[tor-bugs] #15518 [Tor]: Tor considers routers in the same IPv6 /16 to be "in the same subnet"

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#15518: Tor considers routers in the same IPv6 /16 to be "in the same subnet"
-----------------------------------+---------------------
 Reporter:  isis                   |          Owner:
     Type:  defect                 |         Status:  new
 Priority:  normal                 |      Milestone:
Component:  Tor                    |        Version:
 Keywords:  ipv6, path, path-bias  |  Actual Points:
Parent ID:                         |         Points:
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 When `EnforceDistinctSubnets` is enabled, tor uses:

 {{{
 /** Return true iff router1 and router2 have similar enough network
 addresses
  * that we should treat them as being in the same family */
 static INLINE int
 addrs_in_same_network_family(const tor_addr_t *a1,
                              const tor_addr_t *a2)
 {
   return 0 == tor_addr_compare_masked(a1, a2, 16, CMP_SEMANTIC);
 }
 }}}

 to determine if an address is in the same family. For an example IPv6
 address, `2001:1234::0:1`, its /16 representation is `2001::/16`, meaning
 that `2001:ffff::` would be in the same family. A `\16` for IPv6 is
 ''huge'', particularly considering that
 [https://www.iana.org/assignments/ipv6-unicast-address-assignments/ipv6
 -unicast-address-assignments.xhtml only one-eighth of all IPv6 space is
 currently allocated for use on the internet] (`2000::/3`).

 For a related ticket, see #15517 governing how BridgeDB's version of
 `EnforceDistinctSubnets` will work for IPv6. (In that ticket, I proposed
 using IPv6 `/32`s, since that is the
 [https://www.arin.net/resources/request/ipv6_initial_assign.html minimum
 ARIN IPv6 subnet allocation for a LIR].

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15518>
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