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https://github.com/infiniteverma/cpp-tcpip

A TCP/IP implementation over raw sockets as per RFC 793
https://github.com/infiniteverma/cpp-tcpip

tcp

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A TCP/IP implementation over raw sockets as per RFC 793

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README 

        
# cpp-tcpip



This project is a custom implementation of the TCP/IP protocol stack (as per RFC 793), simulating core TCP mechanisms such as the 3-way handshake and basic data transmission. It includes a user-space simulation where socket operations are handled by communicating with a kernel-like thread, which manages connection states and packet transmission.



## Features

 - **3-Way Handshake**: The implementation successfully establishes a TCP connection using the SYN, SYN-ACK, and ACK packets.

 - **Data Transmission**: Sending and receiving data between connected sockets is functional (currently using some workarounds).

 - **User and Kernel Thread Simulation**: Socket operations are initiated by the user thread and executed by a separate "kernel" thread using message queues for inter-thread communication.



https://github.com/user-attachments/assets/4f0875ee-477c-42e6-8804-d0b4579dc0fe



## Current Status



Core mechanism of the TCP protocol are implemented (with some hacks) including connection establishment (3-way handshake) and data transfer. Details are mentioned below in **Plan** section.



## Usage 



Note: Currently, our 3-way handshake doesn't support timeout + retransmission so first run `myserver`, wait for it to reach `LISTEN` state and then run `myclient` to reach `ESTABLISHED` state.



1. Clone the repository



2. Add the two ips in lo loopback interface



```

sudo ip addr add 192.168.1.1/24 dev lo

sudo ip addr add 192.168.1.2/24 dev lo

```



3. Build the project



```

make -j4

```



4. Open two terminals and run below:



```

sudo ./myserver |tee server.log

```



and



```

sudo ./myclient |tee client.log

```



## Plan



Continuously iterating on it since there's so much going on here :)



- [x] Read RFC793 few times

- [x] Boilerplate code

    - [x] Make your own basic socket API calls

    - [x] TCB basic code

- [x] Communication over raw sockets

    - [x] IP Header class

    - [x] Test and verify packet exchange

- [x] 3 way handshake init

    - [x] Add TCP segment header

    - [x] Basic FSM driver logic (using fn pointers?)

    - [x] One way FSM

    - [x] Payload -> TCP Segment + Payload -> IP Header + TCP Segment + Payload

- [x] Reorganize code before it gets out of hand

- [x] Make a different thread for TCP stuff

    - [x] Redesign the socket design for 'userland' and 'tcp' (kernel) thread state

    - [x] Communicate

    - [x] Move socket logic to new thread

    - [x] Implement timer logic

    - [x] Stop the tcp thread gracefully

    - [x] Verify the handshake that worked before

    - [x] Stop threads once handshake is done for now

- [x] Proper handshake

    - [x] Fix timer. Timeout should fail and after receiving ACK, delete timer

    - [x] Implement it as designed in the "Event Processing" chapter in RFC

    - [?] Full state machine

    - [x] Do 3 way handshake and maintain the state (of FSM) properly

- [x] Close socket and free resources

- [x] Send and receive data

    - [x] Insert packet in a buffer and use a bitset with mutex

    - [x] Retrieve packet in tcp thread

    - [x] Add tcp/ip headers and send to socket

    - [x] After ESTABLISHED, server needs to listen for data

    - [x] For now, get the packet and dump it

- [x] Add debug logging

    - [x] write to a logfile to keep it separate from user code

- [ ] Sequence and ACK logic

- [ ] Revisit the ACTION logic of FSM (should be more tightly coupled)

- [ ] Revisit FSM hacking done in recvSegment

- [ ] Retransmission queue

    - [ ] Test 3 way handshake with this

- [ ] Add properly error codes

- [ ] getISN from time module 2**32



End goal: Sending a file as a byte stream over this tcp/ip implementation



## Gochas and lessons learned



 - Assigned a char* pointer from MyMsg into a packet but the MyMsg instance gets deleted so pointer now pointed to undefined area.

 - Assigned and mutated a local copy of an object and then was confused why the actual object wasn't updated.

 - Called join of the second thread in it's own context (instead of main thread) causing an exception



## References

 - https://datatracker.ietf.org/doc/html/rfc791/

 - https://datatracker.ietf.org/doc/html/rfc793