tcp congestion control evolution steven low caltech netlab.caltech.edu
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TCP Congestion Control Evolution
Steven LowCaltech
netlab.CALTECH.edu
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Summary First 25 years of Internet
Networks are several orders of magnitude smaller in size, speed, and heterogeneity
Applications are much simpler
Last 14 years of Internet Networks are much bigger, faster, and
heterogeneous Applications are much more diverse and
demanding
TCP congestion control gradually becomes unsuitable for more and more applications in more and more networks
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Summary Both networks and applications are
evolving much faster in last 14 years than previous 25 Our control mechanisms have shown
remarkable robustness, but also essentially frozen in the last 20 years
TCP congestion control is gradually being stressed more and more Time to change is now
… though there may not be an internet-wide train wreck But why wait for train wreck to change?
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Outline
Where we were Network growth Application growth Control mechanism growth
Where we are Need to change TCP congestion
control
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1969 1974
ARPANet
Internet milestones
19931988
TCP
20061983
TCP/IP
50-56kbps, ARPANet
Backbone speed:
T1 NSFNet
1991
Mosaic
OC12MCI
T3, NSFNet
1996 1999
OC48vBNS
2003
OC192Abilene
HTTP
Tahoe
Napster
Network is exploding
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Internet at birth (1969)
Source: http://www.computerhistory.org/exhibits/internet_history/full_size_images/1969_4-node_map.gif
December, DEC
1 November, IBM
1 October, SDS
2 September, SDS
Routers
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Internet at 31
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Internet growth
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In comparison: car
If car technology has been advancing as fast:
US$ 5 /car [US$ 25,000 /car] 100 km / lt [8 km / lt]
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The Wireless Internet
DoCoMo’s i-mode• E-mail• Internet• Doubles
every half year
• More than 20 million subscribers by early 2000s
Sub
scri
bers
in m
illi
ons
(Courtesy: D. Rutledge)
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Wireless Mesh Network
• Rooftop routers with 0.5-W transmitters• 2Mb/s channels and up to 1-mile range
(Courtesy: D. Rutledge)
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1969 1972
Application milestones
1988
Tahoe
1971
ARPANet
NetworkMail
1973
FileTransfer
Telnet
Simple applications
1993 20051995
InternetPhone
Whitehouseonline
Internet Talk Radio
Diverse & demanding applications
1990
Napstermusic
2004
AT&TVoIP
iTunesvideo
YouTube
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Network Mail (1971)First Internet (ARPANet) application
The first network email was sent by Ray Tomlinson between these two computers at BBN that are connected by the ARPANet.
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First ARPANet applications
First Network Mail: SNDMSG, Ray Tomlinson, 1971 Replaced by SMTP: RFC 821, J. Postel, Aug
1982
Telnet: RFC 318, Jon Postel, April 1972 Started RFC 137, T. C. O’Sullivan, April 1971
FTP: RFC 454, A. McKenzie, Feb 1973 Started in July 1972, RFC 354, Abhay
Bhushan
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Internet applications today
Telephony Music TV & home theatre
Software As A Service
Finding your way
Games
Library at your finger tip Network centric warfare
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1969
Control milestones
1988 20061983
TCP/IP
Tahoe
ARPANet
1973
TCP
Flow control: Prevent overwhelming receiver
+ Congestion control:Prevent overwhelming network
Going forward: provide/facilitate new application requirements
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TCP (1974)
Cerf and Kahn, Trans. Communications (1974) RFC 793, TCP (1981) ARPANet cutover to TCP/IP (Jan 1, 1983)
Window control: ack-clocking, timeout, retransmission For correct delivery of packet sequence over
unreliable networks To prevent overwhelming receiver (through
Advertised Window in TCP header)
“We envision [HOST retransmission capability] will occasionally beinvoked to allow HOST accommodation to infrequent overdemands for limited buffer resources, and otherwise not used much.”
-- Cerf and Kahn, 1974
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TCP (1974)
Key control mechanism Receiver specified max amount of data it
can receive (Advertised Window) Sender maintains no more than that
amount of outstanding packets Ack-clocking adapts to (mild) congestion
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Congestion collapse
October 1986, Internet had its first congestion collapse
Link LBL to UC Berkeley 400 yards, 3 hops, 32 Kbps throughput dropped to 40 bps factor of ~1000 drop!
