First of all, some background on how it works. There are three parts: the node database, the network scanner, and the map display maker.
The node database is read from the HECnet node name database at MIM. The code extracts name, node address, owner's name, name of the node's location, and location latitude/longitude. This information is retrieved daily. If your node location information is missing or incorrect, please contact Johnny Billquist, the keeper of the HECnet node database.
For some nodes, the node database does not give a location. If so, the map data table shows the location as blank, and the map display shows the node in the South Atlantic Ocean, at Inaccessible Island (in honor of Daniel Suarez's novel "Influx").
The network scanner looks at every node mentioned in the node database plus every node it can see as reachable. Roughly speaking, it makes a NICE connection (remote network management protocol) to each node, then asks it three questions: "show exec characteristics ", "show active circuits" and "show active nodes". The first questions answers "what type of node is this" and also gets the node name if we do not already know it. The other two give the adjacent nodes, i.e., the connections from this node to its neighbors. Often but not always, that information includes the node type of the adjacent nodes.
This process continues iteratively until information for all visible nodes has been refreshed.
There are some optimizations and tricks. If a node is reported as reachable in the "show active nodes" reply (whether as a neighbor or farther away) it's recorded as reachable, even if the network scanner can't talk to it. This means that Phase II nodes, Phase III nodes in another area, or Cisco routers will all be shown as reachable so long as there is a node somewhere that answers NICE requests and shows them as reachable.
Also, for optimization, if a node reports a neighbor's type as Phase II or Phase III (in another area) or endnode, the network scanner doesn't try to connect to it because it already has all the information needed. This speeds up the scan. It also avoids a bug in DECnet/E where "show active nodes" on an endnode leaves the request hanging after the reply data is sent (the "no more data" message is missing).
At the completion of the scan, the up/down information is updated. Currently each node, and each adjacency entry for that node, has three timestamps: last up, last down, and last seen. During the scan, when a node is seen as visible, or when an adjacent node is reported, that entry is marked with the current time as "last seen". When the scan is finished, the entire database is examined. Any item not marked with the current timestamp in "last seen" is currently down. If it was previously up (its "last up" time is greater than its "last down") then "last down" is set to the current time. Similarly, any item previously marked "down" ("last down" greater than "last up") has its "last up" set to the current time.
The map display takes as input the node and adjacent-node information collected by the scanner and summarizes it in two steps. First, all the nodes are grouped by latitude/longitude. Each distinct spot on the map is given a marker associated with all the nodes at that location. Nodes without location information are assigned the placeholder coordinates 37° 18′ S, 12° 41′ W (the location of Inaccessible Island).
Next, all the circuits (adjacent node) information is examined, grouped by lat/long of the endpoints. An arc on the map corresponds to all the connections between node pairs at the location markers connected by that arc.
The mapper is an "event monitor" and will react to node or circuit change events send to the DECnet node of which it is part by performing a partial scan of the network. For a node reachable event, it will scan that node; for a circuit change, it will scan the nodes at the endpoints of that circuit. For a node unreachable event, the node is simply marked as unreachable (no scan is done for this case).
Each item (node or circuit) has a "health" which is similar to its up/down status. In the table below, the type color and style for the different "Value" entries are how that health value is shown in the map data table.
| Value | Description |
|---|---|
| Up | Node or circuit is currently visible to the scanner as operational |
| Down | Node or circuit is not currently visible to the scanner, or it is seen in the scan but the reported status is "not operational" |
| Faded | Node or circuit has been "down" for more than two weeks, or node has never been seen as "up". |
A possible future enhancement would add a "warning" status, which would be something like "it is up now but it has been bouncing up/down a lot" but that isn't currently in the code.
Location markers and connection arcs both summarize multiple nodes and circuits. The color used for these comes from the health of the components that go into them.
| Color | Description |
|---|---|
| All nodes at this location, or all circuits on this path, are "up" |
| The majority, but not all, nodes at this location, or circuits on this path, are "up" | | At least half of the nodes at this location, or circuits on this path, are "down" |
| All nodes at this location, or circuits on this path, are "faded". But if a path is "faded" for more than 180 days, it is not plotted on the map at all. |
All location markers have a popup (window that appears when you click the marker) which lists the nodes at that location, grouped by heath status (up in black, down in red, faded in gray italics). Exception: the default marker at Inaccessible Island omits gray nodes because there are too many. A location marker that isn't gray also has a tool tip (appears when you hover over the marker) which lists the up and down nodes only, not the faded ones.
Arcs also have popups, which list the individual circuits for this path by their end point node names/numbers.
The map data table shows the underlying data collected by the mapper, in tabular form sorted by node number. For each node, it shows the address and name, node typoe, location if shown in the database, and time when the node status was last changed to down, and was last changed to up, if applicable. The status is indicated by the color and style of the node address/name text: black text is "up", red is "down", and gray italic is "faded".
If the mapper obtained circuit information from a given node, that is shown as a nested table below the line for the node information. For each circuit, it shows the circuit name, adjacent node address and name, adjacent node location if known, and last down and last up timestamps. As before, the name color and style indicates the status of the circuit.
The zoom in/out is at the top left. Just below it is the "help" button which shows map documentation. The button at the top right is the layers tool, which lets you choose from a number of map sources, and also allows you to turn on/off the location and/or path display separately for the backbone (area routers and connections between area routers) and level 1.
There is also a tabular display, sorted by node number, accessible via a link on the map display. It shows more detail than can comfortably fit on the map itself.
The map images come from various map sources, such as OpenStreetMap. They all use Mercator projection, and there isn't a convenient way to change. The map machinery has some limitations (or bugs). For example, if you zoom way out you'll see the continents more than once, which is proper, but the markers only appear on one copy.