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# RDP
Reliable Datagram Protocol


V00802949
B03

DESIGN:

P2A: project design + IMPLEMENTATION of connection management (handshake)

1. How do you design and implement your RDP header and header fields? Do you use any additional header fields?

The RDP Header appears as follows:
	"CSC361 _type _num1 _num2\r\n\r\n"

	The type field will be one of the following strings and corresponds to the type of packet:
		1. SYN
		2. ACK
		3. DAT
		4. RST
		5. FIN

	For packets from the sender, num1 will correspond to the sequence number and num2 will correspond to the payload length. On the other hand, for packets from the sender, num1 and num2 will correspond to the acknowledgement number and the window size respectively. The header is designed this way as to be as small as possible.


2. How do you design and implement the connection management using SYN, FIN and RST packets? How to choose the initial sequence number?

The designed connection establishment is implemented as a two-way handshake and works as follows:
	1. Sender sends a SYN packet to the reciever
	2. Reciever responds with an ACK packet
	3. Sender may now begin sending data

The designed connection finish is also implemented as a two-way handshake and works as follows:
	1. Sender sends FIN packet
	2. Client responds with an ACK packet
	3. Client closes socket and returns
	4. Sender closes socket and returns
	5. If the FIN packet times out 4 times, it is assumed that the client has shut down and the ack was dropped.


	The initial sequence number is chosen randomly as to avoid collisions with any services that may have previously used the same port and ip.


3. How do you design and implement the flow control using window size? How to choose the initial window size and adjust the size? How to read and write the file and how to manage the buffer at the sender and receiver side, respectively?
	
	As described in the header section above, every ACK packet sent by the receiver will include the window size. This number represents the most amount of data that the receiver can handle at this time. The sender will look at that number, and send the most amount of packets (packets have a max size) it can without overwhelming the receiver.

	The initial window size will be the size of the receiver buffer and will be reduced when the buffer is filled. When the buffer is full, it will be appended to the file in which the data is being stored - after which the buffer can be emptied and the window size reset.

	On the sender side, the entire file is read into memory before the first SYN packet is sent.

4. How do you design and implement the error detection, notification and recovery? How to use timer? How many timers do you use? How to respond to the events at the sender and receiver side, respectively? How to ensure reliable data transfer?

Errors will be detected through timeouts, sequence numbers, acknowledgement numbers, and comparing the payload length to the specified length in the packet header. 

	Timout: If a timeout occurs, the packet that 'timedout' will be resent and the timer restarted (with an increased timeout value). Once the packet is successfully transfered the timeout value will be reset to the original.
	I plan on using 1 timer on either side, keeping track of the most recent 'group' of packets that have been sent out (since the amount of packets being sent at a time will vary based on the advertised window size).

	Sequence Numbers: If the sequence number of the packet received by the receiver is not the one it is expecting, it will drop the packet and resend the previous acknolodgement message sent - prompting the sender to resend the correct packet.

	Acknowledgement Numbers: If the acknowledgment packet received by the sender is acknowledging a packet that the sender has not sent yet, the sender will send a RST packet (as something has clearly gone catastrophically wrong), and, after receiving acknowledgment, will attempt to restart the connection with a SYN packet.

	Length of Payload: If the length of the payload does not match the length specified in the RDP header, then, similarly to the solution for incorrect sequence numbers, the receiver will resend the last ACK packet, prompting the sender to resend the DAT packet.

Using all of the above techniques for error detection and recovery will result in a reliable data transfer.

5. Any additional design and implementation considerations you want to get feedback from your lab instructor?
	
	In your opinion, is keeping multiple timers make implementing this RDP simpler?


 ~ Cole Macdonald