Chapter 7 Packet-Switching Networks
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Chapter 7 Packet-Switching Networks Network Services and Internal Network Operation Packet Network Topology Datagrams and Virtual Circuits Routing in Packet Networks Shortest Path Routing ATM Networks Traffic Management Chapter 7 Packet-Switching Networks Network Services and Internal Network Operation Network Layer z Network Layer: the most complex layer z Requires the coordinated actions of multiple, geographically distributed network elements (switches & routers) z Must be able to deal with very large scales z Billions of users (people & communicating devices) z Biggest Challenges z Addressing: where should information be directed to? z Routing: what path should be used to get information there? Packet Switching t1 t0 Network z Transfer of information as payload in data packets z Packets undergo random delays & possible loss z Different applications impose differing requirements on the transfer of information Network Service Messages Messages Transport Segments Transport layer layer Network Network service service Network Network Network Network layer layer layer layer Data link Data link Data link Data link End End layer layer layer layer system system Physical Physical β α Physical Physical layer layer layer layer z Network layer can offer a variety of services to transport layer z Connection-oriented service or connectionless service z Best-effort or delay/loss guarantees Network Service vs. Operation Network Service Internal Network z Connectionless Operation z Datagram Transfer z Connectionless z Connection-Oriented z IP z Reliable and possibly z Connection-Oriented constant bit rate transfer z Telephone connection z ATM Various combinations are possible z Connection-oriented service over Connectionless operation z Connectionless service over Connection-Oriented operation z Context & requirements determine what makes sense Complexity at the Edge or in the Core? C 1 2 3 2 1 2 2 1 1 End system End system 1 1 1 α 2 2 2 β 4 3 2 1 1 2 3 2 1 1 2 3 2 1 1 2 3 4 Medium 2 A B 1 Network 1 Physical layer entity 3 Network layer entity 3 Network layer entity 2 Data link layer entity 4 Transport layer entity The End-to-End Argument for System Design z An end-to-end function is best implemented at a higher level than at a lower level z End-to-end service requires all intermediate components to work properly z Higher-level better positioned to ensure correct operation z Example: stream transfer service z Establishing an explicit connection for each stream across network requires all network elements (NEs) to be aware of connection; All NEs have to be involved in re- establishment of connections in case of network fault z In connectionless network operation, NEs do not deal with each explicit connection and hence are much simpler in design Network Layer Functions Essential z Routing: mechanisms for determining the set of best paths for routing packets requires the collaboration of network elements z Forwarding: transfer of packets from NE inputs to outputs z Priority & Scheduling: determining order of packet transmission in each NE Optional: congestion control, segmentation & reassembly, security Chapter 7 Packet-Switching Networks Packet Network Topology End-to-End Packet Network z Packet networks very different than telephone networks z Individual packet streams are highly bursty z Statistical multiplexing is used to concentrate streams z User demand can undergo dramatic change z Peer-to-peer applications stimulated huge growth in traffic volumes z Internet structure highly decentralized z Paths traversed by packets can go through many networks controlled by different organizations z No single entity responsible for end-to-end service Access Multiplexing Access MUX To packet network z Packet traffic from users multiplexed at access to network into aggregated streams z DSL traffic multiplexed at DSL Access Mux z Cable modem traffic multiplexed at Cable Modem Termination System Oversubscription z Access Multiplexer r z N subscribers connected @ c bps to mux r z Each subscriber active r/c of time •• • •• •• • •• nc z Mux has C=nc bps to network r z Oversubscription rate: N/n Nr Nc z Find n so that at most 1% overflow probability Feasible oversubscription rate increases with size N r/c n N/n 10 0.01 1 10 10 extremely lightly loaded users 10 0.05 3 3.3 10 very lightly loaded user 10 0.1 4 2.5 10 lightly loaded users 20 0.1 6 3.3 20 lightly loaded users 40 0.1 9 4.4 40 lightly loaded users 100 0.1 18 5.5 100 lightly loaded users Home LANs WiFi Ethernet Home Router To packet network z Home Router z LAN Access using Ethernet or WiFi (IEEE 802.11) z Private IP addresses in Home (192.168.0.x) using Network Address Translation (NAT) z Single global IP address from ISP issued using Dynamic