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Circuit switching in the Circuit switching in the InternetInternet
Ph.D. oral examinationPablo Molinero-Fernández
21st May 2002
2
Traffic doubles every yearTraffic doubles every yearRelative preformance increase
0
250
500
750
1000
2002 2004 2006 2008 2010 2012
Moore’s law
x2/ 1.5 years
Router capacity
x2.2/ 1.5 years
Traffi c growth
x2/ 1 year
1
Cost and complexity of five times as
many central offices is prohibitive
GAP OF 5x!!
3
Optics removes Optics removes bandwidth constraintsbandwidth constraints
But we cannot buffer light!
4
Circuit switchingCircuit switching
•Link bandwidth is reserved•Need signaling•No buffering•No processing in data path
6
ContributionsContributions
The Internet needs circuit
switchingin the core
Provisioning of fat pipes in an all-optical
backbone
TCP Switching: how to
integrate circuit switching in the
core
8
Why the Internet usesWhy the Internet usespacket switching packet switching
• Efficient use of expensive links:– “Circuit switching is rarely used for data
networks, ... because of very inefficient use of the links” – Bertsekas & Gallager ‘92
• Resilience to failure of links & routers:– ”For high reliability, ... [the Internet] was to
be a datagram subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted” – Tanenbaum ‘96
11
Users
Performance criteriaPerformance criteria
Carriers•Cost: Bandwidth efficiency•Reliability and stability•Low complexity
•Response time•Service Level Agreement (QoS)
12
Users •Response time•Service Level Agreement (QoS)
What users wantWhat users want
Carriers•Cost: Bandwidth efficiency•Reliability and stability•Low complexity
13
Response time of Response time of packets and circuitspackets and circuits
Packet swCircuit sw 10 Mb/s1 Gb/sFlow
bandwidth1 s0.51 sAvg response
time1 s1 sMax response
time
All but one of circuits
finish earlier
File = 10Mbit
x 100
1 server100 clients
1 Gb/s
1410.99 sec10.99 sMax response time
Packet swCircuit sw 10Mb/
s;1Gb/s1 Gb/sFlow
bandwidth 1.10 sec10.50 sAvg response
time
A big flow can kill CS if it
blocks the link
File = 10Gbit/10Mbit
x 99
1 server100 clients
1 Gb/s
Response time when Response time when blocking occursblocking occurs
15
Response time with Response time with flow rate limitsflow rate limits
Packet swCircuit sw 1 Mb/s1 Mb/sFlow
bandwidth
10,000 sec 10,000 sMax response time
109.9sec 109.9sAvg response time
No difference between
CS and PS in core
x 99
File = 10Gbit/10Mbit
1 server100 clients
1 Gb/s
1 Mb/s
16
Analytical modelingAnalytical modeling
• Fluid model:
• Many independent arrivals => Poisson• Service policies:
– Packets: Processor Sharing– Circuits: FCFS
• Service time distribution:– Flow size variance: Bimodal– Realistic flow size distrib: Pareto xxf )(
pBPpAP 1)(;)(
17
Fluid model: M/BiModal/NFluid model: M/BiModal/N
•Access link is as fast as the core link
•Hogging by long flows
Flow size variance +
FCFS (load = 0.5)
0.1
1
10
100
0 0.5 1
Res
pons
e ti
me
rel.
to
Proc
esso
r S
har
ing
N=1
18
Fluid model: M/BiModal/NFluid model: M/BiModal/N
• Flow rate limited by access link (1/N)
• Same response time regardless of flow size variance, if N is large
Flow size variance +
FCFS (load = 0.5)
0.1
1
10
100
0 0.5 1
Res
pons
e ti
me
rel.
to
Proc
esso
r S
har
ing
1
32
N=2k
19
Fluid model: M/Pareto/NFluid model: M/Pareto/N
Link hogging becomes
very bad with heavy-tailed traffic, if ratio
N=1
FCFS (alpha = 1.3)
1
10
100
1000
10000
0 0.5 1load
Resp
onse
tim
e r
el.
