Out of Band

The grid is dead. The air isn’t.

First the cloud goes dark. It turns out the cloud was buildings after all: buildings with substations, cooling pumps, fibre routes and people who had been told to do more with less until less finally won. The dashboards remain green for several heroic minutes because the dashboards are dead too. Then the last cached status page expires and the world is briefly spared the opinion of a product manager.

Out-of-band communication means keeping a path separate from the ordinary data path so that failure in the main system does not take your last means of reaching it along for the ride. The word that matters is separate. A backup chat room using the same network, identity provider, power supply and exhausted administrator is not an emergency channel. It is the same corpse wearing a different lanyard.

TCP supplies the traditional technical joke. Implementations call its urgent mechanism out-of-band data, complete with MSG_OOB and SO_OOBINLINE. RFC 6093 says, in the typographical voice of somebody reaching across the table to shake you, that TCP urgent data is NOT a mechanism for sending out-of-band data. It is a marker in the same stream. RFC 9293 keeps the mechanism for compatibility because protocol archaeology rarely permits the luxury of throwing the cursed object into the sea.

The shirt offers a smaller and more credible sort of hope than rebuilding civilisation before lunch. One receiver survived. The aerial is up. A bird has returned before the venture capital. Somewhere off the edge of the print, somebody has found a battery and pressed transmit. The old network may be a smoking crater, but the spectrum has not signed an exclusivity agreement. Listen.

Read it for yourself: RFC 6093: On the Implementation of the TCP Urgent Mechanism, RFC 9293: Transmission Control Protocol