1. Reliable Data Transfer Protocol IMPLEMENTATION TIPS

2. Transport Layer – TCP 2 BBBB Recall  Reliable Data Transfer Mechanisms:  Checksum  Timer  Sequence number  ACK  NAK  Window, pipelining socket door TCP send buffer TCP receive buffer socket door Packet -> application writes data application reads data - Verification of integrity of packet - Signals necessary re-transmission is required - Keeps track of which packet has been sent and received - Indicates receipt of packet in good or bad form - Allows for the sending of multiple yet-to-be-acknowledged packets

3. Client - Server CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER Empty. File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket UDP (User Datagram Protocol) – has no connection establishment - No connection state at servers - less packet overhead than TCP - light error-checking (checksum) - server doesn’t use the listen() function - server doesn’t use the accept() function

4. Lecture-2012-6-Socket Programming-Part2 See Lecture-2012-6-Socket Programming-Part2 (Slide #4) UDP Basics

5. Client - Server CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER Empty. File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket 1235 Rserver 1235 0 0 1235 Rclient 127.0.0.1 1235 0 0 You should run the server first, before running the client. You can test your client and server using the same machine by using the example run above. Sample run: Bits can be corrupted Packets can be lost Port: 1235

6. Client - Server CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER Empty. File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket 1235 Rserver 1235 0 0 1235 Rclient 127.0.0.1 1235 0 0 The client is the sender, while the server is the receiver. File1_Windows.txtFile1_Saved.txt The filenames used for sending and saving have been fixed in the start-up codes (i.e. File1_Windows.txt, File1_Saved.txt).

7. Client - Server CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER Empty. File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket 1235 Rserver 1235 0 0 1235 Rclient 127.0.0.1 1235 0 0 The client sends the contents of the file line by line. One packet contains exactly one line. In order to implement reliable data transfer, you will have to modify the packet header to add more details.

8. Client - Server CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER Empty. File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket 1235 Rserver 1235 0 0 1235 Rclient 127.0.0.1 1235 0 0 The objective is for you to implement a reliable data transfer protocol. You can choose to implement a simple stop-and-wait protocol or any of the pipe-lining protocols (i.e. Go Back-N, Selective Repeat).

9. Client - Server CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket Receives file contents line-by-line, then stores everything into a file Reads file, then sends the contents 1 line at a time UNRELIABLE CHANNEL NETWORK LAYER TRANSPORT LAYER APPLICATION LAYER

10. Client - Server CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket Receives file contents line-by-line, then stores everything into a file Reads file, then sends the contents 1 line at a time UNRELIABLE CHANNEL NETWORK LAYER TRANSPORT LAYER APPLICATION LAYER Simulated by a function named send_unreliably()

11. Unreliable Channel Simulation int send_unreliably( int s, char * send_buffer, struct sockaddr_in remoteaddress) { int fate=packets_fate(); //random number generator: 0, 1, 2 if (fate==0){ //no problem will be introduced bytes = sendto(s, send_buffer,...) printf("<-- SEND: %s \n",send_buffer); } else if (fate== 1){ // introduce corrupted bits send_buffer[damage_bit()]=random_char(); bytes = sendto(s, send_buffer,...) printf("<-- DAMAGED %s \n",send_buffer); … } else if(fate==2){ // lose the packet printf("X-- LOST %s \n",send_buffer); } You are not allowed to modify this function in the assignment.

12. UDP segment structure The optional parameters are not even included in the sendto() function. Optional in IPv4 sendto int sendto ( SOCKET s, char *buf, int msglen, int flags, struct sockaddr *to, int tolen ); The O/S will automatically provide the IP address of the sender of the segment.

