This document proposes a new P2P message to gossip longer node addresses over the P2P network.
This is required to support new-generation Onion addresses, I2P, and potentially other networks
that have longer endpoint addresses than fit in the 128 bits of the current <code>addr</code> message.
===Copyright===
This BIP is licensed under the 2-clause BSD license.
===Motivation===
Tor v3 hidden services are part of the stable release of Tor since version 0.3.2.9. They have
various advantages compared to the old hidden services, among which better encryption and privacy
<ref>[https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt Tor Rendezvous Specification - Version 3]</ref>.
These services have 256 bit addresses and thus do not fit in the existing <code>addr</code> message, which encapsulates onion addresses in OnionCat IPv6 addresses.
Other transport-layer protocols such as I2P have always used longer
addresses. This change would make it possible to gossip such addresses over the
P2P network, so that other peers can connect to them.
==Specification==
<blockquote>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119<ref>[https://tools.ietf.org/html/rfc2119 RFC 2119]</ref>.
</blockquote>
The <code>addrv2</code> message is defined as a message where <code>pchCommand == "addrv2"</code>.
It is serialized in the standard encoding for P2P messages.
Its format is similar to the current <code>addr</code> message format
<ref>[https://bitcoin.org/en/developer-reference#addr Bitcoin Developer Reference: addr message]</ref>, with the difference that the
fixed 16-byte IP address is replaced by a network ID and a variable-length address, and the time and services format has been changed to VARINT.
This means that the message contains a serialized <code>std::vector</code> of the following structure:
To allow for future extensibility, clients MUST ignore address types that they do not know about.
Client MAY store and gossip address formats that they do not know about. Further network ID numbers MUST be reserved in a new BIP document.
Clients SHOULD reject addresses that have a different length than specified in this table for a specific address ID, as these are meaningless.
See the appendices for the address encodings to be used for the various networks.
==Compatibility==
Send <code>addrv2</code> messages only, and exclusively, when the peer has a certain protocol version (or higher):
<source lang="c++">
//! gossiping using `addrv2` messages starts with this version
static const int GOSSIP_ADDRV2_VERSION = 70016;
</source>
For older peers keep sending the legacy <code>addr</code> message, ignoring addresses with the newly introduced address types.
==Reference implementation==
The reference implementation is available at (to be done)
==Acknowledgements==
- Jonas Schnelli: change <code>services</code> field to VARINT, to make the message more compact in the likely case instead of always using 8 bytes.
- Luke-Jr: change <code>time</code> field to VARINT, for post-2038 compatibility.
- Gregory Maxwell: various suggestions regarding extensibility
==Appendix A: Tor v2 address encoding==
The new message introduces a separate network ID for <code>TORV2</code>.
Clients MUST send Tor hidden service addresses with this network ID, with the 80-bit hidden service ID in the address field. This is the same as the representation in the legacy <code>addr</code> message, minus the 6 byte prefix of the OnionCat wrapping.
Clients SHOULD ignore OnionCat (<code>fd87:d87e:eb43::/48</code>) addresses on receive if they come with the <code>IPV6</code> network ID.
==Appendix B: Tor v3 address encoding==
According to the spec <ref>[https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt Tor Rendezvous Specification - Version 3: Encoding onion addresses]</ref>, next-gen <code>.onion</code> addresses are encoded as follows:
- PUBKEY is the 32 bytes ed25519 master pubkey of the hidden service.
- VERSION is an one byte version field (default value '\x03')
- ".onion checksum" is a constant string
- CHECKSUM is truncated to two bytes before inserting it in onion_address
</pre>
Tor v3 addresses MUST be sent with the <code>TORV3</code> network ID, with the 32-byte PUBKEY part in the address field. As VERSION will always be '\x03' in the case of v3 addresses, this is enough to reconstruct the onion address.
==Appendix C: I2P address encoding==
Like Tor, I2P naming uses a base32-encoded address format<ref>[https://geti2p.net/en/docs/naming#base32 I2P: Naming and address book]</ref>.
I2P uses 52 characters (256 bits) to represent the full SHA-256 hash, followed by <code>.b32.i2p</code>.
I2P addresses MUST be sent with the <code>I2P</code> network ID, with the decoded SHA-256 hash as address field.
==Appendix D: Cjdns address encoding==
Cjdns addresses are simply IPv6 addresses in the <code>fc00::/8</code> range<ref>[https://github.com/cjdelisle/cjdns/blob/6e46fa41f5647d6b414612d9d63626b0b952746b/doc/Whitepaper.md#pulling-it-all-together Cjdns whitepaper: Pulling It All Together]</ref>. They MUST be sent with the <code>CJDNS</code> network ID.
