IP routing is the field of routing methodologies of Internet Protocol (IP) packets within and across IP networks. This involves not only protocols and technologies, but includes the policies of the worldwide organization and configuration of Internet infrastructure. In each IP network node, 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 protocol. 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 choosing the next hop.
A routing protocol is a software mechanism by which routers communicate and share information about the topology of the network, and the capabilities of each routing node. It thus implements the network-global rules by which traffic is directed within a network and across multiple networks. Different protocols are often used for different topologies or different application areas. For example, the Open Shortest Path First (OSPF) protocol is generally used for routing packets between subnetworks within an enterprise and the Border Gateway Protocol (BGP) is used on a global scale. BGP, in particular is the de facto standard of worldwide Internet routing.
The IP forwarding algorithm is a specific implementation of routing for IP networks. In order to achieve a successful transfer of data, the algorithm uses a routing table to select a next-hop router as the next destination for a datagram. The IP address that is selected is known as the next-hop address.
When several destinations are matching, the route with the longest subnet mask is chosen (the most specific one). There can be only one default route.
The IP forwarding algorithm states:
Given a destination IP address, D, and network prefix, N:
if ( N matches a directly connected network address )
Deliver datagram to D over that network link;
else if ( The routing table contains a route for N )
Send datagram to the next-hop address listed in the routing table;
else if ( a default route exists )
Send datagram to the default route;
Send a forwarding error message to the originator;
When no route is available, an ICMP error message is sent to the originator of the packet, to inform that host that the packet could not be delivered, and to avoid unnecessary retransmission to avoid network congestion. The sending host should either stop transmitting, or choose another address or route.
The following presents a typical routing table in a Unix-like operating system:
Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 0.0.0.0 126.96.36.199 0.0.0.0 UG 0 0 0 ppp0 10.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 eth0 188.8.131.52 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0 169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0 172.16.0.0 0.0.0.0 255.240.0.0 U 0 0 0 eth0 192.168.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0 192.168.1.0 192.168.96.1 255.255.255.0 UG 0 0 0 eth0 192.168.96.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
The host has several network interfaces. eth0 is the interface name of the network interface card representing an Ethernet port. ppp0 is a PPPoE interface, which is configured as the default route in this example.
|G||Use Gateway (gateway filled in)|
|H||Target is a Host (bit mask of 32 bits)|
|U||Route is Up|
Examples of routing protocols are the Routing Information Protocol (RIP) implemented in the BSD routed software, the Exterior Gateway Protocol (EGP), and the Border Gateway Protocol (BGP), which is the dominant route distribution protocol used on the Internet.
- Comer, Douglas E. (2000). Internetworking with TCP/IP (4 ed.). Prentice Hall: Upper Saddle River, NJ.