In 1995, "always-on" internet was a luxury. Many enterprise sites connected to the corporate backbone via demand circuits — ISDN lines, dial-up modems, or X.25 SVCs that charged by the minute or by the packet. The economic model was clear: keep the circuit down unless there's actual user traffic to send. But there was a problem. If you ran OSPF (the shiny new link-state routing protocol) over such a link, the circuit would never go idle. OSPF chatters constantly: Hello packets every 10 seconds, LSA refreshes every 30 minutes, database synchronization on every neighbor adjacency. Your ISDN bill would be ruinous.

John Moy — the author of OSPF itself (RFC 2328) — wrote RFC 1793 to fix this. The trick is elegant: make OSPF pretend that periodic maintenance traffic isn't necessary on demand circuits, while still guaranteeing that the routing database stays converged.

Three mechanisms make this work:

• Suppression of Hellos. Once an adjacency is fully formed over a demand circuit, Hello packets are suppressed. The neighbor is presumed alive until something (usually the data link layer) says otherwise. No 10-second keepalive means no reason to bring the circuit up.
• The DoNotAge (DNA) bit. Normal LSAs carry an age field that increments every second; when it hits MaxAge (3600s), the LSA must be refreshed by its originator, forcing traffic on the wire. RFC 1793 defines a high bit in the LSA age field — the DoNotAge bit, value 0x8000 — that freezes the age. An LSA flooded across a demand circuit with DNA set stays valid indefinitely, no periodic refresh required.
• Capability negotiation via the Options field. A new DC-bit in the Options field advertises "I understand demand circuits." Every router in the area must set it, or DNA-bearing LSAs get ignored and the whole scheme falls apart. If even one legacy router shows up, the DC extension gracefully disables itself area-wide.

The subtle part is correctness. If Hellos are suppressed and ages are frozen, how do you ever notice a topology change? The answer: real changes still flood normally. A link going down, a new LSA being originated, an SPF-relevant event — these all trigger flooding across the demand circuit, which brings it up briefly. It's only the periodic maintenance that's suppressed. Event-driven routing, not timer-driven.

There's a lovely piece of trivia in the design: the DNA bit lives in the high-order bit of the 16-bit LSA age. Since normal ages max out at 3600 (well under 0x8000 = 32768), this bit was effectively free — a clever piece of protocol archaeology exploiting unused encoding space.

Why should you care in 2026? ISDN is dead, but the pattern lives on. RFC 1793's DoNotAge mechanism inspired similar "quiet the routing protocol" work throughout the IETF: IS-IS added analogous extensions, BGP has its route-refresh and graceful-restart machinery, and modern IoT routing protocols like RPL (RFC 6550) borrowed the philosophy wholesale — event-driven updates over battery-powered links where every transmitted packet costs joules. Anywhere you have an expensive or metered link — cellular backup, satellite, LPWAN, even cloud-provider egress with per-GB charges — the RFC 1793 mindset applies: protocols should stop talking when nothing has changed.

It's also a case study in graceful capability negotiation. The DC-bit's "all-or-nothing per area" rule prevents subtle inconsistency bugs — a design pattern later echoed in TLS extensions, HTTP/2 SETTINGS, and countless other protocols.