Direct and Indirect Delivery

Forwarded IP packets use at least one of two types of delivery based on whether the IP packet is forwarded to the final destination or whether it is forwarded to an IP router. These two types of delivery are known as direct and indirect delivery.

Direct delivery occurs when the IP node (either the sending node or an IP router) forwards a packet to the final destination on a directly attached network. The IP node encapsulates the IP datagram in a frame format for the Network Interface layer (such as Ethernet or Token Ring) addressed to the destination's physical address.

Indirect delivery occurs when the IP node (either the sending node or an IP router) forwards a packet to an intermediate node (an IP router) because the final destination is not on a directly attached network. The IP node encapsulates the IP datagram in a frame format, addressed to the IP router's physical address, for the Network Interface layer (such as Ethernet or Token Ring).

IP routing is a combination of direct and indirect deliveries.

In Figure 1.14, when sending packets to node B, node A performs a direct delivery. When sending packets to node C, node A performs an indirect delivery to Router 1. Router 1 performs an indirect delivery to Router 2. Router 2 performs a direct delivery to node C.

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Figure 1.14 Direct and Indirect Deliveries

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