to
Proc
ess
or S
har
ing
N=1
20
Fluid model: M/Pareto/NFluid model: M/Pareto/N
• Response time similar to IP if core/access ratio N is large
• Typically N >> 1,000
FCFS (alpha = 1.3)
1
10
100
1000
10000
0 0.5 1load
Resp
onse
tim
e r
el.
to
Proc
ess
or S
har
ing
1
2
4
816
256
N =2k
512
21
Users see little difference Users see little difference in response timein response time
•Simulation of full networks
•N is large => same response time1
10
100
1000
1E+2 1E+4 1E+6fl ow size (bytes)
resp
onse
tim
e (s
)
Circuit switching Packet
switching
22
Service Level Service Level AgreementsAgreements
• Packet switching:– Algorithms (WFQ, DRR, …) => not
used– Thus we must overprovisioning =>
used and it works
• Circuit switching:– Simple QoS: guaranteed BW => no
jitter
23
Users •Response time•Service Level Agreement (QoS)
What carriers wantWhat carriers want
Carriers•Cost: Bandwidth efficiency•Reliability and stability•Low complexity
24
Cost: Bandwidth Cost: Bandwidth efficiencyefficiency
• Argument: packets share all link BW => statistical multiplexing gain => more throughput with bursty traffic
• Reality: – Internet avg. link utilization: 5-20%
[Coffman&Odlyzko’02]
– Phone avg. link utilization: ~33% [Odlyzko’99]
– There is a glut of BW in the core [WSJ’00]
• Result:– Packets more efficient, but BW is no longer
a scarce resource
25
OC-12OC-3 OC-48
Carriers peak allocate Carriers peak allocate their networktheir network
• When over-provisioning, link BW is virtually peak allocated
• That is exactly what circuit switching does
Source: Chuck Fraleigh ‘02
26
Reliability and Reliability and stability (I)stability (I)
• Argument: because of the state, rerouting a circuit is more costly than with packets
• Reality:– Internet average availability:
1220 min/year down time [Labovitz’99]
– Phone average availability: 5 min/year down time [Kuhn’97]
27
Reliability and Reliability and stability (II)stability (II)
• Reality (cont.):– IP recovers in about 3 min (median),
sometimes it takes over 15 min [Labovitz’01]
– SONET required to recover in less than 50 ms
• Result: – No evidence packet switching is more
robust
28
Low complexity (I)Low complexity (I)
• Argument: No per-flow state => packet switching is simpler
• Reality: – PS: 8M lines of code in core router
[Cisco’s IOS ‘00]– CS: 18M lines of code in telephone switch
[AT&T/Lucent 5ESS ‘96]– CS: 3M lines of code in transport switch
[’01]
• Result: – Packet switching does not seem
inherently less complex than circuit switching
29
Functions in a Functions in a packet switchpacket switch
Interconnect scheduling
Route lookup
TTL proces
sing
Buffering
Buffering
QoS schedu
ling
Control plane
Ingress linecard Egress linecardInterconnect
Framing
Framing
Data path
Control path
Scheduling path
30
Functions in a Functions in a circuit switchcircuit switch
Interconnect scheduling
Control plane
Interconnect
Framing
Framing
Ingress linecar
d
Egress linecard
Data path
Control path
31
Low complexity (II)Low complexity (II)
• Argument: IP does not have the signaling of circuits switches => Routers go faster• Reality: – IP does almost same operations on every
packet as a circuit switch on the circuit establishment
– CS has no work to do once circuit is established
• Result: – The fastest commercially-available circuit
switches [Ciena ’01, Lucent ‘01] have 5x the capacity of the fastest routers [Cisco ’01, Juniper ’02]
32
Network architectureNetwork architecture
LANs &wireless
WANs
•Use packet switching •Better response time (ratio N small)•Efficient use of the spectrum
•Use circuit switching•More capacity, reliability•Similar response time & QoS
MANs•If metro-to-access BW ratio (N) is
small => use packets•Otherwise use what costs less
33
ContributionsContributions
The Internet needs circuit
switchingin the core
Provisioning of fat pipes in an all-optical
backbone
TCP Switching: how to
integrate circuit switching in the
core
34
Integration of circuits Integration of circuits and packetsand packets
• Create a separate circuit for each user flow
• IP controls circuits• Optimize for the most
common case– TCP (90-95% of traffic)– Data (9 out of 10 pkts)
TCP Switching
36
TCP “creates” a TCP “creates” a connectionconnection
Router Router RouterDestina
-tionSource
SYN
SYN+ACK
DATA
Packets Packets
PacketsPackets
37
Let TCP leave state Let TCP leave state behindbehind
Boundary TCP-SW
Core TCP-SW
Boundary TCP-
SW
Destina-tionSource
SYN
SYN+ACK
DATA
Create circuit
One Circuit PacketsPackets
Create circuit
38
What is a typical flow?What is a typical flow?
