Internet Structure: ISPs, IXPs, and CDNs

• The Internet encompasses Internet Service Providers (ISPs), Internet Exchange Points (IXPs), and Content Delivery Networks (CDNs) to achieve connectivity through a network of networks
• Each network type has business objectives, leading to diverse relationships and interconnection strategies

ISP Tiers

• Tier-3 (Access): ISPs provide local access
• Tier-2 (Regional): ISPs operate at a regional level
• Tier-1 (Global): ISPs are large-scale ISPs forming the Internet's backbone

Internet Exchange Points (IXPs)

• IXPs facilitate interconnection, enabling networks (ISPs, CDNs) to exchange local traffic
• There were approximately 500 IXPs worldwide in 2019

Content Delivery Networks (CDNs)

• CDNs are established by content providers
• Used to control content delivery and minimize connectivity costs
• Examples are Google and Netflix
• They consist of multiple data centers globally distributed

Network Hierarchy, Competition, and Cooperation

• Smaller networks connect to larger ones, forming a hierarchy
• Access ISPs use larger ISPs for Internet connectivity, leading to competition
• Competing ISPs also collaborate for connectivity, using interconnection strategies based on customers and location

Interconnection Options

• Points of Presence (PoPs): Routers in a provider network for customer connections
• Multihoming: ISPs connect to multiple providers
• Peering: Settlement-free agreements between ISPs for direct traffic exchange

Internet Topology

• The Internet structure is hierarchical, evolving to a flatter topology
• IXPs and CDNs are contributing to this shift

Autonomous Systems (AS)

• Networks like ISPs and CDNs may operate
• It's a group of routers under a single administrative authority
• AS have tailored policies for traffic engineering, interconnection, and routing decisions

Autonomous System Operation

• Operated by a single entity,
• Applies BGP policies to manage external traffic and route advertisement
• These policies align with business relationships and peering goals

Routing Protocols

• Border Gateway Protocol (BGP) exchanges routing data between AS border routers
• Internal Gateway Protocols (IGPs), like OSPF, optimize path metrics within an AS
• Focus is on BGP

Autonomous System Relationships

• Provider-Customer (Transit): Financial agreement in which a customer pays a provider to forward traffic to destinations and vice versa
• Peering: ASes share access to subsets of each other's routing tables
• Traffic exchange is generally symmetric
• Peering often involves Tier-1 ISPs

Provider Charges

• Two common methods exist
• Fixed price: Bandwidth usage within a range
• Bandwidth-based: Charges based on periodic measurements

BGP Routing Policies

• AS business relationships and policies dictate route import and export decisions

Exporting Routes

• Deciding which routes to advertise
• Selecting the route for a destination
• This affects traffic flow and is managed with route filters

Route Types

• Routes learned from customers should be widely advertised for increased traffic
• Routes learned from providers are withheld to avoid carrying traffic without financial incentives

Importing Routes

• ASes filter routes based on origin and business relationships
• Routes are received from customers, providers, and peers

Route Preference

• Customer routes are prioritized and prefered
• Peer routes are second
• Provider routes third
• This to minimize costs

BGP Design Goals

• Scalability: Handles Internet growth, managing routes, and convergence

Routing Policies

• Route attributes enable import/export policies, maintaining confidentiality and autonomy

Autonomous System Cooperation

• Each AS makes local decisions while keeping them undisclosed

Security Considerations

• Protection from attacks and early threat detection is important for managing the increasing internet size

BGP Basics

• BGP Session: Routers (BGP peers) exchange route information over TCP
• They exchange announcements from routing tables, which takes seconds or minutes

Session Types

• External BGP (eBGP): Between routers in different ASes
• Internal BGP (iBGP): Session between routers within the same AS

BGP Messages

• BGP has reachability information and routing policies through messages
• Updates are used for route availability or updates with standardized attributes
• Withdrawals remove routes due to failure or policy changes

Keepalive

• Messages exchanged to maintain the connection

BGP Prefix Reachability

• IP Prefixes represent subnets or collections of subnets an AS can reach
• Advertised via eBGP based on the AS’s export policy to neighboring's routers
• iBGP distributes these routes within the AS and used to propagate external routes to internal routers

Path Attributes

• Important BGP route attributes include AS-PATH and NEXT-HOP

AS-PATH Attribute

• It lists the ASes a route traverses
• It is used to prevent loops and select the shortest path to a destination

NEXT-HOP Attribute

• The IP address of the next router towards a destination
• Next-hop enables internal routers to choose the best path

iBGP and eBGP

• Used to disseminate routes for external destinations
• Router learn external prefixes through eBGP an disseminate them with iBGP sessions
• Internal routes are dissiminated through the AS using a full mesh of iBGP
• iBGP disseminates external routes within the AS

BGP Decision Process

• Routers select best routes to advertise
• Apply import policies to exclude routes and select the best routes, and install them to the forwarding table

Route Selection

Routers compare routes and chose routes with values to best apply the policies in order of

• Highest Local Preference
• Shortest AS Path Length
• Lowest origin type
• Lowest MED
• eBGP-learned over iBGP-learned
• Lowest IGP cost to border router
• Lowest router ID

Local Preference

• Route decision is influenced using the LocalPref
• It prefers routes through a preferred AS
• This influences traffic exits

MED (Multi-Exit Discriminator)

• The attribute is used to designate inbound traffic preferences for multiple links
• It is used to tag routes based on business relationship
• Influencing route exports affects traffic entry points

Attribute Control

• LocalPref are set locally
• MED by the neighboring ASs

BGP Challenges

• BGP has scalability and misconfiguration issues
• Misconfiguration of BGP can trigger large updates and router failure

Mitigation

• It is possible to mitigate BGP issues through limiting size and changes

Scaling

• Routing tables can be limited using filtering and aggregating
• ASes can use default routes and aggregate prefixes

Stability

• To limit unstable routes, flapping-dampening applies
• Route updates are tracked, with routes suppressed temporarily if a threshold is reached