Routers

Routers
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Introduction

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

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

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

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

Switching

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

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