WUMP Programming Project

Your assignment is to do the following, by modifying and extending the wclient outline file:

• Implement the basic transfer
• Add all appropriate packet sanity checks: timeouts, host/port, size, opcode, and block number
• Generate output. The transferred file is to be written to System.out. A status message about every packet (listing size and block number) is to be written to System.err. Do not confuse these!
• Terminate after a packet of size less than 512 is received
• Implement an appropriate "dallying" strategy
• send an ERROR packet if it receives a packet from the wrong port. The appropriate ERRCODE in this case is EBADPORT.

An outline of the program main loop, with notes, is available here

1. Latch on to the new port: save the port number from Data[1], and make sure all ACKs get sent to this port. This will mean that the transfer completes. You should also make sure the client stops when a packet with less than 512 bytes of data is received. Unless you properly record the source port for Data[1], you have no place to which to send ACK[1]!
2. For each data packet received, write the data to System.out. All status messages should go to System.err, so the two data streams are separate if stdout is redirected. To write to System.out, use System.out.write:
           System.out.write(byte[] buf, int offset, int length);
   For your program, offset will be 0, buf will typically be dpacket.data(), where dpacket is of type DATA (wumppkt.DATA). The length will be dpacket.size() - wumppkt.DHEADERSIZE (or, equivalently, dg.getLength() - wumppkt.DHEADERSIZE, where dg is a DatagramPacket object).
   
3. Add sanity checks, for (in order) host/port, packet size, opcode, and block number.
4. Handle timeouts, by retransmitting the most recently sent packet when the elapsed time exceeds a certain amount (4 seconds?). One way to do this is to keep a DatagramPacket variable LastSent, which can either be reqDG or ackDG, and just resend LastSent. Note that the response to an InterruptedIOException, a "true" timeout, will simply be to continue the loop again.
5. Add support for an dallying and error packets. After the client has received the file, dallying means to wait 2.0 - 3.0 timeout intervals (or more) to see if the final data packet is retransmitted. If it is, it means that the final ACK was lost. The dally period gives the client an opportunity to resend the final ACK. Error packets are to be sent to any sender of an apparent data packet that comes from the wrong port.

You can test your program by contacting the server, ulam2.cs.luc.edu, and requesting files. If you contact on port 4715, you get a response from a new port (the standard behavior). This doesn't go through NAT firewalls, so if you contact ulam2 on port 4716 you get a response from the same port. No matter what file you ask for, the file you get is always the same. However, by asking port 4715 for the following names, you get the indicated behavior:

vanilla   Normal transfer

lose      Lose everything after the first windowful (min 3). It will be retransmitted when you retransmit the previous ACK.

spray    Constant barrage of data[1]. Implies LOSE too. In this case, no timeout events will occur; you must check for elapsed time.

delay    Delays sending packet 1, prompting a duplicate REQ and thus results in multiple server instances on multiple ports.

reorder  Sends the first windowful in the wrong order. You're probably not implementing windows.

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dupdata2

DATA[2] is sent twice

losedata2

DATA[2] is lost on initial send, until you resend ACK[1]

marspacket

A packet from the wrong port (a "martian" port) arrives

badopcode

a packet arrives with an incorrect opcode

nofile

you get an error packet with a NO FILE error code.

At this point, the only ones that work on port 4716 are vanilla, lose2 (losedata2) and dup2 (dupdata2).