Congestion Control



Aim: Write a program for Congestion Control using :
a. Stop & Wait protocol.
b. Sliding Window protocol.
Apparatus (Software): Eclipse/ Netbeans

Procedure: Following should be studied to understand this practical

Congestion Control Overview

Problem: When too many packets are transmitted through a network, congestion occurs . At very high traffic, performance collapses completely, and almost no packets are delivered

Causes: Bursty nature of traffic is the root cause → When part of the network no longer can cope a sudden increase of traffic, congestion builds upon. Other factors, such as lack of bandwidth, ill-configuration and slow routers can also bring up congestion

Solution: congestion control, and two basic approaches
– Open-loop: try to prevent congestion occurring by good design
– Closed-loop: monitor the system to detect congestion, pass this information to where action can be taken, and adjust system operation to correct the problem (detect, feedback and correct)

A. Stop & Wait protocol.

In this method of congestion and flow control, the sender sends a single frame to receiver & waits for an acknowledgment.

• The next frame is sent by sender only when acknowledgment of previous frame is received.

• This process of sending a frame & waiting for an acknowledgment continues as long as the sender has data to send.

• To end up the transmission sender transmits end of transmission (EOT) frame.

• The main advantage of stop & wait protocols is its accuracy. Next frame is transmitted only when the first frame is acknowledged. So there is no chance of frame being lost.

• The main disadvantage of this method is that it is inefficient. It makes the transmission process slow. In this method single frame travels from source to destination and single acknowledgment travels from destination to source. As a result each frame sent and received uses the entire time needed to traverse the link. Moreover, if two devices are distance apart, a lot of time is wasted waiting for ACKs that leads to increase in total transmission time.




Program

A Java Program implementing Stop and Wait Protocol while sending a message travelling from sender to receiver.

// SENDER SIDE

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;

public class Sender{
Socket sender;
ObjectOutputStream out;
ObjectInputStream in;
String packet,ack,str, msg;
int n,i=0,sequence=0;

Sender(){}

public void run(){
try{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Waiting for Connection....");
sender = new Socket("localhost",2009);
sequence=0;

out=new ObjectOutputStream(sender.getOutputStream());
out.flush();
in=new ObjectInputStream(sender.getInputStream());
str=(String)in.readObject();
System.out.println("reciver > "+str);
System.out.println("Enter the data to send....");
packet=br.readLine();
n=packet.length();
do{
try{
if(i<n){
msg=String.valueOf(sequence);
msg=msg.concat(packet.substring(i,i+1));
}
else if(i==n){
msg="end";out.writeObject(msg);break;
}
out.writeObject(msg);
sequence=(sequence==0)?1:0;
out.flush();
System.out.println("data sent>"+msg);
ack=(String)in.readObject();
System.out.println("waiting for ack.....\n\n");
if(ack.equals(String.valueOf(sequence))){
i++;
System.out.println("receiver > "+" packet recieved\n\n");
}
else{
System.out.println("Time out resending data....\n\n");
sequence=(sequence==0)?1:0;
}
}catch(Exception e){}
}while(i<n+1);
System.out.println("All data sent. exiting.");
}catch(Exception e){}
finally{
try{
in.close();
out.close();
sender.close();
}
catch(Exception e){}
}
}

public static void main(String args[]){
Sender s=new Sender();
s.run();
}
}

// RECEIVER SIDE


import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;


public class Reciever{
ServerSocket reciever;
Socket connection=null;
ObjectOutputStream out;
ObjectInputStream in;
String packet,ack,data="";
int i=0,sequence=0;
Reciever(){}
public void run(){
try{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
reciever = new ServerSocket(2009,10);
System.out.println("waiting for connection...");
connection=reciever.accept();
sequence=0;
System.out.println("Connection established :");
out=new ObjectOutputStream(connection.getOutputStream());
out.flush();
in=new ObjectInputStream(connection.getInputStream());
out.writeObject("connected .");
do{
try{
packet=(String)in.readObject();
if(Integer.valueOf(packet.substring(0,1))==sequence){
data+=packet.substring(1);
sequence=(sequence==0)?1:0;
System.out.println("\n\nreceiver >"+packet);
}
else
{
System.out.println("\n\nreceiver >"+packet +" duplicate data");
}
if(i<3){
out.writeObject(String.valueOf(sequence));i++;
}
else{
out.writeObject(String.valueOf((sequence+1)%2));
i=0;
}
}
catch(Exception e){}
}while(!packet.equals("end"));
System.out.println("Data recived="+data);
out.writeObject("connection ended .");
}
catch(Exception e){}
finally{
try{
in.close();
out.close();
reciever.close();
}
catch(Exception e){}
}
}
public static void main(String args[]){
Reciever s=new Reciever();
while(true){
s.run();
}
}
}

OUTPUT:

//SENDER OUTPUT
Waiting for Connection....
reciver > connected .
Enter the data to send....
myname
data sent>0m
waiting for ack.....
receiver > packet recieved
data sent>1y
waiting for ack.....
receiver > packet recieved
data sent>0n
waiting for ack.....
receiver > packet recieved
data sent>1a
waiting for ack.....
Time out resending data....
data sent>1a
waiting for ack.....
receiver > packet recieved
data sent>0m
waiting for ack.....
receiver > packet recieved
data sent>1e
waiting for ack.....
receiver > packet recieved
All data sent. exiting.

