CSCI466-Reliable Data Transmission Solved

## Learning Objectives

In this programming assignment you will:

- Work with a layered network architecture
- Understand and implement the Stop-and-Wait Protocol with ACKs (Acknowledgments), NACKs (Negative
Acknowledgments), and re-transmissions


## Assignment

During this project, you will implement the Reliable Data Transmission protocols RDT 2.1, and RDT 3.0 discussed in class and the textbook, by extending an RDT 1.0 implementation.


### Starting Code

The starting code for this project provides you with the implementation of several network layers that cooperate to achieve end-to-end communication.

<p align="center">
<img src="images/network.png" alt="Network stack and layer interface calls" width="600">
</p>

The client sends messages to the server, which converts them to [pig latin](https://en.wikipedia.org/wiki/Pig_Latin) and transmits them back.
The client and the server send messages to each other through the transport layer provided by an RDT implementation using the `rdt_1_0_send` and `rdt_1_0_receive` functions.
The starting `rdt.py` provides only the RDT 1.0 version of the protocol, which does not tolerate packet corruption, or loss.
The RDT protocol uses `udt_send` and `udt_receive` provided by `network.py` to transfer bytes between the client and server machines.
The network layer may corrupt packets or lose packets altogether.
`rdt.py` relies on the `Packet` class (in the same file) to form transport layer packets.

Your job will be to extend `rdt.py` to tolerate packet corruption and loss.
The provided code  lists prototype send and receive functions for these protocols.
You may need to modify/extend the `Packet` class to transmit the necessary information for these functions to work correctly.
The provided implementation of `network.py` is reliable, but we will test your code with non-zero probability for packet corruption and loss by changing the values of `prob_pkt_loss` and `prob_byte_corr` of the `NetworkLayer` class.
You should change those variables yourself to test your code and show that your protocol implementations tolerate corruption and loss in your demonstration videos.

Note that unlike full-fledged TCP, RDT is a unidirectional protocol with data transferring from the sender to the receiver, but not in both directions.
To communicate in both directions the client and server execute separate RDT send and receive state machines as described in Section 3.4.1 of your textbook.
However, due to the unidirectional capabilities of RDT, the connection between client RDT send and server RDT receive needs its own UDT channel.
Similarly a separate UDT channel is needed between server RDT send and client RDT receive.
To implement this design, the RDT class sets up two `network` connections.
The `net_snd.udt_send` and `net_snd.udt_receive` functions should be used in `RDT.send`, while `net_rcv.udt_send` and `net_rcv.udt_receive` in `RDT.receive`.

Finally, the `RDT.send` function for the client and server is blocking and may return when the sent data has been confirmed with an ACK by the receiver. 
The `RDT.receive` function however, does not confirm if the ACK it sends to the sender has been received. 
As a result, the receive function returns, while the sender may still need the ACK to be retransmitted. 

The easiest way to allow `RDT.receive` to return while providing ACK retransmission for the sender is to set up a `receive_helper` function that continually runs in a thread to execute the RDT receive finite state machine. 
That way the receive function simply pulls data from a buffer that is fed by `RDT.receive_helper`.
If you are looking for help with setting up threads in Python, look at how it is already implemented in `Network.py` for the `collect` function.


### Program Invocation

To run the starting code you may run:

```
python server.py 5000
```

and

```
python client.py 127.0.0.1 5000
```

in separate terminal windows. 
Be sure to start the server first, to allow it to start listening on a socket used by `network.py`, and start the client soon after, before the server times out.


## BONUS 

We will award __one bonus point__ for each of the following:

* The network layer may also reorder packets.
If you set `prob_pkt_reorder` to a non-zero probability you will start seeing packet that are reordered.
Implement RDT 3.1, which delivers packets in the correct order.

* RDT 3.1 is a stop and wait protocol.
Implements RDT 4.0 - a pipelined reliable data transmission protocol based on either Go-back-N (GBN), or Selective Repeat (SR) mechanisms.