addrv2 BIP proposal

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+<pre>
+  BIP: ???
+  Layer: Peer Services
+  Title: addrv2 message
+  Author: Wladimir J. van der Laan <laanwj@gmail.com>
+  Comments-Summary: No comments yet.
+  Comments-URI: 
+  Status: Draft
+  Type: Standards Track
+  Created: 2019-02-27
+  License: BSD-2-Clause
+</pre>
+
+==Introduction==
+
+===Abstract===
+
+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:
+
+{| class="wikitable" style="width: auto; text-align: center; font-size: smaller; table-layout: fixed;"
+!Type
+!Name
+!Description
+|-
+| <code>VARINT</code> (unsigned)
+| <code>time</code>
+| Time that this node was last seen as connected to the network. A time in Unix epoch time format, up to 64 bits wide.
+|-
+| <code>VARINT</code> (unsigned)
+| <code>services</code>
+| Service bits. A 64-wide bit field.
+|-
+| <code>uint8_t</code>
+| <code>networkID</code>
+| Network identifier. An 8-bit value that specifies which network is addressed.
+|-
+| <code>std::vector<uint8_t></code>
+| <code>addr</code>
+| Network address. The interpretation depends on networkID.
+|-
+| <code>uint16_t</code>
+| <code>port</code>
+| Network port. If not relevant for the network this MUST be 0.
+|}
+
+One message can contain up to 1,000 addresses. Clients SHOULD reject messages with more addresses.
+
+Field <code>addr</code> has a variable length, with a maximum of 32 bytes (256 bits). Clients SHOULD reject
+longer addresses.
+
+The list of reserved network IDs is as follows:
+
+{| class="wikitable" style="width: auto; text-align: center; font-size: smaller; table-layout: fixed;"
+!Network ID
+!Enumeration
+!Address length (bytes)
+!Description
+|-
+| <code>0x01</code>
+| <code>IPV4</code>
+| 4
+| IPv4 address (globally routed internet)
+|-
+| <code>0x02</code>
+| <code>IPV6</code>
+| 16
+| IPv6 address (globally routed internet)
+|-
+| <code>0x03</code>
+| <code>TORV2</code>
+| 10
+| Tor v2 hidden service address
+|-
+| <code>0x04</code>
+| <code>TORV3</code>
+| 32
+| Tor v3 hidden service address
+|-
+| <code>0x05</code>
+| <code>I2P</code>
+| 32
+| I2P overlay network address
+|-
+| <code>0x06</code>
+| <code>CJDNS</code>
+| 16
+| Cjdns overlay network address
+|}
+
+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:
+<pre>
+onion_address = base32(PUBKEY | CHECKSUM | VERSION) + ".onion"
+ CHECKSUM = H(".onion checksum" | PUBKEY | VERSION)[:2]
+
+ where:
+   - 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.
+
+==References==
+
+<references/>