Routing Internet traffic

When an OLSR node communicates with an external host the destined external host will normally send return traffic to the source IP address of the outgoing packet. Thus, for IPv6, a MANET node configures an address under a global prefix managed by one of the gateways and uses this address as source IP address when communicating with external hosts on the Internet. Return traffic from the external nodes on the Internet is therefore routed back to the gateway, which in turn can forward the packets to the MANET node. However, for IPv4 there is a great scarcity of global IPv4 addresses. Thus, the gateway may be equipped with a very limited number of external IPv4 addresses. To allow different MANET nodes to share an address for external communication, the gateway may implement a Network Address Translator (NAT)[32].

All routing within an OLSR routed MANET is host based. This means that there exists one entry in the routing table for every host to which the local node has calculated a route. When using HNA gateways the routing table is aggregated. This means that all traffic to a defined network is sent to a certain gateway. For an OLSR routed MANET this means that IP packets that do not have an IP destination address known locally on the MANET are forwarded along a possible default route out of the MANET through the default gateway.

In addition to using default routes for outgoing packets, a mechanism is required to ensure that return traffic from the Internet gets routed back to the node in the MANET. A gateway that implements NAT will translate the source IP address of outgoing packets from the MANET node. It replaces the source IP address with an address of the NAT gateway which is route-able on the external network. Hence, an external host will return packets using the IP address of the NAT-gateway as destination IP address. The gateway can then replace the destination IP address with the IP address of the MANET node, and inject the return traffic into the MANET.