Download Switching Optical Packets - Motivation and Prospects TransiNet Workshop 2002

TransiNet
2002
Workshop
Heinrich-Hertz-Institut Berlin
08. 10. 2002
Switching Optical Packets Motivation and Prospects
Dipl.-Ing. Thomas Fischer
TU München - Institute of Communication Networks
Dipl.-Ing. Martin Maier
Dipl.-Inform. Hagen Woesner
TU Berlin - Telecommunication Networks Group
Outline
• Motivation
• Difficulties with OPS
• IP traffic characteristics
• Overhead
• Implications
• OPS core node
• Architecture, complexity & scalability
• OPS metro node (distributed router concept)
• Architecture, medium access,
fairness in the TCP context
• Conclusion
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Why use OPS: Motivation
• Efficiency
• Statistical multiplexing - within the optical network layer
• Improved utilization of wavelength channels
• Flexibility
• Variable bandwidth share - packet duration and/or rate
• Bitrate transparency - variable line bitrate for legacy
support
• Inherent support
• Dynamic traffic
• Bitrate asymmetry - bidirectional unicast/multicast
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
IP traffic characteristics
• Packet arrivals
• long-range dependent - correlated across distant
periods
⇒ persisting deviations from mean arrival rate
(burstiness)
• self-similar - similar burstiness across time scales
⇒ compromises statistical multiplexing gain
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
IP traffic characteristics
• Packet lengths
• IP protocol accounts for vast majority of Internet traffic
• Small packets, for the most part from Ethernet LANs, are
predominant
• 40% of packets ~40 Bytes long
• 40 Bytes translates into 32 ns @ 10 Gb/s
⇒ multitude of switching operations
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Overhead
• A matter of principle & chromatic dispersion
• Destination information
• Header synchronization/recognition
• Header-payload alignment (slow & fast jitter)
• Synchronization trade-off (slotted systems)
Time Slot
Guard
Time
Payload
Workshop 2002
Berlin
Sync
Optical Guard
Sync
Pattern
Label
Time
Pattern
Hagen Woesner
[email protected]
Optical
Label
Sync Guard
Pattern Time
Martin Maier
Thomas Fischer
[email protected] [email protected]
Implications (1)
• OPS is statistically constrained
⇒ pure loss systems won‘t do
Loss System
Bursty Packet
Arrivals
Packet Loss
Buffering
Statistical
Multiplexing
< 1%
Source Traffic
Control
Multicasting
Aggregation
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Implications (2)
• Without further measures, OPS is inherently
inefficient
⇒ increase utilization by aggregation
Switching
Principle
Jitter
Overhead
Short Packet
Lenghts
High Bitrate
Efficiency Loss
Many
Switching Ops
Aggregation
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Scalable OPS core node (1)
• Complexity reduction
• Scalability improvement
O
E
OE
Switch
Switch
Control
Control
before after
F·W
F
Taps
F·W
-
Delay Lines (Tunable)
-
F
Delay Lines (Fixed)
F
2·F
WDM Demultiplexers
F·W
F·W
Receivers (Fixed)
F·W
F
Transmitters (Fixed)
F·W -
2x1 Couplers
F
F
WDM Multiplexers
W
-
FxF Switches
-
W
(F-1)x(F-1) Switches
Workshop 2002
Berlin
E
O
EO
Hagen Woesner
[email protected]
O
E
OE
E
O
EO
Martin Maier
Thomas Fischer
[email protected] [email protected]
Scalable OPS core node (2)
•
Unslotted instead of slotted
transmission
O
E
OE
Switch
Switch
Control
Control
• Reduces throughput
• Renders packet alignment
unnecessary
•
E
O
EO
O
E
OE
E
O
EO
Ingress components:
before
after
F·W
-
Delay Lines (Tunable)
-
F·W
Delay Lines (Fixed)
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Scalable OPS core node (3)
• Simplification within ingress stage
(cost trade-off)
O
E
OE
Switch
Switch
Control
Control
• Doubles WDM demultiplexer count
• Yields „one-for-many“ power tapping
& packet retardation
E
O
EO
O
E
OE
E
O
EO
• Ingress components :
before after
F
2·F
WDM Demultiplexers
F·W
F
Taps
F·W
F
Delay Lines (Fixed)
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Scalable OPS core node (4)
• Using out-band headers
• Decrements wavelength channel
count
O
E
OE
Switch
Switch
Control
Control
• Saves many transmitters
• Egress components:
E
O
EO
O
E
OE
E
O
EO
before after
F·W
-
2x1 Couplers
F·W
F
Transmitters (Fixed)
• Switch matrix:
W
-
FxF Switches
-
W
(F-1)x(F-1) Switches
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Distributed packet switching: Sharing the medium
The distributed router concept:
•Use the network as the backplane of a big virtual
router.
