CS 6250 ANSWERS AND QUESTIONS SET A+
✔✔● Describe the relationships between ISPs, IXPs, and CDNs. - ✔✔The Internet is a
complex ecosystem. It is built with a network of networks. These networks include
(ISPs) Internet Service Providers, (IXPs) Internet Exchange Points, (CDNs) Content
Delivery Networks. In 2019, there were apx. 500 IXPs around the world.
large scale Tier-1 ISPs that operate at a global scale, and essentially they form the
"backbone" network over which smaller networks can connect
IXPs are interconnection infrastructures, which provide the physical infrastructure,
where multiple networks (eg ISPs and CDNs) can interconnect and exchange traffic
locally.
CDNs are networks that are created by content providers with the goal of having greater
control of how the content is delivered to the end-users, and also to reduce connectivity
costs. Some example CDNs include Google and Netflix. These networks have multiple
data centers - and each one of them may be housing hundreds of servers - that are
distributed across the world.
✔✔● What is an AS? - ✔✔AS stands for autonomous systems. ISPs, IXPs and CDNs
can all operate as an AS. An AS is a group of routers including the links among them
that operate under the same administrative authority.
✔✔● What kind of relationship does AS have with other parties? - ✔✔An AS is
independent of other ASes. They set their own policies, make its own traffic engineering
decisions and interconnection strategies and also determine how the traffic leaves and
enters the network.
✔✔● What is BGP? - ✔✔BGP stands for Border Gateway Protocol. The border routers
of ASes use BGP to exchange routing information with each other.
✔✔● How does an AS determine what rules to import/export? - ✔✔AS business
relationships drive an AS' routing policies and influence which routes an AS needs to
import or export.
, There are 3 selective transit routes: Transit customer routes, Transit Provider Routes,
Peer Routes. The main rule is financial incentive. Transit customer routes > peer routes
> transit provider routes.
Similarly as exporting, ASes are selective about which routes to import based, primarily,
on which neighboring AS advertises them and what type of business relationship is
established. An AS receives route advertisements from its customers, providers and
peers.
When an AS receives multiple route advertisements towards the same destination, from
multiple ASes, then it needs to rank the routes before selecting which one to import.
The routes that are preferred first are the customer routes, then the peer routes and
finally the provider routes. The reasoning behind this ranking is that an AS...
1. wants to ensure that routes towards its customers do not traverse other ASes
unnecessarily generating costs,
2. uses routes learned from peers since these are usually "free" (under the peering
agreement),
3. and finally resorts to import routes learned from providers as these will add to costs.
✔✔● What are / were the original design goals of BGP? What was considered later? -
✔✔The original design goals of BGP were:
1. Scalability
manage the complications of this growth, while achieving convergence in reasonable
timescales and providing loop-free paths.
2. Express routing policies
BGP has defined route attributes that allow ASes to implement policies (which routes to
import and export), through route filtering and route ranking
3. Allowing cooperation among ASes
individual AS can still make local decisions (which routes to import and export) while
keeping these decisions confidential from other ASes.
Security was considered later as the complexity and size of the Internet has been
increasing. Solutions have not been widely deployed or adopted due to difficulty to
transition to new protocols and lack of incentives.
✔✔● What are the basics of BGP? - ✔✔BGP session. A pair of routers, known as BGP
peers, exchange routing information over a semi-permanent TCP port connection called
a BGP session. To begin a BGP session a router will send an OPEN message to
another router. Then the sending and receiving router will send each other
announcements from their individual routing tables. Depending on the number of routes
being exchanged, this can take from seconds up to several minutes.
A BGP session between a pair of routers in two different ASes is called external BGP
(eBGP) session, and a BGP session between routers that belong to the same AS is
called internal BGP (iBGP) session.
✔✔● Describe the relationships between ISPs, IXPs, and CDNs. - ✔✔The Internet is a
complex ecosystem. It is built with a network of networks. These networks include
(ISPs) Internet Service Providers, (IXPs) Internet Exchange Points, (CDNs) Content
Delivery Networks. In 2019, there were apx. 500 IXPs around the world.
large scale Tier-1 ISPs that operate at a global scale, and essentially they form the
"backbone" network over which smaller networks can connect
IXPs are interconnection infrastructures, which provide the physical infrastructure,
where multiple networks (eg ISPs and CDNs) can interconnect and exchange traffic
locally.
CDNs are networks that are created by content providers with the goal of having greater
control of how the content is delivered to the end-users, and also to reduce connectivity
costs. Some example CDNs include Google and Netflix. These networks have multiple
data centers - and each one of them may be housing hundreds of servers - that are
distributed across the world.
✔✔● What is an AS? - ✔✔AS stands for autonomous systems. ISPs, IXPs and CDNs
can all operate as an AS. An AS is a group of routers including the links among them
that operate under the same administrative authority.
✔✔● What kind of relationship does AS have with other parties? - ✔✔An AS is
independent of other ASes. They set their own policies, make its own traffic engineering
decisions and interconnection strategies and also determine how the traffic leaves and
enters the network.
✔✔● What is BGP? - ✔✔BGP stands for Border Gateway Protocol. The border routers
of ASes use BGP to exchange routing information with each other.
✔✔● How does an AS determine what rules to import/export? - ✔✔AS business
relationships drive an AS' routing policies and influence which routes an AS needs to
import or export.
, There are 3 selective transit routes: Transit customer routes, Transit Provider Routes,
Peer Routes. The main rule is financial incentive. Transit customer routes > peer routes
> transit provider routes.
Similarly as exporting, ASes are selective about which routes to import based, primarily,
on which neighboring AS advertises them and what type of business relationship is
established. An AS receives route advertisements from its customers, providers and
peers.
When an AS receives multiple route advertisements towards the same destination, from
multiple ASes, then it needs to rank the routes before selecting which one to import.
The routes that are preferred first are the customer routes, then the peer routes and
finally the provider routes. The reasoning behind this ranking is that an AS...
1. wants to ensure that routes towards its customers do not traverse other ASes
unnecessarily generating costs,
2. uses routes learned from peers since these are usually "free" (under the peering
agreement),
3. and finally resorts to import routes learned from providers as these will add to costs.
✔✔● What are / were the original design goals of BGP? What was considered later? -
✔✔The original design goals of BGP were:
1. Scalability
manage the complications of this growth, while achieving convergence in reasonable
timescales and providing loop-free paths.
2. Express routing policies
BGP has defined route attributes that allow ASes to implement policies (which routes to
import and export), through route filtering and route ranking
3. Allowing cooperation among ASes
individual AS can still make local decisions (which routes to import and export) while
keeping these decisions confidential from other ASes.
Security was considered later as the complexity and size of the Internet has been
increasing. Solutions have not been widely deployed or adopted due to difficulty to
transition to new protocols and lack of incentives.
✔✔● What are the basics of BGP? - ✔✔BGP session. A pair of routers, known as BGP
peers, exchange routing information over a semi-permanent TCP port connection called
a BGP session. To begin a BGP session a router will send an OPEN message to
another router. Then the sending and receiving router will send each other
announcements from their individual routing tables. Depending on the number of routes
being exchanged, this can take from seconds up to several minutes.
A BGP session between a pair of routers in two different ASes is called external BGP
(eBGP) session, and a BGP session between routers that belong to the same AS is
called internal BGP (iBGP) session.
