Add sequence diagram and start description

Author: emhane

discv5/discv5-theory.md

@@ -525,6 +525,35 @@ advertisement, but samples the average number of results from TOPICQUERY instead
 average time to registration. The radius estimation value can be shared with the
 registration algorithm if the same topic is being registered and searched for.
 
+## Hole punch asymmetric NATs
+
+The protocol introduces the notification packet kind. There are 4 total message
+containers, these are abbreviated in the sequence diagram below as follows:
+- m - message packet
+- whoareyou - WHOEAREYOU packet
+- hm - handshake message packet
+- n - notification packet
+
+```mermaid
+    sequenceDiagram
+        Relay-->>Alice: m(NODES[Bob's ENR])
+        Alice->>Bob: m(nonce,FINDNODE)
+        Note left of Alice:Hole punched in Alice's NAT for Bob
+        Alice->>Relay: n(RELAYINIT[nonce])
+        Relay->>Bob:n(RELAYMSG[nonce])
+        Bob-->>Alice: whoareyou(nonce)
+        Note right of Bob: Hole punched in Bob's NAT for Alice
+```
+Bob is behind a NAT. Bob is in Relay's kbuckets, they have a session together.
+
+As part of a periodic recursive query to fill its kbuckets, Alice sends a FINDNODE
+request to Bob, who's ENR it received from Relay. By making an outgoing request to
+Bob, if Alice is behind a NAT, Alice's NAT adds the filtering rule
+`(Alice's-LAN-ip, Alice's-LAN-port, Bob's-WAN-ip, Bob's-WAN-port)`[^1] to it's UDP table.
+The request to Bob times out as Bob is behind a NAT.
+
+
+
 [EIP-778]: ../enr.md
 [identity scheme]: ../enr.md#record-structure
 [handshake message packet]: ./discv5-wire.md#handshake-message-packet-flag--2
@@ -535,3 +564,4 @@ registration algorithm if the same topic is being registered and searched for.
 [REGTOPIC]: ./discv5-wire.md#regtopic-request-0x07
 [REGCONFIRMATION]: ./discv5-wire.md#regconfirmation-response-0x09
 [TOPICQUERY]: ./discv5-wire.md#topicquery-request-0x0a
+[^1]: https://datatracker.ietf.org/doc/html/rfc4787