• Most traffic are TCP connections:– Taking less than 10 s, 12 packets and
4 KBytes– Obtaining less than 100 Kbps– ~40% of the flows continue
transmitting ACKs after sending a FIN (asymmetrical closures)
39
State management State management feasibilityfeasibility
• Amount of state– Minimum circuit = 56 Kb/s.– 178,000 circuits for OC-192.
• Update rate– About 51,000 entries per sec for
OC-192• Implementable in hardware or
software.
40
TCP Switching can be TCP Switching can be implemented in implemented in
softwaresoftwareTCP Switching boundary router:• Kernel module in Linux 2.4 1GHz
PC • Forwarding latency
– Forward one packet: 21s.– Compare to: 17s for IP. – Compare to: 95s for IP + QoS.
• Time to create new circuit: 57s. Source: Byung-Gon Chun ‘01
41
Slow Start
Congestion Avoidance
Flow duration
Inactivity timeout
Cir
cuit
BW
Inst
an
tan
eou
s b
an
dw
idth
time
Bandwidth inefficienciesBandwidth inefficiencies
Compromise: inactivity timeout of
few seconds
42
Related workRelated work
• IP Switching– Uses ATM virtual circuits (i.e. packets)– Became MPLS (but no longer user
flows)
• Generalized Multi-Protocol Label Switching (GMPLS)– Coarse circuits– Heavy weight signaling
43
ContributionsContributions
The Internet needs circuit
switchingin the core
TCP Switching: how to
integrate circuit switching in the
core
Provisioning of fat pipes in an all-optical
backbone
46
Circuit creation is slowCircuit creation is slow
• We need a safeguard to avoid running out of BW => inefficiency
A slow signaling requires a larger BW safeguard
47
Controlling coarse Controlling coarse circuits with user flowscircuits with user flows
• Should use the fastest optical switching elements
• Should avoid ACKs => no RTT
0.0001
0.001
0.01
0.1
1
1E+04 1E+06 1E+08Bandwidth saf eguard (bps)
Prob
. of
exce
edin
g
safe
guar
d
10 us100us
1 ms
10 ms
100 ms
1 s
Circuit
creation
latency
48
ConclusionsConclusions
• Circuits should be used in the core, packets in the edges
• TCP Switching integrates circuits and packets in an evolutionary way
• User flows can be used to control an all-optical network
49
PapersPapers
• PMF, Nick McKeown, "TCP Switching: Exposing Circuits to IP“, IEEE Micro, 2002
• PMF, NM, "TCP Switching: Exposing Circuits to IP“, Hot Interconnects, 2001
• PMF, NM, “Study of routing behavior trough traffic analysis and traceroute measurements”, NAT Times, 2001
• PMF, NM, Hui Zhang, "Is IP going to take over the world (of communications)?“, submitted