Behind every Rickroll is a technology designed at the height of the Cold War to guarantee nuclear annihilation. Since the late 1990s, this technology has gone by vague names like "differentiated services," "AF41," and "CS5." But the manuals still use their old names: precedence, a system for making certain the most critical messages would always get through; Flash Override, designed to allow the President to reliably control nuclear weapons after a surprise Soviet attack; and CRITIC/ECP, a priority so high that even the US Strategic Air Command never used it.
This technology is built into the Internet Protocol (known as "IP" for short), which defines the most basic unit of internet communications, the "datagram." Each of these contains a small piece of data to be sent and a message header with handling information like the message's source and destination. Datagrams are designed to be sent over any kind of network, from copper telephone lines, to undersea fiber optic cables, to satellites, to "avian carriers," to—as the sticky note below illustrates—simply being left on someone's desk.
Part of the handling information is the "precedence." The definition of the IP protocol simply says that networks may "somehow [treat] high precedence traffic as more important than other traffic," and lists the eight options given on the sticky note. Neither it, nor any other documentation, explains the meaning of these terms further; even the engineers who used this system to develop reliable streaming video and online gaming rarely had any idea what the words meant or where they came from.
Their story goes back to the atomic bomb.
The idea of prioritizing messages is as old as the problem of having too many messages to deal with. Armies, copious producers of communications, had a very bad case of this, and so by World War II the US Army had a well-established notion of ordinary, urgent, and "flash" messages. Urgent messages, like Eisenhower's "mission fulfilled," were to be handled ahead of ordinary ones. Flash messages were brief messages, such as the sighted position of enemy aircraft, which had to be delivered in real-time. The term came from telegraphy, and before that from sending messages with literal flashes of light with mirrors or fires; "news flashes" come from the same origin.
The widespread availability of FM radio relay after World War II opened the possibility of electronic message networks reaching all the way to the front line, which revolutionized warfare. However, it also created a tremendous surge in message traffic, and chaos often followed as many people in close proximity needed to use the same frequencies. By the time of the Korean War, this led to a more organized notion of four priorities, which remain in use to this day: routine, priority, immediate, and flash.
These four precedence levels were given strictly defined meanings:
Routine messages (recognizable in military messages by their first letter being "R") are for ordinary traffic, such as peacetime operations or periodic reports, and should be delivered within 24 hours.
Priority messages ("P"), formerly known as "urgent," are for operations in progress, such as updates on troop movements; they should be delivered within one to six hours, and should only interrupt a routine transmission in progress if that transmission is very long.
Immediate ("O," for its old name "operational immediate") messages are for very urgent matters affecting security, such as requests for additional support or reports of unusual major movements of foreign military forces during peacetime; they can interrupt other messages and are handled in 30 minutes to an hour.
And flash messages ("Z," to indicate that they were to be sent through entirely different means – the wire rather than mail) are "reserved for initial enemy contact messages or operational combat messages of extreme urgency… [such as] messages recalling or diverting friendly aircraft about to bomb targets unexpectedly occupied by friendly forces." Rather than having a time limit, these should be delivered "as fast as humanly possible."
The years after the Korean War saw the Cold War expand to unprecedented proportions. 1953 brought the first hydrogen bomb, and the death of Stalin. 1961 saw the USSR detonate Tsar Bomba, the largest H-bomb ever to be built, and the US fielding its first "TACAMO" ("TAke Charge And Move Out") aircraft, designed to become a mobile command post for the nuclear arsenal should Washington be destroyed. 1962 brought the world to within minutes of Armageddon in the Cuban Missile Crisis.
It was during these years, between 1958 and 1963, that the predecessors of the predecessor of the Internet—a data network called AUTODIN and a phone network called AUTOVON—were being invented.
Recognizing the flood of flash messages which would surely accompany the outbreak of World War III, and those messages' ability to overload mobile communication bases, AUTODIN introduced a new precedence level: Flash Override. Flash Override messages, given the code-letter Y, could interrupt all flash messages, and could only be used by the President and the Secretary of Defense, or by their deputies if they were killed. Their primary purpose was to send Emergency Action Messages: predefined messages to nuclear bombers, missile bases, and submarines, instructing them where to go and what to attack.
For what you might call "in-case-of-no-future"–proofing, AUTODIN went one step further and introduced a sixth precedence level, whose name and meaning were never fully standardized. But it was the five-level system which stuck, for two main reasons.
If priority level four is already for "Oh my God, we're being shot at!" and priority level five is for starting a global thermonuclear war, it's hard to imagine what priority level six might be for
The first was that the very successful AUTOVON phone system only used five levels. AUTOVON was one of the first systems to use touch-tone dialing. If you are old enough to remember land lines, you may remember the sound of the buttons: two-note chords, where the low note represents the row and the high note represents the column on the touch pad. AUTOVON added a fourth column for precedence keys, which could be dialed before the phone number to get a priority line. This fourth column thus had room for four keys—the four priorities above "routine."
