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File Transfer

File Transfer. Transfer a file from one DTE to another DTE. Issues:. - Are file attributes (name, extension, ownership, time, size, etc.) transferred? - What will happen to the file on the destination DTE if the file transfer fails? Left or deleted?

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File Transfer

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  1. File Transfer • Transfer a file from one DTE to another DTE. • Issues: - Are file attributes (name, extension, ownership, time, size, etc.) transferred? - What will happen to the file on the destination DTE if the file transfer fails? Left or deleted? - Does ACK mean successful reception or successful writing to the disk on destination DTE?

  2. File System • File system is responsible for the generation of and access to files stored on some external medium, typically a disk. • Typical file system operations: • Files consist of a series of bytes often grouped into structures known as records. • Each file opened is associated with a handle, or file descriptor. - Creation - Opening - Closing - Reading: sequential or direct - Writing

  3. File Transfer Protocol Three basic operations: • File identification • End-of-file indication - Open existing file for reading on one DTE - Create new file on the other DTE - File name (and path, etc.) must be specified on both machines. - File attributes must be associated with the new file - file transfer can be aborted at this point for reasons like: - file to be copied does not exist - file name is already in use on the receiving DTE - the user does not have the necessary access right to the file - An end-of-file indication must be sent to the destination.

  4. File Transfer Protocol [cont’d] • Record Transfer - Source: 1. Open the file 2. Read a record 3. Pass the record to the communication software 4. Repeat steps 2 and 3 until end-of-file is detected - Destination: 1. Create the file 2. Wait for a record 3. Write the record to the newly created file 4. Repeat steps 2 and 3 until end-of-file is indicated - Need handshake between file transfer and comm. Software. - Many file transfer protocols also implement end-to-end ACK.

  5. Commkit File Transfer • s&wdisk.c: a simple file transfer utility using low-level stop-and-wait softwares&wlow.c. • Two modes: • Restrictions: - Source: read, transmit record one at a time - Destination: wait, receive, write - a maximum of 80 bytes per unit can be transferred - half-duplex communication

  6. s&wdisk( ) s&wdisk( ) 1 4 3a 2 s&wlow( ) s&wlow( ) 3b Implementation • s&wdisk.c: a simple file transfer utility using low-level stop-and-wait softwares&wlow.c. struct disk_msg { char option; /* Message option: EOF:1, MORE_TO_COME: 2 */ char data[REC_LEN]; /* Data to be sent with the option */ };

  7. Main void main(int argc, char *argv[]) { /* - mainline for stop-and-wait protocol example C:\> s&wdisk LineSpeed Port Protocol Filename - the Protocol is either X (xmit) or R (recv) -- the default is receive */ int line_speed; line_speed = get_line_speed(argv[1]); if (line_speed > 0) { initialize(line_speed, argv[2][0], argv[3][0]); if (stop_and_wait_init(argv[3][0])) read_and_send(argv[4]); else receive_and_write(argv[4]); rtn_to_dos(); } }

  8. void read_and_send (char *fname) // opens the specified file (name pointed to by `fname') fd = open(fname, O_RDONLY); while (running) { // up to REC_LEN bytes are read and passed to the handler for transmission if ((size = read(fd, message.data, REC_LEN)) == 0) message.option = EOF; else message.option = MORE_TO_COME; send(APPLICATION, handler_id, &message, size + 1); // the process then stops and waits for an acknowledgement from the remote process (via the handler); this continues until there are no further bytes in the file and the process signals end-of-file (EOF) do { recv(&src, APPLICATION, &ch, 1); if (src == KEYIH && ch == ETX) running = FALSE; } while (running && src != handler_id); /* Exit if ^C or SPxIH */ if (message.option == EOF) running = FALSE; } close(fd);

  9. void receive_and_write (char *fname) // opens a file for writing, file name is pointed to by `fname` fd = open(fname, O_WRONLY | O_CREAT, S_IREAD | S_IWRITE) //records are written to the file as they are received //end-of-file is signalled when the message option equals EOF do { size = recv(&src, APPLICATION, &message, sizeof(struct disk_msg)); if (message. option != EOF) write(fd, message. data, size - 1); } while (message . option != EOF); close(fd); /* CTRL-Z is NOT written to the file */

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