Source Routing-Advance Computer Networking-Lecture Slides, Slides of Computer Networks

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• Lecture No.

Source Routing

• Packet header contains

sequence

of address/ports on path fromsource to destination– One direction per switch: port, nextswitch; (absolute)– Switches read, use, and then discarddirections

Data Transfer in Source Routing• Analogous tofollowingdirections

0

1 (^23)

(^21)

3 0

0

1 3 2

Switch 3

Host B

Switch 2

Switch 1 Host A

datadata^

datadata^

datadata^

datadata^

datadata^

datadata^

Source Routing Model

• Source host needs to know thecorrect and

complete topology

of

the network– Changes must propagate to all hosts• Packet headers may be

large

and

variable in size: the length isunpredictable

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

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 1Interface 2 Interface 3

Bridges and Extended LANs

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

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

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

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^

PortA (^1) B (^1) C (^1) X (^2) Y (^2) Z 2

Learning Bridges

• Problem– Redundancy (desirable to handle failures, but …)– Makes extended LAN structure cyclic– Frames may

cycle forever

• Solution: spanning tree

A^ B C E

D B

B

B B^

K F H B

J B B G I

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

Spanning Tree Algorithm

• Each bridge has a unique,totally-ordered identifier• Select bridge with lowest IDas

root bridge