3.3.1 Loss Propagation Math

A 1500-byte IP packet = 32 ATM Cells. The probability of a successful packet $P_{success}$ with cell loss $p$: $$P_{success} = (1 - p)^{32}$$

Table: The Impact of $p$ on Goodput

3.3.2 Early Packet Discard (EPD) Implementation

EPD is a proactive buffer management technique.

EPD Pseudocode:

#define THRESHOLD 0.85 // 85% full

void on_cell_arrival(Switch *s, Cell *c) {
    if (s->buffer_usage > THRESHOLD) {
        if (c->is_start_of_frame) {
            // Reject the entire packet before it enters
            s->rejected_vci_list.add(c->vci);
            drop(c);
        } else if (s->rejected_vci_list.contains(c->vci)) {
            // Continuation of a rejected packet
            drop(c);
            if (c->is_end_of_frame) s->rejected_vci_list.remove(c->vci);
        }
    } else {
        accept(c);
    }
}

3.3.3 Partial Packet Discard (PPD)

PPD is a reactive strategy. If a buffer becomes 100% full and drops cell $K$, the switch immediately marks the VCI and drops all subsequent cells $K+1...M-1$.

• Result: It frees up buffer space faster than simple Tail Drop, but is less effective than EPD.

3.3.4 Comparative Analysis of Discard Policies

1. Tail Drop: Blindly drops individual cells. Worst performance.
2. PPD: Drops the tail of packets. Moderate performance.
3. EPD: Blocks whole packets during congestion. Best performance.
4. WRED-EPD: EPD with multiple thresholds for different AAL classes.