Hello Everyone!
Today, we will briefly discuss about the concept of forwarding and routing in network layer.
The role of the network layer is deceptively simple—to move packets from a sending host to a receiving host. To do so, two important network-layer functions can be identified:
• Forwarding. When a packet arrives at a router’s input link, the router must move the packet to the appropriate output link. For example, a packet arriving from Host H1 to Router R1 must be forwarded to the next router on a path to H2.
• Routing. The network layer must determine the route or path taken by packets as they flow from a sender to a receiver. The algorithms that calculate these paths are referred to as routing algorithms. A routing algorithm would determine, for
example, the path along which packets flow from H1 to H2.
The terms forwarding and routing are often used interchangeably by authors discussing the network layer. Forwarding refers to the router-local action of transferring a packet from an input link interface to the appropriate output link interface. Routing refers to the network-wide process that determines the end-to-end paths that packets take from source to destination.
Every router has a forwarding table. A router forwards a packet by examining the value of a field in the arriving packet’s header, and then using this header value to index into the router’s forwarding table. The value stored in the forwarding table entry for that header indicates the router’s outgoing link interface to which that packet is to be forwarded. Depending on the network-layer protocol, the header value could be the destination address of the packet or an indication of the connection to which the packet belongs. The below figure provides an example.
Figure. Routing algorithms determine values in forwarding tables
In the above figure, a packet with a header field value of 0111 arrives to a router. The router indexes into its forwarding table and determines that the output link interface for this packet is interface 2. The router then internally forwards the packet to interface 2.
You might now be wondering how the forwarding tables in the routers are configured. This is a crucial issue, one that exposes the important interplay between routing and forwarding. As shown in the above figure, the routing algorithm determines the values that are inserted into the routers’ forwarding tables.
The routing algorithm may be centralized (e.g., with an algorithm executing on a central site and downloading routing information to each of the routers) or decentralized (i.e., with a piece of the distributed routing algorithm running in each router). In either case, a router receives routing protocol messages, which are used to configure its forwarding table.
The distinct and different purposes of the forwarding and routing functions can be further illustrated by considering the hypothetical (and unrealistic, but technically feasible) case of a network in which all forwarding tables are configured directly by human network operators physically present at the routers.
In this case, no routing protocols would be required! Of course, the human operators would need to interact with each other to ensure that the forwarding tables were configured in such a way that packets reached their intended destinations. It’s also likely that human configuration would be more error-prone and much slower to respond to changes in the network topology than a routing protocol. We’re thus fortunate that all networks have both a forwarding and a routing function!
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