Definición y significado de Routing

A rout is commonly defined as a chaotic and disorderly retreat or withdrawal of troops from a battlefield, resulting in the victory of the opposing party, or following defeat, a collapse of discipline, or poor morale. A routed army often degenerates into a sense of "every man for himself" as the surviving combatants attempt to flee to safety. A disorganized rout often results in much higher casualties for the retreating force than an orderly withdrawal. On many occasions, more soldiers are killed in the rout than in the actual battle. Normally, though not always, routs either effectively end a battle, or provide the decisive victory the winner needs to gain the momentum with which to end a battle (or even campaign) in their favor. The opposite of a rout is a rally, in which a military unit that has been giving way and is on the verge of being routed suddenly gathers itself and turns back to the offensive.

  History

Historically, lightly equipped soldiers such as auxiliaries, light cavalry, partisans or militia were important when pursuing a fast-moving, defeated enemy force, and could often keep up the pursuit into the following day, causing the routed army heavy casualties or total dissolution. The slower moving heavy forces could then either seize objectives or pursue at leisure. However, with the advent of armoured warfare and blitzkrieg style operations, an enemy army could be kept more or less in a routed or disorganized state for days or weeks on end.

  Tactics

Routs may be feigned to entice an enemy into pursuing the "retreating" force, with the intent of causing the enemy to abandon a strong defensive position or leading the enemy into a prepared ambush. However this carries some risk; a feigned route can quickly turn into a real one. It is thought that Breton cavalry performed this maneuver at the Battle of Hastings.

  Other uses of the term

A rout is also a synonym for an overwhelming defeat as well as a verb meaning "to put to disorderly retreat" or "to defeat utterly", and is often used in sports to describe a blowout.

In law, a rout is a disturbance of the public peace by three or more persons acting together in a manner that suggests an intention to riot, although they do not actually carry out the intention.^{[citation needed]}

Rout is personified as the eponymous deity in Homer's Iliad as the cowardly son of Ares.

  See also

• Crowd psychology

  References

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2006)

Look up route in Wiktionary, the free dictionary.

Route may refer to:

• Route or thoroughfare for transportation
• Route number or road number
• Trade route, a commonly used path for the passage of goods
• Scenic route, a thoroughfare designated as scenic based on the scenery through which it passes
• Route, County Antrim, an area in Northern Ireland
• route (command), in computing, a program used to configure the routing table
• Route, a term used in woodworking. To cut a channel or groove

  See also

• Acronyms and abbreviations in avionics
• GPS route, a series of one or more GPS waypoints
• Routing (disambiguation)
• Rout

This disambiguation page lists articles associated with the same title. If an internal link led you here, you may wish to change the link to point directly to the intended article.

Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network (Circuit switching), electronic data networks (such as the Internet), and transportation networks. This article is concerned primarily with routing in electronic data networks using packet switching technology.

In packet switching networks, routing directs packet forwarding, the transit of logically addressed packets from their source toward their ultimate destination through intermediate nodes, typically hardware devices called routers, bridges, gateways, firewalls, or switches. General-purpose computers can also forward packets and perform routing, though they are not specialized hardware and may suffer from limited performance. The routing process usually directs forwarding on the basis of routing tables which maintain a record of the routes to various network destinations. Thus, constructing routing tables, which are held in the router's memory, is very important for efficient routing. Most routing algorithms use only one network path at a time, but multipath routing techniques enable the use of multiple alternative paths.

Routing, in a more narrow sense of the term, is often contrasted with bridging in its assumption that network addresses are structured and that similar addresses imply proximity within the network. Because structured addresses allow a single routing table entry to represent the route to a group of devices, structured addressing (routing, in the narrow sense) outperforms unstructured addressing (bridging) in large networks, and has become the dominant form of addressing on the Internet, though bridging is still widely used within localized environments.

  Delivery semantics

Routing schemes differ in their delivery semantics:

• unicast delivers a message to a single specific node;
• broadcast delivers a message to all nodes in the network;
• multicast delivers a message to a group of nodes that have expressed interest in receiving the message;
• anycast delivers a message to any one out of a group of nodes, typically the one nearest to the source.
• geocast delivers a message to a geographic area

Unicast is the dominant form of message delivery on the Internet, and this article focuses on unicast routing algorithms.

