internet congestion control speaker : yi-cheng chan ( 詹益禎 ) assistant professor, department...
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Internet Congestion Internet Congestion ControlControl
Speaker : Yi-Cheng Chan (詹益禎 )Assistant Professor,
Department of Computer Science and Information Engineering,
National Changhua University of Education
2006/10/27
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OpeningOpening
What is the Internet? A network is a group of connected, communicating
devices. An internet is two or more networks that can
communicate with each other. The most notable internet is called the Internet.
How congestion control in the Internet? Hosts & network devices
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OutlineOutline
The Internet Transmission Control Protocol - TCP Active Queue Management - AQM Admission Control Conclusions
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The InternetThe Internet
Internet history 1969: Four-node ARPANET established. 1972: Vint Cerf and Bob Kahn’s Internet Project 1973: Development of TCP/IP begins. 1981: UNIX operating system includes TCP/IP. 1983: TCP/IP became the official protocol for the ARPANET. 1986: NSFNET (sponsored by the National Science
Foundation) 1990: ARPANET retired. 1991: A high-speed Internet backbone called ANSNET was
build by IBM, Merit, and MCI. 1995: Companies known as Internet Service Providers
(ISPs) started.
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Internet TodayInternet Today
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Growth of the InternetGrowth of the Internet
The Internet has grown tremendously. The Internet is still growing.
New Protocols New Technology Increasing use of multimedia
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Internet Congestion Control ComponentsInternet Congestion Control Components
Internet
BackboneRouter
AccessRouterEnd Host
End Host
AccessRouter
Admission Control
Admission Control
Transmission Control Protocol
Transmission Control Protocol
Active Queue Management
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Transmission Control Protocol - TCPTransmission Control Protocol - TCP
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TCP Services (1/2)TCP Services (1/2)
Process-to-process communication
Well-known ports used by TCP
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TCP Services (2/2)TCP Services (2/2)
Virtual connection Three-way handshaking Flow control
It regulates the amount of data a source can send before receiving an ACK from the destination.
Error control Checksum, ACK, retransmission time-out.
Congestion control The mechanism to control the congestion and keep the load
below the capacity.
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Throughput versus Network LoadThroughput versus Network Load
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Congestion Control in TCP (1/5)Congestion Control in TCP (1/5)
Congestion window Slow start: exponential increase
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Congestion Control in TCP (2/5)Congestion Control in TCP (2/5)
Congestion avoidance: additive increase
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Congestion Control in TCP (3/5)Congestion Control in TCP (3/5)
Fast retransmission
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Congestion Control in TCP (4/5)Congestion Control in TCP (4/5)
TCP congestion control policy summary
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Congestion Control in TCP (5/5)Congestion Control in TCP (5/5)
Congestion example
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Evolutions of TCPEvolutions of TCP
RFC 793 (1981) A simple sliding window flow control mechanism.
Tahoe (1988) Slow start, congestion avoidance, fast retransmit.
Reno (1990) Fast recovery.
New Reno (1995) Refine the fast retransmit.
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Advanced EnhancementsAdvanced Enhancements
TCP with selective acknowledgments (1996) TCP Vegas (1994) Compound TCP (2006) And …
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Active Queue Management - AQMActive Queue Management - AQM
Queue management is defined as the algorithms that manage the length of queues by dropping packets when necessary or appropriate.
Why Queue Management is needed? Passive Queue Management (PQM)
Drop packets when router buffer gets full. Active Queue Management (AQM)
Employ preventive packet drop before the router buffer gets full.
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Passive Queue Management (1/2)Passive Queue Management (1/2)
Two states of PQM: No packet drop 100% packet drop
Drop-Tail Drop packets from the tail of the queue. All arriving packets are dropped once the queue size
reaches a certain threshold.
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Passive Queue Management (2/2)Passive Queue Management (2/2)
Drop-Front Drop the packet in the buffer with the oldest age.
Push-Out The latest buffered packet is pushed out from the
queue.
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Problem with PQMProblem with PQM
Lock out Global synchronization Full queue
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Active Queue ManagementActive Queue Management
Provide preventive measures to manage a buffer to eliminate problems associated with PQM.
