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Routers
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Introduction
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  • Role of the Network Layer:
    • Move packets from the sender to the reciever
  • Network Layer Functions:
    • Forwarding - moving a packet from an input link to an output link
    • Routing - determining the route/path packets will take when moving from the sender to the receiver
An Important(?) Distinction
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  • Switch (a.k.a., Packet Switch):
    • A link-layer forwarding device (i.e., a device that makes its forwarding decisions using fields in the link layer frame)
  • Router:
    • A network-layer forwarding device (i.e., a device that makes its forwarding decisions using fields in the network layer frame)
An Abstract View of a Router
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  • Input Link:
    • The incoming physical connection
    • Processing
    • A buffer of incoming packets
  • Output Link:
    • A buffer of outgoing packets
    • Processing
    • The outgoing physical connection
  • Switching Fabric:
    • Connects the input links to the output links
  • Routing Processor:
    • Maintains routing information
    • Executes the routing protocols
The Forwarding Plane
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  • Components:
    • Input Links
    • Switching Fabric
    • Output Links
  • Requirements:
    • Given a 10 Gbps input link and a 64 byte datagram the input port has only about 50 nanoseconds to process the datagram
    • Most systems must have multiple input links
  • Implication:
    • These components/functions are usually implemented in custom hardware (e.g., Cisco, Alcatel-Lucent, Juniper, Huawei, Broadcom)
Input/Output Processing Requirements
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  • Input:
    • Decapsulation
    • Lookup
    • Forwarding
  • Output:
    • Encapsulation
Switching
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  • Memory Switching:
    • The processor reads packets (one at a time) from the input link's memory and writes them to the output link's memory (as in a shared memory computer)
  • Bus Switching:
    • The input link prepends a destination to the packet and transmits it to the bus
    • Only one packet can be on the bus at a time
    • All output ports receive the packet but it is ignored by all but the designated port which strips-off the destination
  • Interconnection Network Switching (e.g., Crossbar Switch):
    • Can forward multiple packets in parallel (to different output links)
Behavior at the Buffers
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  • Queueing:
    • Does not tend to occur at the input links because the transfer rate of the switching fabric tends to be very high
    • Does tend to happen at the output links
  • Packet Loss:
    • Buffers have a finite size so packets can be dropped
There's Always More to Learn
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