When a host using a routable protocol wants to send data to another host, it must first obtain the internetwork address of the destination. The destination internetwork address is obtained through an address resolution process whereby the sending host obtains the destination internetwork address by referencing its logical name. For example, TCP/IP hosts use Domain Name System (DNS) name resolution to resolve a DNS domain name to an IP address. Novell NetWare workstations query the bindery (a database stored on a NetWare server) or directory tree of their default server to resolve a server name to its Internetwork Packet Exchange (IPX) internetwork address.
Once the destination internetwork address has been obtained, the source network and the destination network addresses are compared. When the source and destination hosts are on the same network, the packets are sent directly to the destination host by the source without the use of a router (see Figure 1.3). The source host sends the packet to the destination by addressing the packet to the destination's physical address. This is known as a direct delivery. In a direct delivery, the destination internetwork address and the destination physical address are for the same end system.
Conversely, when the source and destination hosts are on different networks, the packets to the destination cannot be directly delivered by the source. Instead, the source delivers them to an intermediate router (see Figure 1.3) by addressing the packet to the router's physical address. This is known as an indirect delivery. In an indirect delivery, the destination internetwork address and the destination physical address are not for the same end system.
During an indirect delivery, the sending host forwards the packet to a router on its network by determining the router corresponding to the first hop or by discovering the entire path from the source to the destination.
Figure 1.3 Host Routing Process
Host Determination of the First Hop
IP and IPX sending hosts determine the physical address of the first hop router using one of the following processes:
Host routing table A routing table on the host yields the forwarding address of the router to be used to reach the desired destination network ID. An example is the IP routing table on a Microsoft TCP/IP host. See "Routing Tables" later in this chapter for a detailed definition of a routing table.
Dynamic updates of host routing table TCP/IP has a facility to dynamically update the host routing table with better routes, as packets are sent to destinations. The Internet Control Message Protocol (ICMP) Redirect message is sent by an IP router to a sending host informing it of a better route to a destination host. The better route becomes a host route in the routing table. TCP/IP for Windows 2000 supports the dynamic update of the IP routing table based on the receipt of the ICMP Redirect message.
Eavesdropping TCP/IP hosts have the ability to listen to the routing protocol traffic used by routers. This is known as eavesdropping or wiretapping. Eavesdropping hosts have the same routing information as the routers. An example of eavesdropping is Silent RIP. Silent RIP is the ability of a TCP/IP host to listen to RIP for IP routing traffic exchanged by RIP routers and update its routing table. Microsoft® Windows NT® Server 3.51 and Service Pack 2 and later, Microsoft® Windows NT® Workstation 4.0 and Service Pack 4 and later support Silent RIP.
Default route To simplify the configuration of hosts and routers and to reduce the overhead associated with each host having routes for all the networks in the internetwork, a sending host is configured with a single default route. The default route and its forwarding address to the default router are used when no other routes to the destination network are found. The Default Gateway for TCP/IP hosts is a default router.
Querying the network for the best route For hosts without a routing table or a configured default router, the sending host can determine the physical address of the first hop router by querying the routers on the network. A query for the best route to a specified destination network address is sent as a broadcast or multicast packet. The responses from the routers are analyzed by the sending host, and the best router is chosen. An example of this querying process is the RIP GetLocalTarget message sent by an IPX host. The Routing Information Protocol (RIP) GetLocalTarget message contains a desired destination IPX network ID. IPX routers on the sending host's network that can reach the destination IPX network ID send a response to the sending host. Based on the RIP responses from the local routers, the sending host chooses the best router to forward the IPX packet.
Host Determination of the Entire Path
When using some routable protocols, the sending host does more than determine the first hop. The source host goes through a route discovery process and determines the path between the sending host and the destination. The list of networks or routers is then included in the Network layer header and is used by the routers to forward the packet along the indicated path. This process is known as source routing.
In source routing, the routers are only acting as store and forward devices because the routing decisions have already been made by the sending host. Source routing is not typically implemented as a method of routing because the path either needs to be known or discovered. Source route discovery processes tend to be traffic intensive and slow. IP routing is normally done through routing decisions made by sending hosts and IP routers based on local routing tables. However, in network testing and debugging situations, it is sometimes desired to specify an exact route through the IP internetwork that overrides the path that would normally be taken. This is known as IP source routing.
In IP source routing, the entire route is specified by the sending host through the IP addresses of successive IP routers between the source and destination. At each IP router, the IP datagram is addressed to the next router using the Destination IP address field of the IP header.
IP supports two types of source routing. The first type is loose source routing, in which the IP address of the next router can be one or more routers away (multiple hops). The second type is strict source routing, in which the next router must be a neighboring router (single hop).
Token Ring source routing is a Media Access Control (MAC)–sublayer routing scheme and does not apply to the internetwork-based source routing discussed earlier.