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Chaosnet is the name for both an internetworking protocol family, and an early LAN technology, both invented at the MIT AI Laboratory; the latter was the LAN on which the protocol first ran. (This article will use 'Chaosnet' for the latter, and 'Chaos' for the former, to reduce confusion.)

The LAN was a CSMA-CD system modeled on the Xerox PARC 3 megabit/second Experimental Ethernet, running over cable TV coaxial cable (using standard CATV connectors to connect the transceivers; not vampire taps, as on the Experimental Ethernet).

The protocol was later made to run over standard 10 megabit/second DIX Ethernet, which largely supplanted the Chaosnet hardware. (On Ethernet, the Address Resolution Protocol is required to provide mappings from 16-bit Chaos addresses to the 48-bit addresses used by Ethernet.)

The protocol provided a reliable byte stream service, but also had a datagram mode.


Chaosnet was developed at MIT around 1973-1975, for LISP machines (personal workstations developed to run LISP). It was a local network (about 1-2 km cable lengths), without central control (thus “chaos”). It was “cheap, efficient and fast”: circa 10 times the speed of the ARPANET, or 30000 characters per second (or “double this in some favorable cases”).

Chaosnet was initially called CAIOSnet.

Header format of Chaosnet hardware

On the wire, a Chaosnet packet has a 48-bit header imposed by the hardware, followed by a maximum of 4032 payload bits. The header has three 16-bit words, which are handled by the interface hardware. The first word is a CRC checksum. The next words are the source and destination addresses, on the particular subnet the packet is currently traveling on. The interface hardware typically presents the packet data in reverse, so the three words appear last as viewed by software.

Header format of Chaos on various hardware types

Chaos packets on Chaosnet appear immediately after the hardware header (above), with no intervening header. (Some schemes for carrying non-Chaos packets on Chaosnet - e.g. Internet Protocol packets - used non-zero values in the 'pad' low-order byte of the first word in the Chaos header, below.)

Chaos packet format on DIX Ethernet

Chaos traffic on an DIX Ethernet uses EtherType 0x0804.

Chaos packet format on synchronous serial links

Chaos traffic was eventually carried on synchronous serial lines; e.g. inside Symbolics' network. The format has not yet been documented.

Protocol header

On top of the hardware wire format, there is a software defined protocol. The packet data starts with eight 16-bit words, followed by the payload. The header appears first as viewed by software, but is actually transmitted last on the wire on a Chaosnet.

Offset Bits 15-12 Bits 11-8 Bits 7-4 Bits 3-0
0 Opcode 0
1 Forwarding Payload length
2 Source address
3 Source index
4 Destination address
5 Destination index
6 Packet number
7 Acknowledgement

The source and destination addresses are of course those of the original source and ultimate destination, which will, on intermediate links between the two, be unrelated to the source and destination addresses in the hardware LAN packet header on that link.

Packet opcodes

Code Meaning
1 RFC, request for connection
2 OPN, connection opened
3 CLS, connection closed
4 FWD, connection forwarded
5 ANS, answer
6 SNS, sense status
7 STS, status
10 RUT, routing information
11 LOS, lossage
12 LSN, listen for connection
13 MNT, maintenance
14 EOF, end of file
15 UNC, uncontrolled packet
16 BRD, broadcast packet
200 and up DAT, connection data


Historically, the protocol ran only directly on LANs. On hardware Chaosnet, no encapsulation was used. On Ethernet, protocol 0x0804 was allocated for carriage of Chaos protocol traffic.

In addition to the Ethernet implementations described above, several encapsulations have been developed. The standard one is using protocol 16 (decimal) on the Internet Protocol - this was implemented e.g in TOPS-20 and in Cisco routers [1].

Currently, Chaos is encapsulated also in:

  • Unix domain sockets (e.g. for the CADR emulator)
  • UDP (originally developed for KLH10)
  • TLS over TCP/IP (the preferred encapsulation across the Internet, see [2])

There is a bridge/router that understands all these, including Ethernet and IP, written by Bjorn Victor. It also has an API (a "Network Control Program") providing Chaos for modern computers.

Protocol implementations

This table lists both historical and current implementations, along with preservation status, whether code is publicly available, and whether it's currently possible to run it.

Implementation Preserved Public Running
LISP machines Yes Yes Yes
ITS Yes Yes Yes
TOPS-20 Yes Partially Work underway
VAX/VMS Yes Yes Work underway
BSD Unix Yes Yes Yes
MINITS Yes Yes Yes
PDP-11 Unix V7 No - -
Unix V8 Yes Yes No
MagicSix No - -
C Gateway - router only Yes Yes No
Chaosnet Bridge Yes Yes Yes
Oswalds's Python and Lisp implementations Yes Yes Yes?
Linux Yes Yes No?

Hardware, and simulations

See also

External links

  • AI memo 628 - Includes chapters on ITS, TOPS-20, Lisp Machine, and Unix implementations.
  • SYSDOC;CHAORD > - Initial design
  • MOON;AMBER > - Another Moon document, early version of AI Memo 628.
  • Chaosnet - Detailed descriptions of both the hardware system, and the protocol(s)
  • CHAOS; - hardware interface designs, etc.
    • PDP10 CHAOS - notes on the PDP-10 I/O bus interface
    • CHAOS FILE - Chaosnet FILE protocol
    • CHAOS;CHAOS PLANS - interesting details of the physical installation at MIT, plans for building ore boards, etc
  • Chaosnet wiki - central site for a global emulated Chaosnet, also with descriptions of packet formats and (application) protocol details.
  • chaos - Unix Eighth Edition implementation
  • chunix