Inside one postal company (distance-vector, link-state, and ICMP)

One parcel tracked from a laptop to example.com, hop by hop. Most of those hops sit inside one company’s own network. This lesson is how the depots inside one company agree on their roads, and the short notes they send back when a parcel cannot go on.

Before you start. From How a parcel finds any house on earth you have the depot picture: a router is a depot that only knows its own short list of rows, each row saying which way to hand a parcel on, and a handoff counter stamped on every parcel (the TTL) stops a lost one from circling forever. That lesson was about the conversation BETWEEN companies, BGP. This one is about where each company’s own rows come from, inside its own network, where every depot belongs to the same owner and can be told the truth.

The idea in 30 seconds

Inside one network, routers build their rows with a routing protocol of their own; BGP is only the conversation between networks.
Distance-vector routers tell their neighbours their distance to every destination; it is simple, but after a failure routers can bounce traffic between them while their costs count up to “infinity” (16 in RIP).
Link-state routers flood only their own links to every router, so all hold the same map and each runs Dijkstra’s shortest path first on it; OSPF works this way.
Link-state depends on everyone agreeing which update is newest; the ARPANET’s 1980 outage was three corrupted versions of one update that each looked newer than another.
ICMP carries the network’s notes back to a sender: echo for ping, time exceeded for traceroute, destination unreachable, and fragmentation needed.

Read the story · Skip to the numbers