Computer Networking

1. Adaptive Routing Algorithm: These algorithms change their routing decisions to reflect changes in the topology and in traffic as well. These get their routing information from adjacent routers or from all routers. The optimization parameters are the distance, number of hops and estimated transit time. This can be further classified as follows:
2. 1. Centralized: In this type some central node in the network gets entire information about the network topology, about the traffic and about other nodes. This then transmits this information to the respective routers. The advantage of this is that only one node is required to keep the information. The disadvantage is that if the central node goes down the entire network is down, i.e. single point of failure.
   2. Isolated: In this method the node decides the routing without seeking information from other nodes. The sending node does not know about the status of a particular link. The disadvantage is that the packet may be send through a congested route resulting in a delay. Some examples of this type of algorithm for routing are:

• Hot Potato: When a packet comes to a node, it tries to get rid of it as fast as it can, by putting it on the shortest output queue without regard to where that link leads. A variation of this algorithm is to combine static routing with the hot potato algorithm. When a packet arrives, the routing algorithm takes into account both the static weights of the links and the queue lengths.
• Backward Learning: In this method the routing tables at each node gets modified by information from the incoming packets. One way to implement backward learning is to include the identity of the source node in each packet, together with a hop counter that is incremented on each hop. When a node receives a packet in a particular line, it notes down the number of hops it has taken to reach it from the source node. If the previous value of hop count stored in the node is better than the current one then nothing is done but if the current value is better then the value is updated for future use. The problem with this is that when the best route goes down then it cannot recall the second best route to a particular node. Hence all the nodes have to forget the stored informations periodically and start all over again.

1. Flooding: Flooding adapts the technique in which every incoming packet is sent on every outgoing line except the one on which it arrived. One problem with this method is that packets may go in a loop. As a result of this a node may receive several copies of a particular packet which is undesirable. Some techniques adapted to overcome these problems are as follows:

• Sequence Numbers: Every packet is given a sequence number. When a node receives the packet it sees its source address and sequence number. If the node finds that it has sent the same packet earlier then it will not transmit the packet and will just discard it.
• Hop Count: Every packet has a hop count associated with it. This is decremented(or incremented) by one by each node which sees it. When the hop count becomes zero(or a maximum possible value) the packet is dropped.
• Spanning Tree: The packet is sent only on those links that lead to the destination by constructing a spanning tree routed at the source. This avoids loops in transmission but is possible only when all the intermediate nodes have knowledge of the network topology.

Flooding is not practical for general kinds of applications. But in cases where high degree of robustness is desired such as in military applications, flooding is of great help.

1. Random Walk: In this method a packet is sent by the node to one of its neighbours randomly. This algorithm is highly robust. When the network is highly interconnected, this algorithm has the property of making excellent use of alternative routes. It is usually implemented by sending the packet onto the least queued link.

Because it shares the key attributes of a computer. We just don't think about many things as "computers" because they aren't like traditional Personal Computers (PCs) or similar general purpose machines. The attributes a router shares with a computer depend on your viewpoint.

From a Networking point of view:

Computer networks don't really care that much about the function of the devices connected to it, that's not their job. A router has a MAC address and has (or is assigned, from another router) an IP address, just like the computers on the network. For all intents an purposes, it is just another computer.

From a Hardware point of view:

A router contains a processor and runs an operating system (firmware). It is programmable (usually). Input/Output (I/O) can be performed. These are the main attributes of what we call a "computer". A router is a computer, just one with a limited intended purpose.

From a layperson point of view:

You can't play solitaire or minesweeper on it. It is not a computer.

The Client-Server Model

The most common model for distributing a system is the client-server model. The model is fairly simple to explain and use.

The name of the model is quite descriptive. In your distributed system you have one or more servers. These servers provide services to other parts of the system, called clients. When a server is started it first opens up a particular port through which clients can access it. It then sits down and waits until somebody (the client) attempts to connect to it. When that happens, the server and client exchange some messages and ultimately of the two close the connection. This connection takes place using so-called sockets.

*The Server*
The simplest version of such a server is non-threaded. That means that multiple connections are handled sequentially, in other words: clients have to queue up and they are handled one by one. That's fine if connections last only very short and if there are not too many clients connecting at the same time, however if you have many clients connecting at the same time or long-lasting connections you have to handle connections in parallel. You can handle multiple connections in parallel using threads. Each time a connection is established a new thread is created and the connection is handled by that new thread. The server thread then continues accepting new connections. Because creating threads is an expensive process (in terms of CPU cycles) threads are usually kept in a "pool". When a thread finished its job, it is kept alive until a new request has arrived it can handle.

*The Client*
For the client to connect it has to know the server's IP or hostname and the port to connect to. Once the connection is established the client and server can exchange messages. Depending on the distributed system a client may connect to multiple servers. One server to access the database, one for file services, another for e-mail, for example.

Regards

Dipak

siliguri

dipak_slg@rediffmail.com

Application testing is a process, usally automated in enterprise and hand driven in video games, where code or people interact with the application under development in an effort to use it and break it.

What happens if you enter a negative floating point number in the address field? Do you get a proper, user understandable answer like "Hey! Jerk! Gimme a street, not -0.3!" or something that requires a programmer to translate: "wrong data type" or, better yet, "failed row update. data server reject: -4017. floats not permitted. abend."

This can be done in data entry programs, like those found in business, by automatedd testing. Testing software 'uses' the software under development.

IN video games, testing is done by humans, often teenagers, who have unlimited time and interest and will work cheaply. No prior knowledge required.

Application testing produces logs which are routed back to programmers for triage and repair. This code, debug, test, recode, retest cycle is completed when it SEEMS there are few enough bugs to try a release candidate.