IP Routing

Chapter 6. Open Shortest Path First (OSPF)

Last year I flew from New York to Osaka for a conference. My journey began when I hailed a cab on Broadway in downtown New York. “JFK,” I told the cabbie, telling her my destination was John F. Kennedy Airport. I was still pushing my luggage down the seat so I could pull my door shut when the cab started to move. The cabbie changed lanes twice before I got it shut. I did make it to JFK in one piece, where I presented my ticket and boarded a flight to Osaka. At Osaka Airport, the taxi driver bowed to me as he took my luggage from my hand. Once the luggage was properly stowed, he asked for my destination. “New Otani Hotel,” I told him, and he bowed again and closed my side door.

This everyday story of a passenger in transit illustrates how a traveler is able to complete a journey in spite of the fact that the whereabouts of his destination are not known to every element in the system. The cabbie in New York knows only local destinations and so knows how to get to JFK but not to the New Otani Hotel. The airline routes passengers between major airports. The taxi driver in Osaka also knows only local destinations, so, when returning to New York, I tell the driver that my destination is “Osaka Airport,” not “New York.” Any single element of the transportation system knows only the local geography. This leads to obvious efficiencies: the cabbie in New York needs to know only the New York metropolitan area, and the taxi driver in Osaka needs to ...