Lecture10 - COP 5611 Operating Systems Spring 2010 Dan C...

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5/13/10 Lecture 6
COP 5611 Operating Systems Spring 2010
Dan C. Marinescu
Office: HEC 439 B
Office hours: M-Wd 2:00-3:00 PM

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Lecture 10
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Reading Assignment: Chapter 7 from the online textbook
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A progress report for the project is due on every Monday.
n
Last time:
¨
Network properties
n
Today:
¨
Layering
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Link layer
¨
Network layer
n
Next time
¨
Routing

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Networks encounter a v ast range of
Data rates
Propagation, transmission, queuing, and processing delays.
Loads
Numbers of users
Networks traverse hostile envir onments
Noise damages data
Links stop working
Best-effor t networks have
Variable delays
Variable transmission rates
Discarded packets
Duplicate packets
Maximum packet length
Reordered delivery

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A
B
time
X
overloaded
resend request,
send request,
receive response,
send request,
timer expires,
set new timer
receive response,
X
X
set timer
reset timer
set timer
reset timer
forwarder
discards
request
packet.
request 1
response 1
request 2
request 2’
response 2’
Recovery of lost packets

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A
B
X
overloaded forwarder
send request,
duplicate arrives at B
B sends response 3’
timer expires,
resend request,
X
receive response,
set new timer
set timer
reset timer
discards response 3
request 3
request 3’
response 3’
Duplicate requests

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A
B
packet containing response timer expires,
resend
send request,
duplicate arrives at B
B sends response 4’
receive
response
receive
response,
reset timer
X
duplicate
gets delayed
set timer
request 4
response 4
request 4’
response 4’
Delays and recovery lead to duplicate response

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Layering
n
Simplify the design
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Example- RPC

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result ←
FI RE
(#, target , action )
procedure
FIRE
( nmiss , where , react )
Client stub Service stub
proc:
FIRE
args:
3
type: integer
value:
2
type:
string
value:
“Lucifer”
type:
procedure
value:
EVADE
Prepare
request
message.
Send to
service
Receive
request
message.
Call
requested
procedure.
Prepare
response
message.
Send to client.
response:
acknowledgment
type:
string
value:
“destroyed”
request:
Wait for
response.
return result
...
Client-server communication based on RPC

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Main program called procedure application protocol
RPC client stub presentation protocol
RPC service stub

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Client network
Service network
transport protocol
package
package
fire
send_
receive_
(return)
fire
(return)
Main program called procedure
application protocol
RPC client stub presentation protocol RPC service stub
message
message
receive_
message
send_
message

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Multiple protocols may be supported at each layer
1111
Layer One
Layer Two
Layer Three
A B C D
X Y Z
J K L

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Multiplexing and Demultiplexing

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Network layers for store and forward networks
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Every module has three interfaces:
¨
To the layers above
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To the layers below
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To the peer layer on another system
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Each layer hides the operation of the layer below from the layer