1988, Van Jacobson proposed TCP congestion control
throughput
load
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Networks in late 1986
5,089 hosts on Internet (Nov 1986) Backbone speed: 50 – 56 kbps Control mechanism focused only on receiver
congestion, not network congestion
Large number of hosts sharing a slow (and small) network Network became the bottleneck, as opposed to
receivers But TCP flow control only prevents overwhelming
receivers
Jacobson introduced mechanism to deal with network congestion in 1988
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Tahoe and its variants (1988)
Jacobson, Sigcomm 1988 + Avoid overwhelming network + Window control mechanisms
Dynamically adjust sender window based on congestion (as well as receiver window)
Loss-based AIMD Fast Retransmit/Fast Recovery New timeout calculation that incorporates
variance in RTT samples
“… important considering that TCP spans a range from 800 Mbps Cray channels to 1200 bps packet radio links”
-- Jacobson, 1988
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Outline
Where we were Where we are Need to change TCP congestion
control
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Summary
First 25 years of Internet Network is several orders of magnitude
smaller in size, speed, and heterogeneity Applications are simple
Last 14 years of Internet Networks are much bigger, faster, and
heterogeneous Applications are much more diverse and
demanding Both network and applications are
evolving much faster in last 14 years than previous 25
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What is inadequate
New applications demand new requirements that stretch TCP FTP or Telnet vs video streaming
New networks stretch TCP New wireless challenges Solutions to some wireless issues
need more than transport layer mechanisms
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What is inadequate
Wireless networks: 5 demos Video delivery: 4 Large BDP: 2 Short messages: 1 Content upload: 1
TCP congestion control becomes unsuitable for more and more applications in more and more networks
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Real-world evidence
50x delays are common over DSL speed links
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0.1 0.5 1 5 10 20 60
File size (MB)
FTP
thro
ughp
ut (k
bps)
Alternative TCP
32.5x28.1x21.8x17.6x6.3x3.8x1.8x
Reno avg:35Mbps
Alter avg: 233Mbps
Throuput: LA Tokyo Throuput: San Fran MIT
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Another example
San Francisco New YorkJune 3, 2007
Heavy packet loss in Sprint network: Alternative increased throughput by 120x !
TCPthruput: 1Mbps
Alternativethruput: 120Mbps
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Why is evolving TCP important
Pockets of industry are experiencing increasing pain
They will not wait for research or standards communities
MS and Linux are already deploying new TCP’s
The pain will only intensity …… because of powerful industry trends
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Trend 1
Exploding need to deliver large content Video, software, games, business info
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Trend 1: Online content is exploding
CAGR 2005 – 2011: 36% Google worldwide: 46PB/month Library of Congress: 0.136PB
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Trend 1: Video is exploding
Jan 2007
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Trend 1: Multi-year growth
Only ~10% of digital content is currently online
Jan 2007
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Trend 2
Exploding need to deliver large content Video, software, games, business info
Infrastructure growth to support delivery need
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Source: Deutsche Bank, Jan 2007
Trend 2: Broadband penetration Global broadband penetration is accelerating
11 million new subscribers/month globally 83% of US home Internet access is broadband by June 2007
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Trend 3
Exploding need to deliver large content Video, software, games, business info
Infrastructure growth to support delivery need Centralize IT to reduce costs of management, space,
power, cooling Exacerbated by virtualization of infrastructure and
personalization of content
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Trend 3: Centralization Power & cooling cost is escalating, fueling centralization
CAGR (2005-10): power & cooling 11.2%, new server spend 2.7% In 2005, 1,000 servers cost $3.8M to power & cool in 4yrs; 2%
increase in electricity cost raises cost by $200K
Source: IDC Sept 2006
Worldwide Expense(US: $4.5B in 2006; EPA)
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Trend 3: Centralization
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Content Volume & Global Transfer Increasing
Exploding need to deliver large content Video, software, games, business info
Infrastructure growth to support delivery need Centralize IT to reduce costs of management, space,
power, cooling Exacerbated by virtualization of infrastructure and
personalization of content
Implication More large contents over longer distance Served from centralized data centers
http://sramanamitra.com/2007/10/21/speeding-up-the-internet-algorithms-guru-and-akamai-founder-tom-leighton-part-5/
“You could only get that sustained rate if you are delivering within100 miles, due to the way current Internet protocols work.”
Tom Leighton, MIT/Akamai, Oct 2007
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Solution Approaches
Protocol problems degrade performance of long-distance transfers
Current approach: reduce distance
Alternative approach: fix protocol problems
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Content Delivery Networks (CDNs)
CDNs circumvent protocol problems by placing servers all around the world
Protocol limitations inherent in Internet give rise to Content Delivery Networks
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Alternative Solution
Distance is no longer a constraint in IT infrastructure design