Network transports and protocol architecture on CADR and Genera

Packet and service models

The System 46 NCP provides two distinct communication contracts.

• A connection is a reliable, ordered, full-duplex packet conversation. The NCP maintains read, receive, and send queues, connection indices and uniquizers, allocation windows, retransmission state, and probes. The manual's reliability guarantee belongs to an established connection: packets are not knowingly delivered garbled, duplicated, lost, or out of order unless the connection itself breaks.
• A simple transaction sends an RFC and expects an ANS without constructing the full stream state. It is intentionally cheaper and weaker. An application can retransmit a request, so the peer can see duplicates, and receipt of an answer does not retroactively make every request exactly-once.

The pinned opcode set is RFC, OPN, CLS, FWD, ANS, SNS, STS, RUT, LOS, LSN, MNT, EOF, UNC, and the data-opcode range beginning at octal 200. This System 46 file has no BRD broadcast opcode. A packet is at most 252 16-bit words including its eight-word header, leaving 244 words, or 488 bytes, for data. The normal connection receive window is octal 15, which is 13 packets; the declared maximum is octal 200, or 128 packets.

The receiver moves packets from wired hardware buffers into NCP queues. A background process sends probes, retransmits unacknowledged traffic, updates allocations, and ages learned routes. This is why “disable” and “reset” are not synonyms: disabling stops network activity, while resetting closes connections, clears software state, resets hardware state, and leaves the network disabled.

Addresses and routes

A Chaos address is a 16-bit value conventionally written in octal. The high byte identifies a subnet and the low byte a host; the host byte must not be zero for an ordinary host. Contact names such as FILE, STATUS, or SUPDUP name services on that address rather than replacing it.

The System 46 routing table has 32 subnet positions. The inspected initial table contains a costly default route through octal 440 and a special entry through octal 426 for subnet 4. Incoming RUT packets can replace an entry when their cost is no worse than the recorded one, and the background task ages dynamic entries. Those literals describe the historical source's site assumptions; they are not configuration advice for this emulator or a claim about every CADR site.

The implementation assumes one active local Chaos interface. It can route toward other subnets, but it does not yet present Genera's graph of independently typed interfaces and media.

A complete filename/content search of this pinned public snapshot found no standalone Ethernet controller or address-resolution implementation comparable to System 303's simple-ether.lisp and addr-res.lisp. Documentary occurrences of “Ethernet” elsewhere in the tree do not establish a loaded System 46 link driver.

Direct System 46 operator interface

System 46 does not define the later SHOW-ROUTING-TABLE or SHOW-ROUTING-PATH utilities. The complete System 46 Peek mode and menu behavior is documented separately in Peek on the MIT Lisp Machine.

Chaos extensions

System 303 preserves the connection and simple-transaction split, packet size, and ordinary 13-packet window, but its NCP adds the BRD opcode and broadcast APIs. Its routing structures have 256 subnet entries plus parallel cost and route- type information. A local route can be typed as a direct CADR Chaos path or, on a Lambda-family build, as Ethernet.

The NCP source explicitly says that it does not itself contain the Ethernet-II interface. That interface is the separate simple-ether.lisp module. Keeping the modules distinct matters: Chaos remains the network protocol, while Ethernet is one way of carrying its packets.

The 3Com driver

simple-ether.lisp directly implements a Lambda Multibus 3Com controller path. The Chaos NCP selects between that function and a named CADR Unibus Ethernet function, but a definition of the CADR function was not found in the checked System 303 tree. The branch therefore proves a working source path for Lambda and an architectural hook for CADR, not a complete maintained CADR Ethernet implementation. It also does not prove that the preserved CADR System 303 load band loaded the separate ETHERNET system.

The implemented path uses six-byte hardware addresses and 2048-byte controller buffers. The registered Ethernet type values are #x0408 for Chaos packets and #x0608 for the maintained address-resolution packets. The byte order appears reversed relative to the familiar printed EtherType values because the controller and Lisp code exchange low bytes first; the source comments warn about the same diagram-versus-memory-order issue.

The driver meters successful receive and transmit activity as well as collisions, jams, frame-check errors, and unknown packet types. It also retains recent headers for diagnosis. These are controller-level facts; they do not imply Internet IP is present in the System 303 tree inspected for this article.

The maintained Chaos/Ethernet resolver

addr-res.lisp maps a two-byte Chaos protocol address to a six-byte Ethernet address. A cache entry has a Chaos address, Ethernet address, and an age field. Unknown destinations cause a broadcast request and return no hardware address until a reply is learned. Requests directed at a local Chaos address are answered, and addressed request/reply traffic can update the cache.

The source initializes or refreshes age to 1000, but the inspected file contains no routine that decrements or expires it, and the lookup path does not consult it. The field is therefore not evidence of a functioning timeout policy at this revision. The file also contains hard-coded restoration/site exceptions: most subnet-6 lookups are suppressed except for CADR2, and one duplicate-address case is treated specially. They are implementation evidence, not general ARP rules.

Maintained route and Ethernet operator surface

No install, remove, reset, transmit, route query, or external Hostat operation was performed for this study. The maintained Peek adds C Chaosnet and H Hostat modes. Its Chaos display includes connections, packets, queues, meters, and routes; its mouse menus can reset or close live connections and initiate other network applications. Those visible controls and their safety classification are fully inventoried in Peek on the MIT Lisp Machine.

