3.6.1 Westwood: Bandwidth Estimation

TCP Westwood performs better on lossy wireless links because it doesn't assume every loss is congestion.

Westwood Logic:

void on_ack_westwood(TCP_State *s, int acked_bytes) {
    double delta_t = now() - s->last_ack_time;
    s->bw_estimate = (acked_bytes * 8) / delta_t;
    s->bw_estimate = smooth(s->bw_estimate);
}

void on_loss_westwood(TCP_State *s) {
    // Instead of cwnd/2, use the estimated pipe capacity
    s->ssthresh = (s->bw_estimate * s->min_rtt) / MSS;
    s->cwnd = s->ssthresh;
}

3.6.2 BBR: Bottleneck Bandwidth and RTT

Google BBR is the modern replacement for Reno.

1. Probe BW: Cycle through different pacing rates to find the actual delivery rate.
2. Probe RTT: Periodically drop to 4 packets to find the absolute minimum RTT of the path.
3. Control: Paces packets exactly at the bottleneck rate, preventing bufferbloat.

3.6.3 VS/VD: Feedback Latency Reduction

In satellite ABR networks, the RM cell takes 500ms to return.

• VS/VD Switch: Intercepts the RM cell at the mid-point.
• It acts as a Virtual Destination (VD) for the first hop and a Virtual Source (VS) for the second.
• Math: Feedback loop reduced from 500ms to 250ms. Stability is improved by the square of the RTT reduction.