Chapter 11.4
END-TO-END ISSUES
Optical Internet
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Optical technology
Protocol translates availability of gigabit
bandwidth in user-perceived QoS
WDM network architecture
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WDM wide/metropolitan area network
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Access network
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As an Internet backbone
Ex: campus network
Domain border gateway
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High-speed IP router
WDM network architecture
Open issue
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Access network
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Optical backbones
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Packet loss and delay due to congestion
High transmission rates
Extremely low bit error rates
Bridging the gap between access and
backbone network is an issue
End-to-end TCP
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It is not practical, the reasons is below
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TCP slow-start algorithm constrains very
large bandwidth available in the lightpath
until the steady state is reached
Socket buffer is not enough
congestion and flow control features
needed in access network
Split TCP connection models (1/2)
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It is an evolutionary approach to the TCP end-to-end
model to adapt to the specific characteristics of each
of the network segment
It is not an efficient solution for optical networks due
to the wavelength speed
EX: 10-Gbps wavelength bandwidth
10-ms propagation delay
bandwidth delay product (BDP)=25MB
File sizes in the Internet are smaller than such BDP
Because of different round-trip delays, it is difficult to
optimize TCP windows to achieve transmission
efficiency
Split TCP connection models(2/2)
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Most TCP features( congestion and flow
control) unnecessary in optical network
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Extremely low loss rate in the optical
network makes retransmission unlikely to
happen
The optical network can operate in a burstswitched mode in the optical layer, so there
are no intermediate queues in which
overflow occurs
11.4.1 TCP for High-Speed
and Split TCP Connection
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First approach to solving the adaptation
problem between access and backbone
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TCP connection can be split in the optical
backbone edges
TCP extensions for high speed
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a larger transmission window
no slow start
Files over lightpaths (FOL)
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Files are encapsulated in an optical
burst in order to be transmitted across
the optical network.
Simulation
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Topology
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Network parameters
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An optical channel(1Gbps) which connects several
access routers located at the boundaries of optical
networks
Namely, link capacities, propagation delay, and
loss probability
Performance metric
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Connection throughput
Simulation
Simulation Result
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For small file sizes
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For large file sizes
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The connection duration is dominated by setup
time and slow start, which does not allow the
window size to reach a steady-state value
The TCP reach steady state and the throughput is
equal to window size divided by roundtrip time.
Such behavior is expected in a large BDP
network, in which connections are RTT-limited
rather than bandwidth-limited
11.4.2 Performance Evaluation of File
Transfer (WWW) Services over WDM
Networks(1/2)
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11.4.1 is in error-free condition
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second-generation WDM network suffer
blocking probability
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Ex: a first-generation WDM network (static
lightpath between routers)
Limited number of wavelengths
Burst dropping due to limited queueing space in
photonic packet switchs
Split TCP becomes inefficient
Performance Evaluation of File Transfer
(WWW) Services over WDM Networks(2/2)
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In FOL, files are encapsulated in an
optical bursts through the optical
backbone using a simple stop-and-wait
protocol for error control.
Assuming the setup of an optical burst
takes RTT/2
Simulation Result
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TCP congestion avoidance limits
transfer efficiency
This serve to illustrate that the
throughput penalty imposed by the TCP
congestion control mechanism is rather
significant
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The main difference between a simple
FOL protocol and TCP is in interpreting
congestion
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TCP considers loss is produced by
queueing overflow
FOL is aware that loss is due to blocking