TCP Handshake, Flow Control & Congestion Control

TCP is a connection-oriented, reliable, byte-stream transport protocol. Before application data flows, a 3-way handshake establishes sequence numbers and buffer parameters:

1. Step 1 (Client -> Server): Client sends SYN segment (SYN=1, ACK=0) with initial sequence number seq = x. Consumes 1 sequence number.

2. Step 2 (Server -> Client): Server responds with SYN+ACK segment (SYN=1, ACK=1) with its own initial sequence number seq = y and acknowledgment ack = x + 1. Consumes 1 sequence number.

3. Step 3 (Client -> Server): Client sends ACK segment (SYN=0, ACK=1) with seq = x + 1 and ack = y + 1. May carry payload.

Connection Teardown (Modified 4-Way Handshake): Either end can initiate termination by sending a FIN segment. The receiving side sends ACK (half-close), finishes transmitting its remaining data, and sends its own FIN segment. The initiator acknowledges with ACK and enters the TIME_WAIT state (2 * MSL = Maximum Segment Lifetime, typically 2 minutes) to ensure the final ACK arrives.

TCP regulates its sending rate dynamically using two state variables: cwnd (Congestion Window, maintained by sender) and rwnd (Receiver Advertised Window, sent in TCP header).

Actual Transmission Window W = min(cwnd, rwnd).

The three main phases of TCP congestion control are:

1. Slow Start: cwnd starts at 1 MSS (Maximum Segment Size). For EVERY ACK received, cwnd increases by 1 MSS. Thus, over each RTT, cwnd DOUBLES exponentially: 1 -> 2 -> 4 -> 8 -> 16... until cwnd >= ssthresh (Slow Start Threshold).

2. Congestion Avoidance: Once cwnd reaches ssthresh, cwnd increases LINEARLY by 1 MSS per RTT (Additive Increase: cwnd = cwnd + 1/cwnd per ACK).

3. Multiplicative Decrease on Packet Loss:

Case A — Loss detected by Timeout: Severe congestion! Set ssthresh = max(cwnd / 2, 2 MSS). Set cwnd = 1 MSS. Re-enter Slow Start.

Case B — Loss detected by 3 Duplicate ACKs (Fast Retransmit): Mild congestion! Retransmit missing segment immediately without waiting for timeout.

In TCP Tahoe: Set ssthresh = cwnd / 2; reset cwnd = 1 MSS (enters Slow Start).

In TCP Reno (Fast Recovery): Set ssthresh = cwnd / 2; set cwnd = ssthresh + 3 MSS; then continue Congestion Avoidance (linear growth). Avoids slow-start penalty!