Allow IPv6 extend cells

As part of legacy/trac#33817 (moved), we want to allow IPv6 extend cells.

We want clients and relays to send:

• dual-stack EXTEND2 cells
• IPv6-only EXTEND2 cells

We want relays to parse:

• dual-stack EXTEND2 cells
• IPv6-only EXTEND2 cells

changed milestone to %Tor: 0.4.4.x-final in legacy/trac

Trac:
Parent Ticket: legacy/trac#33048 (moved)

added actualpoints::0.5 in Legacy / Trac component::core tor/tor in Legacy / Trac ipv6 in Legacy / Trac milestone::Tor: 0.4.4.x-final in Legacy / Trac owner::teor in Legacy / Trac parent::33048 in Legacy / Trac points::0.5 in Legacy / Trac priority::medium in Legacy / Trac prop311 in Legacy / Trac resolution::fixed in Legacy / Trac reviewer::ahf in Legacy / Trac severity::normal in Legacy / Trac sponsor::55-must in Legacy / Trac status::closed in Legacy / Trac type::enhancement in Legacy / Trac labels

See my PR:

• master: https://github.com/torproject/tor/pull/1863

The unit tests cover the two major cases:

• dual-stack EXTEND2 cells
• IPv6-only EXTEND2 cells

The tests also cover cell parsing and cell encoding.

They don't cover the edge cases of all-zero IP addresses or ports. I'm not particularly worried about those cases.

Trac:
Reviewer: N/A to ahf
Status: assigned to needs_review

I considered moving create_cell_parse() and extend_cell_parse() (and the functions they call) to a new feature/relay/onion_relay.c file.

But there are lots of dependencies between parsing and formatting. (Clients and relays format, only relays parse.)

ahf, nickm, what do you think?

Trac:
Cc: N/A to ahf, nickm

Trac:
Parent: legacy/trac#33817 (moved) to legacy/trac#33220 (moved)

I think we should review this code along with legacy/trac#33817 (moved).

Trac:
Parent: legacy/trac#33220 (moved) to legacy/trac#33817 (moved)
Status: needs_review to closed
Resolution: N/A to fixed

Trac:
Parent: legacy/trac#33817 (moved) to legacy/trac#33048 (moved)

closed

changed time estimate to 4h

added 4h of time spent

mentioned in issue legacy/trac#33048 (moved)

mentioned in issue #33818 (closed)

moved from legacy/trac#33901 (moved)

added 1 deleted label

mentioned in issue #33048 (closed)

added Feature label and removed 1 deleted label

added IPv6 label and removed 1 deleted label