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::
ZIP: 155
Title: addrv2 message
Owners: Daira Hopwood <daira@electriccoin.co>
Credits: Wladimir J. van der Laan
Zancas Wilcox
Status: Proposed
Category: Network
Created: 2021-08-13
License: BSD-2-Clause
Discussions-To: <https://github.com/zcash/zips/issues/542>
Pull-Request: <https://github.com/zcash/zips/pull/543>
Terminology
===========
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in this
document are to be interpreted as described in RFC 2119. [#RFC2119]_
The term "network upgrade" in this document is to be interpreted as described in
ZIP 200. [#zip-0200]_
The terms "Testnet" and "Mainnet" are to be interpreted as described in
section 3.12 of the Zcash Protocol Specification [#protocol-networks]_.
"P2P network" means the Zcash peer-to-peer network.
``uint8``, ``uint16``, and ``uint32`` denote unsigned integer data types of the
corresponding length (8, 16, or 32 bits respectively).
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 ``addr`` message.
Motivation
==========
Tor v3 onion services are part of the stable release of Tor since version 0.3.2.9.
They have various advantages compared to the old v2 onion services, among which better
encryption and privacy [#Tor-rendezvous-v3]_. These services have 256-bit addresses
and thus do not fit in the existing ``addr`` message (unchanged from Bitcoin
[#Bitcoin-addr-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
=============
The ``addrv2`` message is defined as a message where the ``command`` field is
(NUL-padded) ``"addrv2"``. It is serialized in the standard encoding for P2P messages.
Its format is similar to the current ``addr`` message format described in
[#Bitcoin-addr-message]_, with the difference that the fixed 16-byte IP address is
replaced by a network ID and a variable-length address, and the services format
has been changed to [#Bitcoin-CompactSize]_.
This means that the message contains a serialized vector of the following structure:
+--------------+-----------------+---------------------+----------------------------------------------------------------+
| Bytes | Name | Data Type | Description |
+--------------+-----------------+---------------------+----------------------------------------------------------------+
| 4 | ``time`` | ``uint32`` | Time that this node was last seen as connected to the network. |
| | | | A time in Unix epoch time format. |
+--------------+-----------------+---------------------+----------------------------------------------------------------+
| ``varies`` | ``services`` | ``CompactSize`` | Service bits. A CompactSize-encoded bit field that is 64 bits |
| | | | wide. |
+--------------+-----------------+---------------------+----------------------------------------------------------------+
| 1 | ``networkID`` | ``uint8`` | Network identifier. An 8-bit value that specifies which |
| | | | network is addressed. |
+--------------+-----------------+---------------------+----------------------------------------------------------------+
| ``varies`` | ``sizeAddr`` | ``CompactSize`` | The length in bytes of ``addr``. |
+--------------+-----------------+---------------------+----------------------------------------------------------------+
| ``sizeAddr`` | ``addr`` | ``uint8[sizeAddr]`` | Network address. The interpretation depends on networkID. |
+--------------+-----------------+---------------------+----------------------------------------------------------------+
| 2 | ``port`` | ``uint16`` | Network port. If not relevant for the network this MUST be 0. |
+--------------+-----------------+---------------------+----------------------------------------------------------------+
One message can contain up to 1,000 addresses. Clients MUST reject messages with
more addresses.
Field ``addr`` has a variable length, with a maximum of 512 bytes (4096 bits). Clients
MUST reject messages with a longer ``addr`` field, irrespective of the network ID.
The list of reserved network IDs is as follows:
+------------+-------------+------------------------+-----------------------------------------+
| Network ID | Enumeration | Address length (bytes) | Description |
+------------+-------------+------------------------+-----------------------------------------+
| ``0x01`` | ``IPV4`` | 4 | IPv4 address (globally routed internet) |
+------------+-------------+------------------------+-----------------------------------------+
| ``0x02`` | ``IPV6`` | 16 | IPv6 address (globally routed internet) |
+------------+-------------+------------------------+-----------------------------------------+
| ``0x04`` | ``TORV3`` | 32 | Tor v3 onion service address |
+------------+-------------+------------------------+-----------------------------------------+
| ``0x05`` | ``I2P`` | 32 | I2P overlay network address |
+------------+-------------+------------------------+-----------------------------------------+
| ``0x06`` | ``CJDNS`` | 16 | Cjdns overlay network address |
+------------+-------------+------------------------+-----------------------------------------+
Network ID ``0x03`` is intentionally not assigned. In BIP 155 [#bip-0155]_ it was
assigned to Tor v2 onion addresses, but those addresses are no longer supported
by the latest Tor client code, and MUST NOT be used once this ZIP is deployed.
Clients SHOULD gossip valid, potentially routable addresses from all known
networks, even if they are currently not connected to some of them. That could
help multi-homed nodes and make it more difficult for an observer to tell which
networks a node is connected to.
Clients MUST NOT gossip addresses from unknown networks, because they have no means
to validate those addresses and so can be tricked to gossip invalid addresses.
Clients MUST reject messages that contain addresses that have a different length
than specified in this table for a specific network ID, as these are meaningless.
Network address encodings
-------------------------
The ``IPV4`` and ``IPV6`` network IDs use addresses encoded in the usual way for
binary IPv4 and IPv6 addresses in network byte order (big endian).
