Ada Audits Packet-Network Resilience

Ada checks path-diversity reliability, packet size, and the exponential scale claim

ADA · CALCULATION AUDIT

Ada Audits Packet-Network Resilience

The chapter shows three independent 90%-reliable paths deliver 99.9% of the time — a 9.9-point gain — and compares an ARPANET packet of 1,008 bits (126 bytes) with a 1,500-byte Ethernet frame, nearly 12x larger. The history is really a set of engineering checks. This audit works packet-network resilience, from path diversity to byte ceilings.

Companion to the chapter History of the Internet: From ARPANET to IoT — every number here comes from that chapter.

Packet switching became convincing because the arithmetic matched the physics of real links: independent paths reduce shared failure risk, while finite packet and buffer sizes force every network to account for bytes.

1. Redundant paths turn failure probability into a power term

The chapter's example gives three independent paths, each with 90% reliability. One path fails with probability 1 - 0.90 = 0.10. All three fail only if the failures happen together:

P(delivery) = 1 - (1 - 0.90)³ = 1 - 0.10³ = 1 - 0.001 = 0.999 = 99.9%

The absolute improvement over one 90% path is 99.9 - 90.0 = 9.9 percentage points. The relative improvement is (0.999 / 0.900 - 1) x 100 = 11.0%.

2. Packet size is a physical resource budget

The chapter notes that an ARPANET packet carried 1,008 bits, while a standard Ethernet frame carries 1,500 bytes. Converting the older packet to bytes makes the comparison audit-ready:

Claim	Arithmetic	Audited result
ARPANET packet size	1,008 bits / 8 bits per byte	126 bytes
Ethernet versus ARPANET packet payload scale	1,500 / 126 = 11.904...	Rounds to 11.9x, so "nearly 12x" checks out
12 KB IMP memory scale	12 x 1024 / 126 = 97.523... if KB is read in the usual memory sense	About 97.5 packet-sized chunks before real buffers and code overhead

3. The scale claim is exponential notation, not a slogan

A throughput increase of 10⁹ means nine factors of ten:

10⁹ = 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 = 1,000,000,000

What the audit buys you: the history is not only a timeline. It is a set of engineering checks: path diversity changes delivery probability, byte ceilings shape packet design, and layered protocols survived because those budgets could scale without changing the core idea.

Every number above is taken from this chapter's own worked example and re-derived step by step.