13. Send data through a socket Send data through a socket: sendto(SOCKET s, char *msg, int msglen, int flags, struct sockaddr *to, int *tolen);sendto() Example: sendto(s, sbuffer, strlen(sbuffer),0,(struct sockaddr*) to, &len); s = socket (inside the socket descriptor: port and IP address...) msg = a pointer to a buffer (could be a string) msglen = the length of the buffer flags = 0 (forget about them for this exercise...) to=structure of address with the IP / port # tolen=length of the structure PARAMETERS

14. 14  Receive data int recvfrom(SOCKET s, char *msg, int msglen, int flags, struct sockaddr *from, int *fromlen)recvfrom() Example: recvfrom(s, rbuffer, 1, 0,(struct sockaddr *) &from, &len); s = socket (inside the socket descriptor: port and IP address...) msg = a pointer to a buffer msglen = the length of the buffer flags = 0 from =structure of address with the IP / port # fromlen=length of the structure PARAMETERS

15. Client - Server CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket Receives file contents line-by-line, then stores into a file Destination Port: 1235DATAchecksumLength of segment Reads file, then sends the contents 1 line at a time UDP SEGMENT

16. Client - Server CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket Receives file contents line-by-line, then stores into a file Destination Port: 1235DATA 0 ABCDEF checksumLength of segment Reads file, then sends the contents 1 line at a time PACKET 0 ‘\r’ ‘\n’ CRC_NUM User-defined

18. Let’s have a look at the CRC function provided as part of the start-up codes and the CRC test program. CRC

19. //******************************************************************* // CREATE CLIENT'S SOCKET //******************************************************************* s = socket(AF_INET, SOCK_DGRAM, 0); if (s < 0) { printf("socket failed\n"); WSACleanup(); exit(1); } //nonblocking option u_long iMode=1; ioctlsocket(s, FIONBIO, &iMode); Socket (in non-blocking mode) We need to set our socket in non-blocking mode of operation. This prevents the recvfrom() function from stopping and waiting for a packet. Remember, packets could be lost in our simulation of the unreliable channel.

20. Start-up Codes (Client – Server) CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket Receives file contents line-by-line, then stores into a file Reads file using fgets(), then sends the contents 1 line at a time Loop: data send_unreliably(data) recvfrom() Loop: recvfrom() ACK send_unreliably(ACK) fopen(“file1_Windows.txt”, r) fopen(“file1_Saved”, w)

21. Reading the file contents The client reads the file contents line by line using fgets() fgets(send_buffer, SEGMENT_SIZE, fin) stops reading the file when it encounters either: new line character a new line character (copied into send_buffer) EOF EOF (End-Of-File) character a NULL-termination character (‘\0’) is automatically appended this is counted as one of the characters strlen() – counts the number of characters excluding the NULL-character CLIENT A B C ‘\n’ ‘\0’ send_buffer strlen()=4 ‘\0’ send_buffer strlen()-1 0 1 2 3 4 0 1 2 3 4 A B C

22. Data Format CLIENT A B C ‘\n’ ‘\0’ send_buffer strlen()=4 ‘\0’ send_buffer strlen()-1 0 1 2 3 4 0 1 2 3 4 A B C Destination Port: 1235DATA ABC checksumLength of segment PACKET 0 ‘\r’ ‘\n’ CRC_NUM User-defined Remove the line feed character from the row of data read from the file

23. CLIENT 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt SERVER 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Saved.txt Rclient_UDP.cppRserver_UDP.cpp socket Loop: send_unreliably(data) recvfrom() Loop: recvfrom() send_unreliably(ACK) r fopen(“file1_Windows.txt”, r) w fopen(“file1_Saved”, w) send_unreliably(”CLOSE”) closesocket() file1_Saved.txt Write everything into file1_Saved.txt fclose() closesocket() Start-up Codes (Client – Server)

25. CLIENTSERVER Loop: read one line from file if(not EOF){ create packet with header fields store packet into send_buffer send_unreliably(send_buffer) Sleep(1); recvfrom(receive_buffer) trim ‘\r’, ‘\n’ from receive_buffer } else { fclose() send_unreliably(”CLOSE”) } Loop: recvfrom(receive_buffer) trim ‘\r’, ‘\n’ from receive_buffer process receive_buffer if(receive_buffer contains DATA){ create ACK packet send_unreliably(ACK) save_line_without_header } else { fclose() } r fopen(“file1_Windows.txt”, r) w fopen(“file1_Saved”, w) closesocket() 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF...

