Network Protocols

Network Protocols Mechanics

TCP State Machine

TCP is a connection-oriented, reliable transport protocol driven by a complex state machine defined in RFC 793.

• Connection Establishment: Transitions through SYN_SENT, SYN_RCVD, to ESTABLISHED via the 3-way handshake.
• Data Transfer: In ESTABLISHED, sequencing and ACKs manage reliability.
• Teardown: Involves FIN_WAIT_1, FIN_WAIT_2, TIME_WAIT, CLOSE_WAIT, LAST_ACK. TIME_WAIT ensures delayed packets are not mistakenly delivered to a new incarnation of the connection.

Congestion Control Algorithms

Congestion control modulates the Congestion Window (cwnd) to avoid network collapse.

• Loss-based (Cubic): Uses a cubic function of time since the last congestion event to dictate cwnd growth. It aggressively probes for bandwidth, resulting in a convex growth profile that scales well in high-BDP (Bandwidth-Delay Product) networks. Relies on packet loss as the primary congestion signal.
• Delay-based (BBR - Bottleneck Bandwidth and Round-trip propagation time): Models the network path. It continuously measures the maximum delivery rate (Bottleneck Bandwidth) and minimum RTT. BBR controls sending rate to match the estimated BDP, minimizing queue buildup (bufferbloat) and operating near the optimal Kleinrock point, rather than reacting solely to loss.

%%{init: {"theme": "default", "flowchart": {"useMaxWidth": true}}}%%
flowchart TD
    subgraph TCPStateTCPStateMachine ["TCP State Machine<br><br><br>"]
        CLOSED -->|"Send(SYN)"| SYN_SENT
        SYN_SENT -->|"Recv(SYN-ACK)"| ESTABLISHED
        ESTABLISHED -->|"Send(FIN)"| FIN_WAIT_1
    end
    
    subgraph CongestionCongestionControl ["Congestion Control<br><br><br>"]
        ESTABLISHED --> CC_Algo["Algorithm Selection"]
        CC_Algo --> Cubic["CUBIC (Loss-based)"]
        CC_Algo --> BBR["BBR (Model-based)"]
        
        Cubic -->|"Update(cwnd)"| Network
        BBR -->|"Update(pacing_rate)"| Network
    end