//RECEIVER OUTPUT
waiting for connection...
Connection established :
receiver >0m
receiver >1y
receiver >0n
receiver >1a
receiver >1a duplicate data
receiver >0m
receiver >1e
Data recived=myname
waiting for connection...


B. Sliding Window protocol.

A sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the Data Link Layer (OSI model) as well as in the Transmission Control Protocol (TCP).

  • Frames have sequence number 0 to maximum 2n - 1 (n bit field).
  • At any moment, the sender maintains a list of sequence numbers it is permitted to send - these fall within the sending window. These are frames sent-but-no-ack and frames not-yet-sent.
  • When new packet from Network layer comes in to send, it is given highest no, and upper edge of window advanced by 1.
  • When ack comes in, lower edge of window advanced by 1.
  • Receiver has receiving window - the frames it is permitted to accept.

Sliding window size 1. Sequence nos. 0 to 7.
(a) At start. Receiver waits for 0.
(b) Sender sends 0.
(c) Receiver receives 0. Waits for 1.
(d) Sender got ack for 0. Hasn't got 1 from its Network layer yet.


More complex Data Link layer, as more freedom about the order in which it sends and receives frames. Sender may have n unacknowledged frames at any time (window size n).

Needs n buffers to hold them all for possible re-transmit. If window grows to its maximum size, DA must shut off NA. This is all hidden from NB - still receives packets in exact same order.

1. Sender window might grow as it receives more frames to send and still has ones un-ack'ed.
Starts with nothing to send, then NA gives it frames to send.
Later, window may shrink as frames are ack-ed and NA has no more.

2. Receiver window constant size.
Receiver window size 1 means will only accept them in order.
Size n means will receive out of order (e.g. receive later ones after earlier frame is lost)
and then must buffer them before sending to NB (must send to NB in order).


e.g. DB has buffers to receive frames 0..7
Receives 1..7 in varying orders. Still waiting on 0. Can't send frames to NB yet.
0 got lost and was re-sent. Eventually gets 0.
Can now send all of 0..7 to NB
and re-use these slots.

e.g. consider frames numbered 0..7 but DB only has 2 buffers
Currently the sliding window is over 4,5
If get 4 can send it to NB and move window to 5,6
If get 5 have to wait for 4, then send both, and advance window to 6,7

Program

//CLIENT PROGRAM

import java.io.DataInputStream;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;


public class slidsender
{
public static void main(String a[])throws Exception
{

ServerSocket ser=new ServerSocket(7870);
Socket s=ser.accept();
DataInputStream in=new DataInputStream(System.in);
DataInputStream in1=new DataInputStream(s.getInputStream());
String sbuff[]=new String[8];
PrintStream p;
int sptr=0,sws=8,nf,ano,i;
String ch;
do
{
p=new PrintStream(s.getOutputStream());
System.out.print("Enter the no. of frames : ");
nf=Integer.parseInt(in.readLine());
p.println(nf);
if(nf<=sws-1)
{

System.out.println("Enter "+nf+" Messages to be send\n");
for(i=1;i<=nf;i++)
{
sbuff[sptr]=in.readLine();
p.println(sbuff[sptr]);
sptr=++sptr%8;
}
sws-=nf;
System.out.print("Acknowledgment received");
ano=Integer.parseInt(in1.readLine());
System.out.println(" for "+ano+" frames");
sws+=nf;
}
else
{
System.out.println("The no. of frames exceeds window size");
break;
}
System.out.print("\nDo you wants to send some more frames : ");
ch=in.readLine(); p.println(ch);
}
while(ch.equals("yes"));
s.close();
}
}

//RECEIVER PROGRAM

import java.io.DataInputStream;
import java.io.PrintStream;
import java.net.Socket;

class slidreceiver
{
public static void main(String a[])throws Exception
{

Socket s=new Socket("localhost",7870);

DataInputStream in=new DataInputStream(s.getInputStream());
PrintStream p=new PrintStream(s.getOutputStream());
int i=0,rptr=-1,nf,rws=8;
String rbuf[]=new String[8];
String ch; System.out.println();
do
{
nf=Integer.parseInt(in.readLine());
if(nf<=rws-1)
{
for(i=1;i<=nf;i++)
{
rptr=++rptr%8;
rbuf[rptr]=in.readLine();
System.out.println("The received Frame " +rptr+" is : "+rbuf[rptr]);
}
rws-=nf;
System.out.println("\nAcknowledgment sent\n");
p.println(rptr+1); rws+=nf; }
else
break;
ch=in.readLine();
}
while(ch.equals("yes"));
}
}

OUTPUT:

//SENDER OUTPUT
Enter the no. of frames : 4
Enter 4 Messages to be send

hiii
how r u
i am fine
how is evryone
Acknowledgment received for 4 frames

Do you wants to send some more frames : no

//RECEIVER OUTPUT
The received Frame 0 is : hiii
The received Frame 1 is : how r u
The received Frame 2 is : i am fine
The received Frame 3 is : how is evryone

Acknowledgment sent








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