IP Routing and Packet Delivery Process

- The Transmission Medium

The data is first broken into small packets of information, as it cannot be sent whole. The data then travels through a physical route to its destination, commonly consisting of a possible mix of  co-axial cables, a twisted-pair cable, or fibre optics. This physical route of cables is called the transmission medium. Each transmission medium is restricted to a fixed capacity known as the the bandwidth. In order to successfully transmit data, a minimum bandwidth is required which depends on the data being transported. Data being transported at 300 Mb/s, for instance, requires a minimum of 1 gigabit of ethernet transport. 

- IP Routing on the Internet

When an Internet router receives an IP packet, that packet carries no information beyond a destination IP address. There is no instruction on how that packet should get to its destination or how it should be treated along the way. 

Each router has to make an independent forwarding decision for each packet based solely on the packet’s network-layer header. Thus, every time a packet arrives at a router, the router has to “think through” where to send the packet next. The router does this by referring to complex routing tables. 

The process is repeated at each hop along the route until the packet eventually reaches its destination. All of those hops and all of those individual routing decisions result in poor performance for time-sensitive applications like video-conferencing or voice over IP (VoIP). 

- IP Routing and Packet Delivery Process

To minimize unnecessary traffic load and to provide efficient movement of frames from one location to another, the interconnected hosts are grouped into separate networks. As a result of this grouping (which is determined by the network designer and administrator) it is possible for an interconnect device (router) to determine the best path between two networks. 

This interconnect device (router) operates at Layer 3, the Network Layer, forms the boundary between one network and another network. When a frame crosses a router it is in a different network. A frame that travels from source to destination without crossing a router has remained in the same network. A network is a group of communicating machines bounded by routers.

The router will use some of the bits in the IP address to identify the network location to which the frame is destined. The remaining bits in the address will uniquely identify the host on that network that will ultimately receive the frame.

It is necessary to differentiate between the bits used to identify the network and those used to identify the host. The sender of a frame must make this differentiation because it must decide whether it is on the same network as the destination or on a different network. If the sender is on the same network as the destination, it will determine the data link address of the destination machine. Then it will send the frame directly to the destination machine. 

On the other hand, if the destination is on a different then the originator must send the frame to a router and let the router forward the frame on to the ultimate destination network. At the ultimate destination network the last router must determine the data link address of the host and forward the frame directly to that host on that ultimate destination network.

When a router receives an incoming data frame it masks the destination address to create a lookup key that is compared to the entries in its routing table. The routing table indicates how the frame should be processed. 

The frame might be delivered directly on a particular port on the router. The frame might have to be sent on to the next router in line for ultimate delivery to some remote network. The routing table contains this information. The routing table is created by the combination of direct configuration by the administrator or dynamically through the periodic broadcasting of router update frames. Protocols like RIP (Routing Information Protocol), OSPF (Open Shortest Path First), and Cisco’s IGRP (Internet Gateway Routing Protocol) are sent from all routers at periodic intervals. As a result, all routers become aware of how to reach all other networks.

[More to come ...]