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CS 5565 Network Architecture and Protocols

CS 5565 Network Architecture and Protocols. Godmar Back. Lecture 28. Announcements. Project 2A due Apr 5 11:59pm Reading Assignment Chapter 4.1-4.6. Network Layer. no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection”

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CS 5565 Network Architecture and Protocols

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  1. CS 5565Network Architecture and Protocols Godmar Back Lecture 28

  2. Announcements • Project 2A due Apr 5 11:59pm • Reading Assignment Chapter 4.1-4.6 CS 5565 Spring 2006

  3. Network Layer

  4. 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 transport network data link physical application transport network data link physical Datagram Networks 1. Send data 2. Receive data CS 5565 Spring 2006

  5. Internet data exchange among computers “elastic” service, no strict timing req. “smart” end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at “edge” many link types different characteristics uniform service difficult ATM evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service “dumb” end systems telephones complexity inside network Internet vs ATM CS 5565 Spring 2006

  6. Router Architectures;Packet Forwarding & Classification

  7. Router Architecture Overview Two key router functions: • run routing algorithms/protocol (RIP, OSPF, BGP) • forwarding datagrams from incoming to outgoing link CS 5565 Spring 2006

  8. Input Port Functions Decentralized switching: • given datagram dest., lookup output port using forwarding table in input port memory • goal: complete input port processing at ‘line speed’ • queuing: if datagrams arrive faster than forwarding rate into switch fabric Physical layer: bit-level reception Data link layer: e.g., Ethernet CS 5565 Spring 2006

  9. Forwarding Tables Destination Address RangeLink 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 CS 5565 Spring 2006

  10. Longest Prefix Matching Prefix MatchLink Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3 Examples Which interface? DA: 11001000 00010111 00010110 10100001 Which interface? DA: 11001000 00010111 00011000 10101010 Which interface? DA: 11001000 00010111 00011010 00100101 CS 5565 Spring 2006

  11. Packet Classification Source [Gupta & McKeown 2001] CS 5565 Spring 2006

  12. Performance Metrics for Packet Classification (Gupta/McKeown) • Search speed • 10 Gbps will have 31.25 Mp/s for min-TCP • Low storage requirements • The smaller, the faster (SRAM) • Ability to handle large real-life classifiers • Fast updates • Scalability in number of header fields used for classification • Flexibility in specification • Not just prefixes: ranges, operators, wildcards, etc. CS 5565 Spring 2006

  13. Example for 2 Fields • Geometry problem: N number of regions, d number of dimensions; regions are prioritized • Best worst case time O(log N) time with O(Nd) space • Best worst case space O(N) with O((log N)d-1) time Priority CS 5565 Spring 2006

  14. Packet Classification Solutions • Basic data structures • Linear search, caching, hierarchical tries, set-pruning tries • Geometry-based structures • Grid-of-tries, AQT (area-based quadtree), FIS (fat-inverted segment tree) • Heuristics • RFC (recursive flow classification), hierarchical cuttings, tuple-space search • Hardware • Ternary CAM (content-addressable memory), bitmap-intersection CS 5565 Spring 2006

  15. Three Types of Switching Fabrics CS 5565 Spring 2006

  16. Memory Input Port Output Port System Bus Switching Via Memory First generation routers: • traditional computers with switching under direct control of CPU • packet copied to system’s memory • speed limited by memory bandwidth (2 bus crossings per datagram) CS 5565 Spring 2006

  17. Switching Via a Bus • datagram from input port memory to output port memory via a shared bus • bus contention: switching speed limited by bus bandwidth • 1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone) CS 5565 Spring 2006

  18. Switching Via An Interconnection Network • Overcome bus bandwidth limitations • Banyan/Butterfly networks, other interconnection nets initially developed to connect processors in multiprocessor machines • Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. • Cisco 12000: switches Gbps through the interconnection network • See Cisco Whitepaper for more information & background CS 5565 Spring 2006

  19. Output Ports • Buffering required when datagrams arrive from fabric faster than the transmission rate • Possibility of queueing (delay) and loss due to output port buffer overflow! • Scheduling discipline chooses among queued datagrams for transmission CS 5565 Spring 2006

  20. Scheduling Disciplines • Single or multiple queues • If multiple: Flow-based or Class-based • FCFS, RR (Round-Robin), WRR • Various priority schemes: • expedited packets, assured forwarding • WFQ (Weighted Fair Queuing) • proportional sharing of link between queues • HFSC (Hierarchical Fair Service Curve) • hierarchical extension, better queuing delay bounds CS 5565 Spring 2006

  21. Active Queue Management (AQM) • When should packets be dropped? • Goal: avoid congestion • Simplest policy: drop-tail • If queue is full, drop new arrivals • More sophisticated: • Random Early Detection (RED) • Before queue fills up, mark some packets randomly for drop • Idea: force TCP congestion control to throttle rate • Lots of research in this area CS 5565 Spring 2006

  22. Input Port Queuing • Fabric slower than input ports combined  queueing may occur at input queues • Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward • queueing delay and loss due to input buffer overflow! CS 5565 Spring 2006

  23. Summary • Basics of Network Layer • Routing (path selection) vs Forwarding (switching) • Service models • Datagram Networks vs VC Networks • Basics of routers • Packet classification, Packet Scheduling, AQM • Next: Routing Algorithms CS 5565 Spring 2006

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