the ns3 namespace.
Step 2: Run with FACK disabled (Standard Reno+SACK behavior)
./ns3 run "fack-example --fack=false"
Step 3: Compare the output traces 'bytesInFlight.dat' and 'cwnd.dat' from both runs.

SPECIFIC OBSERVATIONS:

A. Preservation of Self-Clock (Compare 'bytesInFlight.dat'):

WITHOUT FACK: When multiple packet losses occur (approx t=0.5s), the inflight data drops significantly (often stalling near zero). This indicates a loss of the ACK clock, matching the behavior of "Reno+SACK" described in Section 4.2 and Figure 3 of the paper.
WITH FACK: The inflight data remains elevated even during recovery. This demonstrates that FACK successfully decouples congestion control from data recovery, using SACK blocks to inject new data and maintain the self-clock. This matches the behavior in Figure 4 of the paper.

B. Overdamping and Queue Draining (Compare 'cwnd.dat' vs 'bytesInFlight.dat'):

In the FACK run (between t=1.0s and t=2.0s), observe that 'cwnd' drops to a low value (~10 packets) while 'bytesInFlight' remains high (~50 packets).
This divergence is the "Overdamping" mechanism (Section 4.4). FACK detects that the actual data in the network (inflight) exceeds the target window (cwnd) due to the initial Slow-Start overshoot.
Consequently, FACK inhibits transmission (visible as a throughput dip) to allow the excess queue to drain.

Definition in file fack-example.cc.

Function Documentation

◆ BytesInFlightTracer()

void BytesInFlightTracer	(	Ptr< OutputStreamWrapper >	stream,
		uint32_t	oldval,
		uint32_t	newval )

static

Trace BytesInFlight in segments.

Parameters

stream	The output stream
oldval	Old value
newval	New value

Definition at line 181 of file fack-example.cc.

References g_segmentSize, ns3::Time::GetSeconds(), and ns3::Simulator::Now().

Referenced by TraceBytesInFlight().

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◆ CheckQueueSize()

void CheckQueueSize ( Ptr< QueueDisc > qd )

Check and trace the queue size.

Parameters

qd	The queue disc

Definition at line 133 of file fack-example.cc.

References CheckQueueSize(), g_queueSize, ns3::Time::GetSeconds(), ns3::Simulator::Now(), ns3::Simulator::Schedule(), and ns3::Seconds().

Referenced by CheckQueueSize(), CheckQueueSize(), CheckQueueSize(), and experiment().

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◆ CwndTracer()

void CwndTracer	(	Ptr< OutputStreamWrapper >	stream,
		uint32_t	oldval,
		uint32_t	newval )

static

Trace congestion window.

Parameters

stream	The output stream
oldval	Old value
newval	New value

Definition at line 168 of file fack-example.cc.

References g_segmentSize, ns3::Time::GetSeconds(), and ns3::Simulator::Now().

Referenced by ConnectSocketTraces(), TraceCwnd(), TraceCwnd(), TraceCwnd(), and TraceCwnd().

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◆ FackAwndTracer()

void FackAwndTracer	(	Ptr< OutputStreamWrapper >	stream,
		uint32_t	oldval,
		uint32_t	newval )

static

Trace FACK's inflight(AWND) in segments.

Parameters

stream	The output stream
oldval	Old value
newval	New value

Definition at line 194 of file fack-example.cc.

References g_segmentSize, ns3::Time::GetSeconds(), and ns3::Simulator::Now().

Referenced by TraceFackAwnd().

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◆ TraceBytesInFlight()

void TraceBytesInFlight	(	uint32_t	nodeId,
		uint32_t	socketId )

Bind the BytesInFlight trace source.

Parameters

nodeId	The node ID.
socketId	The socket ID.

Definition at line 222 of file fack-example.cc.

References BytesInFlightTracer(), ns3::Config::ConnectWithoutContext(), ns3::AsciiTraceHelper::CreateFileStream(), g_dir, and ns3::MakeBoundCallback().

