PyDECnet is an implementation of the Digital Network Architecture
(commonly called "DECnet") implemented as a pure Python module.  It
requires Python 3.3 or later.  Python 3.7 or later is recommended;
note that the Python team no longer supports versions older than 3.5,
and with Python 3.6 certain features of PyDECnet are not available.

Standards compliance

PyDECnet implements the DECnet Phase IV architecture, from the
specifications published by Digital. 

PyDECnet was implemented from the published specifications, as a pure
Python module.  It implements DECnet routing, both level 1 and level 2
(area) routing.  It also implements other aspects of DNA, specifically
the MOP (Maintenance Operations) protocols.  It will interoperate with
Phase IV as well as Phase III nodes, as required by the
specifications.

PyDECnet also interoperates with Phase II nodes.  While this is not
found in the Phase IV specifications, it is described in the Phase III
specifications, and making it work was a straightforward extension.
Phase II support includes the ability to provide "intercept" service.
With this, a Phase II node that requests intercept (such as TOPS-20
V4.1) will be able to reach any node, not just adjacent nodes.

In addition, PyDECnet can be configured to operate as a Phase III or a
Phase II node, in the sense that it runs the routing and NSP protocols
according to those versions.  This is primarily a test tool.

Supported datalinks

Supported datalinks include Ethernet, DDCMP over TCP, UDP,
asynchronous serial connections, or synchronous serial connections
with the use of the USB connected DDCMP framer board, "Multinet"
encapsulation of point to point datalink messages over UDP or TCP, and
GRE encapsulation of Ethernet packets.

Note that Multinet encapsulation over UDP is not recommended because
it violates the DNA specification badly enough that it is extremely
unreliable except in the most ideal circumstances.  All the other data
link types, including Multinet over TCP, are fine.

Major components

PyDECnet contains the following major components:

- DNA layers:
  - Datalink
  - MOP
  - Routing
  - NSP
  - Session Control
  - Network Management   ("show" and "loop" support only)
  - Application layer servers ("objects" in DECnet terminology)
    - File transfer listener
    - NML
    - EVL
    - Mirror
    - Passthrough ("Poor Man's Routing)
- API for MOP and Session Control, using JSON over a Unix domain
  socket).  Two types of "connectors" to wrap this API with a simple
  programming interface are also supplied.
  
- HTTP(S) based monitoring interface
- Applications:
  - File transfer (NFT)
  - Network management (NCP)
  - Sample MOP remote console client
  - Several other tools intended more as examples

The API allows external processes to access protocol services in
PyDECnet, via JSON carried over Unix domain sockets.

Application features

PyDECnet includes several standard DECnet application layer
components, currently just those implementing network management
services.  These are all implemented as Python modules that run as
part of the PyDECnet process (rather than as external application
processes).  The following are currently available:

Mirror object (MIRROR, object 25, "node loopback")
Event listener object (EVL, object 26)
Network management listener (NML, object 19)
Poor man's routing (PSTHRU, object 123)
File access listener (FAL, object 17)
Topology server (TOPOL, used by TOPS-20 V4.1)

Multiple node support

In typical use, a PyDECnet invocation starts a single DNA node.  For
testing, multiple systems (bridges or nodes) may be started by a
single invocation.  The main advantage of doing so is that there is a
single logging stream, so the entire sequence of events in the test
configuration will show up, in correct order, in that log.  If this is
done, the home page in the monitoring HTTP service will list the
systems, and the pages for each system will have a URL argument of the
form "?system=<name>" appended, for example "/routing?system=NI1D".

Documentation

The following documents are currently provided.  Note that
documentation is very much a work in progress.  Comments would be
appreciated. 

1. overview.txt - this document
2. install.txt - installing PyDECnet
3. run.txt - how to configure, run, and monitor PyDECnet
4. config.txt - detailed description of the configuration files
5. api.txt - programmer's documentation of the JSON API
6. internals.txt - overview of the design of PyDECnet