Source Routing - Advanced Computer Networks - Lecture Slides, Slides of Computer Networks

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Lecture No. 15

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Source Routing

• Packet header contains sequence

of address/ports on path from

source to destination

• One direction per switch: port, next

switch; (absolute)

• Switches read, use, and then discard

directions

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Data Transfer in Source Routing

• Analogous to following directions 0 1

3

2

2 (^13) 0

0 3 1 2

Switch 3 Host B

Switch 2 Switch 1

Host A

data 0 1 3 data 3 0 1

data 1 3 0

data 3 0 1

data 1 0 3 data 2 3 0 1

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Source Routing Model

• Source host needs to know the

correct and complete topology of

the network

• Changes must propagate to all hosts
• Packet headers may be large and

variable in size: the length is

unpredictable

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Forwarding Performance

• Assume switch is
  • General-purpose workstation
  • With DMA support
  • Multiple network adapters (NIC’s)
• Switching process
  • Packet arrives on NIC 1
  • NIC 1 DMA’s packet into memory
  • CPU looks at header, decides to send on NIC 2
  • NIC 2 DMA’s packet into NIC 2 memory
  • Packet leaves via NIC 2 Docsity.com

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Implementation and Performance

• Packet arriving at interface 1 has to go on interface 2
• Point of contention for packets: I/O and memory bus

CPU

Main memory

I/O bus Interface 1

Interface 2

Interface 3

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Bridges and Extended LANs

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Building Extended LANs

• Traditional LAN
  • Shared medium (e.g., Ethernet)
  • Cheap, easy to administer
  • Supports broadcast traffic
• Problem
  • Want to scale LAN concept
    • Larger geographic area (> O(1 km))
    • More hosts (> O(100))
  • But retain LAN-like functionality
• Solution: bridges Docsity.com

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Bridges vs. Switches

• Switch
  • Receive frame on input port
  • Translate address to output port
  • Forward frame
• Bridge
  • Connect shared media
  • All ports bidirectional
  • Repeat subset of traffic
    • Receive frame on one port
    • Send on all other ports Docsity.com

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Uses and Limitations of Bridges

• Extend LAN concept
• Limited scalability
  • To O(1,000) hosts
  • Not to global networks
• Not heterogeneous
  • Some use of address, but
  • No translation between frame

formats Docsity.com

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Learning Bridges

• Learn table entries based on source address
  • Timeout entries to allow movement of hosts
• Table is an optimization ; need not be complete
• Always forward broadcast frames
• Uses datagram or connectionless forwarding

A

Bridge

B C

X Y Z

Port 1 Port 2

Host Port A 1 B 1 C 1 X 2 Y 2 Z 2 Docsity.com

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Learning Bridges

• Problem
  • Redundancy (desirable to handle failures, but …)
  • Makes extended LAN structure cyclic
  • Frames may cycle forever
• Solution: spanning tree

B

A C E B2^ D

B

B B7 (^) K F

H B J

B B

G I

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Spanning Tree Concept

• LAN’s and bridges make a bipartite graph
• Ports are edges connecting LAN’s to bridges
• Spanning tree required
  • Connect all LAN’s: all vertices of graph are covered
  • Can leave out bridges: all edges may not be covered

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Spanning Tree Algorithm

• Each bridge has a unique,

totally-ordered identifier

• Select bridge with lowest ID

as root bridge

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