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TODO:
eBGP/iBGP session
Graphs
/**********************************************/
/* A no-bullshit guide to a bullshit protocol */
/**********************************************/
We can think of the Internet as a network of networks which are all connected in some way. We will refer to
these networks as Autonomous Systems(AS) in this article.
Now, how do we connect these AS's in a sane way? This is where BGP comes in. BGP, which stands for `Border
Gateway Protocol`, enables different AS's to exchange information with each other, e.g. communicating with each
other.
There are three main challenges BGP has to solve:
- Scalability: The Internet is BIG. BGP needs to scale well to be feasible in a large scale.
- Privacy: Networks don't want to divulge internal topologies (topology = how a network is structured)
- Policy enforcement: The Networks themselves need to have control over where to send and recieve traffic.
Other protocols, like Link-Sate routing, do not solve these challenges.
BGP's key concept, which it revolves about, is called *path-vector routing*, where it adertises the entire
AS-level path.
Now, what is an AS-level path? We'll get to that in a minute.
Before that, why do we need even BGP? The problem is that sending data over the internet costs money. If we want
to minimize the costs of sending data, we need to find routes which cost as little as possible. This is also why
BGP is a "follow the money" protocol. Different ASes only connect with each other if they can save money that
way.
There are two policies we can use to define this routing:
- Selection: Which path to use (only relevant for outbound traffic)
- Export: Which path to advertise (only relevant for inbount traffic)
TODO: add graphs here
===============================================
eBGP and iBGP
===============================================
BGP comes in two flavors: eBGP and iBGP.
The e/i stands for external/internal
External BGP sessions connect border routers in different ASes. These are therefore use to learn routes to
external destinatons
TODO: add graph
Internal BGP sessions connect the routers in the same AS. These are used to split up externally-learned routes
internally. These are tehn announces exernally(to other ASes) again, using eBGP sessions.
BGP itself is a pretty simple protocol composed of four basic messages:
- open: establish a BGP session (using TCP)
- notification: report unusual conditions
- update: inform neighbor of a new best route
- can be a change or a removal of a best route
- keepalive: inform neighbor that connection is alive
update messages carry an IP prefix together with a set of attributes:
/*********************/
/* IP prefix */
/*********************/
/* Attributes */
/* */
/* */
/* */
/* */
/*********************/
There are four types of different attributes:
- NEXT-HOP
- AS-PATH
- LOCAL-PREF
- MED
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JJ is is a new version control system, made to be compatible with git and _much_ less painful than it.