core

A step-by-step explanation of TCP and implementation using Python’s socket module

Introduction to TCP

The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It provides reliable, ordered, and error-checked delivery of a stream of bytes between applications running on hosts communicating via an IP network.

TCP is: - Connection-oriented: A connection is established before data is sent - Reliable: It guarantees delivery of data to the destination - Ordered: If packets arrive in the wrong order, TCP reorders them - Error-checked: Corrupted packets are retransmitted

The Three-Way Handshake

TCP establishes a connection through a process called the three-way handshake:

  1. SYN: Client sends a SYN (synchronize) packet with a random sequence number x
  2. SYN-ACK: Server responds with a SYN-ACK packet with its own sequence number y and acknowledgment number x+1
  3. ACK: Client sends an ACK packet with acknowledgment number y+1 to establish the connection
from IPython.display import SVG
from IPython.display import display
svg_data = '''
<svg width="500" height="300" xmlns="http://www.w3.org/2000/svg">
    <rect width="100%" height="100%" fill="white"/>
    
    <!-- Entities -->
    <rect x="50" y="20" width="100" height="50" fill="lightblue" stroke="black"/>
    <text x="100" y="50" text-anchor="middle">Client</text>
    <rect x="350" y="20" width="100" height="50" fill="lightgreen" stroke="black"/>
    <text x="400" y="50" text-anchor="middle">Server</text>
    
    <!-- Lines -->
    <line x1="100" y1="70" x2="100" y2="280" stroke="gray" stroke-dasharray="5,5"/>
    <line x1="400" y1="70" x2="400" y2="280" stroke="gray" stroke-dasharray="5,5"/>
    
    <!-- Arrows -->
    <line x1="100" y1="120" x2="380" y2="150" stroke="black" stroke-width="2"/>
    <polygon points="380,150 370,145 370,155" fill="black"/>
    <text x="240" y="125" text-anchor="middle">SYN (seq=x)</text>
    
    <line x1="400" y1="180" x2="120" y2="210" stroke="black" stroke-width="2"/>
    <polygon points="120,210 130,205 130,215" fill="black"/>
    <text x="260" y="185" text-anchor="middle">SYN-ACK (seq=y, ack=x+1)</text>
    
    <line x1="100" y1="240" x2="380" y2="270" stroke="black" stroke-width="2"/>
    <polygon points="380,270 370,265 370,275" fill="black"/>
    <text x="240" y="245" text-anchor="middle">ACK (ack=y+1)</text>
</svg>
'''

display(SVG(data=svg_data))

Socket API in Python

Python’s socket module provides a simple API for creating TCP connections. Here are the key elements we’ll use:

We will create a simplified wrapper around Python’s socket API to make it easier to understand TCP concepts.

source

get_free_port

 get_free_port ()

Get an available port number by creating and closing a temporary socket.

Let’s define some constants for our implementation:

Socket State Information

TCP sockets go through various states during their lifecycle:

  • CLOSED: No connection
  • LISTEN: Server is waiting for incoming connections
  • SYN_SENT: Client has sent SYN packet
  • SYN_RECEIVED: Server has received SYN and sent SYN-ACK
  • ESTABLISHED: Connection is established
  • FIN_WAIT: Connection termination in progress
  • CLOSE_WAIT: Waiting for application to close
  • CLOSING: Both sides have initiated connection termination
  • TIME_WAIT: Waiting to ensure remote TCP received connection termination
  • LAST_ACK: Waiting for last acknowledgement

We’ll create utility functions to help us track these states in our implementation.

source

SocketState

 SocketState ()

Constants for socket states.

Now let’s define a basic connection structure that will help us manage TCP connections:

source

TCPConnection

 TCPConnection (sock:Optional[socket.socket]=None, state:str='CLOSED',
                remote_address:Optional[Tuple[str,int]]=None,
                connection_id:Optional[str]=None)

Represents a TCP connection with state information.

Note that I’ve renamed the field from socket to sock to avoid confusion with the module name.

TCP Connection Lifecycle

A TCP connection goes through the following phases: 1. Connection establishment (three-way handshake) 2. Data transfer 3. Connection termination (four-way handshake)

In our implementation, we’ll focus on making these phases explicit and easy to understand.