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1、Chapter 4 The Network Layer,Computer Networking A Top-Down Approach Featuring the Internet 计算机网络-自顶向下方法与Internet特色,Introduction,Chapter goals understand principles behind network layer services: routing (path selection) dealing with scale how a router works advanced topics: IPv6, mobility instantiat
2、ion and implementation in the Internet,roadmap,4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 Whats inside a router 4.4 IP: Internet Protocol 4.5 Routing algorithms Link state、Distance Vector、Hierarchical routing 4.6 Routing in the Internet RIP、OSPF、BGP 4.7 Broadcast and multicast r
3、outing Summary,4.1 Network layer introduction,Services description Key Network-Layer Functions Routing Forwarding Connection setup Network service model,1、 Key Network-Layer Functions,Forwardingmove packets from routers input to appropriate router output Routingdetermine the route (path) taken by pa
4、ckets from source to dest. routing algorithms,4.1 introduction,1、 Key Network-Layer Functions,Connection setup 3rd important function in some network architectures: ATM, frame relay, X.25 Before datagrams flow, two hosts and intervening routers establish virtual connection Routers get involved,4.1 i
5、ntroduction,Network and transport layer service: Network Layer: between two hosts point to point. Transport Layer: between two processes end to end.,4.1 introduction,2、 Network service model,2、 Network service model,4.1 introduction,Q: Whats the service model of the channel connecting the transporti
6、ng layer in the sending and receiving hosts? Example services for individual datagrams: guaranteed delivery Guaranteed delivery with less than 40 msec delay Example services for a flow of datagrams: In-order datagram delivery Guaranteed minimum bandwidth to flow Restrictions on changes in inter-pack
7、et spacing (delay jitter),2、 Network service model,4.1 introduction,Network ArchitectureInternetATMATMATMATM,Service Modelbest effortCBRVBRABRUBR,Bandwidthnoneconstant rate guaranteed rate guaranteed minimum none,Lossnoyesyesnono,Ordernoyesyesyesyes,Timingnoyesyesnono,Congestion feedbackno (inferred
8、 via loss) no congestion no congestion yesno,Guarantees ?,roadmap,4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 Whats inside a router 4.4 IP: Internet Protocol 4.5 Routing algorithms Link state、Distance Vector、Hierarchical routing 4.6 Routing in the Internet RIP、OSPF、BGP 4.7 Broadc
9、ast and multicast routing Summary,4.2 Virtual circuit and datagram networks,Network layer connection and connection-less service Virtual-Circuit Networks Datagram Networks Datagram or VC network: why?,Introduction,Datagram network provides network-layer connectionless service VC network provides net
10、work-layer connection service Analogous to the transport-layer services, but: Service: host-to-host (aka. End to End) No choice: network provides one or the other Implementation: in the network core,4.2 Virtual circuit and datagram networks,1、Virtual-Circuit Networks,4.2 Virtual circuit and datagram
11、 networks,“source-to-dest path behaves much like telephone circuit”, but there are crucial differences，so called virtual circuit rather than circuit.shared or dedicated resource actions along source-to-dest pathRouter determines the packets path according to the information in the packet itself,call
12、 setup for each call before data can flow teardown the connection each packet carries VC identifier (not destination host address) every router on source-dest path maintains “state” for each passing connection link, router resources (bandwidth, buffers) may be allocated to a VC,1、Virtual-Circuit Net
13、works,4.2 Virtual circuit and datagram networks,VC implementation,A VC consists of: A path from source to destination VC numbers, one number for each link along path Entries in forwarding tables in routers along path Packets belonging to a VC carries the VC number. VC number must be changed on each
14、link. VC switching New VC number comes from forwarding table,4.2 Virtual circuit and datagram networks,Virtual circuits: signaling protocols,4.2 Virtual circuit and datagram networks,used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in todays Internet (packet switching netw
15、orks),2、 Datagram Networks,4.2 Virtual circuit and 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 packets between same source-dest pair may take different paths,
16、Forwarding table of Datagram Net,Destination Address Range output Link Interface11001000 00010111 00010000 00000000through 0 11001000 00010111 00010111 1111111111001000 00010111 00011000 00000000through 111001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000through 211001000 00010111 00011111 11111111 otherwise 3,
