add autonatv2 docs

content/concepts/nat/autonatv2.md

@@ -0,0 +1,64 @@
+---
+title: "AutoNAT v2"
+description: "Detailed documentation for the AutoNAT v2 protocol in libp2p."
+weight: 201
+aliases:
+    - "/concepts/autonat"
+    - "/concepts/nat/autonat"
+---
+
+Overview
+Nodes in a peer-to-peer network cannot easily determine their reachability status. A node may be reachable on some addresses but not others due to NATs or firewalls. Determining which addresses are publicly reachable helps nodes:
+
+Avoid advertising unreachable addresses.
+
+Reduce unnecessary dial attempts from other peers.
+
+Proactively enable connectivity through relay servers if necessary.
+
+AutoNAT v2 addresses these challenges by providing per-address reachability checks.
+
+How AutoNAT v2 Works
+AutoNAT v2 lets nodes verify the reachability of individual addresses:
+
+The client node sends a prioritized list of addresses (DialRequest) along with a nonce.
+
+The server selects exactly one address from the list to dial back.
+
+If the dial-back address is different from the observed IP, the server initiates an Amplification Attack Prevention mechanism.
+
+The server opens a /libp2p/autonat/2/dial-back stream, sends the nonce, and awaits a response (DialBackResponse) from the client.
+
+The server sends a final DialResponse with the dial outcome.
+
+AutoNAT v2 helps clients build pipelines for testing addresses from various sources like identify, UPnP mappings, and circuit addresses.
+
+Visualization
+
+Client                                          Server
+|                                                 |
+| DialRequest (nonce, addr list)                  |
+|------------------------------------------------>|
+|                                                 | Chooses an address
+|                   Dial selected address         |
+|<----------------------------------------------- |
+| DialBack (nonce verification)                   |
+|------------------------------------------------>|
+| DialBackResponse (nonce confirmed)              |
+|<----------------------------------------------- |
+|                                                 | Final dial status
+| DialResponse (address index, dial outcome)      |
+|<------------------------------------------------|
+
+
+
+Differences from AutoNAT v1
+Address-specific reachability checks rather than overall node reachability.
+
+Nonce-based verification ensures the correct address was dialed.
+
+Amplification Attack Prevention allows dialing addresses different from the observed IP without risking amplification attacks.
+
+Amplification Attack Prevention
+When dialing an address different from the client's observed IP, the server requests a significant data transfer (30–100 KB) from the client before proceeding, making attacks impractical.
+