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Network Layer - PowerPoint PPT Presentation


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Network Layer. Network layer. Physical layer: move bits between physically connected stations Data link layer: move frames between physically connected stations Network layer: move packets from source A to destination B where A and B can be world-apart. Main Network Layer Design Issues.

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network layer1
Network layer
  • Physical layer: move bits between physically connected stations
  • Data link layer: move frames between physically connected stations
  • Network layer: move packets from source A to destination B where A and B can be world-apart
main network layer design issues
Main Network Layer Design Issues
  • Service interface: what kind of services to be supported at the network layer?
  • Routing: Where to send the packets received?
service interface
Service Interface
  • Should be independent of router technology
  • The upper layer shouldn’t worry about the topology
  • Network address should be uniform
routing
Routing
  • Routing – Determining the path from the source to the destination. At a router, it means which output port should a packet to be sent to
  • Desirable properties of the routing algorithms:
    • correctness: of course
    • simplicity: for efficiency
    • robustness: must be able to sustain the changes in the networks (cannot just rely on reboot)
    • stability: when run long enough, should converge to equilibrium
    • fairness: every one gets to send
    • optimality: as efficient as possible, of course
  • sometimes, these properties may conflict
flooding
Flooding
  • Flooding
    • send all the incoming packet to all outgoing links except the one the packet arrived on
    • Reliable, fast, large overhead, not scalable
    • user: military, distributed database(update everything), highly reliable packets.
flooding1
Flooding
  • Problems?
  • A packet will be sent for infinite number of times
  • Hop count. Don’t forward the same packet for more than H times.
    • How to set H?
shortest path
Shortest Path
  • find the shortest path from the source to all other nodes.
  • Dijkstra algorithm: finding the shortest paths from the source s to all other nodes in the network.

1) Initial set = empty,

2) maintain the distance from s to all other nodes (distance(s, s) = 0, distance(s, t) = infinite)

3) repeat until all nodes are included in the set

4) find a node d currently no in the set with shortest distance

5) include d in the set

6) update the distance from s to all other nodes using

7) if distance(s, m) > distance(s, d) + dist(d, m) then

8) distance(s, m) = distance(s, d) + dist (d, m)

slide9

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Dijkstra’s algorithm: 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

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Step

0

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start N

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ADE

ADEB

ADEBC

ADEBCF

D(F),p(F)

infinity

infinity

4,E

4,E

4,E

shortest path1
Shortest Path
  • Why this gives the shortest path?
  • Node added to the set has found its minimum distance to the source. Suppose this is not true. At a step, we add node W to the set. If there is another path s---Z---W with distance shorter than d(W), where Z is the first node in the path currently not in the set. d(Z) must be less than d(W) (why?) and we would have added Z to the set at this step rather than W.
link state algorithm
Link State Algorithm
  • Each router independently computes optimal paths
    • From itself to every destination
    • Routes are guaranteed to be loop free if
      • Each router sees the same cost for each link
      • Uses the same algorithm (shortest path algorithm for OSPF) to compute the best path
topology dissemination
Topology Dissemination
  • Each router creates a set of link state packets
    • Describing its links to neighbors
    • LSP contains
      • Router id, neighbor’s id, and cost to its neighbor
  • Copies of LSPs are distributed to all routers
    • Using controlled flooding
  • Each router maintains a topology database
    • Database containing all LSPs