Opened 8 months ago

Last modified 4 months ago

#33223 assigned enhancement

Prop 311: 4.3.1. Don't Publish Descriptor if IPv6 ORPort is Unreachable

Reported by: teor Owned by:
Priority: Medium Milestone:
Component: Core Tor/Tor Version:
Severity: Normal Keywords: ipv6, prop311, 044-deferred
Cc: Actual Points:
Parent ID: #33221 Points: 3
Reviewer: Sponsor: Sponsor55-must

Description (last modified by teor)

This ticket depends on the protocol version ticket #33226.

Since it stops relays publishing their descriptors, it needs to be
tested on the public tor network, see #33229 and #33230.

If IPv6 reachability checks fail, relays (and bridges) should refuse to
publish their descriptors, if:

  • enough existing relays support IPv6 extends, and
  • the IPv6 address was explicitly configured by the operator (rather than guessed using [Proposal 312: Relay Auto IPv6 Address]).

Directory authorities may perform reachability checks, and warn if those
checks fail. But they always publish their descriptors.

We set a threshold of consensus relays for reliable IPv6 ORPort checks:

  • at least 30 relays, and
  • at least 1% of the total consensus weight,

must support IPv6 extends.

We chose these parameters so that the number of relays is triple the
number of directory authorities, and the consensus weight is high enough
to support occasional reachability circuits.

In small networks with:

  • less than 2000 relays, or
  • a total consensus weight of zero,

the threshold should be the minimum tor network size to test reachability:

  • at least 2 relays, excluding this relay.

(Note: we may increase this threshold to 3 or 4 relays if we discover a
higher minimum during testing.)

If the current consensus satisfies this threshold, testing relays (and
bridges, but not directory authorities) that fail IPv6 ORPort reachability
checks should refuse to publish their descriptors.

To ensure an accurate threshold, testing relays should exclude:

  • the testing relay itself, and
  • relays that they will not use in testing circuits,

from the:

  • relay count, and
  • the numerator of the threshold percentage.

Typically, relays will be excluded if they are in the testing relay's:

  • family,
  • IPv4 address /16 network,
  • IPv6 address /32 network (a requirement as of Tor,

unless EnforceDistinctSubnets is 0.

As a useful side-effect, these different thresholds for each relay family
will reduce the likelihood of the network flapping around the threshold.

If flapping has an impact on the network health, directory authorities
should set the AssumeIPv6Reachable consensus parameter. (See the next

See proposal 311, section 4.3.1:

Child Tickets

Change History (6)

comment:1 Changed 6 months ago by teor

Summary: Prop 311: 4.3.1. Refusing to Publish Descriptor if IPv6 ORPort is UnreachableProp 311: 4.3.1. Don't Publish Descriptor if IPv6 ORPort is Unreachable

comment:2 Changed 6 months ago by teor

Description: modified (diff)

comment:3 Changed 6 months ago by teor

Description: modified (diff)

comment:4 Changed 6 months ago by teor

Owner: teor deleted

Un-assign myself from future Sponsor 55 tasks.

comment:5 Changed 4 months ago by nickm

Keywords: 044-deferred added
Milestone: Tor: 0.4.4.x-finalTor: unspecified

Bulk-remove tickets from 0.4.4. Add the 044-deferred label to them.

comment:6 Changed 4 months ago by nickm

Milestone: Tor: unspecified

Bulk-move prop311 and prop312 to 0.4.5

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