Cheap VPS -Routing,Topology distribution

Topology distribution

In a practice known as static routing (or non-adaptive routing), small networks may use manually configured routing tables. Larger networks have complex topologies that can change rapidly, making the manual construction of routing tables unfeasible. Nevertheless, most of the public switched telephone network (PSTN) uses pre-computed routing tables, with fallback routes if the most direct route becomes blocked (see routing in the PSTN). Adaptive routing, or dynamic routing, attempts to solve this problem by constructing routing tables automatically, based on information carried by routing protocols, and allowing the network to act nearly autonomously in avoiding network failures and blockages.

Examples of adaptive-routing algorithms are the Routing Information Protocol (RIP) and the Open-Shortest-Path-First protocol (OSPF). Adaptive routing dominates the Internet. However, the configuration of the routing protocols often requires a skilled touch; networking technology has not developed to the point of the complete automation of routing.

Distance vector algorithms:

Distance vector algorithms use the Bellman-Ford algorithm. This approach assigns a number, the cost, to each of the links between each node in the network. Nodes will send information from point A to point B via the path that results in the lowest total cost .

The algorithm operates in a very simple manner. When a node first starts, it only knows of its immediate neighbours, and the direct cost involved in reaching them. (This information, the list of destinations, the total cost to each, and the next hop to send data to get there, makes up the routing table, or distance table.) Each node, on a regular basis, sends to each neighbour its own current idea of the total cost to get to all the destinations it knows of. The neighbouring node examine this information, and compare it to what they already 'know'; anything which represents an improvement on what they already have, they insert in their own routing table. Over time, all the nodes in the network will discover the best next hop for all destinations, and the best total cost.

When one of the nodes involved goes down, those nodes which used it as their next hop for certain destinations discard those entries, and create new routing-table information. They then pass this information to all adjacent nodes, which then repeat the process. Eventually all the nodes in the network receive the updated information, and will then discover new paths to all the destinations which they can still "reach".

Link-state algorithms:

When applying link-state algorithms, each node uses as its fundamental data a map of the network in the form of a graph. To produce this, each node floods the entire network with information about what other nodes it can connect to, and each node then independently assembles this information into a map. Using this map, each router then independently determines the least-cost path from itself to every other node using a standard shortest paths algorithm such as Dijkstra's algorithm. The result is a tree rooted at the current node such that the path through the tree from the root to any other node is the least-cost path to that node. This tree then servers to construct the routing table, which specifies the best next hop to get from the current node to any other node.

Optimised Link State Routing algorithm:

A link-state routing algorithm optimised for mobile ad-hoc networks is the Optimised Link State Routing Protocol (OLSR). OLSR is proactive; it uses Hello and Topology Control (TC) messages to discover and disseminate link state information through the mobile ad-hoc network(Server). Using Hello messages, each node discovers 2-hop neighbor information and elects a set of multipoint relays (MPRs). MPRs distinguish OLSR from other link state routing protocols.

Path vector protocol:

Distance vector and link state routing are both intra-domain routing protocols. They are used inside an autonomous system, but not between autonomous systems. Both of these routing protocols become intractable in large networks and cannot be used in Inter-domain routing. Distance vector routing is subject to instability if there are more than a few hops in the domain. Link state routing needs huge amount of resources to calculate routing tables. It also creates heavy traffic because of flooding.

Path vector routing is used for inter-domain routing. It is similar to distance vector routing. In path vector routing we assume there is one node (server) in each autonomous system which acts on behalf of the entire autonomous system. This node is called the speaker node. The speaker node creates a routing table and advertises it to neighboring speaker nodes in neighboring autonomous systems. The idea is the same as distance vector routing except that only speaker nodes in each autonomous system can communicate with each other. The speaker node advertises the path, not the metric of the nodes, in its autonomous system or other autonomous systems. Path vector routing is discussed in RFC 1322; the path vector routing algorithm is somewhat similar to the distance vector algorithm in the sense that each border router advertises the destinations it can reach to its neighboring router. However, instead of advertising networks in terms of a destination and the distance to that destination, networks are advertised as destination addresses and path descriptions to reach those destinations. A route is defined as a pairing between a destination and the attributes of the path to that destination, thus the name, path vector routing, where the routers receive a vector that contains paths to a set of destinations. The path, expressed in terms of the domains (or Server) traversed so far, is carried in a special path attribute that records the sequence of routing domains through which the reachability information has passed.

Comparison of routing algorithms:

Distance-vector routing protocols are simple and efficient in small networks and require little, if any, management. However, traditional distance-vector algorithms have poor convergence properties due to the count-to-infinity problem.

This has led to the development of more complex but more scalable algorithms for use in large networks. Interior routing mostly uses link-state routing protocols such as OSPF and IS-IS.

A more recent development is that of loop-free distance-vector protocols (e.g., EIGRP). Loop-free distance-vector protocols are as robust and manageable as naive distance-vector protocols, but avoid counting to infinity, and have good worst-case convergence times.

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