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1 COMP 445 Lab tutorial Lab assignment 2 Fall 2006

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1 1 COMP 445 Lab tutorial Lab assignment 2 Fall 2006

2 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 3 Lab 1 Client server TCP guarantees that all packets are delivered.

4 4 Stop and wait Client server If a packet is dropped, the server will not know

5 5 Stop and wait Client server Client waits for an Ack for every packet packet Ack

6 6 Stop and wait Client server Client waits for an Ack for every packet packet Packet LOST packet Ack Wait for an ack. After a timeout, send the same packet again

7 7 Stop and wait Client server How will the server know that the third packet is the same as the second packet. Packet Ack Wait for an ack. After a timeout, send the same packet again Ack LOST

8 8 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 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

9 9 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 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.

10 10 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.

11 11 Three way hand shake 36 Clientserver 36 36, 55 Client generates random number let it be equal to 36 The server save this number, and generate its own number

12 12 send 36 Clientserver 36 36, 55 Client generates random number let it be equal to 36 The server save this number, and generate its own number packet1 ack1 0 1

13 13 receive 36 Clientserver 36 36, 55 Client generates random number let it be equal to 36 The server save this number, and generate its own number packet0 ack0 0 1

14 14 Program architecture Client port 5000 port 7000 | port 7001 Router discard delay Server port 5001

15 15 Main 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)

16 16 SOCKET s; SOCKADDR_IN sa; // port 5000, client IP address SOCKADDR_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 socket if (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

17 17 SOCKET s; SOCKADDR_IN sa; // port 5001, server IP address SOCKADDR_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 socket if (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

18 18 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));

19 19 Main differences - receiving Receiving 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 type FD_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

20 20 How to define a timer with 300ms for select? #define UTIMER #define STIMER 0 struct timeval timeouts; timeouts.tv_sec=STIMER; timeouts.tv_usec=UTIMER;

21 21 Stop and wait protocol Client Send a packet if time_out send the packet again else //ack received send a new packet

22 22 Stop and wait protocol Server wait for the first packet send acknowledge while(!EOF) Get a packet; Send an ack;

23 23 Packet structure used #define STOPNWAIT 1 //Bits width For stop and wait struct 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;

24 24 Three way hand-shaking #include 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

25 25 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”


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