1. Network Layer

1. Network Layer - Why network layer? Network layer role is...

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Why network layer? Network layer role is to move packets from sending host to a receiving host. transport segment from sending to receiving host on sending side encapsulates segments into datagrams on rcving side, delivers segments to transport layer network layer protocols in every host, router router examines header fields in all IP datagrams passing through it What does network layer enable? Two Key Network-Layer functions: Forwarding : move packets from router’s input to appropriate router output Routing : determine route taken by packets from source to destination. Routing algorithms Analogy: Routing : process of planning trip from source to destination. Forwarding : process of getting through single interchange. Datagram (IP) network versus Virtual Circuit (VC) network Datagrams and virtual circuits represent the two major approaches to network layer service that have been taken over the years. The Internet is a datagram network , in which each network-layer datagram carries the IP address of the final destination of the datagram . This address is used by a router in forwarding the datagram toward its final destination. In a virtual circuit (VC) network, each packet of data (called a “cell” in ATM VC networks) carries a VC number, which is used by a switch in forwarding the packet of data toward its destination . In a VC network, when a call is made between a source and destination, a call setup procedure is needed to create state in each switch on the endto-end path that matches the VC number for this call with the outgoing switch port to which the VC’s packets of data will be forwarded. Similarly, a call-teardown procedure is needed in VC networks. Datagram network provides network-layer connectionless service VC network provides network-layer connection service Virtual Circuits “source-to-dest path behaves much like telephone circuit” Performance-wise Network actions along source-to-dest path each packet carries VC identifier(not the destination host address)
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call setup, teardown for each call before data can flow every router on source-dest path maintains “state” for each passing connection link, router resources bandwidth, buffers) may be allocated to VC(deciated resources = predictable service) VC Implementation a Virtual Circuits consists of: 1. path from source to destination 2. VC numbers, one number for each link along path 3. entries in forwarding tables in routers along path Packet belonging to VC carries VC number (rather than dest address) VC number can be changed on each link. New VC number comes from forwarding table 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 no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection” packets forwarded using destination host address
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1. Network Layer - Why network layer? Network layer role is...

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