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Circuit Switching, Packet Switching, the Network Layer Data Communications & Networks Session 7 – Main Theme Networks: Part I Circuit Switching, Packet Switching, The Network Layer Dr. Jean-Claude Franchitti New York University Computer Science Department Courant Institute of Mathematical Sciences Adapted from course textbook resources Computer Networking: A Top-Down Approach, 5/E Copyright 1996-2009 J.F. Kurose and K.W. Ross, All Rights Reserved 1 Agenda 11 SessionSession OverviewOverview 22 NetworksNetworks PartPart 11 33 SummarySummary andand ConclusionConclusion 2 What is the class about? Course description and syllabus: »http://www.nyu.edu/classes/jcf/g22.2262-001/ »http://www.cs.nyu.edu/courses/spring10/G22.2262- 001/index.html Textbooks: » Computer Networking: A Top-Down Approach (5th Edition) James F. Kurose, Keith W. Ross Addison Wesley ISBN-10: 0136079679, ISBN-13: 978-0136079675, 5th Edition (03/09) 3 Course Overview Computer Networks and the Internet Application Layer Fundamental Data Structures: queues, ring buffers, finite state machines Data Encoding and Transmission Local Area Networks and Data Link Control Wireless Communications Packet Switching OSI and Internet Protocol Architecture Congestion Control and Flow Control Methods Internet Protocols (IP, ARP, UDP, TCP) Network (packet) Routing Algorithms (OSPF, Distance Vector) IP Multicast Sockets 4 Networks Part 1 Session in Brief Understand principles behind network layer services: Network layer service models Forwarding versus routing How a router works Instantiation, implementation in the Internet Conclusion 5 Icons / Metaphors Information Common Realization Knowledge/Competency Pattern Governance Alignment Solution Approach 66 Agenda 11 SessionSession OverviewOverview 22 NetworksNetworks PartPart 11 33 SummarySummary andand ConclusionConclusion 7 Networks Part 1 Agenda Introduction Virtual circuit and datagram networks What’s inside a router IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 8 Switching Networks Long distance transmission is typically done over a network of switched nodes Nodes not concerned with content of data End devices are stations Computer, terminal, phone, etc. A collection of nodes and connections is a communications network Data routed by being switched from node to node 9 Technology Two different switching technologies Circuit switching Packet switching 10 Simple Switched Network 11 Circuit Switching Dedicated communication path between two stations (during conversation) Three phases Establish Transfer Disconnect Must have switching capacity and channel capacity to establish connection Must have intelligence to work out routing 12 Circuit Switching - Issues Circuit switching is inefficient (designed for voice) Resources dedicated to a particular call Much of the time a data connection is idle Data rate is fixed Both ends must operate at the same rate Set up (connection) takes time Once connected, transfer is transparent 13 Packet Switching – Basic Operation Data transmitted in small packets Typically 1000 octets Longer messages split into series of packets Each packet contains a portion of user data plus some control info Control info Routing (addressing) info Packets are received, stored briefly (buffered) and passed on to the next node Store and forward 14 Use of Packets 15 Network layer application transport segment from transport network sending to receiving host data link physical on sending side network network data link data link network physical physical encapsulates segments data link physical network network into datagrams data link data link physical physical on receiving side, delivers network network data link data link segments to transport physical physical network data link layer physical application network layer protocols in network transport network data link network data link network physical data link every host, router physical data link physical router examines header physical fields in all IP datagrams passing through it 16 Two Key Network-Layer Functions forwarding: move analogy: packets from router’s input to routing: process of appropriate router planning trip from output source to dest routing: determine forwarding: process route taken by of getting through packets from source single interchange to dest. » routing algorithms 17 Interplay between routing and forwarding routing algorithm local forwarding table header value output link 0100 3 0101 2 0111 2 1001 1 value in arriving packet’s header 0111 1 3 2 18 Connection setup 3rd important function in some network architectures: » ATM, frame relay, X.25 before datagrams flow, two end hosts and intervening routers establish virtual connection » routers get involved network vs transport layer connection service: » network: between two hosts (may also involve intervening routers in case of VCs) » transport: between two processes 19 Network service model Q: What service model for “channel” transporting datagrams from sender to receiver? Example services for Example services for a individual flow of datagrams: datagrams: in-order datagram guaranteed delivery delivery guaranteed delivery guaranteed with less than 40 minimum bandwidth msec delay to flow restrictions on changes in inter- packet spacing 20 Network layer service models Guarantees ? Network Service Congestion Architecture Model Bandwidth Loss Order Timing feedback Internet best effort none no no no no (inferred via loss) ATM CBR constant yes yes yes no rate congestion ATM VBR guaranteed yes yes yes no rate congestion ATM ABR guaranteed no yes no yes minimum ATM UBR none no yes no no 21 Networks Part 1 Agenda Introduction Virtual circuit and datagram networks What’s inside a router IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 22 Network layer connection and connection-less service datagram network provides network-layer connectionless service VC network provides network-layer connection service analogous to the transport-layer services, but: » service: host-to-host » no choice: network provides one or the other » implementation: in network core 23 Virtual circuits “source-to-dest path behaves much like telephone circuit” » performance-wise » network actions along source-to-dest path call setup, teardown for each call before data can flow 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 VC (dedicated resources = predictable service) 24 VC implementation a VC 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 25 Forwarding table VC number 12 22 32 1 3 2 Forwarding table in interface northwest router: number Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 …… … … Routers maintain connection state information! 26 Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet application 6. Receive data application transport 5. Data flow begins transport network 4. Call connected 3. Accept call network data link 1. Initiate call 2. incoming call data link physical physical 27 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 application application transport transport network network data link 1. Send data 2. Receive data data link physical physical 28 Forwarding table 4 billion possible entries Destination Address Range Link Interface 11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111 otherwise 3 29 Longest prefix matching Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3 Examples DA: 11001000 00010111 00010110 10100001 Which interface? DA: 11001000 00010111 00011000 10101010 Which interface? 30 Datagram or VC network: why? Internet (datagram) ATM (VC) data exchange among evolved from telephony computers human conversation: » “elastic” service, no strict » strict timing, reliability timing req. requirements “smart” end systems » need for guaranteed (computers) service » can adapt, perform control, “dumb” end systems error recovery » telephones » simple inside network, complexity at “edge” » complexity inside network many link types » different characteristics » uniform service difficult 31 Networks Part 1 Agenda Introduction Virtual circuit and datagram networks What’s inside a router IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 32 Router Architecture Overview Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link 33 Input Port Functions Physical layer: bit-level reception Data link layer: Decentralized switching: e.g., Ethernet given datagram dest., lookup output port (see Textbook using forwarding table in input port memory
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