Prop 312: 3.2.5. Make Relays do IPv4 and IPv6 Directory Fetches

Relays should start fetching directory documents over IPv4 and IPv6, so they can use X-Your-Address-Is HTTP headers to find their IPv6 addresses.

Bridges must only fetch directory documents over IPv4, because they try to imitate clients. (So they can't find their IPv6 addresses in this way.)

We propose to use a simple load balancing scheme for IPv4 and IPv6 directory requests:

choose between IPv4 and IPv6 directory requests at random.

We do not expect this change to have any load-balancing impact on the public tor network, because the number of relays is much smaller than the number of clients. However, the 6 directory authorities with IPv6 enabled may see slightly more directory load, particularly over IPv6.

See proposal 312, section 3.2.5, directory fetch part: https://gitweb.torproject.org/torspec.git/tree/proposals/312-relay-auto-ipv6-addr.txt#n429

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