Directions for project JTRN3 TCP Timeout revised 25 Nov 2011
with simplifying edits 1 May 2012

Project JTRN3 combines JNW with the transport protocol from the older
C++ Network Workbench. The project is very close to the one described 
in chapter 10 of Understanding Internet Protocols. The requirement is 
to complete case 3 (segment timeout handling) under method 
send_rtl_segments().

You will need to run JNWS as loaded for JDLC3, with your code solutions
to JDLC3 and JWAN2:
  ModelDataLink/HDLC.java
  ModelEvents/RoutePacketAtNodeOrProcessAtDestination.java
  ModelNetwork.Routing.java
  
Then study the whole module TCPConnection.java to understand how the NW 
TCP-like reliable transport protocol works in the context of JNW. Some of 
the variables and methods used are described in TCPState.java and 
TCPUtilities.java. Note that the NW protocol does not perform congestion 
control by varying its send window as TCP does, nor does it support 
multiple data flows simultaneously.

You also will need to use the file BookRouterNetworkHDLC.txt for this
assignment. After you get the "email" to transfer, switch to file
BookRouterNetworkHDLC-errors.txt for final submission.

To test your code:
1. Run JNWS by selecting StartJNW under Run in the NetBeans Properties tab. 
2. Use the T button to load a topology - use BookRouterNetworkHDLC.txt
3. Use the folder button to load an email file - use file EMAIL_TCP_1.txt.
   (There is a file EMAIL_TCP_1short.txt with a shorter email that may be
   easier to work with)
4. To see the email input, click on the second function button from the
   left edge and select "Print Email Inputs".
5. Click on the locomotive icon button to run the simulation.
6. When your program works, the selected email will print out as 
   "E-Mail TCP Message RECEIVED" at end of run.
7. For a final test, use network BookRouterNetworkHDLC-errors.txt and
   email file EMAIL_TCP_2.txt.
8. Submit TCPConnection.java and a file containing the text output from 
   (7) for grading, via Moodle.

Here are some methods that could prove useful:

JNWBitSet.convertJNWBitSetToString(); produces ASCII equivalent string

JNWBitSet.toString(); produces a string of ones a zeros with separator blanks
Try printing the output of this method at the beginning of the receiving case
of rtrans_receive. (HINT: the ACK/SYN/FIN bits are in the second octet of the 
third word of the TCP header).

GetTCPACKBitFromTCPSegment(JNWBitSet segment);

GetTCPSYNBitFromTCPSegment(JNWBitSet segment);

GetTCPFINBitFromTCPSegment(JNWBitSet segment);

GetSequenceNumberFromTCPSegment(JNWBitSet segment);

getTCPSegmentPayloadLength(JNWBitSet segment);// length in bits

ModelDES.DiscreteEventSimulator.getSimulationTime(); // the simulation "clock" 
(note that JNW has a variable ModelData.GlobalVariables.simulationTime 
which plays a different role and should not be used as the current value 
of simulation time)

sendNewFrame(int interfaceNumber, JNWBitSet packetBuffer);
  (hint: use hostNode.MessageBuffer[segmtno] for the packetBuffer)

various data in ModelData.GlobalVariables

NOTE: do not use .length to determine the number of bits in
an instance of JNWBitSet. Instead use getJNWLength().


QUESTIONS TO BE ANSWERED AS PART OF JTRN3 SUBMISSION

Include your answers at the end of output.txt.

- What would you expect to happen if the JNWS TCP receiver were to 
reply with an ACK where the value of acknowledgementNumber is always zero?

- Suppose we were going to add a congestion window to JNWS TCP. What 
variable in the current code could be used to impose a congestion window 
in TCPConnection.java?

- If we had slow-start congestion avoidance in JNWS TCP, what should 
be the window size at startup?

- When the checksum contents of the TCP packet is being calculated, what
value is used for the checksum itself?