Link layer:context datagram transferred by transportation analogy: different link protocols over trip from Princeton to Lausanne different links: limo:Princeton to JFK e.g.,Ethernet on first link, ·plane:JFK to Geneva frame relay on train:Geneva to Lausanne intermediate links,802.II ■tourist=datagram on last link transport segment each link protocol provides communication link different services ■ transportation mode link 。eg,may or may not layer protocol provide rdt over link travel agent routing algorithm Link Layer and LANs 6-5Link layer: context ▪ datagram transferred by different link protocols over different links: • e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link ▪ each link protocol provides different services • e.g., may or may not provide rdt over link transportation analogy: ▪ trip from Princeton to Lausanne • limo: Princeton to JFK • plane: JFK to Geneva • train: Geneva to Lausanne ▪ tourist = datagram ▪ transport segment = communication link ▪ transportation mode = link layer protocol ▪ travel agent = routing algorithm Link Layer and LANs 6-5