  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.^{[citation needed]}

  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 (i.e. the sum of the costs of the links between the nodes used).

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(s) 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(s). 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 serves 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).^[1] 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, 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 due to 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 (there can be many) 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 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.

  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.

  Path selection

Path selection involves applying a routing metric to multiple routes, in order to select (or predict) the best route.

In the case of computer networking, the metric is computed by a routing algorithm, and can cover such information as bandwidth, network delay, hop count, path cost, load, MTU, reliability, and communication cost (see e.g. this survey for a list of proposed routing metrics). The routing table stores only the best possible routes, while link-state or topological databases may store all other information as well.

Because a routing metric is specific to a given routing protocol, multi-protocol routers must use some external heuristic in order to select between routes learned from different routing protocols. Cisco's routers, for example, attribute a value known as the administrative distance to each route, where smaller administrative distances indicate routes learned from a supposedly more reliable protocol.

A local network administrator, in special cases, can set up host-specific routes to a particular machine which provides more control over network usage, permits testing and better overall security. This can come in handy when required to debug network connections or routing tables.

  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 which minimizes their own travel time. With such routing, the equilibrium routes can be longer than optimal for all drivers. In particular, Braess paradox shows that adding a new road can lengthen travel times for all drivers.

In another model, for example used 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.^[2]

The Internet is partitioned into autonomous systems (ASs) such as internet service providers (ISPs), each of which has control over routes involving its network, at multiple levels. First, AS-level paths are selected via the BGP protocol, which produces a sequence of ASs through which packets will flow. Each AS may have multiple paths, offered by neighboring ASs, from which to choose. Its decision often involves business relationships with these neighboring ASs,^[3] 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, in part because two ISPs may be connected in multiple locations. 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.

Consider two ISPs, A and B, which each have a presence in New York, connected by a fast link with latency 5 ms; and which each have a presence in London connected by a 5 ms link. Suppose both ISPs have trans-Atlantic links connecting their two networks, but A's link has latency 100 ms and B's has latency 120 ms. When routing a message from a source in A's London network to a destination in B's New York network, A may choose to immediately send the message to B in London. This saves A the work of sending it along an expensive trans-Atlantic link, but causes the message to experience latency 125 ms when the other route would have been 20 ms faster.

A 2003 measurement study of Internet routes found that, between pairs of neighboring ISPs, more than 30% of paths have inflated latency due to hot-potato routing, with 5% of paths being delayed by at least 12 ms. Inflation due to AS-level path selection, while substantial, was attributed primarily to BGP's lack of a mechanism to directly optimize for latency, rather than to selfish routing policies. It was also suggested that, were an appropriate mechanism in place, ISPs would be willing to cooperate to reduce latency rather than use hot-potato routing.^[4]

Such a mechanism was later published by the same authors, first for the case of two ISPs^[5] and then for the global case.^[6]

  Route analytics

As the Internet and IP networks 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, flapping and/or downtime. Monitoring routing in a network is achieved using Route analytics tools and techniques.

  See also

  Routing algorithms and techniques

  Routing in specific networks

• National Routeing Guide: passenger routing in the UK rail network
• Open Source Routing Machine (Road map routing)
• Route assignment in transportation networks
• Routing in the PSTN
• Small world routing - the internet is approximately a small world network

  Routing protocols

• Routing protocol
• Classless inter-domain routing (CIDR)
• MPLS routing
• ATM routing
• RPSL
• RSMLT
• Routing in optical mesh networks

  Alternative methods for network data flow

• Peer-to-peer
• Network coding

  Router Software and Suites

• GNU Zebra
• Quagga (software)
• Iproute2

  Router Platforms

• Network operating system - NOS
• XORP - the eXtensible Open Router Platform
• Junos
• Cisco IOS
• NX-OS
• CatOS

  References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations. (October 2011)

  External links

• Count-To-Infinity Problem
• "Stability Features" are ways of avoiding the "count to infinity" problem.
• Cisco IT Case Studies about Routing and Switching
• good example at event-helix