Send an extend cell to relay A, listing the address and identity key of relay B but the wrong port. Relay A calls `circuit_extend()` for the new cell, which calls `channel_get_for_extend()`, which tries to figure out if there's a canonical connection already established. To do that, it asks ``` if (!channel_is_canonical(chan) && channel_is_canonical_is_reliable(chan) && !channel_matches_target_addr_for_extend(chan, target_addr)) { ++n_noncanonical; continue; } ``` and `channel_matches_target_addr_for_extend()` turns into `channel_tls_matches_target_method()` which basically is ``` return tor_addr_eq(&(tlschan->conn->real_addr), target); ``` It doesn't consider the port. So if there is a canonical channel open, bingo we use it. But if there isn't one open, then off we go to make one: ``` n_chan = channel_connect_for_circuit(&ec.orport_ipv4.addr, ec.orport_ipv4.port, (const char*)ec.node_id); ``` where `ec.orport_ipv4.port` was set from `extend_cell_parse()`, i.e. it came from our extend cell. If we specify the wrong port, that connect attempt will fail. Now we can distinguish, remotely, which situation we're in.