Describe the relationships between ISPs, IXPs, and CDNs.
ISP: Internet Service Provider - 3 tiers, smaller ISP is customer of larger ISP (Tier-1)
IXP: Internet Exchange Points - physical infrastructure where ISPs interconnect (~500 in the world)
CDN: Content Delivery Network - networks created by content providers (e.g. Netflix), goal of having greater control of how the content is delivered to end-users
a network of networks make up the internet ecosystem.
What is an AS?
Autonomous System - a group of routers (including the links among them) that operate under the same administrative authority, has its own traffic policies. An ISP can be its own AS or be under multiple ASes.
What kind of relationship does AS have with other parties?
- Provider-Customer relationship (or transit): financial settlement that determines how much the customer will pay the provider. The provider forwards the customer’s traffic to destinations found in the provider’s routing table
- Peering relationship: formed between Tier-1 ISPs but also between smaller ISPs, share access to a subset of each other’s routing tables. beneficial when a significant amount of traffic is destined for each other
What is BGP?
Border Gateway Protocol - routes traffic between and within ASes
How does an AS determine what rules to import/export?
Exporting Rules:
AS learns routes from:
1. Customers - advertised routes from customers, more incentive to move traffic toward customers
2. Providers - no financial incentive to carry traffic for its provider’s routes
3. Peers - no revenue to advertise back to peers
Importing Rules:
In order of preference, the imported routes are the customer, peer, and provider routes (in order of most to least revenue)
What were the original design goals of BGP? What was considered later?
- Scalability -manage the complications of internet growth while achieving convergence in reasonable timescales and providing loop-free paths
- Express routing policies - which routes to import and export through route filtering and route ranking. policies can be independent and private
- Allow cooperation among ASes - local decisions are independent from other ASes
- Security - ongoing effort that was considered later
What are the basics of BGP?
- BGP Session - pair of routers, BGP Peers, exchange routing information (from their routing tables) over a semi-permanent TCP port connection
- eBGP Session - pair of routers in two different ASes
- iBGP Session - pair in same ASes
2 Types of BGP messages sent between BGP peers:
1. UPDATE - announcements (new routes), withdrawals
2. KEEPALIVE - keep a session going
In BGP protocol, destinations are represented by IP prefixes; Gateway routers running eBGP advertise the IP prefixes
Advertised BGP routes contain:
ASPATH - the IDs of the ASes are collected as announcement passes through, this information prevents looping
NEXT HOP - next-hop router’s IP address (interface) along the path towards the destination
What is the difference between iBGP and eBGP?
eBGP - routers learn routes to external prefixes and disseminate them to all routers within the AS. Each eBGP speaking router has an iBGP session
iBGP - only used to disseminate external routes within the AS
What is the difference between iBGP and IGP-like protocols (RIP or OSPF)?
IGP (Interior Gateway Protocols, e.g. OSPF, RIP) -like protocols are used to establish paths between the internal routers of an AS based on specific costs within the AS. In contrast, iBGP is only used to disseminate external routes within the AS.
How does a router use the BGP decision process to choose which routes to import?
when a router receives multiple route advertisements, it compares a pair of routes by going through the list of attributes. In simplistic practice, it selects the route with the attribute value that will help apply the policy. Attributes are set either (a) locally by the AS (e.g., LocalPref), (b) by the neighboring AS (e.g., MED - multi exit discriminator), (c) or by the protocol (e.g., if a route is learned through eBGP or iBGP).
What are the 2 main challenges with BGP? Why?
- misconfigurations
- faults
can result in an excessively large number of updates, leading to router failures. solution: limit routing table size and routing changes.
What is an IXP?
Internet Exchange Points - physical infrastructures (usually a network of switches) that provide the means for ASes to interconnect (peering) and directly exchange traffic with one another. The router of each AS must use BGP
What are four reasons for IXP’s increased popularity?
- handling large traffic volumes
- mitigating DDoS attacks
- “Real-world” infrastructures with a plethora of research opportunities
- active marketplaces and technology innovation hubs
Which services do IXPs provide?
- Public Peering - two networks use the IXP’s network infrastructure to establish a connection to exchange traffic. one time cost, then monthly/annual fee
- Private Peering - doesn’t use the IXP’s public peering infrastructure
- Route servers and Service level agreements (SLA) - allows participants to arrange instant peering with many co-located participant
- Remote peering through resellers - Third parties resell IXP ports wherever they have infrastructure connected to the IXP
- Mobile peering - interconnection of mobile networks
- DDoS blackholing - mitigate DDoS attacks
- **Free value-added services **
How does a route server work?
IXPs operate a route server to help to make peering more manageable
* collects and shares routing information from its peers or participants of the IXP that connect to the RS.
* executes its own BGP decision process and re-advertises the resulting information (e.g., best route selection) to all RS’s peer routers.
a Route Server can maintain multi-lateral peering sessions via import and export filters.
