IP forwarding

IP routing is the field of routing methodologies of (IP) packets within and across IP networks. This involves not only a large array of technologies developed since the installation of TCP/IP on the ARPANET, but includes the policies of the worldwide organization of Internet infrastructure, including the methods and practices of Classless Inter-Domain Routing.

At the technical level in each IP network host, IP routing involves the determination of a suitable path for a network packet from a source to its destination in an IP network. The process uses static configuration rules or dynamically obtained status information to select specific packet forwarding methods to direct traffic to the next available intermediate network node one hop closer to the desired final destination, a total path potentially spanning multiple computer networks.

Networks are separated from each other by specialized hosts, called gateways or routers with specialized software support optimized for routing. In routers, packets arriving at any interface are examined for source and destination addressing and queued to the appropriate outgoing interface according to their destination address and a set of rules and performance metrics. Rules are encoded in a routing table that contains entries for all interfaces and their connected networks. If no rule satisfies the requirements for a network packet, it is forwarded to a default route. Routing tables are maintained either manually by a network administrator, or updated dynamically with a . Routing rules may contain other parameters than source and destination, such as limitations on available bandwidth, expected packet loss rates, and specific technology requirements.

IP forwarding algorithms take into account the size of each packet, the type of service specified in the header, as well as characteristics of the available links to other routers in the network, such as link capacity, utilization rate, and maximum datagram size that is supported on the link. In general, most routing software determines a route through a shortest path algorithm. However, other routing protocols may use other metrics for determining the best path. Based on the metrics required and present for each link, each path has an associated cost. The routing algorithm attempts to minimize the cost when deciding the next hop.

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