西安交通大学计算机网络与通讯课件第四章 Network Layer_A.pptVIP

Chapter 4: Network Layer Chapter goals: understand principles behind network layer services: routing (path selection) dealing with scale how a router works advanced topics: IPv6, multicast instantiation and implementation in the Internet Overview: network layer services hierarchical routing IP what’s inside a router? IPv6 Network layer functions transport packet from sending to receiving hosts network layer protocols in every host, router three important functions: path determination: route taken by packets from source to dest. Routing algorithms switching: move packets from router’s input to appropriate router output call setup: some network architectures require router call setup along path before data flows Network service model Q: What service model for “channel” transporting packets from sender to receiver? guaranteed bandwidth? preservation of inter-packet timing (no jitter)? loss-free delivery? in-order delivery? congestion feedback to sender? Virtual circuits call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host OD) every router on source-dest path s maintain “state” for each passing connection transport-layer connection only involved two end systems link, router resources (bandwidth, buffers) may be allocated to VC to get circuit-like perf. “source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet Datagram networks: the Internet model no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection” packets typically routed using destination host ID packets between same source-dest pair may take different paths Network layer service models: Datagram or VC network: why? Internet data exchange among computers “elastic” service, no strict timing req. “s