OpenCyber AcademyLearning outcomes
You will (1) model a cumulative quantity in an assessment as an integral of a rate; (2) use a derivative to find when a rate peaks; (3) turn both into a rate limit Sam Ito can defend in the rules of engagement (ROE).
1.1 Rates and accumulations in an engagement
Every authorized test produces traffic, and the client cares about two things: how fast it arrives and how much accumulates. If an enumeration job sends requests at rate r(t) requests per second, the total sent between times a and b is the integral ∫ r(t) dt from a to b. A ROE that says "no more than 5 requests per second" limits r(t); one that says "no more than 20,000 requests per day against the ticketing API" limits the integral. They are different constraints and a schedule can satisfy one while breaking the other.
1.2 Finding the peak
Suppose TrackScope ramps a scan up and then down, with r(t) = 6t − t² requests per second for 0 ≤ t ≤ 6 minutes (t in minutes, synthetic). The derivative r′(t) = 6 − 2t is zero at t = 3, where r(3) = 9 requests per second. If the ROE ceiling is 5 per second, this schedule breaches it between the two roots of 6t − t² = 5, that is t = 1 and t = 5 minutes. Calculus tells you not just whether a plan breaks the limit but for how long, which is what the client's operations team asks first.
1.3 From total to schedule
The total for that ramp is ∫₀⁶ (6t − t²) dt = [3t² − t³/3]₀⁶ = 108 − 72 = 36 request-minutes per second, i.e. 36 × 60 = 2,160 requests. A flat schedule at 5 per second would need 2,160 / 5 = 432 seconds, 7.2 minutes, to send the same volume without any breach. Engineers prefer the flat schedule because its peak equals its average and is trivial to verify from logs.
Synthetic case — Riverstone Transit. Sam's draft ramp for the fare-gateway test peaked at 9 req/s; the ROE allowed 5. Sam replaced it with a 5 req/s token bucket (see Chapter 36) and attached the integral calculation to the change request.
1.4 Errors and limits
Real traffic is bursty, not smooth; a continuous model gives an average-case picture, and the ROE limit must be enforced by a mechanism, not by the calculation. Units cause most mistakes: keep seconds and minutes separate.
Enforcement belongs in the scanner configuration and in a network rate limiter the client can see, so a mistake in the schedule cannot silently exceed the agreed limit. When converting between minutes and seconds, write the units next to every number and check the final figure against a rough estimate. Burstiness means a short spike can break a per-second limit even when the hourly average is fine, so Sam Ito sets both a short-window and a long-window ceiling in the ROE and tests them before the engagement starts.
Exercises
- EX001 — Breach window. For r(t) = 8t − t² req/s (t in minutes, 0–8) and a ceiling of 7 req/s, find the peak and the interval during which the ceiling is breached. Rubric: 3 = peak 16 at t=4 and breach between t=1 and t=7 (6 minutes); 2 = one slip; 1 = peak only; 0 = none.
- EX002 — Two kinds of limit. Give one ROE clause that limits a rate and one that limits a total, for the same API, and say which one a log-based check verifies more easily. Rubric: 3 = both clauses with units and justified answer; 2 = clauses without justification; 1 = one clause; 0 = none.
- EX003 — Flat equivalent. How long does a flat 4 req/s schedule take to send the 2,160 requests of §1.3? Rubric: 3 = 540 s (9 minutes) with working; 2 = right number, no units; 1 = method only; 0 = none.
Synthesis
A rate limit and a volume limit are a derivative and an integral of the same curve. Checking both turns a vague promise of "low impact" into numbers the client can audit.