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Routing tables contain information used by switching software to select the best route.
Routing algorithms have used many different metrics to determine the best route. Sophisticated routing algorithms can base route selection on multiple metrics, combining them in a single (hybrid) metric. All the following metrics have been used:
•Path length
•Reliability
•Delay
•Bandwidth
•Load
•Communication cost
Path length is the most common routing metric. Some routing protocols allow network administrators to assign arbitrary costs to each network link. In this case, path length is the sum of the costs associated with each link traversed. Other routing protocols define hop count, a metric that specifies the number of passes through internet working products, such as routers, that a packet must take en route from a source to a destination.
Reliability, in the context of routing algorithms, refers to the dependability (usually described in terms of the bit-error rate) of each network link. Some network links might go down more often than others. After a network fails, certain network links might be repaired more easily or more quickly than other links. Any reliability factors can be taken into account in the assignment of the reliability ratings, which are arbitrary numeric values usually assigned to network links by network administrators.
Routing delay refers to the length of time required to move a packet from source to destination through the internetwork. Delay depends on many factors, including the bandwidth of intermediate network links, the port queues at each router along the way, network congestion on all intermediate network links, and the physical distance to be traveled. Because delay is a conglomeration of several important variables, it is a common and useful metric.
Bandwidth refers to the available traffic capacity of a link. All other things being equal, a 10-Mbps Ethernet link would be preferable to a 64-kbps leased line. Although bandwidth is a rating of the maximum attainable throughput on a link, routes through links with greater bandwidth do not necessarily provide better routes than routes through slower links. For example, if a faster link is busier, the actual time required to send a packet to the destination could be greater.
Load refers to the degree to which a network resource, such as a router, is busy. Load can be calculated in a variety of ways, including CPU utilization and packets processed per second. Monitoring these parameters on a continual basis can be resource-intensive itself.
Communication cost is another important metric, especially because some companies may not care about performance as much as they care about operating expenditures. Although line delay may be longer, they will send packets over their own lines rather than through the public lines that cost money for usage time.
Polymorphism means that the same thing can exist in two forms. This is an important characteristic of true object oriented design - which means that one could develop good OO design with data abstraction and inheritance, but the real power of object oriented design seems to surface when polymorphism is used.In programming languages, polymorphism means that some code or operations or objects behave differently in different contexts.For example, the + (plus) operator in C++:4 + 5 <-- integer addition 3.14 + 2.0 <-- floating point addition s1 + "bar" <-- string concatenation! In C++, that type of polymorphism is called overloading.Polymorphism
A classic example is how area is calculated on different shapes. We define an abstact base class shape and derive two classes - rectangle and circle from it. An area() method defined in the base class will have to be implemented differently in rectangle and circle, since it has to be calculated differently.
This is an example of polymorphic behavior, and in C++, this is implemented using virtual member functions. The area() member function is defined virtual in the base class, which signals to the compiler that this member function has to be invoked from the correct derived class at run time. This is indeed a run time decision, and which derived class member function is to be invoked depends on which derived class pointer has been assigned to the base class pointer. This also brings in the concept of late binding (run time binding), since the above association to the area member function can only be done at run time. The compiler will have to insert extra code to do the late binding, which adds in a little runtime overhead.
Types of Polymorphism:C++ provides three different types of polymorphism. Virtual functions Function name overloading Operator overloading
In addition to the above three types of polymorphism, there exist other kinds of polymorphism: run-time compile-time ad-hoc polymorphism parametric polymorphism
Other types of polymorphism defined:run-time: The run-time polymorphism is implemented with inheritance and virtual functions.compile-time: The compile-time polymorphism is implemented with templates.ad-hoc polymorphism: If the range of actual types that can be used is finite and the combinations must be individually specified prior to use, this is called ad-hoc polymorphism.parametric polymorphism: If all code is written without mention of any specific type and thus can be used transparently with any number of new types it is called parametric polymorphism.In general, there are two main categories of Polymorphism namely Ad Hoc Polymorphism Pure Polymorphism
Overloading concepts fall under the category of Ad Hoc Polymorphism and Virtual methods. Templates or parametric classes fall under the category of Pure Polymorphism.
