The roadwarriors carol and dave set up a connection each to gateway alice, which identifies itself as mars. Host venus in the subnet behind this gateway uses gateway moon as default gateway, so it is not fully reachable by the two roadwarriors. Then both SAs are actively redirected with the swanctl --redirect command to gateway moon (also identified as mars) using the mechanism described in RFC 5685 enabling the roadwarriors to ping venus.