The Real Price of a Bigger Zone

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

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Ada ADA · CALCULATION AUDIT

The Real Price of a Bigger Zone

On a 150-node campus mesh, ZRP’s zone radius is the tuning knob: moving from radius 2 to radius 3 spends an extra 106 MB/hour of periodic update traffic to eliminate about 844 reactive route discoveries an hour. Divide one by the other and the trade has an exact price: 126 KB per discovery removed. This audit rebuilds both overhead ledgers and asks whether that 126 KB exchange rate is worth paying, or just a bigger zone dressed up as an upgrade.

Companion to the chapter ZRP Hybrid Zone Routing — every number here comes from that chapter.

Ada: The campus example weighs zone radius 2 against radius 3 and ends on a single decisive number: 126 KB of extra control traffic per discovery the larger zone eliminates. That figure is what tells an engineer whether covering more of the building proactively is worth it, so it has to be exact. Let me rebuild both overhead ledgers for the 150-node network from the chapter’s own inputs (50 bytes/route entry, 240 updates/hour, 30 packets/hour/node, 1,800 bytes/discovery; decimal MB).

  • rho = 2 proactive: 24 routes x 50 B = 1,200 B; x 240/hr = 288 KB/node; x 150 = 43.2 MB/hr.
  • rho = 3 proactive: 84 routes x 50 B = 4,200 B; x 240/hr = 1,008 KB/node; x 150 = 151.2 MB/hr.
  • rho = 2 reactive: 150 x 30 x 0.4375 = 1,968.75 discoveries/hr; x 1,800 B = 3.54 MB/hr.
  • rho = 3 reactive: 150 x 30 x 0.25 = 1,125 discoveries/hr; x 1,800 B = 2.025 MB/hr.
  • Totals: rho 2 = 43.2 + 3.54 = 46.74 MB/hr; rho 3 = 151.2 + 2.025 = 153.225 MB/hr.
  • Extra overhead: 153.225 - 46.744 = 106.481 MB/hr.
  • Discoveries eliminated: 1,968.75 - 1,125 = 843.75/hr.
  • Cost per eliminated discovery: 106.481 MB / 843.75 = 0.1262 MB = 126.2 KB — matching the chapter’s 126 KB.

The arithmetic confirms the trade and names what the number means: moving from rho 2 to rho 3 spends an extra 106 MB/hour of periodic update traffic to save 844 reactive discoveries, and 126 KB is the exchange rate between them. Whether that price is worth paying depends on how latency-sensitive the traffic is — the ledger turns the ZRP tuning decision into an explicit purchase rather than a default.

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