1 comp 445 lab tutorial lab assignment 2 fall 2006 ae_abdal@cs.concordia.ca

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COMP 445COMP 445

Lab tutorialLab assignment 2

Fall 2006ae_abdal@cs.concordia.ca

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Lab 2 Specification

• Transfer files using UDP.

• Packets might be lost during the transfer. (router.cpp code drops packets)

• Output of program is the same as Lab1. There are couple of new concepts:– Stop and Wait protocol: to guarantee delivery– Three way handshake: to start the S/W

protocol.

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Lab 1

Client server

TCP guarantees that all packets are delivered.

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Stop and wait

Client server

If a packet is dropped, the server will not know

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Stop and wait

Client server

Client waits for an Ack for every packet

packet

packet

packet

Ack

Ack

Ack

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Stop and wait

Client server

Client waits for an Ack for every packet

packet

Packet LOST

packet

Ack

Ack

Wait for an ack.

After a timeout, send the same packet again

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Stop and wait

Client server

How will the server know that the third packet is the same as the second packet.

Packet

Packet

Packet

Ack

Ack

Wait for an ack.

After a timeout, send the same packet again

Ack LOST

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Stop and wait

Client server

How will the server know that the third packet is the same as the second packet.

Solution: number the packets

Packet1

Packet2

Packet2

Ack1

Ack2

Wait for an ack.

After a timeout, send the same packet again

Ack2 LOST

After checking the number, the client ignore this packet

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Stop and wait numbering system

Client server

How will the server know that the third packet is the same as the second packet.

Solution: number the packets

Packet0

Packet1

Packet1

Ack0

Ack1

Wait for an ack.

After a timeout, send the same packet again

Ack1 LOST

Client expects packet0, when packet1 is received, it ignores it.

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Three way hand shake

• To fix the starting numbers at the source and destination, its good idea to add some hand shake above all this process.

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Three way hand shake36

Client server

36

36, 55

Client generates random number let it be equal to 36

The server save this number, and generate its own number 55.

55

36

55

36

55 36

55

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send36

Client server

36

36, 55

Client generates random number let it be equal to 36

The server save this number, and generate its own number 55.

55

36

55

36

55

36

55packet1

ack1

0

1

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receive36

Client server

36

36, 55

Client generates random number let it be equal to 36

The server save this number, and generate its own number 55.

55

36

55

36

55

36

55

packet0

ack0

0

1

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Program architectureProgram architecture

Clientport 5000

port 7000 | port 7001

Router

discard delay

Serverport 5001

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Main differences - socketMain differences - socket

Creating a UDP socket• Instead of:

s = socket(AF_INET,SOCK_STREAM,0)

• Use:s = socket(AF_INET,SOCK_DGRAM,0)

Binding: Client: binds its socket with a physical address (IP : INADDR_ANY

and port number : 5000)

Server: binds its socket with a physical address (IP : INADDR_ANY and port number : 5001)

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SOCKET s;SOCKADDR_IN sa; // port 5000, client IP addressSOCKADDR_IN sa_in; // router info, IP, port(7000)if((s = socket(AF_INET,SOCK_DGRAM,0))==INVALID_SOCKET)

throw "Socket failed\n";

memset(&sa,0,sizeof(sa));sa.sin_family = AF_INET;sa.sin_port = htons(port);sa.sin_addr.s_addr = htonl(INADDR_ANY); //host to network//bind the port to the socketif (bind(s,(LPSOCKADDR)&sa,sizeof(sa)) == SOCKET_ERROR)

throw "can't bind the socket";

cin >> remotehost;rp=gethostbyname(remotehost);memset(&sa_in,0,sizeof(sa_in));memcpy(&sa_in.sin_addr, rp->h_addr, rp->h_length);sa_in.sin_family = rp->h_addrtype; sa_in.sin_port = htons(7000);

CLIENT

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SOCKET s;SOCKADDR_IN sa; // port 5001, server IP addressSOCKADDR_IN sa_in; // router info, IP, port(7001)if((s = socket(AF_INET,SOCK_DGRAM,0))==INVALID_SOCKET)

throw "Socket failed\n";

memset(&sa,0,sizeof(sa));sa.sin_family = AF_INET;sa.sin_port = htons(port);sa.sin_addr.s_addr = htonl(INADDR_ANY);//bind the port to the socketif (bind(s,(LPSOCKADDR)&sa,sizeof(sa)) == SOCKET_ERROR)

throw "can't bind the socket";

router IP and port numbers are obtained from the recvfrom(…) function.

SERVER

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Main differences - sending Main differences - sending Sending frames – you pass the destination

address to the send() function.• Instead of:ibytessent = send(s, (char*)&message_frame, sizeof(message_frame), 0);

• Use:ibytessent = sendto(s, (const char*)&message_frame, sizeof(message_frame), 0,(struct sockaddr*) &sa_in, sizeof(sa_in));

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Main differences - receivingMain differences - receivingReceiving frames (you receive the socket address of the

sender)• Instead of:

ibytesrecv = recv(s, (char*)& message_frame, sizeof(message_frame),0)

• Use:fd_set readfds; //fd_set is a typeFD_ZERO(&readfds); //initialize FD_SET(s, &readfds); //put the socket in the set

if(!(outfds = select (1 , &readfds, NULL, NULL, & timeouts)))

{//timed out, return}

if (outfds == 1) //receive frame

ibytesrecv = recvfrom(s, (char*)& message_frame, sizeof(message_frame),0, (struct sockaddr*)&fromAddr, &fromAddrSize);

If this parameter is NULL, you are listening forever

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How to define a timer with 300ms for select?

#define UTIMER 300000

#define STIMER 0

struct timeval timeouts;

timeouts.tv_sec=STIMER;

timeouts.tv_usec=UTIMER;

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Stop and wait protocolStop and wait protocolClient

Send a packet

if time_out

send the packet again

else //ack received

send a new packet

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Stop and wait protocolStop and wait protocolServer

wait for the first packet

send acknowledge

while(!EOF)

Get a packet;

Send an ack;

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Packet structure usedPacket structure used#define STOPNWAIT 1 //Bits width For stop and waitstruct MESSAGE_FRAME {unsigned char header;unsign int snwseq:STOPNWAIT;char data[MAX_SIZE];//or error message

} message_frame;

struct THREE_WAY_HS{int client_number;int server_number;int direction;char file_name[FILE_NAME_SIZE];

} three_way_hs;

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Three way hand-shakingThree way hand-shaking#include <stdlib.h>THREE_WAY_HS three_way_hs;srand((unsigned)time(NULL));

three_way_hs.client_number = rand();

CLIENT SERVER

three_way_hs.server_number = rand();

sequence_number = three_way_hs.server_number % 2;

Send an acknowledgment with sequence_number

You may send the file name and the direction here

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If the client sends out its first packet, the router has received it and forwarded it to the server side. Then the Router displays an error”Socket error 10054” and the server seems to receive nothing. What’s wrong with my program?

check your receiving code of the server side, make sure you did every thing right(e.g when you are binding the server socket, you give the right port number to the server socket address)

””Socket error 10054”Socket error 10054”