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◆ TraceCwnd()

void TraceCwnd	(	uint32_t	nodeId,
		uint32_t	socketId )

Bind the Congestion Window trace source.

Parameters

nodeId	The node ID.
socketId	The socket ID.

Definition at line 206 of file fack-example.cc.

References ns3::Config::ConnectWithoutContext(), ns3::AsciiTraceHelper::CreateFileStream(), CwndTracer(), g_dir, and ns3::MakeBoundCallback().

Referenced by experiment(), and InstallBulkSend().

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◆ TraceFackAwnd()

void TraceFackAwnd	(	uint32_t	nodeId,
		uint32_t	socketId )

Bind the FackAwnd trace source.

Parameters

nodeId	The node ID.
socketId	The socket ID.

Definition at line 238 of file fack-example.cc.

References ns3::Config::ConnectWithoutContext(), ns3::AsciiTraceHelper::CreateFileStream(), FackAwndTracer(), g_dir, and ns3::MakeBoundCallback().

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◆ TracePacketDrop()

void TracePacketDrop	(	Ptr< OutputStreamWrapper >	stream,
		Ptr< const QueueDiscItem >	item )

static

Trace packets dropped at Queue.

Parameters

stream	The output stream
item	The queue disc item dropped

Definition at line 146 of file fack-example.cc.

References g_segmentSize, ns3::Time::GetSeconds(), ns3::TcpHeader::GetSequenceNumber(), ns3::SequenceNumber< NUMERIC_TYPE, SIGNED_TYPE >::GetValue(), and ns3::Simulator::Now().

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◆ TraceThroughput()

void TraceThroughput ( Ptr< FlowMonitor > monitor )

static

Calculate and trace throughput.

Parameters

monitor The flow monitor

Definition at line 108 of file fack-example.cc.

References g_prev, g_prevTime, g_throughput, ns3::Time::GetSeconds(), ns3::Now(), ns3::Simulator::Schedule(), ns3::Seconds(), TraceThroughput(), and ns3::Time::US.

Referenced by TraceThroughput(), TraceThroughput(), and TraceThroughput().

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Variable Documentation

◆ g_dir

std::string g_dir

Output directory.

Definition at line 94 of file fack-example.cc.

Referenced by TraceBytesInFlight(), TraceCwnd(), and TraceFackAwnd().

◆ g_individualLostSegments

std::ofstream g_individualLostSegments

Lost segments output stream.

Definition at line 97 of file fack-example.cc.

◆ g_prev

uint32_t g_prev = 0

Previous throughput byte count.

Definition at line 99 of file fack-example.cc.

Referenced by TraceThroughput().

◆ g_prevTime

Time g_prevTime

Previous throughput measurement time.

Definition at line 100 of file fack-example.cc.

Referenced by TraceThroughput().

◆ g_queueSize

std::ofstream g_queueSize

Queue size output stream.

Definition at line 96 of file fack-example.cc.

Referenced by CheckQueueSize().

◆ g_segmentSize

uint32_t g_segmentSize = 1448

Segment size.

Definition at line 101 of file fack-example.cc.

Referenced by BytesInFlightTracer(), CwndTracer(), FackAwndTracer(), and TracePacketDrop().

◆ g_throughput

std::ofstream g_throughput

Throughput output stream.

Definition at line 95 of file fack-example.cc.

Referenced by TraceThroughput().

Functions

Variables

Detailed Description

Function Documentation

◆ BytesInFlightTracer()

◆ CheckQueueSize()

◆ CwndTracer()

◆ FackAwndTracer()

◆ TraceBytesInFlight()

◆ TraceCwnd()

◆ TraceFackAwnd()

◆ TracePacketDrop()

◆ TraceThroughput()

Variable Documentation

◆ g_dir

◆ g_individualLostSegments

◆ g_prev

◆ g_prevTime

◆ g_queueSize

◆ g_segmentSize

◆ g_throughput