Lesson 4 - AS relationships and interdomain routing Flashcards
Describe the relationships between ISPs, IXPs, and CDNs.
ISPs (Internet Service Providers) are the “backbone” network over which smaller networks can connect.
IXPs (Internet Exchange Points) are physical interconnection infrastructures that are used by multiple networks (such as ISPs and CDNs) to interconnect.
CDNs (Content Delivery Networks) are networks created by content providers (such as Shopify/Netflix) to reduce connectivity costs and provide greater control for the content provider on how the content is delivered to the end-users. They may have multiple data centers with hundreds of servers distributed across the world.
What is AS?
AS = Autonomous System (such as ISPs/CDNs) - a group of routers that operate under the same administrative authority. An ISP (or CDN) can operate as a single AS or multiple. Each AS has its own set of policies/strategies based on their needs and doesn’t need to share this info with other ASes.
What kind of relationship does AS have with other parties?
Competition and cooperation. T1 ISPs compete with each other, T2 with each other, etc - there is a hierarchy but this hierarchy is becoming more flat over time thanks to evolutions such as IXPs and CDNs. Competing ISPs also need to cooperate so that the Internet can work on a global scale.
What is BGP?
Protocol use to route traffic between ASes, BGP or Border Gateway Protocol is used which is primarily based on incentives (money) - ASes do whatever makes the most sense for them from a financial standpoint.
How does an AS determine what rules to import/export?
Basically - Customers > Peers > Providers
What were the original design goals of BGP? What was considered later?
Scalability - achieving reasonable convergence time and loop-free paths
Express routing policies - ASes can define their own policies/rules and don’t need to share them
Allow cooperation among ASes - local decisions (which routes to import/export) can be made and kept confidential but cooperation is still possible
Security was considered later with varying success - mostly they have not been widely adopted as it is difficult to transition to new protocols and there is a lack of incentives to do so.
What are the basic of BGP?
A pair of routers, BGP peers, exchange routing info over a semi-permanent TCP port connection (BGP session). This starts with an OPEN message and is followed by the routers sending each other announcements from their own routing tables.
There are two types of BGP messages:
UPDATE - announcements of new routes/updates to existing routes; withdrawal of previous routes due to a failure or change in routing policy
KEEPALIVE - message exchanged to keep a current session going
BGP routes - main components: reachable IP prefix field, AS-PATH (route passed through from destination), and NEXT-HOP (IP of next-hop router along the path towards the destination). In iBGP the NEXT-HOP is the address of the nearest border router.
What is the difference between iBGP and eBGP?
eBGP = external BGP = BGP session between pair of routers in two different ASes
iBGP = internal BGP = BGP session between routers in the same AS
What is the difference between iBGP and IGP-like protocols (RIP or OSPF)?
IGP-like protocols is used to establish internal paths based on costs within the AS.
iBGP is only used to disseminate external routes within the AS
iBGP is used to propagate information about what ASes can be reached by the gateway routers in the current AS. IGP-like protocols are used to establish paths between the internal routers of an AS based on specific costs within the AS (lesson 3).
How does BGP decision process to select routes at a router work?
The actual policies are based on the business goals of the AS, but they all follow the same process to select the best routes based on the policy in place. The router compares a list of attributes, between a pair of routes, in top down order - if two attributes are equal then it moves down to the next attribute and so on.
- Local routes
- Lowest AS Path length
- Lower MED (Multi-exit descriminator)
- Shortest IGP path
- Tiebreaker with ID or reliability.
Attributes with higher values are preferred
(i.e if MED A < MED B .. MED B is preferred)
What are the main challenges with BGP? How to mitigate these challanges?
Scalability and misconfigurations or faults. An error can result in an excessively large number of updates which can lead to route instability, overloading, outages, etc.
ASes can reduce this risk by limiting routing table size with filtering to encourage route aggregation and limiting the number of route changes with flap damping (suppresses route updates for a period of time when a threshold is reached which can be set individually for prefixes according to a specific strategy).
What is an IXP?
Internet Exchange Points - physical infrastructures that provide the means for ASes to interconnect and directly exchange traffic with one another.
What are the 4 reasons of IXPs increased popularity?
They can handle large traffic volumes comparable to T1 ISPs
Play an important role in mitigating DDoS attacks as they can play the role of a “shield” such as with BGP blackholing
Provide a plethora of research opportunities into the evolution of the Internet landscape
Active marketplaces with many services beyond interconnection - they have been evolving from simple interconnection hubs to technology innovation hubs
Keeps traffic local when both on the same IXP, lower costs, incentives from big players.
Which services do IXPs provide?
Peering (private, public, mobile, remote through resellers)
route servers and service level agreements (many participants on a network can use it with a single agreement/BGP session)
DDoS blackholing (customer cna trigger this to alleviate DDoS effects)
and free value-added services such as nameservers, local time, DNS root name servers, etc.
How does a route server work?
Purpose - make IXPs more scalable/manageable by allowing multi-lateral BGP sessions. RSes maintain two types of route filters:
Import filters - ensure that each member AS can only advertise routes that it should advertise
Export filters - each IXP member can restrict the set of other IXP member ASes that receive its routes
AS X advertises a prefix (route) to RS which gets added to the route server’s AS X specific RIB. The route server uses the import filter to check whether X can advertise this route and if so it adds it to the master RIB. The router server applies the export filter from X to see if AS Z can receive this route. If so, it adds that route to Z’s RIB. Now the RS advertises this route to Z with X as the next hop. There is no direct advertising - must go through the router server.
