IP Routing

Routing is the process of moving a packet of data from the source host network, across one or more gateway systems, to the destination host network. The terms Internet Gateway and IP Router are commonly used interchangeably. These terms refer to a device that moves an IP datagram from one network to another based on the destination address. An IP router can be a standalone device that only forwards datagrams between networks. It might also be a host computer that is connected to more than one network that forwards IP datagrams in addition to running other applications. When a packet is transmitted from one application to another across one or more gateways, the data is processed through all four layers of the protocol stack by the originating and destination systems. The intermediate systems process the packets only up to the Internet Layer where the routing decisions are made. Systems using TCP/IP can only deliver packets to devices that are connected to the same physical network. If the destination of a packet is not the same physical network as the source then it must be passed to a gateway which is on the same physical network for routing. If the destination is on a network connected to the gateway then the packet is delivered. If the destination is not on a network connected to the gateway then the gateway will forward the packet to another gateway. Typically an IP router has two or more networks connected to it. For each network connected to the router there is a defined range of valid IP addresses which define what devices are local to that network. A router normally has a table which defines other gateway systems that it can reach on the networks to which it is connected. Each of these table entries identifies a range of IP addresses and lists the gateway to use to reach these addresses. This table is used by the routing system to determine where to send packets that are addressed to destinations that are not on a locally connected network. There is one special routing table entry called a Default Gateway. This entry essentially says "If there is no other route defined for a destination, send the packet to this gateway for delivery". If the routing system processes the routing table and cannot find any other entry that matches the destination for a packet, it passes the packet to the default gateway. On a network that is connected to the Internet, the default gateway is usually the address of the router that connects the network to the Internet Service Provider. Internet Routing Architecture Traditional Internet routing is based on a hierarchical structure. The top of the structure is a single system called the core and the gateways that are connected to the core are called core gateways or core routers. In this traditional structure all routing information is gathered at the core, processed there, then passed back to the core gateways and down the hierarchy. The protocol used to exchange information with core gateways is called Gateway-to-Gateway Protocol or GGP. GGP is only used by core gateways. Connected to the core are independent networks called autonomous systems, each of which is identified by an autonomous system number (ASN). An autonomous system is a specific TCP/IP term that means a collection of networks and gateways with an independent system for managing their own routing information and for exchanging this information with other autonomous systems. The information exchanged between autonomous systems is called reachability information, and it defines which networks can be reached through that autonomous system. The protocol used to pass reachability information between autonomous systems is called Exterior Gateway Protocol or EGP. The core hierarchical structure requires all routing information to be processed at the central location, and routing decisions to made by the core. This places a large burden on the core system and does not scale well in the rapidly expanding environment of the Internet. A newer routing scheme, called Domain Routing has replaced the core model for the majority of the Internet. A routing domain is similar to an autonomous system under the core model, but each routing domain may connect to other routing domains, exchanging packets and routing information, at more than one point. Unlike the core model, each routing domain makes its own routing decisions rather than relying on a single central system to choose the "best" route. A gateway that connects two or more routing domains is called a border gateway, and the protocol used to exchange routing information between routing domains is called Border Gateway Protocol or BGP. There are several primary points in North America where major routing domains meet. These central meeting points were originally ethernet network segments with border gateway routers from each major routing domain. These border gateway routers exchange routing information and data packets across the network. The name of two of these centers was derived from the use of ethernet network segments: the Metro Area Ethernet - East (MAE-East) on the east coast, and the Metro Area Ethernet - West (MAE-West) on the west coast. As technology advanced and the need for higher bandwidth grew, higher speed network segments were added to these networking centers, such as 100 Megabit Ethernet, FDDI, HSSI, and ATM. The name Metro Area Ethernet was replaced by the more generic term National Access Point or NAP. The third primary domain network meeting point is in Chicago and is usually referred to as simply the Chicago NAP. These three NAPs exchange routing information and data packets via high speed communications lines and currently form important pieces of the primary Internet backbone in North America. As the Internet expands, more NAPs will be brought on line to handle the growing networking needs.