docs: add docs file for the peer exchange (#6665)

internal/p2p/pex/doc.go

@@ -0,0 +1,34 @@
+/*
+Package PEX (Peer exchange) handles all the logic necessary for nodes to share
+information about their peers to other nodes. Specifically, this is the exchange
+of addresses that a peer can use to discover more peers within the network.
+
+The PEX reactor is a continuous service which periodically requests addresses
+and serves addresses to other peers. There are two versions of this service
+aligning with the two p2p frameworks that Tendermint currently supports.
+
+V1 is coupled with the Switch (which handles peer connections and routing of
+messages) and, alongside exchanging peer information in the form of port/IP
+pairs, also has the responsibility of dialing peers and ensuring that a
+node has a sufficient amount of peers connected.
+
+V2 is embedded with the new p2p stack and uses the peer manager to advertise
+peers as well as add new peers to the peer store. The V2 reactor passes a
+different set of proto messages which include a list of
+[urls](https://golang.org/pkg/net/url/#URL).These can be used to save a set of
+endpoints that each peer uses. The V2 reactor has backwards compatibility with
+V1. It can also handle V1 messages.
+
+The V2 reactor is able to tweak the intensity of it's search by decreasing or
+increasing the interval between each request. It tracks connected peers via a
+linked list, sending a request to the node at the front of the list and adding
+it to the back of the list once a response is received. Using this method, a
+node is able to spread out the load of requesting peers across all the peers it
+is currently connected with.
+
+With each inbound set of addresses, the reactor monitors the amount of new
+addresses to already seen addresses and uses the information to dynamically
+build a picture of the size of the network in order to ascertain how often the
+node needs to search for new peers.
+*/
+package pex