Host Configuration Protocol (DHCP) LAN Concentration Switch / Router zzzzzz zzzzzz z LAN Hubs and switches in the access network also aggregate packet streams that flows into switches and routers Servers have Campus Network redundant connectivity to backbone Organization To Internet or Servers wide area network s s Gateway Backbone R R R S S S Departmental R R Server R s s s OnlyHigh-speed outgoing packetscampus leave s s s s LANbackbone through net s s routerconnects dept routers Connecting to Internet Service Provider Internet service provider Border routers Campus Border routers Network Interdomain level Autonomous system or domain Intradomain level s LAN network administered s s by single organization Internet Backbone National Service Provider A National Service Provider B NAP NAP National Service Provider C Private peering z Network Access Points: set up during original commercialization of Internet to facilitate exchange of traffic z Private Peering Points: two-party inter-ISP agreements to exchange traffic (a) National Service Provider A National Service Provider B NAP NAP National Service Provider C Private peering (b) NAP RA Route RB Server LAN RC Key Role of Routing How to get packet from here to there? z Decentralized nature of Internet makes routing a major challenge z Interior gateway protocols (IGPs) are used to determine routes within a domain z Exterior gateway protocols (EGPs) are used to determine routes across domains z Routes must be consistent & produce stable flows z Scalability required to accommodate growth z Hierarchical structure of IP addresses essential to keeping size of routing tables manageable Chapter 7 Packet-Switching Networks Datagrams and Virtual Circuits The Switching Function z Dynamic interconnection of inputs to outputs z Enables dynamic sharing of transmission resource z Two fundamental approaches: z Connectionless z Connection-Oriented: Call setup control, Connection control Backbone Network Switch Access Network Packet Switching Network Packet switching network User z Transfers packets between users Transmission line z Transmission lines + packet switches Network (routers) Packet switch z Origin in message switching Two modes of operation: z Connectionless z Virtual Circuit Message Switching z Message switching invented for telegraphy Message z Entire messages Message multiplexed onto shared Message lines, stored & forwarded z Headers for source & Source destination addresses Message z Routing at message switches z Connectionless Switches Destination Message Switching Delay Source T t Switch 1 t τ Switch 2 t t Destination Delay Minimum delay = 3τ + 3T Additional queueing delays possible at each link Long Messages vs. Packets 1 Mbit message source dest BER=p=10-6 BER=10-6 How many bits need to be transmitted to deliver message? z Approach 1: send 1 Mbit z Approach 2: send 10 message 100-kbit packets z Probability message z Probability packet arrives arrives correctly correctly −6 106 −10610−6 −1 −6 105 −10510−6 −0.1 Pc = (1−10 ) ≈ e = e ≈1/ 3 Pc′ = (1−10 ) ≈ e = e ≈ 0.9 z On average it takes about z On average it takes about 1.1 transmissions/hop 3 transmissions/hop z Total # bits transmitted ≈ z Total # bits transmitted ≈ 6 Mbits 2.2 Mbits Packet Switching - Datagram z Messages broken into smaller units (packets) z Source & destination addresses in packet header Packet 1 z Connectionless, packets routed independently Packet 1 (datagram) Packet 2 z Packet may arrive out of order z Pipelining of packets across Packet 2 network can reduce delay, increase throughput Packet 2 z Lower delay that message switching, suitable for interactive traffic Packet Switching Delay Assume three packets corresponding to one message traverse same path t 1 2 3 t 1 2 3 t 1 2 3 t Delay Minimum Delay = 3τ + 5(T/3) (single path assumed) Additional queueing delays possible at each link Packet pipelining enables message to arrive sooner Delay for k-Packet Message over L Hops Source t 1 2 3 Switch 1 t τ 1 2 3 Switch 2 t 1 2 3 t Destination 3 hops L hops 3τ + 2(T/3) first bit received Lτ + (L-1)P first bit received 3τ + 3(T/3) first bit released Lτ + LP first bit released 3τ + 5 (T/3) last bit released Lτ + LP + (k-1)P last bit released where T = k P Routing Tables in Datagram Networks Destination Output z Route determined by table address port lookup z Routing decision involves 0785 7 finding next hop in route to given destination 1345 12 z Routing table has an entry for each destination 1566 6 specifying output port that leads to next hop z Size of table becomes impractical for very large number of destinations 2458 12 Example: Internet Routing z Internet protocol uses datagram packet switching across networks z Networks are treated as data links z Hosts have two-port IP address: z Network address + Host address z Routers do table lookup