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Overview

The ReliableClientProtocol class provides client-side implementation of the Selective Repeat ARQ protocol over UDP. It handles connection establishment via three-way handshake, reliable data transmission with windowing, and automatic retransmission of lost packets.

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Connection Establishment

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connection()

Establishes a three-way handshake connection with the server.
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socket
DatagramSocket
required
The UDP socket to use for communication
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serverAddr
InetSocketAddress
required
The server’s address and port number
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port
int
The server’s data port assigned for this connection
DatagramSocket socket = new DatagramSocket();
InetSocketAddress serverAddress = new InetSocketAddress("127.0.0.1", 8000);
int dataPort = ReliableClientProtocol.connection(socket, serverAddress);
The connection process implements a three-way handshake:
  1. Client sends SYN packet (type=2)
  2. Server responds with SYN-ACK (type=3)
  3. Client sends ACK (type=1)

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Data Transmission

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requestresponse()

Sends data reliably to the server and waits for a response using the Selective Repeat protocol.
​
socket
DatagramSocket
required
The UDP socket for communication
​
serverAddr
InetSocketAddress
required
The server’s data port address
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data
String
required
The data payload to send to the server
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response
String
The complete response received from the server
String httpRequest = "GET /index.html HTTP/1.0\r\n\r\n";
String response = ReliableClientProtocol.requestresponse(
    socket, 
    new InetSocketAddress("127.0.0.1", dataPort), 
    httpRequest
);
The method automatically:
  • Fragments data into 1013-byte chunks
  • Sends the total packet count first
  • Transmits packets using a sliding window (size: 100)
  • Handles ACKs and retransmits on timeout or NACK
  • Receives the server’s response using the same protocol

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Internal Methods

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sendreq()

Handles the sliding window transmission logic with selective repeat.
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nofpackets
int
required
Total number of data packets to send
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serverAddr
InetSocketAddress
required
Destination server address
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socket
DatagramSocket
required
Socket for transmission
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chunks
List<byte[]>
required
List of data chunks to send
This is an internal method called by requestresponse(). Direct use is not recommended.

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isAcked()

Timeout handler that checks if a packet has been acknowledged and retransmits if necessary.
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seq
int
required
Sequence number of the packet to check
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socket
DatagramSocket
required
Socket for retransmission
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retries
int
required
Current retry attempt count
Packets are retransmitted up to 10 times before being abandoned.

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response()

Waits for and receives the complete response from the server.
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response
String
The assembled response data from all received packets

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getRequestFromPacket()

Receives and assembles packets using the Selective Repeat protocol.
​
socket
DatagramSocket
required
Socket to receive packets from
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totalPackets
int
required
Expected number of packets to receive
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request
String
The complete assembled data from all packets

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Protocol Constants

static int lastack = 0;           // Last acknowledged packet sequence number
static int window = 100;          // Sliding window size
static InetSocketAddress routerAddr = 
    new InetSocketAddress("127.0.0.1", 3000);  // Router address

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Error Handling

When waiting for ACKs, the socket uses a 50ms timeout. On timeout, the size packet is retransmitted.
The protocol handles packet loss through:
  • Cumulative ACKs for in-order packets
  • NACKs (type=4) for out-of-order packets
  • Timeout-based retransmission with exponential backoff
When a packet with type=6 is received, the connection is closed gracefully.

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Example Usage

import java.net.DatagramSocket;
import java.net.InetSocketAddress;
import client.ReliableClientProtocol;

public class Example {
    public static void main(String[] args) {
        try {
            // Create socket
            DatagramSocket socket = new DatagramSocket();
            
            // Establish connection
            InetSocketAddress serverAddress = 
                new InetSocketAddress("192.168.1.100", 8000);
            int dataPort = ReliableClientProtocol.connection(
                socket, serverAddress
            );
            
            // Send request and get response
            String request = "GET /data HTTP/1.0\r\n\r\n";
            String response = ReliableClientProtocol.requestresponse(
                socket,
                new InetSocketAddress("192.168.1.100", dataPort),
                request
            );
            
            System.out.println("Response: " + response);
            
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

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See Also

ServerTcpProtocl

Server-side protocol implementation

Packet Format

Packet structure and builder pattern

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