Internet Routing and Traffic Engineering.Internet Routing.Internet routing today is handled through the use of a routing protocol known as BGP Border Gateway Protocol.Individual networks on the Internet are represented as an autonomous system AS.An autonomous system has a globally unique autonomous system number ASN which is allocated by a Regional Internet Registry RIR, who also handle allocation of IP addresses to networks.A Software Metric For Python Language Wikipedia' title='A Software Metric For Python Language Wikipedia' />A Software Metric For Python Language Wikipedia The FreeEach individual autonomous system establishes BGP peering sessions to other autonomous systems to exchange routing information.A BGP peering session is a TCP session established between two routers, each one in a particular autonomous system.This BGP peering session rides across a link, such as a 1.Gigabit Ethernet interface between those routers.The routing information contains an IP address prefix and subnet mask.Interactive geometry software IGS or dynamic geometry environments DGEs are computer programs which allow one to create and then manipulate geometric.This translates which IP addresses are associated with an autonomous system number AS origin.Routing information propagates across these autonomous systems based upon policies that individual networks define.This is where things get a bit interesting because various factors influence how routing is handled on the Internet.There are two main types of relationships between autonomous systems today Transit and Peering.Transit is where an autonomous system will pay an upstream network known as a transit provider for the ability to forward traffic towards them who will forward that traffic further.It also provides for the autonomous system purchasing who is the customer in this relationship to have their routing information propagated to their adjacencies.Transit involves obtaining direct connectivity from a customer network to an upstream transit provider network.These sorts of connections can be multiple 1.Gigabit Ethernet links between each others routers.Transit pricing is based upon network utilization in a particular dominant direction with 9.A transit provider will look at a months worth of utilization and in the traffic dominant direction they will bill on the 9.The unit used in billing is measured in bits per second bps and is communicated in a price per Mbps for example 2 per Mbps.Peering is where an autonomous system will connect to another autonomous system and agree to exchange traffic with each other and routing information of their own networks and any customers transit customers they have.With peering, there are two methods that connectivity is formed on.The first is where direct connectivity is established between individual networks routers with multiple 1.Gigabit Ethernet or 1.Gigabit Ethernet links.This sort of connectivity is known as private peering or PNI Private Network Interconnect.This sort of connection provides both parties with clear visibility into the interface utilization of traffic in both directions inbound and outbound.Another form of peering that is established is via Internet Exchange switches, or IXs.With an Internet Exchange, multiple networks will obtain direct connectivity into a set of Ethernet switches.Individual networks can establish BGP sessions across this exchange with other participants. File Seem To Be Patched Lol Dolls . The benefit of the Internet Exchange is that it allows multiple networks to connect to a common location and use it for one to many connectivity.A downside is that any given network does not have visibility into the network utilization of other participants.Most networks will deploy their network equipment routers, Dense Wave Division Multiplexing DWDM transport equipment into colocation facilities where networks will establish direct connectivity to each other.This can be via Internet Exchange switches which are also found in these colocation facilities or direct connections which are fiber optics cables ran between individual suitesracks where the network gear is located.Routing Policy. Networks will define their routing policy to prefer routing to other networks based upon a variety of items.The BGP best path decision process in a routers operating system dictates how a router will prefer one BGP path over another.Network operators will write their policy to influence that BGP best path decision process based upon factors such as the cost to deliver traffic to a destination network in addition to performance.A typical routing policy within most networks will dictate that internal their own and routes learned from their own customers are to be preferred over all other paths.After that, most networks will then prefer peering routes since peering is typically free and often times can provide a shorteroptimal path to reach a destination.Finally the least preferred route to a destination is over paid transit links.When it comes to transit paths, both cost and performance are typically factors in determining how to reach a destination network.Routing policies themselves are defined on routers in a simple text based policy language that is specific to the router operating system.They contain two types of functions matching on one or multiple routes and an action for that match.The matching can include a list of actual IP prefixes and subnet lengths, ASN origins, AS Paths or other types of BGP attributes communities, next hop, etc.The actions can include resetting BGP attributes such as local preference, Multi Exit Discriminators MED and various other values communities, Origin, etc.Below is a simplified example of a routing policy on routes learned from a transit provider.It has multiple terms to permit an operator to match on specific Internet routes to set a different local preference value to control what traffic should be forwarded through that provider.There are additional actions to set other BGP attributes related to classifying the routes so they can be easily identified and acted upon by other routers in the network.TRANSIT 1 IN. PREFER OVER PEERING.TRANSIT 1 OVERRIDE.TRANSIT. community add LOCATION.PREFER OVER OTHER TRANSIT.TRANSIT 1 HIGH PREF.TRANSIT. community add LOCATION.DEPREF OTHER TRANSIT.TRANSIT 1 LOW PREF.TRANSIT. community add LOCATION.DEFAULT TERM. metric 1.TRANSIT. community add LOCATION.Network operators will tune their routing policy to determine how to send traffic and how to receive traffic through adjacent autonomous systems.This practice is generally known as BGP traffic engineering.Making outbound traffic changes is by far the easiest to implement because it involves identifying the particular routes you are interested in directing and increasing the routing preference to egress through a particular adjacency.Operators must take care to examine certain things before and after any policy change to understand the impact of their actions.Inbound traffic engineering is a bit more difficult as it requires a network operator to alter routing information announcements leaving your network to influence how other autonomous systems on the Internet prefer to route to you.While influencing the directly adjacent networks to you is somewhat trivial, influencing networks further beyond those directly connected can be tricky.This technique requires the use of features that a transit provider can grant via BGP.In the BGP protocol, there is a certain type of attribute known as Communities.Communities are strings you can pass in a routing update across BGP sessions.Most networks use communities to classify routes as transit vs.The transit customer relationship usually gives certain capabilities to a customer to control the further propagation of routes to their adjacencies.This grants a network with the ability to traffic engineer further upstream to networks it is not directly connected to.Traffic engineering is used for several reasons today on the Internet.The first reason might be to reduce bandwidth costs by preferring particular paths different transit providers.The other is for performance reasons, where a particular transit provider may have less congestedlower latency path to a destination network.Network operators will view a variety of metrics to determine if there is a problem and start to make policy changes to examine the outcome.Of course on the Internet, the scale of the traffic being moved around counts.Moving a few Gbps of traffic from one path to another may improve performance, but if you move tens of Gbps over you may encounter congestion on this newly selected path.
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