in the beginning was floodsub

pubsub/gossipsub/README.md

@@ -2,12 +2,12 @@
 
 Revision:
 
-Authors: vyzo
+Author: vyzo
 
 This is the specification for an extensible baseline pubsub protocol,
 based on randomized topic meshes and gossip. It is a general purpose
 pubsub protocol with moderate amplification factors and good scaling
-properties.  The protocol is designed to be extensible by more
+properties. The protocol is designed to be extensible by more
 specialized routers, which may add protocol messages and gossip in
 order to provide behaviour optimized for specific application
 profiles.
@@ -17,9 +17,12 @@ profiles.
 - [Implementation status](#implementation-status)
 - [General purpose pubsub for libp2p](#general-purpose-pubsub-for-libp2p)
 - [In the beginning was floodsub](#in-the-beginning-was-floodsub)
-- [From floodsub to gossipsub](#from-floodsub-to-gossipsub)
-  * [Controlling the flood: meshsub](#controlling-the-flood-meshsub)
-  * [Gossip propagation: augmenting the mesh](#gossip-propagation-augmenting-the-mesh)
+  * [Ambient Peer Discovery](#ambient-peer-discovery)
+  * [Flood routing](#flood-routing)
+  * [Retrospective](#retrospective)
+- [Controlling the flood](#controlling-the-flood)
+  * [A random mesh algorithm](#a-random-mesh-algorithm)
+  * [Gossip propagation](#gossip-propagation)
 - [The gossipsub protocol](#the-gossipsub-protocol)
 - [Protobuf](#protobuf)
 
@@ -37,12 +40,69 @@ profiles.
 
 ## In the beginning was floodsub
 
+The initial pubsub experiment in libp2p was floodsub.
+It implements pubsub in the most basic manner, with two defining aspects:
+- ambient peer discovery.
+- most basic routing; flooding.
 
-## From floodsub to gossipsub
+### Ambient Peer Discovery
 
-### Controlling the flood: meshsub
+With ambient peer discovery, the function is pushed outside the scope
+of the protocol. Instead, it relies on ambient connection events to
+perform peer discovery via protocol identification. Whenever a new
+peer is connected, the protocol checks to see if the peer implements
+floodsub, and if so it sends a hello packet announcing the topics it
+is currently subscribing.
 
-### Gossip propagation: augmenting the mesh
+This allows the peer to maintain soft overlays for all topics of
+interest. The overlay is maintained by exchanging subscription
+control messages whenever there is a change in the topic list. The
+subscription messages are not propagated further, so each peer
+maintains a topic view of its direct peers only. Whenever a peer
+disconnects, it is removed from the overlay.
+
+Ambient peer discovery can be driven by arbitrary external means, which
+allows orthogonal development and no external dependencies for the protocol
+implementation.
+
+These are a couple of options we are exploring as canonical approaches
+for the discovery driver:
+- DHT lookups using provider records.
+- Rendezvous through known or dynamically discovered rendezvous points.
+
+### Flood routing
+
+With flooding, routing is almost trivial: for each incoming message,
+forward to all known peers in the topic. There is a bit of logic, as
+the router maintains a timed cache of known messages, so that seen
+messages are not further forwarded. It also never forwards a message
+back to the source or the peer that forward the message.
+
+### Retrospective
+
+Evaluating floodsub as a viable pubsub protocol reveals the following
+highly desirable properties:
+- it is straightforward to implement.
+- it minimizes latency; messages are delivered across minimum latency
+  paths, modulo overly connectivity.
+- it is highly robust; there is very little maintenance logic or state.
+
+The problem however is that messages don't just follow the minimum
+latency paths; they follow all edges, thus creating a flood. The
+outbound degree of the network is unbounded, restricted solely from
+connectivity. This creates a problem for individual densely connected
+nodes, as they may have large number of connected peers and cannot
+afford the bandwidth to forward all these pubsub messages.  Similary,
+the amplification factor is only bounded by the sum of degrees of all
+nodes in the overlay, which creates a scaling problem for densely
+connected overlays at large.
+
+
+## Controlling the flood
+
+### A random mesh algorithm
+
+### Gossip propagation
 
 
 ## The gossipsub protocol