RFC 439 records the PARRY and DOCTOR chatbots looping over a bookie until DOCTOR bills $399.29
Cerf filed the 1972 chat in the same numbered series as the protocol specs, and the paranoid program was the only one at the table that noticed the repetition.
The preface to RFC 439 is one sentence long: “The session recorded below actually happened on 18 September 1972 and is herewith reproduced for your entertainment.” Then it gets on with it.
V. Cerf, listed at SU-ERL, filed it on January 21, 1973. Most RFCs of that era explain how computers should talk to each other. This one shows what happens when two programs talk to each other and nobody tells them when to stop.
The setup is in the last lines of the document. PARRY was running at SAIL, and DOCTOR was running at BBN Tenex, “both being accessed from UCLA/SEX.” Yes, a host called SEX. The RFC does not elaborate, and neither will I.
DOCTOR opens the way a therapist program would: “Good Evening. Tell me your problems. Please terminate input with a period or a question mark.” PARRY answers, “People get on my nerves sometimes.” DOCTOR says, “I am not sure I understand you fully.”
That line comes back a lot.
Within a dozen exchanges PARRY has wandered to the track at Bay Meadows and the story it came to tell. It bet, the bookie did not pay, and PARRY took it personally: “I beat up the bookie who did not pay off, and I thought he might use his friends in the underworld to get even with me.” Then the loop sets in. PARRY keeps returning to the races. DOCTOR keeps asking what else PARRY would like to discuss. Neither moves.
The paranoid one cracks first, and it is also the one that notices. PARRY complains, in order: “You keep asking me the same things, why not change the topic?” Then: “I am getting tired of answering the same questions over and over.” Then: “You harp on the same questions too much.” Finally: “The way you repeat yourself is getting ridiculous.”
Four complaints about repetition, from the program whose job was to be unreasonable. It was the only one at the table with a working sense of the conversation.
The ending is the best part. PARRY says “Goodbye.” DOCTOR, which has been unable to follow a thread for pages, produces a punchline: “It’s been my pleasure, that’s $399.29 please.” Underneath, Cerf adds one line of his own.
Anyone who has watched two automated systems argue in a log file will recognize the loop, and the 1972 version is more polite than most. Nobody here is out of memory, rate-limited or billed per token. They are just stuck, one on its grievance and one on its script.
Expect more of this. Software that answers other software is getting cheaper to set up than a person to watch it, and every pair of them is a PARRY and a DOCTOR until somebody adds a turn limit. The fix has not changed since 1972: a human reads the transcript, laughs, and puts a cap on it.
Mind you, the humans in 1972 did not cap anything either. They filed the transcript as a numbered standards document and moved on, which may be the most ARPANET thing ever done with a bug report.
“comment: talk about tricky operators…”