Why a One-Hour Lease Cannot Survive a Two-Hour Outage

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

ADA · CALCULATION AUDIT

Why a One-Hour Lease Cannot Survive a Two-Hour Outage

A DHCP client tries to renew its lease at T1 (50 percent of the lease) and rebinds at T2 (87.5 percent) before surrendering the address at expiry. The chapter drops 300 fixed readers holding one-hour leases into a two-hour server-maintenance window and reports lost connectivity. This audit turns those renewal fractions into minutes to show why a one-hour lease cannot survive a two-hour outage.

Companion to the chapter DHCP and Address Resolution — every number here comes from that chapter.

Ada: The chapter pins two renewal fractions — clients try to renew at T1, “normally 50 percent” of the lease, and retry more broadly at T2, “normally 87.5 percent.” The knowledge check then puts 300 fixed readers on one-hour leases through a two-hour DHCP maintenance window and reports lost connectivity. Let me turn the fractions into minutes and show why that outcome is arithmetic, not bad luck.

For the one-hour (60-minute) lease:

• T1: 0.5 x 60 = 30.0 min — first renewal attempt
• T2: 0.875 x 60 = 52.5 min — rebinding, broadcast to any server
• Expiry: 60 min — the address is surrendered

During a 120-minute outage with the DHCP server down, a reader passes T1 (30 min, no answer), passes T2 (52.5 min, no answer), and hits expiry at 60 minutes — halfway through the window. Every reader loses its address inside the outage; with lease ages spread uniformly across the hour, the average reader drops about 30 minutes in.

Now the same outage against a 7-day (10,080-minute) lease:

• T1: 0.5 x 10080 = 5040 min, which is 3.5 days
• The 120-minute outage reaches only 120 / 5040 = 0.0238, about 2.4% of the way to the first renewal attempt.

Nothing even tries to renew, so nothing fails. The tolerance is linear in lease length: a lease rides out any maintenance window shorter than its remaining time-to-expiry, so matching lease duration to the maintenance risk — long leases for fixed sensors — is what keeps a routine DHCP restart from looking like a fleet outage.

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