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Overview

The Packet class represents a simulated network packet used in the Selective Repeat protocol. It provides immutable packet objects with a builder pattern for construction and methods for serialization/deserialization.

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Packet Structure

Each packet consists of the following fields:
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type
int
required
Packet type identifier:
  • 0: Data packet
  • 1: ACK (Acknowledgment)
  • 2: SYN (Connection request)
  • 3: SYN-ACK (Connection acknowledgment)
  • 4: NACK (Negative acknowledgment)
  • 6: Connection close
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sequenceNumber
long
required
Packet sequence number for ordering and acknowledgment (unsigned 32-bit)
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peerAddress
InetAddress
required
IPv4 address of the destination peer (4 bytes)
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peerPort
int
required
Destination port number (unsigned 16-bit)
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payload
byte[]
required
Packet payload data (0-1024 bytes)

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Constants

public static final int MIN_LEN = 11;        // Minimum packet size (header only)
public static final int MAX_LEN = 11 + 1024; // Maximum packet size (header + payload)
Packet header is 11 bytes: 1 (type) + 4 (sequence) + 4 (address) + 2 (port) = 11 bytes

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Constructor

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Packet()

Creates a new packet instance.
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type
int
required
Packet type (0-6)
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sequenceNumber
long
required
Sequence number
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peerAddress
InetAddress
required
Destination IP address
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peerPort
int
required
Destination port
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payload
byte[]
required
Payload data
Packet packet = new Packet(
    0,                                          // Data packet
    42L,                                        // Sequence number
    InetAddress.getByName("192.168.1.100"),   // Peer address
    8000,                                       // Peer port
    "Hello World".getBytes()                   // Payload
);
Direct constructor usage is discouraged. Use the Builder pattern instead for better readability and flexibility.

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Getter Methods

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getType()

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type
int
Returns the packet type

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getSequenceNumber()

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sequenceNumber
long
Returns the sequence number

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getPeerAddress()

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peerAddress
InetAddress
Returns the destination IP address

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getPeerPort()

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peerPort
int
Returns the destination port number

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getPayload()

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payload
byte[]
Returns the payload data as a byte array

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Builder Pattern

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Packet.Builder

Provides a fluent API for constructing packets. 
Packet packet = new Packet.Builder()
    .setType(0)
    .setSequenceNumber(1L)
    .setPortNumber(8000)
    .setPeerAddress(InetAddress.getByName("127.0.0.1"))
    .setPayload("Request data".getBytes())
    .create();

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Builder Methods

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type
int
required
Sets the packet type
​
builder
Builder
Returns the builder instance for chaining
builder.setType(1)  // ACK packet
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sequenceNumber
long
required
Sets the sequence number
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builder
Builder
Returns the builder instance for chaining
builder.setSequenceNumber(42L)
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peerAddress
InetAddress
required
Sets the destination IP address
​
builder
Builder
Returns the builder instance for chaining
builder.setPeerAddress(InetAddress.getByName("192.168.1.100"))
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portNumber
int
required
Sets the destination port number
​
builder
Builder
Returns the builder instance for chaining
builder.setPortNumber(8000)
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payload
byte[]
required
Sets the payload data
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builder
Builder
Returns the builder instance for chaining
builder.setPayload("Data".getBytes())
Constructs and returns the final Packet object.
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packet
Packet
The constructed packet instance
Packet packet = builder.create();

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toBuilder()

Creates a builder from an existing packet for modification.
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builder
Builder
A new builder initialized with the current packet’s values
Packet original = new Packet.Builder()
    .setType(0)
    .setSequenceNumber(1L)
    .setPortNumber(8000)
    .setPeerAddress(InetAddress.getByName("127.0.0.1"))
    .setPayload("Data".getBytes())
    .create();

// Create modified version
Packet ack = original.toBuilder()
    .setType(1)              // Change to ACK
    .setPayload(new byte[0]) // Empty payload
    .create();

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Serialization

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toBytes()

Converts the packet to a byte array for network transmission.
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bytes
byte[]
Raw byte representation of the packet in BigEndian format
Packet packet = new Packet.Builder()
    .setType(0)
    .setSequenceNumber(1L)
    .setPeerAddress(InetAddress.getByName("127.0.0.1"))
    .setPortNumber(8000)
    .setPayload("Hello".getBytes())
    .create();

byte[] data = packet.toBytes();
DatagramPacket dgram = new DatagramPacket(
    data, 
    data.length, 
    routerAddress, 
    routerPort
);
socket.send(dgram);

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toBuffer()

Converts the packet to a ByteBuffer in BigEndian format.
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buffer
ByteBuffer
ByteBuffer flipped and ready for reading
ByteBuffer buffer = packet.toBuffer();

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Deserialization

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fromBytes()

Creates a packet from a byte array.
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bytes
byte[]
required
Raw packet data received from network
​
packet
Packet
Reconstructed packet object
DatagramPacket dgram = new DatagramPacket(new byte[1024], 1024);
socket.receive(dgram);

Packet packet = Packet.fromBytes(dgram.getData());
System.out.println("Received sequence: " + packet.getSequenceNumber());
Throws IOException if the data length is invalid (not between MIN_LEN and MAX_LEN).

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fromBuffer()

Creates a packet from a ByteBuffer.
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buf
ByteBuffer
required
ByteBuffer containing packet data in BigEndian format
​
packet
Packet
Reconstructed packet object
ByteBuffer buffer = ByteBuffer.allocate(MAX_LEN)
    .order(ByteOrder.BIG_ENDIAN);
buffer.put(receivedData);
buffer.flip();

Packet packet = Packet.fromBuffer(buffer);

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Byte Format

Packets are encoded in BigEndian byte order: 
Byte Layout:
┌─────────────────────────────────────────────────────────┐
│ Type (1)  │ Seq (4)   │ IP (4)    │ Port (2) │ Payload  │
│  byte     │   int     │  4 bytes  │  short   │ 0-1024   │
└─────────────────────────────────────────────────────────┘
  0         1           5           9          11         MAX
All multi-byte fields use BigEndian (network byte order) encoding.

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String Representation

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toString()

Returns a human-readable string representation.
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string
String
Formatted string with sequence number, peer address, port, and payload size
System.out.println(packet.toString());
// Output: #42 peer=192.168.1.100:8000, size=5

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Packet Type Reference

Data (0)

Regular data packet containing payload

ACK (1)

Acknowledgment of received packet

SYN (2)

Connection initiation request

SYN-ACK (3)

Connection acknowledgment

NACK (4)

Negative acknowledgment

CLOSE (6)

Connection termination

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Example Usage

Packet dataPacket = new Packet.Builder()
    .setType(0)
    .setSequenceNumber(1L)
    .setPeerAddress(InetAddress.getByName("127.0.0.1"))
    .setPortNumber(8000)
    .setPayload("Request data".getBytes())
    .create();

byte[] bytes = dataPacket.toBytes();

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See Also

ReliableClientProtocol

Client-side protocol using packets

ServerTcpProtocl

Server-side protocol using packets

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