add autonat v2 spec (#538)

autonat/README.md

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+# NAT Discovery  <!-- omit in toc -->
+> How we detect if we're behind a NAT.
+
+
+Specifications:
+- [autonat v1](autonat-v1.md)
+- [autonat v2](autonat-v2.md)

autonat/autonat-v1.md

@@ -1,6 +1,3 @@
-# NAT Discovery  <!-- omit in toc -->
-> How we detect if we're behind a NAT.
-
 | Lifecycle Stage | Maturity       | Status | Latest Revision |
 |-----------------|----------------|--------|-----------------|
 | 3A              | Recommendation | Active | r1, 2023-02-16  |

autonat/autonat-v2-amplification-attack-prevention.plantuml

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+@startuml
+participant Cli
+participant Srv
+
+skinparam sequenceMessageAlign center
+skinparam defaultFontName monospaced
+
+
+== Amplification Attack Prevention ==
+
+Cli -> Srv: [conn1: stream: dial] DialRequest:{nonce: 0xabcd, addrs: (addr1, addr2, addr3)}
+Srv -> Cli: [conn1: stream: dial] DialDataRequest:{addrIdx: 1, numBytes: 120k}
+Cli -> Srv: [conn1: stream: dial] DialDataResponse:{data: 4k bytes},DialDataResponse:{data: 4k bytes},...
+Srv -> Cli: [conn2: stream: dial-back]addr2 DialBack:{nonce: 0xabcd}
+Srv -> Cli: [conn1: stream: dial] DialResponse:{status: OK, addrIdx: 1, dialStatus: DIAL_STATUS_OK} 
+@enduml

