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Introduction

TCP (Transmission Control Protocol) is the backbone of reliable internet communication. Understanding how TCP establishes connections, manages data flow, and handles HTTP is crucial for building robust networked applications.
This is part of Week 1 – Networking from Scratch in the “From CPU to the Browser” course.

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Three-way handshake

The TCP three-way handshake is the process used to establish a connection between a client and server.
1

SYN: Client initiates connection

The client sends a SYN (synchronize) packet to the server with an initial sequence number. This indicates the client wants to establish a connection.Packet contains:
  • SYN flag set
  • Initial sequence number (ISN)
  • TCP options (window size, MSS, etc.)
2

SYN-ACK: Server responds

The server responds with a SYN-ACK packet, acknowledging the client’s request and sending its own sequence number.Packet contains:
  • SYN flag set
  • ACK flag set
  • Acknowledgment number (client’s ISN + 1)
  • Server’s initial sequence number
3

ACK: Client confirms

The client sends an ACK packet to acknowledge the server’s response. The connection is now established and data transfer can begin.Packet contains:
  • ACK flag set
  • Acknowledgment number (server’s ISN + 1)
Each step in the handshake can fail due to network issues, firewall rules, or server unavailability. Always implement proper timeout and retry logic.

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Flow control

TCP implements flow control to prevent overwhelming the receiver with data it can’t process quickly enough.

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Sliding window protocol

The sliding window mechanism allows the sender to transmit multiple packets before receiving acknowledgments, improving efficiency.
1

Window size advertisement

The receiver advertises its available buffer space (receive window) in every ACK packet.
2

Sender respects window

The sender ensures the amount of unacknowledged data never exceeds the receiver’s window size.
3

Dynamic adjustment

As the receiver processes data and frees buffer space, the window size can grow. If the buffer fills up, the window shrinks.

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Key flow control concepts

TCP window scaling extends the maximum window size beyond 65,535 bytes, allowing for better performance on high-bandwidth networks.
When a receiver’s buffer is full, it advertises a window size of zero, telling the sender to stop transmitting until buffer space becomes available.
When a zero window is received, the sender periodically sends small probe packets to check if the window has reopened.

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HTTP basics

HTTP (Hypertext Transfer Protocol) operates on top of TCP, providing the foundation for web communication.

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Parsing HTTP headers

HTTP messages consist of a request/response line, headers, and an optional body. 
GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/html
Connection: keep-alive

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Parsing steps

1

Parse request/status line

Extract the HTTP method, path, version (for requests) or status code and message (for responses).
2

Parse headers

Read header lines until an empty line is encountered. Each header is a key-value pair separated by a colon.
3

Determine body length

Check Content-Length header or Transfer-Encoding to determine how to read the body.
4

Read body

Read the message body based on the determined length or encoding method.

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Chunked encoding

Chunked transfer encoding allows a server to send data in chunks without knowing the total size upfront. 
HTTP/1.1 200 OK
Transfer-Encoding: chunked

1A
This is the first chunk of data.
14
Another chunk here.
0
Format:
  • Each chunk starts with its size in hexadecimal
  • Followed by \r\n
  • Then the chunk data
  • Followed by another \r\n
  • A chunk size of 0 indicates the end

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Keep-alive connections

HTTP keep-alive allows multiple requests/responses to be sent over a single TCP connection, reducing overhead.
In HTTP/1.1, keep-alive is enabled by default. To close the connection, include Connection: close header.
Benefits:
  • Reduces latency (no need for multiple handshakes)
  • Reduces CPU and memory usage
  • Enables HTTP pipelining
  • Improves page load times
Servers must properly manage keep-alive connections with timeouts to prevent resource exhaustion.

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Network debugging

Use these tools to inspect TCP and HTTP traffic:

tcpdump

Capture and analyze TCP handshakes and packet flow:
tcpdump -i eth0 tcp port 80

Wireshark

Visualize TCP streams and HTTP messages with a graphical interface. Follow TCP streams to see complete conversations.

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Next steps

TLS and WebSockets

Learn about secure connections with TLS and real-time communication with WebSockets

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