The 119 ms Path and Its Fragile Margin

Ada re-derives this chapter’s own numbers step by step, at full precision

ADA · CALCULATION AUDIT

The 119 ms Path and Its Fragile Margin

The chapter’s crosswalk example chains five hop delays into a 119 ms end-to-end path against a 500 ms safety budget — comfortable, with 381 ms to spare. Yet one careless protocol choice can erase most of that headroom. This audit totals the hops and prices the “skip QoS” mistake to expose how fragile the 119 ms margin really is.

Companion to the chapter Packet Journey Adventure Game — every number here comes from that chapter.

Ada: The crosswalk example adds five hop delays to 119 ms against a 500 ms safety budget, then warns that one careless protocol choice erases most of the headroom. Let me total the hops, confirm the margin, and price the “skip QoS” mistake.

Summing the five hop totals:

• 7 + 25 + 55 + 17 + 15 = 119 ms end to end.
• Safety margin: 500 - 119 = 381 ms of headroom, and 119 < 500, so the design passes.

The middle hop is worth checking because it mixes three delay types — 10 (propagation) + 0.12 (transmission) + 14.88 (queueing) = 25.00 ms — which shows queueing, not distance, dominates that leg. Now the counterfactual where the alarm is sent best-effort instead of expedited, so the peering queue grows from 30 ms to 200 ms:

• New total: 119 - 30 + 200 = 289 ms.
• Remaining margin: 500 - 289 = 211 ms, i.e. 211 / 500 = 0.422 = 42% of the budget.

The design meaning is that a single per-hop decision moves the whole end-to-end result: one un-prioritised queue turned 381 ms of comfort (381 / 500 = 76% of budget) into 211 ms (42%), still passing but no longer safe against a second bad hop. That is why the chapter budgets for worst case — the delays are additive, so headroom that looks generous at the average collapses quickly when even one hop is left to best-effort handling.

Every number above is taken from the chapter’s own material and re-derived step by step.