contextdatagram transferred by different link protocols over different links:e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last linkeach link protocol provides different servicese.g., may or may not provide rdt over linktransportation analogy:trip from Princeton to Lausannelimo: Princeton to JFKplane: JFK to Genevatrain: Geneva to Lausannetourist = datagramtransport segment = communication linktransportation mode = link layer protocoltravel agent = routing algorithm