1 COMP 445 Lab tutorial Lab assignment 2 Fall 2006 [email protected].
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Transcript of 1 COMP 445 Lab tutorial Lab assignment 2 Fall 2006 [email protected].
2
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.
3
Lab 1
Client server
TCP guarantees that all packets are delivered.
4
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”