26. Let’s have a look at the Start-up Codes (downloadable from our website) Start-up Codes

27. Parameters Settings

28. Parameters (Client – Server) CLIENTSERVER Rclient_UDP.cppRserver_UDP.cpp 1235 Rserver 1235 0 0 1235 Rclient 127.0.0.1 1235 0 0 Sample run: Bits can be corrupted Packets can be lost

29. Ultimate Test CLIENTSERVER Rclient_UDP.cppRserver_UDP.cpp 1235 Rserver 1235 1 1 1235 Rclient 127.0.0.1 1235 1 1 Ultimate Test: Bits can be corrupted Packets can be lost You can simply inspect the contents of File1_Windows.txt and File1_Saved.txt, to check to see if they are exactly the same. 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Windows.txt 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... File1_Saved.txt

30. Extending the codes

31. Let’s have a look at the Go-Back N Protocol Specifications… Lecture-2012-7-Transport Layer-Part-1 Slide #37 See Lecture-2012-7-Transport Layer-Part-1 (Slide #37) Go-Back N

32. Pipelining Protocol (Go Back-N) CLIENT (sender) base base + (N-1) ABC PACKET 0 ‘\r’ ‘\n’ CRC_NUM ABC PACKET 1 ‘\r’ ‘\n’ CRC_NUM ABC PACKET 2 ‘\r’ ‘\n’ CRC_NUM Send a Window’s worth of packets N=Window size = 4 Sequence Number Space

33. Pipelining Protocol (Go Back-N) base base + (N-1) ABC PACKET 0 ‘\r’ ‘\n’ CRC_NUM ABC PACKET 1 ‘\r’ ‘\n’ CRC_NUM ABC PACKET 2 ‘\r’ ‘\n’ CRC_NUM Send a Window’s worth of packets N = Window size = 4 nextSequenceNum-1 Packets sent but not yet ACKed... nextSequenceNum CLIENT (sender)

34. Pipelining Protocol (Go Back-N) base base + (N-1) N = Window size = 4 nextSequenceNum-1 Packets sent but not yet ACKed nextSequenceNum base baseMax ACKnum We need to keep track of the ACK number received CLIENT (sender)

35. Pipelining Protocol (Go Back-N) N = Window size = 4 Upon receipt of an ACK, slide the window forward base=0 baseMax=3 ACKnum=1 base=ACKnum+1 baseMax=base+(N-1) 0123 0123 45 time=0 time=1 CLIENT (sender)

36. Pipelining Protocol (Go Back-N) N = Window size = 4 Transmit more packets (up to baseMax) base=0 baseMax=3 ACKnum=1 base=ACKnum+1 baseMax=base+(N-1) 0123 0123 45 time=0 time=2 CLIENT (sender)

37. WARNING: The following pseudo codes are not complete. They are meant just to give you an idea of how to implement a sliding Window protocol. red The statements highlighted in red corresponds to the suggested routines that need to be incorporated.

38. 38 Calculating the Elapsed Time clock() void wait ( int seconds ) { clock_t clock_t endwait; endwait = clock () + seconds * CLOCKS_PER_SEC ; while (clock() < endwait) {} } clock_t clock_t startTime, elapsedTime; startTime = clock(); …... elapsedTime = (clock() - startTime) / CLOCKS_PER_SEC;

39. Extending the codes Loop: read one line from file if(not EOF){ create packet with header fields store packet into send_buffer send_unreliably(send_buffer) Sleep(1); recvfrom(receive_buffer) trim ‘\r’, ‘\n’ from receive_buffer extract ACK number update base, baseMax } else { fclose() send_unreliably(”CLOSE”) } Loop: recvfrom(receive_buffer) trim ‘\r’, ‘\n’ from receive_buffer extract CRC1 from receive_buffer extract data from receive_buffer calc CRC2 using data if(CRC1 == CRC2){ extract Packet Num if(PacketNum is in-order){ if(receive_buffer contains DATA){ create ACK packet send_unreliably(ACK) save_line_without_header update expectedSeqNum, base } else if(receive_buffer contains CLOSE) { fclose(); closesocket(); } r fopen(“file1_Windows.txt”, r) w fopen(“file1_Saved”, w) closesocket() 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... 0 ABCDEF 1 ABCDEF 2 ABCDEF 3 ABCDEF... SERVER CLIENT

40. Other Helpful Functions strchr const char * strchr ( const char * str, int character ); char * strchr ( char * str, int character ); Locate first occurrence of character in string Returns a pointer to the first occurrence of character in the C string str. The terminating null-character is considered part of the C string. Therefore, it can also be located to retrieve a pointer to the end of a string.