The second reason was simply that nobody knew what to do with the higher level. After all, if priority level four is already for "Oh my God, we're being shot at!" and priority level five is for starting a global thermonuclear war, it's hard to imagine what priority level six might be for. As a result, most documentation either ignored this level outright, or told people to treat it as a synonym for Flash Override. It was widely forgotten.
Today, AUTODIN and AUTOVON have been replaced, but the five-level system remains a part of both telephony standards and every modern military phone. But outside of telephones, Flash Override has largely receded from view; modern international military communications manuals don't even mention it. Emergency Action Messages (EAMs) remain in widespread use, and can routinely be heard on "numbers stations," radio frequencies which read out coded messages to aircraft and submarines. Most of these, we may presume, are training exercises or logistics messages.
(Here too we can see precedence at work: EAMs are sometimes interrupted by "Skyking messages," easily recognizable by their format "Skyking, skyking, do not answer. [Three letters]. Time [two-digit minutes]. Authorization [two letters]." These are widely believed to be instructions to airborne alert forces, both nuclear and otherwise; they were heavily used in the months following 9/11. For obvious reasons, the Air Force is not saying more.)
The success of the AUTOVON phone system had another consequence: its five-level precedence system became widely known among "phone phreaks," the early hacker community which specialized in getting phones to do what they wanted. From there, it made its way into newsletters like YIPL (a publication of the Yippies, a "countercultural revolutionary" movement), a forerunner of the still-active magazine 2600. The levels up to Flash Override were thus known in the hacker community into the 1990s, and were obliquely referred to in the 1995 movie Hackers by the protagonist's handle, "Crash Override."
This brings us to the Internet Protocol, which was invented between 1979 and 1981 as part of a joint academic and Defense Department program. With compatibility with military networks a key goal, IP included a precedence system which "came straight from the defense department practices on AUTODIN," according to Vint Cerf, who led the project. Early versions experimented with as many as 16 precedence levels, but by 1981 it was clear that five levels were all anyone wanted—or understood.
"Flash Override was at the top of the heap," Cerf said, and his colleague Lauren Weinstein added that "the original field specs were taken directly from AUTOVON['s]… fourth (red) column of buttons." If the sixth level had any purpose, even the Internet's original authors didn't know it.
But this level would be preserved by mathematical necessity. Computers store numbers in binary; a two-bit number allows only four possibilities (00, 01, 10, and 11), while a three-bit number allows eight. That meant that there would have to be eight precedence values in the system, not five, and names would be needed for the extra three.
Next time you're streaming an old X-Files episode, remember that you're using a mechanism designed to ensure nuclear war could be reliably fought
The sixth level was thus named after the two synonyms it most often got in various generations of AUTODIN documentation: "Critical" and "Emergency Command Precedence." Assuming that the only people who would care were people who already knew this documentation inside and out, it was simply given the cryptic label "CRITIC/ECP."
The top two levels came not from the military, but from the lessons learned from civilian telephone networks. When a network is overloaded, the most important message to get through isn't any user's message at all; it's a control message telling the network how to manage that load. These network control messages were split into two levels partially because there were still two slots to fill, and partially because of the nature of the new protocol.
Computer networks weren't, after all, new in 1981. What was being developed was a common way to express the idea of "send this piece of data to that computer over there," even when the computers were on different networks like AUTODIN, SATNET, and ARPANET. That is, it was an "inter-net protocol."
Since managing cross-network junctions is only useful if your own network is working in the first place, control codes for those junctions should be assigned lower priority than control of the local network. Thus level seven was named "Internetwork Control," and level eight was "Network Control."
Thus we ended up with the modern IP precedence system: four levels for ordinary military traffic, one for launching nuclear missiles, one for backwards compatibility, and two for network control.
For many years, this precedence system was barely used, because one thing the Internet Protocol didn't do was establish clear guidelines on what these levels meant. Without that, nobody could rely on their effects on real networks. These guidelines weren't developed until the late 1990s, and network hardware didn't support them until well into the 2000s. By this time, even top engineers in the field knew little of the origins. Amin Vahdat, Google's head of network engineering, said he could only make "some reasonable guesses, based on an understanding of [their] military origin."
This also meant that the first people to seriously use IP precedence were not strategic bombing crews, but the largest and most latency-sensitive users of computer networks in the modern era: streaming video, online gaming, and videoconferencing. Today, Priority traffic is most likely a cat video, and Flash Override or CRITIC/ECP traffic is your boss video-calling you to ask why you're watching cats at work.
So remember next time you're streaming an old X-Files episode on Netflix that you're actually using a mechanism designed to ensure that nuclear war could be reliably fought, even if it had to be done from a modified Boeing 707 after Washington was destroyed. The truth, in this case, is in there.
The author wishes to thank US Army communications historian Floyd Hertweck for his help in discovering the relevant military lore.