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  • 1. Routing and Switching Fabrics
    • Outline
      • Link state routing
      • Link weights
      • Switching Fabrics
  • 2. Intradomain Routing Summary
    • Intradomain routing protocols determine how forwarding tables are maintained in routers
      • Least cost algorithms
    • Distance vector routing
      • Algorithm based on building forwarding table by distributing vector of distances to neighbors
      • Completely distributed and based only on knowledge of immediate neighbors
      • Known to converge under static conditions
      • Count to infinity problem
      • Limited network diameter
  • 3. Link State (Dijkstra’s algorithm,OSPF)
    • Find SP from a given node by sending path data to all nodes and developing paths in order of increasing length
    • Strategy
      • Route calculation is based on sum of all accumulated link state information
      • All nodes forward all information to all directly connected links
    • Link State Packet (LSP) – created by each node
      • id of the node that created the LSP
      • cost of the link to each directly connected neighbor
      • sequence number (SEQNO)
      • time-to-live (TTL) for this packet
  • 4. Link State contd.
    • Send out LSP based on timer and triggers
    • Timer should be coarse grained
    • Seqno’s do not wrap around
      • Since when routers reboot – they start at seqno 0
      • 64 bit field
    • Routing table is not computed until LSP’s from all nodes have been received
  • 5. Route Calculation
    • Dijkstra’s shortest path algorithm
    • Let
      • N denotes set of nodes in the graph
      • l ( i , j ) denotes non-negative cost (weight) for edge ( i , j )
      • s denotes this node
      • M denotes the set of nodes incorporated so far
      • C ( n ) denotes cost of the path from s to node n
    • M = { s }
    • for each n in N - { s }
      • C ( n ) = l ( s , n ) /* Costs of directly connected nodes */
    • while ( N != M )
      • M = M union { w } such that C ( w ) /* Add a node */
      • is the minimum for all w in ( N - M )
      • for each n in ( N - M ) /* Recalculate costs */
        • C ( n ) = MIN( C ( n ), C ( w ) + l ( w, n ))
  • 6. Example
    • Itrn M B Path C Path D Path E Path F Path G Path
    • 1 {A} 2 A-B 5 A-C 1 A-D Inf. Inf. 1 A-G
    • 2 {A,D} 2 A-B 4 A-D-C 1 A-D 2 A-D-E Inf. 1 A-G
    • 3 {A,D,G} 2 A-B 4 A-D-C 1 A-D 2 A-D-E Inf. 1 A-G
    • 4 {A,B,D,G} 2 A-B 4 A-D-C 1 A-D 2 A-D-E Inf. 1 A-G
    • 5 {A,B,D,E,G} 2 A-B 3 A-D-E-C 1 A-D 2 A-D-E 4 A-D-E-F 1 A-G
    • 6 {A,B,C,D,E 2 A-B 3 A-D-E-C 1 A-D 2 A-D-E 4 A-D-E-F 1 A-G
    • G}
    • 7 {A,B,C,D,E 2 A-B 3 A-D-E-C 1 A-D 2 A-D-E 4 A-D-E-F 1 A-G
    • F,G}
    D G A B C E F 5 1 1 1 2 1 3 5 2 2 3
  • 7. Link State Routing Summary
    • One of the oldest algorithm for routing
    • Finds SP by developing paths in order of increasing length
      • Requires each node to have complete information about the network
      • Nodes exchange information with all other nodes in the network
      • Known to converge quickly under static conditions
      • Does not generate much network traffic
    • Other possible routing algorithms?
  • 8. Metrics for link cost
    • Simplest method is to simply assign 1 to each link
    • Original ARPANET metric
      • link cost = number of packets enqueued on each link
        • This moves packets toward shortest queue not the destination!!
      • took neither latency or bandwidth into consideration
    • New ARPANET metric
      • link cost = average delay over some time period
      • stamp each incoming packet with its arrival time ( AT )
      • record departure time ( DT )
      • when link-level ACK arrives, compute
        • Delay = (DT - AT) + Transmit + Latency
        • Transmit and latency are static for the link
      • if timeout, reset DT to departure time for retransmission
    • Fine Tuning
      • compress range over which costs can span using static function
      • smooth variation of cost over time using averaging
  • 9. Introduction to switching fabrics
    • Switches must not only determine routing but also do forwarding quickly and efficiently
      • If this is done on a general purpose computer, the I/O bus limits performance
        • This means that a system with 1Gbps I/O could not handle OC12
      • Special purpose hardware is required
      • Switch capabilities drive protocol decisions
    • Context – a “router” is defined as a datagram “switch”
    • Switching fabrics are internal to routers and facilitate forwarding
  • 10. Goals in switch design
    • Throughput
      • Ability to forward as many pkts per second as possible
    • Size
      • Number of input/output ports
    • Cost
      • Minimum cost per port
    • Functionality
      • QoS
  • 11. Throughput
    • Consider a switch with n inputs and m outputs and link speed of s n
      • Typical notion of throughput:  s n
        • This is an upper bound
        • Assumes all inputs get mapped to a unique output
      • Another notion of throughput is packets per second (pps)
        • Indicates how well switch handles fixed overhead operations
    • Throughput depends on traffic model
      • Goal is to be representative
      • This is VERY tricky!
  • 12. Size/Scalability/Cost
    • Maximum size is typically limited by HW constraints
      • Eg. fanout
    • Cost is related to number of inputs/outputs
      • How does cost scale with inputs/outputs?
  • 13. Ports and Fabrics
    • Ports on switches handle the difficult functions of signaling, buffering, circuits, RED, etc.
      • Most buffering is via FIFO on output ports
        • This prevents head-of-the-line blocking on input ports which is possible if only one input port can forward to one output port at a time
    • Switching fabrics in switches handle the simple function of forwarding data from input ports to output ports
      • Typically fabric does not buffer (but it can)
      • Contention is an issue
      • Many different designs