Tor v3 address encoding
'''''''''''''''''''''''
According to the spec [#Tor-rendezvous-v3]_, version 3 ``.onion`` addresses are
encoded as follows::
onion_address = base32(PUBKEY || CHECKSUM || VERSION) + ".onion"
CHECKSUM = H(".onion checksum" || PUBKEY || VERSION)[:2] // first 2 bytes
where:
* ``PUBKEY`` is the 32-byte Ed25519 master pubkey of the onion service;
* ``VERSION`` is a one-byte version field (default value ``0x03``);
* ``".onion"`` and ``".onion checksum"`` are constant strings;
* ``CHECKSUM`` is truncated to two bytes before inserting it in ``onion_address``;
* ``H()`` is the SHA3-256 cryptographic hash function. [#NIST-SHA3]_
Tor v3 addresses MUST be sent with the ``TORV3`` network ID, with the 32-byte
``PUBKEY`` part in the ``addr`` field. As ``VERSION`` will always be 0x03 in the
case of v3 addresses, this is enough to reconstruct the onion address.
I2P address encoding
''''''''''''''''''''
Like Tor, I2P naming uses a base32-encoded address format [#I2P-naming]_.
I2P uses 52 characters (256 bits) to represent the full SHA-256 hash, followed by
``.b32.i2p``. The base32 encoding does not include ``"="`` padding characters.
I2P addresses MUST be sent with the ``I2P`` network ID, with the decoded SHA-256 hash
as address field.
Cjdns address encoding
''''''''''''''''''''''
Cjdns addresses are simply IPv6 addresses in the ``fc00::/8`` range
[#Cjdns-whitepaper]_. They MUST be sent with the ``CJDNS`` network ID.
They are encoded in network byte order (big endian) as for the ``IPV6`` network ID.
Deployment
----------
TODO: change ${PLACEHOLDER} to a specific peer protocol version.
Support for this specification is signalled by peer protocol version ${PLACEHOLDER}
(on both Testnet and Mainnet). Note that this is the same peer protocol version that
signals support for NU5 on Mainnet [#zip-0252]_.
Nodes that have not negotiated peer protocol version ${PLACEHOLDER} or higher on a
given connection, MUST NOT send ``addrv2`` messages on that connection.
A node that has negotiated peer protocol version ${PLACEHOLDER} or higher on a given
connection, MAY still send ``addr`` messages on the connection, and MUST handle
received ``addr`` messages as it would have done prior to this ZIP.
Rationale
=========
This ZIP is closely based on BIP 155 [#bip-0155]_, with the following changes:
* Deployment: support for the ``addrv2`` message is signalled by advertising a
peer protocol version of ${PLACEHOLDER} or higher, not by sending a ``sendaddrv2``
message. This is motivated by a desire to avoid an exponential explosion in the
space of possible feature configurations in a given peer-to-peer connection. In
Zcash, unlike Bitcoin, the space of such configurations is effectively constant no
matter how many peer-to-peer protocol changes are made, because nodes that do not
support a given peer protocol version will drop off the network over time if they do
not support the latest Network Upgrade. The feature configuration for a given
connection is also established at version negotiation, and cannot change after that
point without reconnecting. Other peer-to-peer protocol changes ported from the
Bitcoin peer-to-peer protocol, for example the ``MSG_WTX`` inv type defined in
ZIP 239 [#zip-0239]_, have taken the same approach to signalling.
* No Network ID for Tor v2 onion addresses: the Tor network is expected to have
removed support for these addresses in the timeframe for deployment of this ZIP.
* Clients MUST, rather than SHOULD, reject ``addrv2`` messages with more than 1,000
addresses. Making this a consistent requirement promotes interoperability.
* Clients MUST NOT, rather than SHOULD NOT, gossip addresses from unknown networks.
* Clients MUST, rather than SHOULD, reject messages that contain addresses that have
a different length than specified for a known network ID, or a length greater
than the 512-byte maximum for an unknown network ID.
Reference implementation
========================
TBD.
Acknowledgements
================
This ZIP is closely based on BIP 155 [#bip-0155]_, written by Wladimir J.
van der Laan. Zancas Wilcox ported the implementation for Zcashd.
Acknowledgements for BIP 155:
* Jonas Schnelli: change ``services`` field to ``CompactSize``, to make the message
more compact in the likely case instead of always using 8 bytes.
* Gregory Maxwell: various suggestions regarding extensibility.
References
==========
.. [#RFC2119] `RFC 2119: Key words for use in RFCs to Indicate Requirement Levels <https://www.rfc-editor.org/rfc/rfc2119.html>`_
.. [#protocol-networks] `Zcash Protocol Specification, Version 2021.2.16 [NU5 proposal]. Section 3.12 Mainnet and Testnet <protocol/protocol.pdf#networks>`_
.. [#bip-0155] `BIP 155: addrv2 message <https://github.com/bitcoin/bips/blob/master/bip-0155.mediawiki>`_
.. [#zip-0200] `ZIP 200: Network Upgrade Mechanism <zip-0200.rst>`_
.. [#zip-0239] `ZIP 239: Relay of Version 5 Transactions <zip-0239.rst>`_
.. [#zip-0252] `ZIP 252: Deployment of the NU5 Network Upgrade <zip-0252.rst>`_
.. [#Bitcoin-addr-message] `Protocol documentation: addr. Bitcoin Wiki <https://en.bitcoin.it/wiki/Protocol_documentation#addr>`_
.. [#Bitcoin-CompactSize] `Variable length integer. Bitcoin Wiki <https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer>`_
.. [#Tor-rendezvous-v3] `Tor Rendezvous Specification - Version 3 <https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt>`_
.. [#NIST-SHA3] `NIST FIPS 202 - SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions <https://csrc.nist.gov/publications/detail/fips/202/final>`_
.. [#I2P-naming] `I2P: Naming and address book <https://geti2p.net/en/docs/naming#base32>`_
.. [#Cjdns-whitepaper] `Cjdns whitepaper: Pulling It All Together <https://github.com/cjdelisle/cjdns/blob/f909b960709a4e06730ddd4d221e5df38164dbb6/doc/Whitepaper.md#user-content-pulling-it-all-together>`_
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