Delivery schemes
Routing schemes differ in how they deliver messages: Unicast is the dominant form of message delivery on the Internet. This article focuses on unicast routing algorithms.Topology distribution
WithDistance vector algorithms
Distance vector algorithms use theLink-state algorithms
When applying link-state algorithms, a graphical map of the network is the fundamental data used for each node. To produce its map, each node floods the entire network with information about the other nodes it can connect to. Each node then independently assembles this information into a map. Using this map, each router independently determines the least-cost path from itself to every other node using a standard shortest paths algorithm such asOptimized Link State Routing algorithm
A link-state routing algorithm optimized for mobile ad hoc networks is the optimized 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. 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 (Internet), 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 significant resources to calculate routing tables. It also creates heavy traffic due to flooding. Path-vector routing is used for inter-domain routing. It is similar to distance vector routing. Path-vector routing assumes that one node (there can be many) in each autonomous system 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. The path-vector routing algorithm is 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. The path, expressed in terms of the domains (or confederations) traversed so far, is carried in a special path attribute that records the sequence of routing domains through which the reachability information has passed. 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; The routers receive a vector that contains paths to a set of destinations.Path selection
Path selection involves applying a Metrics (networking), routing metric to multiple routes to select (or predict) the best route. Most routing algorithms use only one network path at a time. Multipath routing and specifically equal-cost multi-path routing techniques enable the use of multiple alternative paths. In computer networking, the metric is computed by a routing algorithm, and can cover information such as Bandwidth (computing), bandwidth, network delay, hop count, path cost, load, maximum transmission unit, reliability, and communication cost. The routing table stores only the best possible routes, while Link-state routing protocol, link-state or topological databases may store all other information as well. In case of overlapping or equal routes, algorithms consider the following elements in priority order to decide which routes to install into the routing table: #''Prefix length'': A matching route table entry with a longer subnet mask is always preferred as it specifies the destination more exactly. #''Metrics (networking), Metric'': When comparing routes learned via the same routing protocol, a lower metric is preferred. Metrics cannot be compared between routes learned from different routing protocols. #''Administrative distance'': When comparing route table entries from different sources such as different routing protocols and static configuration, a lower administrative distance indicates a more reliable source and thus a preferred route. Because a routing metric is specific to a given routing protocol, multi-protocol routers must use some external heuristic to select between routes learned from different routing protocols. Cisco routers, for example, attribute a value known as the administrative distance to each route, where smaller administrative distances indicate routes learned from a protocol assumed to be more reliable. A local administrator can set up host-specific routes that provide more control over network usage, permits testing, and better overall security. This is useful for debugging network connections or routing tables. In some small systems, a single central device decides ahead of time the complete path of every packet. In some other small systems, whichever edge device injects a packet into the network decides ahead of time the complete path of that particular packet. In either case, the route-planning device needs to know a lot of information about what devices are connected to the network and how they are connected to each other. Once it has this information, it can use an algorithm such as A* search algorithm to find the best path. In high-speed systems, there are so many packets transmitted every second that it is infeasible for a single device to calculate the complete path for each and every packet. Early high-speed systems dealt with this with circuit switching by setting up a path once for the first packet between some source and some destination; later packets between that same source and that same destination continue to follow the same path without recalculating until the circuit teardown (communications), teardown. Later high-speed systems inject packets into the network without any one device ever calculating a complete path for packets. In large systems, there are so many connections between devices, and those connections change so frequently, that it is infeasible for any one device to even know how all the devices are connected to each other, much less calculate a complete path through them. Such systems generally use hop (networking)#Next hop, next-hop routing. Most systems use a deterministic dynamic routing algorithm. When a device chooses a path to a particular final destination, that device always chooses the same path to that destination until it receives information that makes it think some other path is better. A few routing algorithms do not use a deterministic algorithm to find the best link for a packet to get from its original source to its final destination. Instead, to avoid congestion hot spots in packet systems, a few algorithms use a randomized algorithm—Valiant's paradigm—that routes a path to a randomly picked intermediate destination, and from there to its true final destination. In many early telephone switches, a randomizer was often used to select the start of a path through a 1ESS switch#Switching fabric, multistage switching fabric. Depending on the application for which path selection is performed, different metrics can be used. For example, for web requests one can use minimum latency paths to minimize web page load time, or for bulk data transfers one can choose the least utilized path to balance load across the network and increase throughput. A popular path selection objective is to reduce the average completion times of traffic flows and the total network bandwidth consumption. Recently, a path selection metric was proposed that computes the total number of bytes scheduled on the edges per path as selection metric. An empirical analysis of several path selection metrics, including this new proposal, has been made available.Multiple agents
In some networks, routing is complicated by the fact that no single entity is responsible for selecting paths; instead, multiple entities are involved in selecting paths or even parts of a single path. Complications or inefficiency can result if these entities choose paths to optimize their own objectives, which may conflict with the objectives of other participants. A classic example involves traffic in a road system, in which each driver picks a path that minimizes their travel time. With such routing, the Nash equilibrium, equilibrium routes can be longer than optimal for all drivers. In particular, Braess's paradox shows that adding a new road can ''lengthen'' travel times for all drivers. In a single-agent model used, for example, for routing automated guided vehicles (AGVs) on a terminal, reservations are made for each vehicle to prevent simultaneous use of the same part of an infrastructure. This approach is also referred to as context-aware routing. The Internet is partitioned into autonomous system (Internet), autonomous systems (ASs) such as internet service providers (ISPs), each of which controls routes involving its network. Routing occurs at multiple levels. First, AS-level paths are selected via the Border Gateway Protocol, BGP protocol that produces a sequence of ASs through which packets flow. Each AS may have multiple paths, offered by neighboring ASs, from which to choose. These routing decisions often correlate with business relationships with these neighboring ASs, which may be unrelated to path quality or latency. Second, once an AS-level path has been selected, there are often multiple corresponding router-level paths to choose from. This is due, in part, because two ISPs may be connected through multiple connections. In choosing the single router-level path, it is common practice for each ISP to employ hot-potato routing: sending traffic along the path that minimizes the distance through the ISP's own network—even if that path lengthens the total distance to the destination. For example, consider two ISPs, ''A'' and ''B''. Each has a presence in New York City, New York, connected by a fast link with latency —and each has a presence in London connected by a 5 ms link. Suppose both ISPs have trans-Atlantic links that connect their two networks, but ''A''Route analytics
As the Internet and IP networks have become mission critical business tools, there has been increased interest in techniques and methods to monitor the routing posture of networks. Incorrect routing or routing issues cause undesirable performance degradation, route flapping, flapping or downtime. Monitoring routing in a network is achieved using route analytics tools and techniques.Centralized routing
In networks where a logically centralized control is available over the forwarding state, for example, using software-defined networking, routing techniques can be used that aim to optimize global and network-wide performance metrics. This has been used by large internet companies that operate many data centers in different geographical locations attached using private optical links, examples of which include Microsoft's Global WAN, Facebook's Express Backbone, and Google's B4. Global performance metrics to optimize include maximizing network utilization, minimizing traffic flow completion times, maximizing the traffic delivered prior to specific deadlines and reducing the completion times of flows. Work on the later over private WAN discusses modeling routing as a graph optimization problem by pushing all the queuing to the end-points. The authors also propose a heuristic to solve the problem efficiently while sacrificing negligible performance.See also
* Collective routing * Deflection routing * Edge disjoint shortest pair algorithm * Flood search routing * Fuzzy routing * Geographic routing * Heuristic routing * Path computation element (PCE) * Policy-based routing * Wormhole routing * Small-world routing * Turn restriction routingReferences
Further reading
*External links