Re: [tor-dev] Proposal: Move IPv6 ORPorts to the Microdesc Consensus

On Tue, Oct 24, 2017 at 11:35 PM, teor <teor2345@xxxxxxxxx> wrote:

Hi all,

We would like to move IPv6 ORPorts from microdescriptors to the
microdesc consensus. This makes it easier for IPv6 clients to bootstrap
and choose reachable guards.

The proposal is inlined below, it is also available with the corresponding
dir-spec updates in my torspec branch bug23826-23828 on GitHub:

https://github.com/teor2345/torspec.git

The tor code that implements these new consensus methods is in my tor
branch on bug23826-23828 on GitHub:

https://github.com/teor2345/tor.git

The parent ticket for these related changes is #20916. The code changes are
being tracked in #23826 and #23828, and the spec changes and proposal in
#23898:

https://trac.torproject.org/projects/tor/ticket/20916

If we've spoken about this, and I've left you out as an author, please let
me know!

Hi, Tim! I promised you a quick review here, so here goes. I have some questions, but nothing looks like a showstopper here.

Here is the proposal text:

Filename: xxx-ipv6-in-micro-consensus.txt
Title: Move IPv6 ORPorts from microdescriptors to the microdesc consensus
Author: Tim Wilson-Brown (teor)
Created: 18-Oct-2017
Status: Open
Target: 0.3.3.x

1. Summary

Moving IPv6 ORPorts from microdescs to the microdesc consensus will make
it easier for IPv6 clients to bootstrap and select reachable guards.

Since consensus method 14, authorities have voted for IPv6 address/port
pairs (ORPorts) in "a" lines. Unreachable IPv6 ORPorts are dropped from the
full consensus. But for clients that use microdescriptors (the default),
IPv6 ORPorts are 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.

This proposal puts reachable relay IPv6 ORPorts in an "a" line in the
microdesc consensus. This allows clients to discover unreachable IPv6
ORPorts, even if a minority of voting authorities set
AuthDirHasIPv6Connectivity 1.

To me, this motivation makes a little less sense than the bootstrapping improvements in 4.3 do. Don't get me wrong: it's cool that we can get IPv6 online-ness detection "for free" on existing clients... but there are other ways we could IPv6 online-status advertising too (like a status flag, or conditionally omitting "a" lines from microdescriptors, or something else).

But the bootstrapping considerations discussed in 4.3 below are something we really _can't_ do without moving the "a" lines into the consensus. So to my mind, that's the major reason we should do this.

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 do not
contain any IPv6 ORPorts.

Let's say that with method N, we start omitting them. Let's not say that we commit to omitting them forever. Perhaps we will someday have a reason to put more "a" lines in microdescriptors again.

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,
* if a majority of voting authorities set AuthDirHasIPv6Connectivity 1,
relays with unreachable IPv6 ORPorts will be dropped from the consensus.

We will document this behaviour in the tor manual page, see #23870.

So, there's an alternative here: we could let the HasIPV6 authorities vote on a flag to indicate "reachable/unreachable with IPv6," and let all the authorities vote on the "a" lines. Then, in the consensus, we could omit the "a" lines unless the router has the reachable-with-ipv6 flag; and include them otherwise.

This way, we wouldn't need to have a majority of IPv6 authorities in order to have meaningful "a" lines that tell you whether the router is reachable. (But of course, the more we had, the more reliable the information would be.)

This change could be done as part of consensus method M, I think. Do you think it's worthwhile?

3.2. Full Consensus IPv6 ORPorts

The full consensus will continue to contain reachable IPv6 ORPorts.

By "full" consensus, do you mean "NS" consensus? I don't think we use "full" elsewhere.

3.3. Clients that use Full Descriptors

Tor clients that use full descriptors already ignore unreachable IPv6
ORPorts, and have done so since at least 0.2.8.x.

Wow. I'd forgotten this. How does this work?

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.

How far away are we from this today? How long do the authority operators think it would take?

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.

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.

When consensus method M is used, they will be able to bootstrap over IPv6
only connections using microdescriptors, without any code changes.

(How does the behavior of 0.3.0.x and onward differ here?)

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. (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. Previously, IPv6-only clients would use fallback directory
mirrors to download microdescs, because there were no IPv6 ORPorts in the
microdesc consensus. (See #23827.)

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. (See #23975.)

5. Data Size

This change removes 2-50 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 full
consensus. Their "a" lines take up 19 KB, or 33 bytes each on average.
The microdesc consensus is 1981 KB, so this represents about 1% of its
uncompressed size.

Most tor clients are already running 0.3.1.7, which implements consensus
diffs. We expect that most directory mirrors will also implement consensus
diffs by the time 2/3 of authorities are voting for consensus method M.

So we expect that this change will have a minimal impact, which is made
even smaller by compression and consensus diffs.

Let's look at a worst-case analysis, though. How would the impact be if 100% of the relays had IPv6 ORPorts? I'm not very interested in the uncompressed size; it's the gzip-compressed size that determines the worst-case impact.

6. External Impacts

We don't expect this change to impact Onionoo and similar projects, because
they typically use the full consensus.

Metrics doesn't currently graph IPv6 usage in Tor, but would like to in
future.

--
Tim / teor

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