This table is complete at the lifecycle, Hostat, route-query/print, and explicit 3Com operator-tool grain: the read-only controller printers/meters plus the named install/remove controls. It excludes internal register I/O, buffer-arm, receiver, and transmitter helpers. The same sources also contain raw packet-object printers and invasive developer tests. For example, CHAOS:DUMP-GUTS combines an expanded Peek-style dump with recent headers; CHAOS:PRINT-BAD-PKTS and CHAOS:PRINT-RECENT-HEADERS inspect retained failures/history; CHAOS:CD-SEND, CHAOS:CD-RECEIVE, and CHAOS:SETUP disable, construct, receive, or transmit test traffic. These are implementation-debugger entry points, not omitted Command Processor commands, and the mutating/transmitting forms were not exercised.

Objects and path construction

Genera does not require an application to name a wire protocol directly. The core model separates the following objects:

The selector begins with the requested host service and locally installed protocol definitions. It recursively composes steps through networks, gateways, services, media, and local paths, rejects cycles, and sorts the resulting candidates by the product of protocol and medium desirability. Unavailable hosts and missing routes are demerited. If an attempt fails, another path can be tried.

Two implementation details are easy to miss in the conceptual manual. Equal-best paths are selected randomly rather than always taking the first declaration, and small desirability adjustments favor local-site and namespace relationships. Those policies make selection adaptive but mean that declaration order alone is not a complete prediction of the chosen path.

The three generic communication contracts

The public Networks manual says TCP can support a marked stream. The direct TCP medium declaration itself supplies a byte stream; the generic token-stream layer wraps that stream to provide marks. These descriptions are compatible only when the composition layer is kept visible.

Lifecycle semantics

Generic network lifecycle operations are deliberately different:

• Enable makes a network available and starts its implementation activity.
• Disable makes it unavailable without promising to destroy every extant stream immediately; implementation-specific activity stops or quiesces.
• Reset is recovery, not a harmless refresh. It can break connections, clear routes and caches, reset interfaces, and rebuild state.

Specific networks refine this contract. Genera Chaos enablement rebuilds directly known routes from interfaces and starts its background process. Chaos disablement stops that process. Chaos reset closes connections and streams, frees queued and pending packets, clears histories and demerits, and then reconstructs state. The source's CHAOS:ASSURE-ENABLED differs importantly from its System 46 namesake: it waits in process state Await Chaosnet enabled; it does not turn Chaosnet on.

System boundary and addresses

The IP-TCP system loads Internet globals and packet processing, routing, ICMP, UDP, EGP, TCP, service paths, debugging, and applications. Internet addresses use four-byte dotted decimal. IP-TCP is a separately declared package, so its presence in the purchased media does not prove it was loaded or configured on every Genera machine.

An Internet interface records its local address, subnet and mask, gateways, MTU, and whether its configuration is dynamic. Route objects retain source, interface, gateway, packet-size constraints, and meters. Initialization constructs loopback, broadcast, directly connected subnet, and namespace-derived gateway routes.

Packet processing and routing

The inspected implementation covers IPv4 header validation and checksum, output fragmentation, input reassembly, ICMP, UDP, EGP, and TCP. Default IP maximum packet size is 576 bytes and default maximum reassembly is 10,240 bytes. Those defaults are IP-layer limits; a particular Ethernet interface can support a larger frame.

A background gateway process pings gateways, marks them alive or dead, and recomputes routes. ICMP redirects can update routing. Candidate gateways are ranked by desirability and then cost. The generic access-path layer can choose TCP carried directly over Internet or a path bridged through Chaos, and the source exposes a preference variable for bridged-IP-versus-Chaos choices. This is path composition, not a claim that both paths exist in the unconfigured museum world.

TCP stream interface

The TCP medium directly supplies :BYTE-STREAM; the generic token wrapper supplies marks when a caller requests :BYTE-STREAM-WITH-MARK. Streams can be binary or character-oriented and can apply native, ASCII, or Unix translation.

Connection timeout handling offers restarts for a longer timeout or indefinite waiting. Listening can either wait for a SYN and accept it or expose an earlier state according to the two controls above. These are implementation APIs, not additional Command Processor commands.

UDP and low-level IP diagnostics

UDP supplies the generic :DATAGRAM medium, with port allocation, checksums, raw connection objects, and retry behavior at the generic transport-agent layer. The following directly callable diagnostics are the bounded user-facing surface found in the inspected IP sources:

The IP/TCP package manual warns that a first echo attempt can return no result while ARP resolution is still occurring. No echo, UDP packet, TCP connection, listener, or debug mode was started for this article. FTP, TFTP, terminal login, mail, domain, Talk, and other end-user clients and servers are application/service layers; see SUPDUP, Telnet, and the Genera Terminal program and the dedicated network-services dossier rather than treating their presence as proof of a reachable IP stack.

Configuration is not the same surface

Addresses, hosts, sites, gateways, and service declarations are normally stored in the namespace and edited through namespace administration. Historical physical Symbolics machines could also set a boot-time Chaos address in the FEP. Open Genera uses its VLM/interface and namespace context instead; the manual's historical FEP instruction must not be copied blindly into the VLM harness. See Genera Namespace administration and editor for the complete configuration object and command inventory.

Public System 46 source

All paths are under Git commit 8e978d7d1704096a63edd4386a3b8326a2e584af.

Maintained LM-3 System 303 source

All paths are under Fossil check-in 4df393c68d7f083ce42d5c377039d26043cc18a9031ace28258dc97f4137eb91.

Licensed Genera source and public manuals

These checksums identify locally inspected licensed files without publishing their contents. The table selects the defining files for each claim rather than listing every application that can use the network substrate.