autonat/autonat-v2.md

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+# AutonatV2: spec
+
+
+| Lifecycle Stage | Maturity                 | Status | Latest Revision |
+|-----------------|--------------------------|--------|-----------------|
+| 1A              | Working Draft            | Active | r2, 2023-04-15  |
+
+Authors: [@sukunrt]
+
+Interest Group: [@marten-seemann], [@marcopolo], [@mxinden]
+
+[@sukunrt]: https://github.com/sukunrt
+[@marten-seemann]: https://github.com/marten-seemann
+[@mxinden]: https://github.com/mxinden
+[@marcopolo]: https://github.com/marcopolo
+
+
+## Overview
+
+A priori, a node cannot know if it is behind a NAT / firewall or if it is
+publicly reachable. Moreover, the node may be publicly reachable on some of its
+addresses and not on others. Knowing reachability for its addresses is essential
+for the node to be well-behaved in the network: A node doesn't need to advertise
+its unreachable addresses to the rest of the network, preventing superfluous
+dials from other peers. Furthermore, in case it has no publicly reachable
+addresses, it might actively seek to improve its connectivity by finding a relay
+server, which would allow other peers to establish a relayed connection. 
+
+In `autonat v2` client sends a request with a priority ordered list of addresses
+and a nonce. On receiving this request the server dials the first address in the
+list that it is capable of dialing and provides the nonce. Upon completion of
+the dial, the server responds to the client with the response containing the
+dial outcome.
+
+As the server dials _exactly_ one address from the list, `autonat v2` allows
+nodes to determine reachability for individual addresses. Using `autonat v2`
+nodes can build an address pipeline where they can test individual addresses
+discovered by different sources like identify, upnp mappings, circuit addresses
+etc for reachability. Having a priority ordered list of addresses provides the
+ability to verify low priority addresses. Implementations can generate low
+priority address guesses and add them to requests for high priority addresses as
+a nice to have. This is especially helpful when introducing a new transport.
+Initially, such a transport will not be widely supported in the network.
+Requests for verifying such addresses can be reused to get information about
+other addresses
+
+The client can verify the server did successfully dial an address of the same
+transport as it reported in the response by checking the local address of the
+connection on which the nonce was received on.  
+
+Compared to `autonat v1` there are three major differences
+1. `autonat v1` allowed testing reachability for the node. `autonat v2` allows
+testing reachability for an individual address.
+2. `autonat v2` provides a mechanism for nodes to verify whether the peer
+actually successfully dialled an address.
+3. `autonat v2` provides a mechanism for nodes to dial an IP address different
+from the requesting node's observed IP address without risking amplification
+attacks. `autonat v1` disallowed such dials to prevent amplification attacks.
+
+
+## AutoNAT V2 Protocol
+
+![Autonat V2 Interaction](autonat-v2.svg)
+
+A client node wishing to determine reachability of its addresses sends a
+`DialRequest` message to a server on a stream with protocol ID
+`/libp2p/autonat/2/dial-request`. Each `DialRequest` is sent on a new stream.
+
+This `DialRequest` message has a list of addresses and a fixed64 `nonce`. The
+list is ordered in descending order of priority for verification. AutoNAT V2 is
+primarily for testing reachability on Public Internet. Client SHOULD NOT send any
+private address as defined in [RFC
+1918](https://datatracker.ietf.org/doc/html/rfc1918#section-3) in the list. The Server SHOULD NOT dial any private address.
+
+Upon receiving this request, the server selects an address from the list to
+dial. The server SHOULD use the first address it is willing to dial. The server
+MUST NOT dial any address other than this one. If this selected address has an
+IP address different from the requesting node's observed IP address, server
+initiates the Amplification attack prevention mechanism (see [Amplification
+Attack Prevention](#amplification-attack-prevention) ). On completion, the
+server proceeds to the next step. If the selected address has the same IP
+address as the client's observed IP address, server proceeds to the next step
+skipping Amplification Attack Prevention steps.
+
+The server dials the selected address, opens a stream with Protocol ID
+`/libp2p/autonat/2/dial-back` and sends a `DialBack` message with the nonce
+received in the request. The client on receiving this message replies with
+a `DialBackResponse` message with the status set to `OK`. The client MUST 
+close this stream after sending the response. The dial back  response provides
+the server assurance that the message was delivered so that it can close the 
+connection.
+
+Upon completion of the dial back, the server sends a `DialResponse` message to
+the client node on the `/libp2p/autonat/2/dial-request` stream. The response
+contains `addrIdx`, the index of the address the server selected to dial and
+`DialStatus`, a dial status indicating the outcome of the dial back. The
+`DialStatus` for an address is set according to [Requirements for
+DialStatus](#requirements-for-dialstatus). The response also contains an
+appropriate `ResponseStatus` set according to [Requirements For
+ResponseStatus](#requirements-for-responsestatus).
+
+The client MUST check that the nonce received in the `DialBack` is the same as
+the nonce it sent in the `DialRequest`. If the nonce is different, it MUST
+discard this response.
+
+The server MUST close the stream after sending the response. The client MUST
+close the stream after receiving the response.
+
+
+### Requirements for DialStatus
+
+On receiving a `DialRequest`, the server first selects an address that it will
+dial. 
+
+If server chooses to not dial any of the requested addresses, `ResponseStatus`
+is set to `E_DIAL_REFUSED`. The fields `addrIdx` and `DialStatus` are
+meaningless in this case. See [Requirements For
+ResponseStatus](#requirements-for-responsestatus).
+
+If the server selects an address for dialing, `addrIdx` is set to the
+index(zero-based) of the address on the list and the `DialStatus` is set
+according to the following consideration:
+
+If the server was unable to connect to the client on the selected address,
+`DialStatus` is set to `E_DIAL_ERROR`, indicating the selected address is not
+publicly reachable. 
+
+If the server was able to connect to the client on the selected address, but an
+error occured while sending an nonce on the `/libp2p/autonat/2/dial-back`
+stream, `DialStatus` is set to `E_DIAL_BACK_ERROR`. This might happen in case of
+resource limited situations on client or server, or when either the client or
+the server is misconfigured. 
+
