Triangular routing is routing that sends a packet to a proxy system before transmission to the intended destination. Triangular routing is a problem in mobile IP, however it finds applications in other networking scenarios, for example to avoid problems associated with network address translation (NAT), implemented for example by Skype.
In mobile IP, packets that are sent to a mobile node are first routed to the mobile node's home subnet and then forwarded to the mobile node at its current location by its home agent. However, packets that are sent from the mobile node are not handled in this way, but are instead sent straight to their destination.
This may lead to problems when using services that do ingress filtering, since the source address on the packet will be the home address of the mobile node, not the care-of address assigned to the node on its guest network. To avoid this, many Mobile IP implementations offer the option of tunneling packets from the mobile node through the home agent too.
Unlike in mobile IPv4, mobile IPv6 avoids triangular routing and is therefore as efficient as native IPv6.