1 Network Layer Network Layer Recall: Circuit Switching vs. Packet - PDF document

Network Layer Network Layer Chapter 4: Network Layer Mobile network Chapter goals: Global ISP  understand principles behind network layer Network Layer services: Home network  network layer service models Regional ISP  forwarding versus routing Yanmin Zhu  how a router works Institutional network Department of Computer  routing (path selection) Science and Engineering  dealing with scale  advanced topics: IPv6, mobility  instantiation, implementation in the Internet Network Layer Network Layer 1 2 Network Layer Introduction Network Layer Network vs transport layer Network Layer connection service:  Introduction  Routing algorithms  network: between two hosts  Virtual circuit and  Link state  Distance Vector datagram networks  may also involve intervening routers  Hierarchical routing  What’s inside a router in case of VCs  Routing in the  IP: Internet Protocol Internet  Datagram format  RIP  IPv4 addressing  transport: between two  OSPF  ICMP  BGP processes  IPv6  only involve two end hosts Network Layer Network Layer 3 4 Network Layer Network Layer Network layer Two Key Network-Layer Functions  transport segment from application sending to receiving host transport network  forwarding: move analogy: data link  on sending side physical packets from router’s network network data link encapsulates segments data link network  routing: process of physical physical data link input to appropriate into datagrams physical network network planning trip from data link data link router output physical physical  on rcving side , delivers source to dest segments to transport network network data link data link  routing: determine physical physical layer network  forwarding: process data link route taken by physical  network layer protocols application of getting through network transport packets from source network in every host, router data link network data link physical network data link single interchange physical to dest. data link physical  router examines header physical fields in all IP datagrams  routing algorithms passing through it Network Layer Network Layer 5 6 1

Network Layer Network Layer Recall: Circuit Switching vs. Packet Interplay between routing and forwarding Switching routing algorithm  Fundamental question: how is local forwarding table header value output link data transferred 0100 3 through net? 0101 2 0111 2 1001 1  Circuit switching: dedicated circuit value in arriving per call: telephone packet’s header net 1 0111  Packet switching: 2 3 data sent thru net in discrete “chunks” Network Layer Network Layer 7 8 Network Layer Network Layer Connection setup (VC-Switching) Network service model Q: What service model for “channel” transporting  Important function in some network datagrams from sender to receiver? architectures: Example services for a Example services for  ATM, frame relay, X.25 flow of datagrams: individual datagrams:  in-order datagram  guaranteed delivery delivery  guaranteed delivery  guaranteed minimum  before datagrams flow, two end hosts and with less than 40 msec bandwidth to flow delay intervening routers establish virtual  restrictions on connection changes in inter-  routers get involved packet spacing Network Layer Network Layer 9 10 Network Layer Network Layer Virtual circuit and datagram networks Network layer service models: Network Layer  Introduction  Routing algorithms Guarantees ? Network Service Congestion  Virtual circuit and  Link state Model Bandwidth Loss Architecture Order Timing feedback datagram networks  Distance Vector Internet best effort none no no no  Hierarchical routing no (inferred  What’s inside a router via loss)  Routing in the  IP: Internet Protocol ATM CBR constant yes yes yes no Internet rate  Datagram format congestion  RIP ATM VBR guaranteed yes yes yes no  IPv4 addressing  OSPF rate congestion  ICMP ATM ABR guaranteed no yes no yes  BGP  IPv6 minimum UBR none no ATM yes no no Network Layer Network Layer 11 12 2

Network Layer Network Layer Network layer connection and Virtual circuits connection-less service “source -to-dest path behaves much like telephone datagram network VC network circuit”  performance-wise provides network- provides network-  network actions along source-to-dest path layer connectionless layer connection service service  call setup, teardown for each call before data can flow  each packet carries VC identifier (not destination host analogous to the transport-layer services, address) but:  every router on source- dest path maintains “ state ” for  service: host-to-host each passing connection  link, router resources (bandwidth, buffers) may be  no choice: network provides one or the other allocated to VC (dedicated resources = predictable service)  implementation: in network core Network Layer Network Layer 13 14 Network Layer Network Layer Forwarding table VC implementation VC number A B 22 32 12 a VC consists of: 3 1 2 path from source to destination 1. Forwarding table router A: interface VC numbers, one number for each link along 2. C D number path entries in forwarding tables in routers along 3. Incoming interface Incoming VC # Outgoing interface Outgoing VC # path 1 12 3 22  packet belonging to VC carries VC number 2 63 1 18 (rather than dest address) 3 7 2 17  VC number can be changed on each link. 1 97 3 87 … … … … New VC number comes from forwarding table  Routers maintain connection state information! Network Layer Network Layer 15 16 Network Layer Network Layer Datagram networks Virtual circuits: signaling protocols  no call setup at network layer  routers: no state about end-to-end connections  used to setup, maintain, teardown VC  no network- level concept of “connection”  used in ATM, frame-relay, X.25  packets forwarded using destination host address  not used in today’s Internet  packets between same source-dest pair may take different paths application application 6. Receive data 5. Data flow begins transport application transport application 4. Call connected network 3. Accept call transport network transport 1. Initiate call network data link 2. incoming call 1. Send data network data link 2. Receive data physical data link physical data link physical physical Network Layer Network Layer 17 18 3