Elijah's Notes

The Global Internet

2 min read

Purpose

Explain how routing scales from a single network to the whole internet. Protocols like RIP and OSPF require every router to know about every network they can reach, and no router can hold a complete list of all the networks in the internet. The fix is hierarchy.

Structure of the internet

The modern internet is mainly composed of end-user sites and service providers. An end-user site is a collection of devices reaching the internet through a single IP address (NAT in home networks) or through switched ethernet in an enterprise LAN. Service providers build the infrastructure and route traffic between end-user sites. A provider runs many high-performance routers in metro areas, connected to each other and to other providers over high-speed links.

Routing areas

OSPF already divides a network into areas to shrink routing tables (see routing). The global internet applies the same move at a much larger scale. It is divided into routing domains called Autonomous Systems (ASes). An AS is a collection of routers and networks under the control of a single organization. Each AS picks its own routing protocol and handles routing within itself. Routers inside an AS connect to each other over high-speed links and reach other ASes through border routers.

Inter-domain routing

Routing between ASes is a separate problem from routing within one, because ASes are independently managed and don’t share a cost metric. The interior routers handle traffic within the AS, the border routers handle traffic between ASes, and the protocol the border routers speak is BGP.

flowchart LR
    subgraph AS1 [AS 1, runs OSPF internally]
        A1[interior router] --- A2[interior router]
        A2 --- AB[border router]
    end

    subgraph AS2 [AS 2, runs RIP internally]
        BB[border router] --- B1[interior router]
        B1 --- B2[interior router]
    end

    AB ---|BGP| BB

    style AB fill:#e3f2fd
    style BB fill:#e3f2fd

Each AS picks its own interior protocol and metric, and BGP between the border routers is the only thing they have to agree on.

Graph View