Monday, April 1, 2019

Routing Policy -- interconnecting with other networks

The next few posts about Joe's Internet (J-Inet) will start with our routing policy, and refine it based on some ideas concerning how to describe the interconnections between networks.  We'll get a look at J-Inet's network soon, and eventually we'll see how the router configurations enact the routing policy.  For now, let's get a grounding in how we can describe links between networks, and how they relate with routing policy.

J-Inet has a simple management philosophy when it comes to interconnecting with other networks.  In summary:
  • keep things as simple as possible
  • minimize costs
  • maximize revenue

These thoughts are going to be the basis of our routing policy -- make money while keeping things simple and cheap.

Types of interconnection

The types of networks we are discussing are referred to as Autonomous Systems (ASes).  Within any AS are destinations referred to as prefixes or routes.  When an AS interconnects with another, they advertise their prefixes to the other -- essentially offering a path to that destination.

It turns out that if you learn about three simple types of business relationships, you can understand the vast majority of AS interconnection types:

  • Transit  -- If you buy Internet connectivity from someone, they are a transit provider.  You can hand them traffic, and they will deliver it to the desired recipient, or will pass that traffic across their network to a provider that can get the traffic closer to its destination.  For our purposes, if you pay people to take your traffic, you're buying transit.
  • Customer -- If you supply transit to another network, they are a customer.  The simplest customer relies on you completely, and more complex customers look at you in the same way you look at your transit provider(s).  For our purposes, customers pay you to be their transit provider.
  • Peering -- Sometimes, networks find that it is mutually advantageous to interconnect, and they decide to do so without either side paying for the privilege.  Common customers, shared geographic regions, and other factors may influence if networks will become peers.  We aren't going to worry about *why* some of our neighbors a peers -- we will just know that they are.

Egress Policy

As the number of interconnections increase, the odds of having more than a single path to a given prefix grows.  Our policy will give us guidance about which type of interconnection we wish to use, and we will rank prefix advertisements in this order:

  1. Customer -- Goal:  Maximize revenue.  Always use this path if available.
  2. Peers -- Goal: Minimize cost: Use this path before using one to a transit provider.
  3. Transit -- Goal: Minimize cost.  Use this path only if no better path exists.

Prefix Advertisement Policy

Business policy shaped how we prioritize the choice of multiple egress paths.  Likewise, what we choose to advertise across an interconnection point is shaped by our policy as well.  In this case the reasoning becomes:
  • Transit  -- Goal: Minimize costs.  Only advertise internal prefixes and those that your are paid to carry (e.g. prefixes learned from Customers), thus minimizing traffic.
  • Customer -- Goal: Maximize revenue.  Advertise every prefix you know about to maximize traffic.
  • Peering -- Goal: Minimize costs.  Advertise the same set of prefixes as sent to Transit providers, in the hope of reducing transit costs.

Parting thoughts and a glimpse of the future

By describing AS interconnections as one of three different types, we've been able to express some routing goals that will support our business goals.  In the next installment, we'll look at a map of J-Inet, start to see actual prefixes, and classify the inter-AS interconnects.

In other future installments, we will start to put together router configurations that will allow us to enact this routing policy.  At first, we'll use Cisco vocabulary, and stick with IPv4, but the plan is to use a single network to show the use of other vendor's gear as well. Stick around, this should be fun...

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