feat: identify (#286)

Author: salmad3

content/concepts/introduction/protocols/identify.md

@@ -1,7 +1,40 @@
 ---
 title: "Identify"
-description: "A way for nodes in libp2p to query and inform other nodes about their metadata."
+description: "A way for nodes in libp2p to query and inform remote nodes about their metadata."
 weight: 23
 ---
 
-Coming soon!
+The libp2p Identify protocol allows nodes in the libp2p network to share
+and query information about each other. It is usually run right after a new libp2p
+connection has been established and, at certain times, during the lifetime of the
+connection.
+
+The information exchanged includes:
+
+- the protocol version: a network identifier, e.g. `/my-network/0.1.0`.
+- the user agent: a free-form string identifying the local node's implementation, usually
+  in the format "agent-name/version".
+- the public key: cryptographic identifier of the local node in the format of a peer ID.
+- the observed address: the remote node's public IP address as observed by the local node.
+- the listen addresses: the multiaddresses at which the local node can be reached.
+- the list of [protocols](/concepts/introduction/protocols/overview) that the
+  local node supports.
+
+There are two variations of the protocol: `identify` and `identify/push`.
+
+The Identify protocol, for historical reasons identified by the protocol ID `/ipfs/id/1.0.0`,
+is used to actively query a remote node for its metadata. The identify/push protocol, identified
+by the protocol ID `/ipfs/id/push/1.0.0`, allows a node to push Identify data to the remote node
+without querying. This is used whenever changes to the data sent in Identify happen,
+e.g., when the node starts listening on a new address and adds or removes support
+for a protocol.
+
+{{< alert icon="" context="note">}}
+It's worth noting that go-libp2p used to include a feature called 'Identify Delta'
+(<insert protocol ID>) for some time. This protocol was used to reduce the size of
+Identify Pushes, but proved to be less useful (and a lot more complex) than expected.
+It has since been deprecated, but many nodes on the IPFS network still support
+this protocol.
+{{< /alert >}}
+
+{{< alert icon="💡" context="note" text="See the Identify <a class=\"text-muted\" href=\"https://github.com/libp2p/specs/tree/master/identify\">technical specification</a> for more details." />}}