2. [Intra-Domain Routing] (12 marks) a) (6 marks) So far in this lecture series,

Question

2. [Intra-Domain Routing] (12 marks) a) (6 marks) So far in this lecture series, for intra domain routing scenarios, we have assumed that the shortest path is the least cost path where cost may be defined in terms of number of hops, delay etc. This scheme simply builds the overall cost of a path by adding individual link costs along the path until all links for the path are considered. There are many networking environments in which this strategy of selecting the best path using least cost is not applicable. Identify and explain one scenario where this routing strategy may not work. (200 words)

Explanation / Answer

Intradomain routing protocols support one of the two planses, control and forwarding, are needed to deliver IP packets over a network. Intradomain routing protocols provide information to the control plane, which determines and chooses the path to a destination based on metrics such as number of hops, delay, and bandwidth. The forwarding process does the actual insertion of the IP packet into a frame and forwards the frame to the next hop.

The major intradomain routing protocols in current use are:

Shortest path routing refers to the process of finding paths through a network that have a minimum of distance or other cost metric. Routing of data packets on the Internet is an example involving millions of routers in a complex, worldwide, multilevel network. Optimum routing on the Internet has a major impact on performance and cost.

This article will explain a basic routing algorithm [1] commonly used in routing protocols for small to mid-sized networks. A firm understanding of this algorithm will help in studying these protocols.

The algorithm is presented here in Python, a computer language designed for maximum readability. Computer networks texts often use pseudocode or C to explain algorithms. The problem with pseudocode is it can give you a temporary feeling of understanding, which is lost when you try to actually implement the algorithm. Then you may stumble on the ambiguities you didn't notice in the pseudocode, or find that real programs just don't work that way. The problem with C is that it is too low level. It's great for speed and efficiency, but you may get lost in the details of pointers and indices. You can follow every statement, and still not understand the algorithm. If you are not familiar with Python, see Dijkstra59.py for a more heavily-annotated version of this program, or Dijkstra59.c for the same thing in C.

Limitations

The main limitations of simple shortest-path routing have to do with real-world problems that occur in large networks. We can't just keep adding nodes to a huge routing table at each and every node. As shown in Figure 2, the time to build (or re-build) a table increases as the square of the number of nodes. Also, as nodes are added, the number of failing links, changes in topology, and other events that trigger re-builds throughout the network - these events will occur more frequently.

Aside from these technical limitations, there are administrative headaches that come with huge networks, and these often set a size limit far short of what is technically possible. A campus-wide network might have 100 nodes with only two connections to the outside world, and no desire to keep track of nodes for businesses in the same city. Those businesses might be better served by a city-wide network that includes one of the campus "border" routers. The city-wide network might include a few hundred nodes that all have routing table entries for each other, but only one entry for all the nodes on campus. Similarly, the city network might look like just one node in a larger regional network.Hierarchical routing is one way to partition a network into smaller, more manageable pieces.

A routing algorithm is not enough to design a network. We need a complete routing protocol to deal with real-world issues. The Routing protocol articles will discuss how we handle issues such as:

partitioning - strictly hierarchical is not the only possibility
rapid recovery - minimize the time that routing tables are "out of sync" with actual topology
preferential routing - voice packets must not have a noticeable delay
load balancing - don't overload the shortest path
security - keep the bad guys from diverting traffic
policy overrides - block our competitors, even if they are coming through one of our customers

Disadvantages

Shortest Paths Routing Problem in MANETs

: The need for communication services is rapidly increasing, because the mobile communication service is synonymous with an ideal communication style realizing communication anytime, anywhere and with anyone. The availability of a path depends on the number of links and the reliability of each link forming the path. Many routing metrics in terms of number of links have been proposed, such as the shortest path routing. Shortest path routing selects a path having minimum cost to forward the data to the destination node. Shortest path routing algorithm selection depends on direct traffic form source to destination, maximizing the network performance and minimizing the cost. Performance of the network can be enhanced through shortest path routing but it also depends upon the functionality of the routing protocol and the parameters that are selected for the shortest path routing. The primary goal of such an adhoc network routing protocol is correct and efficient route establishment between a pair of nodes so that messages may be delivered in a timely manner. Route construction should be done with a minimum of cost, overhead and bandwidth consumption. Some of researchers explored the concept of shortest path routing over ad hoc network. Each one uses his own parameters with different topology. No one uses all parameters.

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