Reliable Data Transfer

Project for CS305: Reliable Data Transfer.

This project is based on the existing UDP socket to achieve a reliable data transmission protocol, and with a packet loss rate of 10%, the rate reaches 60%. This project mainly builds a finite state machine, defines the different states of the socket, simulates the TCP protocol to achieve three-way handshake, four waved hands, and retransmission mechanism; at the same time, set the timeout time by estimating the round-trip time of the data packet, and modify the sliding window size To achieve congestion control;

Principle

Connection

Process

Flow chart
Log
- Connect
- Close

Data Transmission

Process
- Send (Like GBN)
  - Send payloads from nextseqnum to base + N
  - Resend payloads from base to nextseqnum
- Receive
  - Receive individual payload in rcv_base to rcv_base + N (Like SR)
  - send accumulate ACK payload of rcv_base (Like GBN)
Flow chart
- Send
- Receive
Log

Congestion Control

Process
- CWDN
  - Slow Start:
    - cwdn = 1 (Time out)
    - cwdn = cwdn * 2
  - Congestion Avoidance:
    - cwdn = cwdn + 1
- Fast Retransmit
- Timeout Interval
  - Receive payload:
    - TimeoutInterval = EstimatedRTT + 4 * DevRTT
    - EstimatedRTT = (1 - α) * EstimatedRTT + α * SampleRTT
    - DevRTT = (1 - β) * DevRTT + β * |SampleRTT – EstimatedRTT|
  - Time out
    - Doubling the Timeout Interval
Flow chart
- Congestion Control

Pseudocode

When Timeout:

ssthresh = cwnd / 2 
cwnd = 1 
dupACKcount = 0
RESEND

When duplicate ACK:

dupACKcount++
if dupACKcount = 3 :
	ssthresh = cwnd/2
	Resend missing segment

When new ACK:

if cwdn < ssthresh:
	cwnd += 1
else if ACKcount = cwnd:
	cwnd += 1
	ACKcount = 0
else:
	ACKcount += 1

Feature