``` Filename: 283-ipv6-in-micro-consensus.txt Title: Move IPv6 ORPorts from microdescriptors to the microdesc consensus Author: Tim Wilson-Brown (teor), Nick Mathewson Created: 18-Oct-2017 Status: Closed Target: 0.3.3.x Implemented-In: 0.3.3.1-alpha Ticket: #20916 1. Summary Moving IPv6 ORPorts from microdescs to the microdesc consensus will make it easier for IPv6 clients to bootstrap and select reachable guards. Tor clients on IPv6-only connections currently have to use IPv6 Fallback Directory Mirrors to fetch their microdescriptors. This does not scale well. After this change, they will be able to fetch microdescriptors from any IPv6-enabled directory mirror in the consensus. Tor clients on versions 0.2.8.x and 0.2.9.x are currently unable to bootstrap over IPv6-only connections when using microdescriptors. After this consensus change, they will be able to bootstrap without any client code changes. For clients that use microdescriptors (the default), IPv6 ORPorts are always placed in microdescriptors. So these clients can only tell if an IPv6 ORPort is unreachable when a majority of voting authorities mark the relay as not Running. After this proposal, clients will be able to discover unreachable ORPorts, even if a minority of voting authorities set AuthDirHasIPv6Connectivity 1. 2. Proposal We add two new consensus methods, here represented as M and N (M < N), to be allocated when this proposal's implementation is merged. These consensus methods move IPv6 ORPorts from microdescs to the microdesc consensus. We use two different methods because this allows us to modify client code based on each method. Also, if a bug is discovered in one of the methods, authorities can be patched to stop voting for it, and then we can implement a fix in a later method. 2.1. Add Reachable IPv6 ORPorts to the Microdesc Consensus We specify that microdescriptor consensuses created with methods M or later contain reachable IPv6 ORPorts. 2.2. Remove IPv6 ORPorts from Microdescriptors We specify that microdescriptors created with methods N or later start omitting IPv6 ORPorts. 3. Retaining Existing Behaviour The following existing behaviour will be retained: 3.1. Authority IPv6 Reachability Only authorities configured with AuthDirHasIPv6Connectivity 1 will test IPv6 ORPort reachability, and vote for IPv6 ORPorts. This means that: * if no voting authorities set AuthDirHasIPv6Connectivity 1, there will be no IPv6 ORPorts in the consensus, * if a minority of voting authorities set AuthDirHasIPv6Connectivity 1: unreachable IPv6 ORPort lines will be dropped from the consensus, but the relay will still be listed as Running, and reachable IPv6 ORPort lines will be included in the consensus. * if a majority of voting authorities set AuthDirHasIPv6Connectivity 1, relays with unreachable IPv6 ORPorts will not be listed as Running. Reachable IPv6 ORPort lines will be included in the consensus. (To ensure that any valid majority will vote relays with unreachable IPv6 ORPorts not Running, 75% of authorities must set AuthDirHasIPv6Connectivity 1.) We will document this behaviour in the tor manual page, see #23870. 3.2. NS Consensus IPv6 ORPorts The NS consensus will continue to contain reachable IPv6 ORPorts. 4. Impact and Related Changes 4.1. Directory Authority Configuration We will work to get a super-majority (75%) of authorities checking relay IPv6 reachability, to avoid Running-flag flapping. To do this, authorities need to get IPv6 connectivity, and set AuthDirHasIPv6Connectivity 1. 4.2. Relays and Bridges Tor relays and bridges do not currently use IPv6 ORPorts from the consensus. We expect that 2/3 of authorities will be voting for consensus method N before future Tor relay or bridge versions use IPv6 ORPorts from the consensus. 4.3. Clients 4.3.1. Legacy Clients 4.3.1.1. IPv6 ORPort Circuits Tor clients on versions 0.2.8.x to 0.3.2.x check directory documents for ORPorts in the following order: * descriptors (routerinfo, available if using bridges or full descriptors) * consensus (routerstatus) * microdescriptors (IPv6 ORPorts only) Their behaviour will be identical to the current behaviour for consensus methods M and earlier. When consensus method N is used, they will ignore unreachable IPv6 ORPorts without any code changes, as long as they are using microdescriptors. 4.3.1.2. IPv6 ORPort Bootstrap Tor clients on versions 0.2.8.x and 0.2.9.x are currently unable to bootstrap over IPv6-only connections when using microdescriptors. This happens because the microdesc consensus does not contain IPv6 ORPorts. (IPv6-only Tor clients on versions 0.3.0.2-alpha and later use fallback directory mirrors to fetch their microdescriptors.) When consensus method M is used, 0.2.8.x and 0.2.9.x clients will be able to bootstrap over IPv6-only connections using microdescriptors, without any code changes. 4.3.2. Future Clients 4.3.2.1. Ignoring IPv6 ORPorts in Microdescs Tor clients on versions 0.3.3.x and later will ignore unreachable IPv6 ORPorts once consensus method M or later is in use. This requires some code changes, see #23827. 4.3.2.2. IPv6 ORPort Bootstrap If a bootstrapping IPv6-only client has a consensus made with method M or later, it should download microdescriptors from one of the IPv6 ORPorts in that consensus. This requires some code changes, see #23827. Previously, IPv6-only clients would use fallback directory mirrors to download microdescs, because there were no IPv6 ORPorts in the microdesc consensus. 4.3.2.3. Ignoring Addresses in Unused Directory Documents If a client doesn't use a particular directory document type for a node, it should ignore any addresses in that document type. This requires some code changes, see #23975. 5. Data Size This change removes 7-50 bytes from the microdescriptors of relays that have an IPv6 ORPort, and adds them to reachable IPv6 relays' microdesc consensus entries. As of October 2017, 600 relays (9%) have IPv6 ORPorts in the NS consensus. Their "a" lines take up 19 KB, or 33 bytes each on average. The gzip-compressed microdesc consensus is 564 KB, and adding the existing IPv6 addresses makes it 576 KB (a 2.1% increase). Adding IPv6 addresses to every relay makes it 644 KB (a 14% increase). zstd-compressed microdesc consensuses show smaller increases of 1.7% and 8.0%, respectively. Most tor clients are already running 0.3.1.7, which implements consensus diffs and zstd compression. We expect that most directory mirrors will also implement consensus diffs and zstd compression by the time 2/3 of authorities are voting for consensus method M. Consensus diffs will reduce the worst-case impact of this change for clients and relays that have a recent consensus. 6. External Impacts We don't expect this change to impact Onionoo and similar projects, because they typically use the NS consensus. 7. Monitoring OnionOO has implemented an "unreachable IPv6 address" attribute: https://trac.torproject.org/projects/tor/ticket/21637 Metrics is working on IPv6 relay graphs: https://trac.torproject.org/projects/tor/ticket/23761 Consensus-health implements a ReachableIPv6 pseudo-flag for authorities and relays: https://consensus-health.torproject.org/ ```