doc: add page arch/net/tracker.md on P2P trackers

doc/src/SUMMARY.md

@@ -40,7 +40,8 @@
   - [Transactions](arch/tx_lifetime.md)
   - [Bridge](arch/bridge.md)
   - [Tooling](arch/tooling.md)
-  - [P2P Network](arch/p2p-network.md)
+  - [P2P Network](arch/net/p2p-network.md)
+    - [Tracker](arch/net/tracker.md)
   - [Services](arch/services.md)
   - [Smart Contracts](arch/sc/sc.md)
     - [Transaction lifetime](arch/sc/tx-lifetime.md)

doc/src/arch/net/tracker.md

@@ -0,0 +1,54 @@
+# Tracker
+
+Trackers are run by the community. They allow nodes to dynamically discover
+swarms by querying the tracker for info.
+
+Imagine you are in a channel on the chat. The chat may group media for that day
+in a single swarm which is accessed by nodes in that channel. To access the
+media, nodes will query the tracker, and then be able to spawn a unique `P2P`
+instance.
+
+Resource -> tracker -> P2P instance
+
+## Creating a Network
+
+Networks are spawned on the tracker in a lazy way. That is quering the tracker
+will automatically spawn a network if not already available.
+
+## Dropping Networks
+
+Networks are auto-pruned when they become stale. Assume 1 week without updates
+although this can be configured.
+
+## Query Tracker
+
+We have an identifier string, like `/darkirc/dev/1134/media`. Here `1134` refers
+to the ID of the event graph instance.
+
+Before creating the `P2P` instance, we must get the `Settings` struct.
+We then query the tracker like this:
+
+1. Send the tracker our external IP, and a port we listen on (optional - this is
+   for inbound nodes only) together with the identifier.
+2. If the tracker has no existing info for that identifier, then it will create
+   a new table.
+3. Add our node info to the table.
+4. The tracker returns a list of nodes back to us.
+
+This impl replaces the PeerDiscovery mechanism inside OutboundSession.
+
+```
+trackers = [
+    # list of trackers to query for resources ...
+]
+```
+
+## Role of Seeds
+
+Seeds may continued to be used in place of trackers such as the main DarkIRC
+event graph or darkfid where the networks are statically instanced.
+
+For dynamic instance (swarming), we may eventually replace trackers with a swarm
+network. Seed nodes will then be used to discover the swarm network.
+Trackers allow p2p net discovery to be dynamic.
+