Lab 3 SOlution

Write a file transfer application using UDP sockets in Python. The file transfer protocol includes a server called ftps.py and a client called ftpc.py. In the following, we assume the client is running on beta.cse.ohio-state.edu and the server is running on gamma.cse.ohio-state.edu.1 First, start the server on gamma.cse.ohio-state.edu using the command




python3 ftps.py <local-port-on-gamma




Then, start troll on beta.cse.ohio-state.edu with the command (on one line)




troll -C <IP-address-of-beta -S <IP-address-of-gamma -a <client-port-on-beta -b <server-port-on-gamma -r -s 1 -t -x 0 <troll-port-on-beta




Next, start ftpc.py on beta.cse.ohio-state.edu with the command (on one line)




python3 ftpc.py <IP-address-of-gamma <remote-port-on-gamma




<troll--port-on-beta <local-file-to-transfer




The ftpc.py client will send all bytes of the local file to the troll process, which should for-ward the packets to ftps.py. The ftps.py server should receive the file and then store it. Ensure that you do the following:




– Send all segments to troll, not the ftps.py server. If you send to the server, troll will receive no segments;

– On the client, bind() to the <client-port-on-beta. Normally, in UDP socket pro-gramming, only the server calls bind(), but troll requires binding client and server ports beforehand.

– The new file created by ftps.py must be in a different directory (i.e., recv) to avoid over-writing the original file, since all the CSE machines have your root directory mounted.




The file transfer application will use a simple protocol. The payload of each UDP segment will contain the remote IP (4 bytes), remote port (2 bytes), and a flag (1 byte), followed by a data/-control field as explained below. The flag takes 3 possible values depending on the data/control field:




– First segment (4 bytes): The first segment should contain the number of bytes in the file to follow (in network byte order). The flag is set to a value of 1.

– Second segment (20 bytes): The second segment should contain 20 bytes which is the name of the file (assume the name can fit in 20 bytes). The flag is set to a value of 2.




1In your lab, you can substitute any machine on stdlinux (e.g., zeta.cse.ohio-state.edu or epsilon.cse.ohio-state.edu) for beta.cse.ohio-state.edu and gamma.cse.ohio-state.edu, respec-tively. You should do so to avoid “overloading” these machines.







1
– Other segments: The other segments will contain data bytes from the file to be trans-ferred. Each segment can have up to 1,000 data bytes. The flag is set to a value of 3.




The troll process will drop packets with a drop rate percentage as indicated in the command line. With a drop rate of 0, the file should be delivered intact to ftps.py. Submit well-documented code using the following command:




submit c3461aa lab3 <code-directory-name




No buffer, array, or data structure should exceed 1,000 bytes. You’ll need to “chunk” the file into “pieces,” each of which is at most 1,000 bytes. Submit a README.txt file with your lab. Your program should work for binary files (images, etc.) of arbitrary size and the client and server should be running on different machines on stdlinux. You can use the sample images on the course webpage to test your program. Use diff or md5sum at the command line to ensure that the transferred file is bitwise identical to the original one. To avoid overrunning UDP buffers, you can call sleep() for a short time between sending packets.