•Reduced complexity of the nodes in the network
•Shared medium
•Medium access protocols
•Basically two approaches
•Single-hop Networks
•Multihop Networks
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Packet Switching in Single-Hop WDM Networks
•
•
•
TX and/or RX tunable
Sources of inefficiency:
• Tuning latency
• Chromatic dispersion D
• Most MAC protocols for single-hop WDM networks
are slotted
• Due to different wavelength travel velocities padding
required at beginning (or end) of each slot
• Padding = D • ∆λ • L = 59.5 ns for 17 ps/nm km, 35 nm,
100 km
⇒ Inefficient packet switching in single-hop WDM
networks
Multihop WDM networks with fixed-tuned transceivers
⇒ No inefficiency due to tuning latency and chromatic
dispersion
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Spatial Reuse Protocol - SRP: Overview
•
•
•
•
SRP uses a bi-directional dual ring topology
Both rings transport data and SRP control packets
Insertion buffer MAC protocol
Control packets for topology discovery, protection
switching and bandwidth control.
Outer Ring
Data
Layer 3 Switch/Router
Outer Ring
Control
Control
Packets
Inner Ring
Data
Inner Ring
Control
Low Pri
Rx Data
Queue
Queues
SRP MAC
Transit Queue
Receive Fiber
Transmit Fiber
Low Pri
second ring
Workshop 2002
Berlin
Hi Pri
High Pri
Queue
Fairness signaling
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Fibre delay line - node architecture
• Only one transit buffer of
length 1 packet.
• Fixed packet size of 9216 byte
(= 7 µs @ 10Gbit/s = 1.4 km FDL)
• Aggregation of incoming IP
packets (VOQ is an option)
• Thresholds for transmit queues.
• Definition of fairness
algorithm
depending on thresholds and
counters.
From the
ring
10%
Header
extraction
Fibre
delay
line
Low
priority
transmit
data
queue
To the
ring
LP HP
Hi priority
transmitReceive
data
queue
queue
• Calculation of counters for
my_usage and fd_rate
in MAC layer.
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
How the protocol works
• A node is only allowed to transmit when its FDL
is empty.
• CSMA ring access
• Data rate is controlled by token bucket algorithm
• Variable parameters r (fill rate) and b (# of tokens) which
are function of my_usage and allowed_usage
• Threshold in the low priority transmit queue installed
and observed.
• If the LP-threshold is reached,
• the MAC layer has to tell his upstream node to reduce
the traffic sourced onto the ring.
• Send the long term average of my_usage counter
• Upstream node reduces its rate to the value received
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Why is Fairness needed?
• Given a 5- node dual ring architecture
• Only nodes 0 and 4 get their data through
• Exponential On/Off traffic with 50% 40 byte, 30% 576
byte and 20 % 1500 byte long packet
Outer Ring
Data
1
2
Outer Ring
Control
3
Inner Ring
Data
0
4
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Fairness for unidirectional traffic
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Unfairness for TCP!
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Reasons and consequences
• TCP is a dynamic system!
• Slow start algorithm initially transmits only 1 (or 2)
fragments
• Node will never be “congested”, since the
threshold will not be reached
• Move towards a timer based solution
• HOL (head-of-line) timer decides when the node is
“congested”
• Timer setting depending on the own average traffic load
HOL_timer= 1/(lp_my_usage+ε) -1
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Fairness for TCP
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]
Conclusions
• Reduction of switching complexity
• Discarding non-vital - and smart use of the remaining functionality, or
• Distributing functionality over the network
• Switch & protocol design with underlying traffic
in mind
• OPS nodes alone will not suffice
• Development of link layer protocols may lead to design
errors without bearing TCP in mind
• Destination stripping networks offer spatial reuse
• E.g. bi-directional rings
• But: Fairness becomes an issue
• Global and local fairness
Workshop 2002
Berlin
Hagen Woesner
[email protected]
Martin Maier
Thomas Fischer
[email protected] [email protected]