Routing across hops: how a packet finds a server three routers away

The idea

A laptop in an office reaches a web server in a data center through three routers: officeRouter, coreRouter and dataCenterRouter. No router knows the whole path; each knows only the next hop, and that is enough to cross the network, find a broken link and find a slow one.

Steps

1. The laptop only knows one thing

   A packet's journey starts with the sender's own choice, so begin at the laptop. The web server is three routers away; read what the laptop's routing table knows about it.

   laptop:~# ip route

2. Each router knows one step more

   The laptop trusted officeRouter, so the next question is what officeRouter knows. Read its table, then coreRouter's, and follow the data center's network, 10.0.3.0/24.

   officeRouter:~# ip route
   coreRouter:~# ip route

3. The way back

   A page needs an answer, and the server's reply is a new packet to the laptop, routed again by every router on the way. So each router needs a line back too.

   dataCenterRouter:~# ip route

4. Map the path

   Reading three tables is slow; one command shows the whole path from the laptop. Count the routers to the web server with traceroute.

   laptop:~# traceroute -n webServer.lab

5. How traceroute works

   traceroute must make each router show itself, and it uses a counter every packet carries, which each router lowers by one. ping -t 2 starts it at 2.

   laptop:~# ping -c1 -t 2 webServer.lab
   laptop:~# ping -c1 -t 3 webServer.lab
   laptop:~# ping -c1 webServer.lab

6. Break the link in the middle

   When a page stops loading, the first question is where the path breaks. The lab cuts the link between coreRouter and dataCenterRouter: find the break; then the link comes back.

   laptop:~# ping -c2 -W1 webServer.lab
   laptop:~# traceroute -n -m5 webServer.lab
   laptop:~# ping -c1 -W2 webServer.lab

7. Make that link a satellite hop

   A slow page needs the same hunt: which link adds the time? The lab makes that link a satellite hop, 600 ms each way. Find it from the hop times.

   laptop:~# ping -c3 webServer.lab
   laptop:~# traceroute -n webServer.lab
   laptop:~# ping -c1 webServer.lab

Run these steps on real machines