OSCAR protocol

~ the last words ~

I used this for six years and never once typed its name, which is true of everyone who used it. You installed ICQ in 1998 because everyone you wanted to talk to already had a UIN, and the thing actually carrying the messages was a proprietary AOL protocol called OSCAR. It stands for Open System for Communication in Realtime, a name that reads like it was assembled backwards out of the acronym, and it was never open in any sense that mattered. AOL published nothing about it for eleven years.

Underneath, it was a tidy enough design. Every packet went inside a FLAP container, a five-byte header starting with the hex byte 2A, carrying a channel number for what kind of traffic it was, a sequence counter, and a length. Channel 1 was login, channel 2 was the real message layer, channel 4 was the disconnect. Integers were big-endian and unsigned throughout. Reverse engineers mapped most of it years before AOL admitted any of it existed, which is how Linux users got a working AIM client in the first place.

OSCAR started as AIM's plumbing in 1997, then AOL bought Mirabilis the following year and moved ICQ onto it as well, so one undocumented AOL protocol ended up carrying both of the instant messengers that counted. Its reach got stranger from there. In 2002 AOL licensed it to Apple for iChat, which meant Mac users were talking to ICQ accounts over AOL's wire, and .Mac subscribers could sign into the AIM network with their .Mac addresses. Google Talk inside Gmail could log into AIM too. For most of the 2000s a surprising amount of the world's chat traffic was moving over a specification that officially did not exist.

~ the record ~

~ the client wars ~

The defining fact about OSCAR is how hard AOL fought to keep other people off it. When Microsoft wired MSN Messenger into the AIM network, AOL changed the protocol until it stopped working. When Cerulean Studios shipped Trillian, which let you keep AIM and ICQ and MSN in one window, AOL blocked it repeatedly through 2002 and the trade press covered each round like a boxing match. The stated reason was always security. The actual stake was the buddy list, because whoever owned the buddy list owned the network, and a multi-protocol client dissolved exactly that.

The thaw came late and came in pieces. AOL put out an AIM SDK and a developer site on 6 March 2006, which let people build plugins and custom clients without telling them how the protocol worked. Two years later, on 5 March 2008, it finally published portions of the actual OSCAR documentation. By then Pidgin and Miranda and QIP had been speaking it fluently for most of a decade, from notes written by people who had read the packets instead of the manual.

~ the day it stopped ~

AIM was announced dead on 6 October 2017 and switched off on 15 December. A former AOL employee put the remaining user base in the single-digit millions and said the cost of keeping the OSCAR servers running had stopped making sense, which is a quiet way of describing twenty years of infrastructure being decommissioned because the bill arrived.

The protocol was not finished, though, because Mail.ru had owned ICQ since 2010 and was still answering OSCAR connections in Moscow. In December 2018 old ICQ users got a notice in the client: from 28 December, old versions of ICQ and unofficial clients would stop being supported, and you had to move to the current one. A Habr post on 21 December reproduced the message verbatim under the headline "ICQ is dead. Long live ICQ?", written by somebody with a five-digit UIN. Mail.ru had replaced OSCAR with a protocol of its own called WIM, which shared almost nothing with it. That was the end of the line: the last network speaking AOL's protocol was Russian, and it hung up at the end of December 2018.

~ who still speaks it ~

People rebuilt the servers. There is now more than one open-source OSCAR implementation, written mostly in Go, that will accept a connection from Windows AIM 1.x through 7.x, from ICQ 2000b, from Miranda and Pidgin and TiK. You run one yourself and point a twenty-five-year-old binary at it, and the little door-slam noise works again. The protocol is in better shape than it has been since 2008, and there is nobody on the other end of it.

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