Difference between adaptive and non - adaptive algorithms?

- Routing is the process of sending information from one point of network to another. 

- The originating point is called the source and the last point is called the destination. 

- Through the way a number of intermediate nodes might or might not be encountered. 

- Routing is sometimes compared with bridging. 

- Both of these accomplish the same purpose for the casual observer. But it is not so. 

- The basic difference between the two is that the routing is done at the layer 3 i.e., the network layer of the OSI model and the bridging takes place at the layer 2 i.e., data link layer of the OSI model. 

- Because of this distinction, the input supplied to the two processes is different and thus the task of path selection occurs in different ways. 

- The routing algorithm is included as a part of the network layer software. 

- The primary responsibility of this software is to decide on which line the incoming traffic must be forwarded i.e., what will be the next node. 

- Certain metrics are used by the routing protocols for the evaluation of the path that is most appropriate for the transmission of a packet. 

- These metrics include reliability, path bandwidth, current load, delay and so on. 

- These metrics help in determining the optimal path towards a destination. 

- Routing tables are created and maintained by the routing algorithms in order to aid the path determination process.

- The tables will contain what routing information is entirely based up on the routing algorithm that is being used. 

- The routing tables are filled by a variety of information by the routing algorithms. 

- If the internal subnet used is the datagram subnet, then for every datagram that arrives, a new decision has to be taken since the routes keep changing in this case after every transmission.

- On the other hand in virtual circuit subnet, all the decisions are taken with the setting up of the virtual circuit. 

- Once the connection or the links are established, the same path is followed by all the packets. 

The routing algorithms can be classified in to two major categories namely:

1. Non – adaptive algorithms and
2. Adaptive algorithms

- Another name for non – adaptive algorithms is the static algorithms. 

- Here the computation regarding the various routes is done in advance and the same routes are followed by all the packets. 

- The adaptive algorithms are better known as the dynamic algorithms. 

- In this type of algorithms, the routes are not computed in advance, rather the route is decided up on the arrival of a particular packet depending up on the traffic and the topology of the network. 

We have three different types of algorithms under the category of non – adaptive algorithms as mentioned below:

1. Shortest path routing: This algorithm makes use of the Dijkstra’s algorithm for computing the shortest path where nodes and communication links are represented by vertices and edges of the graph respectively.
2. Flooding: Here, the arriving data packet is transmitted on all the outgoing lines save the one on which it arrived. Its selective flooding variation is commonly used.
3. Flow based routing: This algorithm takes in to consideration the present flow of the network before deciding on which line the packet must be transmitted.

And following are some of the adaptive algorithms:

1. Distance vector routing: It requires knowledge about the whole network and is associated with the count  to infinity problem.
2. Link state routing: It requires knowledge about neighborhood.
3. Hierarchical routing: It is used for very large networks.
4. Optimized link state routing: It is used for mobile hosts. 

What are network layer design issues?

- The network layer i.e., the third layer of the OSI model is responsible for facilitating the exchange of the individual information or data pieces between hosts over the network. 

- This exchange only takes place between the end devices that are identified. 

- For accomplishing this task, 4 processes are used by the network layer and these are:

Ø  Addressing

Ø  Encapsulation

Ø  Routing

Ø  Decapsulation

In this article we focus up on the design issues of the network layer. 

- For accomplishing this task, the network layer also need s to have knowledge about the communication subnet’s topology and select the appropriate routes through it. 

- Another thing that the network layer needs to take care of is to select only those routers that do not overload the other routers and the communication lines while leaving the other lines and router in an idle state.

Below mentioned are some of the major issues with the network layer design:

1. Services provided to the layer 4 i.e., the transport layer.
2. Implementation of the services that are connection oriented.
3. Store – and  - forward packet switching
4. Implementation of the services that are not connection oriented.
5. Comparison of the data-gram sub-nets and the virtual circuits.

- The sender host sends the packet to the router that is nearest to it either over a point-to-point carrier link or LAN. 

- The packet is stored until its complete arrival for the verification of the check sum. 

- Once verified, the packet is then transmitted to the next intermediate router. 

- This process continues till the packet has reached its destination. 

- This mechanism is termed as the store and forward packet switching.

The services that are provided to the transport layer are designed based up on the following goals:

1. They should be independent of the router technology.
2. Shielding from the type, number and topology of the routers must be provided to the transport layer.
3. The network addresses that are provided to the transport layer must exhibit a uniform numbering plan irrespective of whether it’s a LAN or a WAN.

Now based up on the type of services that are offered, there is a possibility for two different organizations.

Offered service is Connection-less: 

- The packets are individually introduced in to the sub-net and the routing of the packets is done independently of each other. 

- It does not require any advance set up. 

- The sub-net is referred to as the data gram sub-net and the packets are called data-grams.

Offered service is connection-oriented: 

- In this case the router between the source and the destination must be established prior to the beginning of the transmission of the packets. 

- Here, the connection is termed as the virtual circuit and subnet as the “virtual circuit subnet” or simply VC subnet.

- Choosing a new router every time is a thing to be avoided and this is the basic idea behind the use of the virtual circuits. 

- Whenever we establish a connection, a route has to be selected from source to destination. 

- This is counted as a part of the connection setup only. 

- This route is saved in the routers tables that are managed by the routers and is then used by the flowing traffic. 

- On the release of connection, the VC is automatically terminated. 

- In case of the connection oriented service, an identifier is contained in each packet which tells the virtual circuit to which it belongs.

- In data-gram sub-net circuit setup is not required whereas it is required in the VC circuit. 

- The state info is not held by the routers in the data gram subnet whereas router table space is required for each VC for each connection.