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ECE453 – Introduction to Computer Networks

ECE453 – Introduction to Computer Networks. Lecture 9 - The Network Layer (I) - Routing. Recap. Framing Error detection and correction Reliable or unreliable data transfer Channel allocation protocol. network data link physical. network data link physical. logical end-end transport.

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ECE453 – Introduction to Computer Networks

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  1. ECE453 – Introduction to Computer Networks Lecture 9 - The Network Layer (I) - Routing

  2. Recap • Framing • Error detection and correction • Reliable or unreliable data transfer • Channel allocation protocol

  3. network data link physical network data link physical logical end-end transport Network Core and Network Edge Network core application transport network data link physical Network edge application transport network data link physical

  4. Network Core – Information Transmission • Circuit switching • Telephone system • Message switching • Mail delivery • The message travels as a complete unit. At any one time, it completely exists in one place. • Packet switching • The Internet

  5. Design Issues • Store-and-forward packet switching • Services to the transport layer • Connection-oriented vs. Connectionless • Quality of service (QoS)

  6. Connectionless Best-effort No guarantee The Internet No advance setup is needed Datagram subnet Connection-oriented Guaranteed delivery ATM A path from the source router to the destination router is established before any data packets can be sent Virtual circuit Network Layer Services

  7. ICMP protocol • error reporting • router “signaling” • IP protocol • addressing conventions • datagram format • packet handling conventions • Routing protocols • path selection • RIP, OSPF, BGP routing table The Internet Network Layer Transport layer: TCP, UDP Network layer Link layer physical layer

  8. Routing - In the Middle of Intersection …

  9. Nonadaptive algorithms (or static routing) Adaptive algorithms (or dynamic routing) Global algorithm (have a global knowledge – a map) Decentralized algorithm (get information only from neighbor) Different Strategies

  10. 5 3 5 2 2 1 3 1 2 1 A D B E F C Graph Abstraction for Routing Algorithms • Graph nodes  Routers • Graph edge  Physical links • Edge weight  Link cost • Link cost • Delay, power consumption, congestion level, $cost, etc. • Good path or optimal path • Minimum link cost

  11. Two Fundamental Routing Algorithms • Link state routing • A global algorithm • Distance vector routing (or Bellman-Ford routing, Ford-Fulkerson routing) • A decentralized algorithm • The original ARPANET routing algorithm, replaced by LS routing in 1979

  12. A Link State Routing Algorithm Dijkstra’s algorithm • Net topology, link costs known to all nodes • accomplished via “link state broadcast” • all nodes have the same info • computes least cost paths from the source to all other nodes • Iterative algorithm

  13. 5 3 5 2 2 1 3 1 2 1 A D E B F C Dijkstra’s Algorithm: An Example D(B),p(B) 2,A 2,A 2,A D(D),p(D) 1,A D(C),p(C) 5,A 4,D 3,E 3,E D(E),p(E) infinity 2,D Step 0 1 2 3 4 5 start N A AD ADE ADEB ADEBC ADEBCF D(F),p(F) infinity infinity 4,E 4,E 4,E

  14. A A A A D D D D B B B B C C C C 2+e 2+e 0 0 1 1 1+e 1+e 0 e 0 0 Dijkstra’s Algorithm: Discussion Algorithm complexity: n nodes • each iteration: need to check all nodes, not in N • n*(n+1)/2 comparisons: O(n**2) • more efficient implementations possible: O(nlogn) Oscillations possible: • e.g., link cost = amount of carried traffic 1 1+e 0 2+e 0 0 0 0 e 0 1 1+e 1 1 e … recompute … recompute routing … recompute initially

  15. cost to destination via E D () A B C D A 1 7 6 4 B 14 8 9 11 D 5 5 4 2 destination distance from X to Y, via Z as next hop X = D (Y,Z) Z c(X,Z) + min {D (Y,w)} = w DV Routing • Each router maintains a distance table • Initialization • 0 for itself • Infinity for non-neighbor • Link cost for neighbor • Message exchange between neighbors • When a neighbor first comes up • When information changes (e.g. change in link cost) • Distance vector calculation • Minimize cost to each destination Iterative routing Distributed routing

  16. 2 1 7 X Z Y DV: Example Y X Z

  17. 1 4 1 50 X Z Y DV: Link Cost Changes Y “good news travels fast” Z X algorithm terminates

  18. 60 4 1 50 X Z Y DV: Link Cost Changes “bad news travels slow” Count to infinity problem algorithm continues on!

  19. cost to destination via E D () A B C D A 1 7 6 4 B 14 8 9 11 D 5 5 4 2 1 7 2 8 1 destination 2 A D E B C E E E D (C,D) D (A,D) D (A,B) D B D c(E,B) + min {D (A,w)} c(E,D) + min {D (A,w)} c(E,D) + min {D (C,w)} = = = w w w = = = 8+6 = 14 2+2 = 4 2+3 = 5 Distance Table: An Example loop! loop!

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