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CS 5565 Network Architecture and Protocols

CS 5565 Network Architecture and Protocols. Godmar Back. Lecture 16. Announcements. Midterm April 1 (Wednesday) Project 2A due Apr 8 Required Reading: DCCP by Koehler et al, SIGCOMM 2006. Handling multiple clients using multiple execution contexts. Q.: When would you use which?. A/B:

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CS 5565 Network Architecture and Protocols

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  1. CS 5565Network Architecture and Protocols Godmar Back Lecture 16

  2. Announcements • Midterm April 1(Wednesday) • Project 2A due Apr 8 • Required Reading: • DCCP by Koehler et al, SIGCOMM 2006 CS 5565 Spring 2009

  3. Handling multiple clients using multiple execution contexts Q.: When would you use which? A/B: # grows & shrinks A B C/D: fixed # C D CS 5565 Spring 2009

  4. Network Layer

  5. transports segment from sending to receiving host on sending side encapsulates segments into datagrams on receiving side, delivers segments to transport layer network layer protocols in every host, router router examines header fields in all IP datagrams passing through it network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical The Network Layer CS 5565 Spring 2009

  6. Key Network-Layer Functions • forwarding: move packets from router’s input to appropriate router output • routing: determine route taken by packets from source to dest. • Routing algorithms • analogy: • routing: process of planning trip from source to dest • forwarding: process of getting through single interchange CS 5565 Spring 2009

  7. routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 1 0111 2 3 Interplay between Routing and Forwarding CS 5565 Spring 2009

  8. 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-packet spacing Network Service Model Q: What service model for “channel” transporting datagrams from sender to receiver? CS 5565 Spring 2009

  9. Network Layer Service Models: Guarantees ? Network Architecture Internet ATM ATM ATM ATM Service Model best effort CBR VBR ABR UBR Congestion feedback no (inferred via loss) no congestion no congestion yes no Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes yes no no Order no yes yes yes yes Timing no yes yes no no CS 5565 Spring 2009

  10. Datagram vs VC Networks • Datagram network provides network-layer connectionless service • VC network provides network-layer connection service • Analogous to the transport-layer services, but different in: • Service: host-to-host • No choice: network provides one or the other • Implementation: in the core CS 5565 Spring 2009

  11. 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 “source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path Virtual Circuits CS 5565 Spring 2009

  12. used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet (at least not end-to-end) application transport network data link physical application transport network data link physical Virtual Circuits: Signaling Protocols 6. Receive data 5. Data flow begins 4. Call connected 3. Accept call 1. Initiate call 2. incoming call CS 5565 Spring 2009

  13. VC Implementation A VC consists of: • Path from source to destination • VC numbers, one number for each link along path • Entries in forwarding tables in routers along path • Packet belonging to VC carries a VC number. • VC number must be changed on each link. • New VC number comes from forwarding table CS 5565 Spring 2009

  14. VC number 22 32 12 3 1 2 interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 2 22 2 63 1 18 3 7 2 17 1 97 3 87 … … … … Forwarding Tables in VCN Forwarding table in northwest router: Routers maintain connection state information! CS 5565 Spring 2009

  15. 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 2009

  16. 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 2009

  17. Router Architectures;Packet Forwarding & Classification

  18. Router Architecture Overview Two key router functions: • run routing algorithms/protocol (RIP, OSPF, BGP) • forwarding datagrams from incoming to outgoing link CS 5565 Spring 2009

  19. 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 2009

  20. 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 2009

  21. 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 2009

  22. Packet Classification Source [Gupta & McKeown 2001] CS 5565 Spring 2009

  23. 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 2009

  24. 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 2009

  25. 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 2009

  26. Three Types of Switching Fabrics CS 5565 Spring 2009

  27. 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) • Newer routers: processing done via local processors on line cards CS 5565 Spring 2009

  28. 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 • 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers (not regional or backbone) CS 5565 Spring 2009

  29. 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 2009

  30. 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 2009

  31. 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 2009

  32. HFSC • Decouple latency and bandwidth allocations Source: [Zhang] CS 5565 Spring 2009

  33. 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 2009

  34. 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 2009

  35. 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 2009

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