+If the server was able to connect to the client and successfully send a nonce on
+the `/libp2p/autonat/2/dial-back` stream, `DialStatus` is set to `OK`. 
+
+### Requirements for ResponseStatus
+
+The `ResponseStatus` sent by the server in the `DialResponse` message MUST be
+set according to the following requirements
+
+`E_REQUEST_REJECTED`: The server didn't serve the request because of rate
+limiting, resource limit reached or blacklisting.
+
+`E_DIAL_REFUSED`: The server didn't dial back any address because it was
+incapable of dialing or unwilling to dial any of the requested addresses.
+
+`E_INTERNAL_ERROR`: Error not classified within the above error codes occured on
+server preventing it from completing the request.
+
+`OK`: The server completed the request successfully. A request is considered
+a success when the server selects an address to dial and dials it, successfully or unsuccessfully.
+
+Implementations MUST discard responses with status codes they do not understand. 
+
+### Amplification Attack Prevention
+
+![Interaction](autonat-v2-amplification-attack-prevention.svg)
+
+When a client asks a server to dial an address that is not the client's observed
+IP address, the server asks the client to send some non trivial amount of bytes
+as a cost to dial a different IP address. To make amplification attacks
+unattractive, servers SHOULD ask for 30k to 100k bytes. Since most handshakes
+cost less than 10k bytes in bandwidth, 30kB is sufficient to make attacks
+unattractive.
+
+On receiving a `DialRequest`, the server selects the first address it is capable
+of dialing. If this selected address has a IP different from the client's
+observed IP, the server sends a `DialDataRequest` message with the selected
+address's index(zero-based) and `numBytes` set to a sufficiently large value on
+the `/libp2p/autonat/2/dial-request` stream
+
+Upon receiving a `DialDataRequest` message, the client decides whether to accept
+or reject the cost of dial. If the client rejects the cost, the client resets
+the stream and the `DialRequest` is considered aborted. If the client accepts
+the cost, the client starts transferring `numBytes` bytes to the server. The
+client transfers these bytes wrapped in `DialDataResponse` protobufs where the
+`data` field in each individual protobuf is limited to 4096 bytes in length.
+This allows implementations to use a small buffer for reading and sending the
+data. Only the size of the `data` field of `DialDataResponse` protobufs is
+counted towards the bytes transferred. Once the server has received at least
+numBytes bytes, it proceeds to dial the selected address. Servers SHOULD allow
+the last `DialDataResponse` message received from the client to be larger than
+the minimum required amount. This allows clients to serialize their
+`DialDataResponse` message once and reuse it for all Requests.
+
+If an attacker asks a server to dial a victim node, the only benefit the
+attacker gets is forcing the server and the victim to do a cryptographic
+handshake which costs some bandwidth and compute. The attacker by itself can do
+a lot of handshakes with the victim without spending any compute by using the
+same key repeatedly. The only benefit of going via the server to do this attack
+is not spending bandwidth required for a handshake. So the prevention mechanism
+only focuses on bandwidth costs. There is a minor benefit of bypassing IP
+blocklists, but that's made unattractive by the fact that servers may ask 5x
+more data than the bandwidth cost of a handshake.
+
+#### Related Work
+
+UDP based protocol's, like QUIC and DNS-over-UDP, need to prevent amplification attacks caused by IP spoofing. To verify that received packets don't have a spoofed IP, the server sends a random token to the client, which echoes the token back. For example, in QUIC, an attacker can use the victim's IP in the initial packet to make it process a much larger `ServerHello` packet. QUIC servers use a Retry Packet containing a token to validate that the client can receive packets at the address it claims. See [QUIC Address Validation](https://datatracker.ietf.org/doc/html/rfc9000#name-address-validation) for details of the scheme. 
+
+## Implementation Suggestions
+
+For any given address, client implementations SHOULD do the following
+- Periodically recheck reachability status.
+- Query multiple servers to determine reachability.
+
+The suggested heuristic for implementations is to consider an address reachable
+if more than 3 servers report a successful dial and to consider an address
+unreachable if more than 3 servers report unsuccessful dials. Implementations
+are free to use different heuristics than this one
+
+Servers SHOULD NOT reuse their listening port when making a dial back. In case
+the client has reused their listen port when dialing out to the server, not
+reusing the listen port for attempts prevents accidental hole punches. Clients
+SHOULD only rely on the nonce and not on the peerID for verifying the dial back
+as the server is free to use a separate peerID for the dial backs.
+
+Servers SHOULD determine whether they have IPv6 and IPv4 connectivity. IPv4 only servers SHOULD refuse requests for dialing IPv6 addresses and IPv6 only
+servers SHOULD refuse requests for dialing IPv4 addresses.
+
+
+## RPC Messages
+
+All RPC messages sent over a stream are prefixed with the message length in
+bytes, encoded as an unsigned variable length integer as defined by the
+[multiformats unsigned-varint spec][uvarint-spec]. 
+
+All RPC messages on stream `/libp2p/autonat/2/dial-request` are of type
+`Message`. A `DialRequest` message is sent as a `Message` with the `msg` field
+set to `DialRequest`. `DialResponse` and `DialDataRequest` are handled
+similarly. 
+
+On stream `/libp2p/autonat/2/dial-back`, a `DialAttempt` message is sent
+directly
+
+```proto3
+
+message Message {
+    oneof msg {
+        DialRequest dialRequest   = 1;
+        DialResponse dialResponse = 2;
+        DialDataRequest dialDataRequest = 3;
+        DialDataResponse dialDataResponse = 4;
+    }
+}
+
+
+message DialRequest {
+    repeated bytes addrs = 1;
+    fixed64 nonce = 2;
+}
+
+
+message DialDataRequest {
+    uint32 addrIdx = 1;
+    uint64 numBytes = 2;
+}
+
+
+enum DialStatus {
+    UNUSED            = 0;
+    E_DIAL_ERROR      = 100;
+    E_DIAL_BACK_ERROR = 101;
+    OK                = 200;
+}
+
+
+message DialResponse {
+    enum ResponseStatus {
+        E_INTERNAL_ERROR   = 0;
+        E_REQUEST_REJECTED = 100; 
+        E_DIAL_REFUSED     = 101;
+        OK  = 200; 
+    }
+
+    ResponseStatus status = 1;
+    uint32 addrIdx        = 2; 
+    DialStatus dialStatus = 3;
+}
+
+
+message DialDataResponse {
+    bytes data = 1;
+}
+
+
+message DialBack {
+    fixed64 nonce = 1;
+}
+
+message DialBackResponse {
+    enum DialBackStatus {
+        OK = 0;
+    }
+
+    DialBackStatus status = 1;
+}
+```
+
+[uvarint-spec]: https://github.com/multiformats/unsigned-varint
+