Characteristics: Preventive random packet drop is performed before
the buffer is full The probability of preventive packet drop increases
with the increasing level of congestion. Goals:
Reduce dropped packets Support low-delay interactive services Improve the fairness
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Random Early Detection (RED)Random Early Detection (RED)
A router maintains two thresholds: Minth:
Accept all packets until the queue reaches Minth
Drop packets with a linear drop probability when the queue is greater than Minth
Maxth: All packets are dropped with probability of Maxdrop when the
queue exceeds this threshold
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Selection of MaxSelection of Maxdropdrop for RED for RED
Selection of Maxdrop significantly affects the performance of RED Too small: Active packet drops not enough to prevent
global synchronisation Too large: Decreases the throughput Optimal value depends on number of connections,
round trip time, etc.
Selection of an optimal value for Maxdrop remains an open issue
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Calculation of the Average Queue LengthCalculation of the Average Queue Length
The average queue length controls the active packet drop. Accumulate short term congestion and trace long term
congestion.
Average queue length works as a low pass filter (LPF).
LPF/ODA
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RED VariantsRED Variants
Aggregate control Modifying the calculation of the control variable and/or
drop function. Determines packet drop probability. SRED, DSRED, REM, BLUE, …
Per-flow control Configuring and setting RED’s parameters. Addresses fairness problem. FRED, FB-RED, XRED, …
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Admission ControlAdmission Control
Why admission control? Expressing QoS can be done in flow
specification.
Characteristics of the Input Service Required
maximum data unit size (bytes) Token bucket rate (bytes/sec) Toke bucket size (bytes) Maximum transmission rate (bytes/sec)
Loss sensitivity (bytes) Loss interval (sec) Burst loss sensitivity (data units) Minimum delay noticed (sec) Maximum delay variation (sec) Quality of guarantee
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Leaky Bucket (1/2)Leaky Bucket (1/2)
At the host-network interface, allow packets into the network at a constant rate.
Packets may be generated in a bursty manner, but after they pass through the leaky bucket, they enter the network evenly spaced.
LeakyBucket
Network
Packets from host
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Leaky Bucket (2/2)Leaky Bucket (2/2)
The leaky bucket is a “traffic shaper”: It changes the characteristics of packet stream.
Traffic shaping makes traffic more manageable and more predictable.
In some cases, we may want to allow short bursts of packets to enter the network without smoothing them out.
For this purpose we use a token bucket, which is a modified leaky bucket.
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Token Bucket (1/2)Token Bucket (1/2)
The bucket holds tokens instead of packets. Tokens are generated at a constant rate. When a packet arrives at the token bucket, it is
transmitted if there is a token available. Otherwise it is buffered until a token becomes available.
Tokens are buffered in a bucket with limited capacity. Bucket is full, tokens are dropped.
Data are passed to network in a relatively constant rate.
Allows for some bursty traffic from the application.
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Token Bucket (2/2)Token Bucket (2/2)
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Token Bucket vs. Leaky Bucket (1/2)Token Bucket vs. Leaky Bucket (1/2)
Case 1: Short burst arrivals
6543210
Arrival time at bucket
Departure time from a leaky bucketLeaky bucket rate = 1 packet / 2 time unitsLeaky bucket size = 2 packets
6543210
6543210
Departure time from a token bucketToken bucket rate = 1 tokens / 2 time unitsToken bucket size = 2 tokens(There are two tokens in the bucket at time 0.)
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Token Bucket vs. Leaky Bucket (2/2)Token Bucket vs. Leaky Bucket (2/2)
Case 2: Large burst arrivals
6543210
Arrival time at bucket
Departure time from a leaky bucketLeaky bucket rate = 1 packet / 2 time unitsLeaky bucket size = 2 packets
6543210
6543210
Departure time from a token bucketToken bucket rate = 1 token / 2 time unitsToken bucket size = 2 tokens(There are two tokens in the bucket at time 0.)
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ConclusionsConclusions
How congestion control in the Internet? Challenges
Increasing traffic volume Varied QoS requirements Complicated networking environment
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Thank Youand
Good-bye !!
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選擇題 選擇題 (1/2)(1/2)
( ) 下列何者不是執行網際網路壅塞控制的元件 (1) end host (2) access router (3) backbone router (4) network link
( ) 下列何者接替了 ARPANET 成為網際網路的骨幹網路 (1) NSFNET (2) ANSNET (3) ETHERNET (4) FDDI
( ) TCP/IP 最先被內建在哪一個作業系統中 (1) Windows (2) Dos (3) Mac (4) UNIX
( ) 下列何者不是 Internet Service Provider (1) HiNet (2) SeedNet (3) AppNet (4) TaNet
( ) 下列何者不是應用層協定 (1) SNMP (2) DNS (3) ARP (4) SMTP
( ) HTTP (Hyper Text Transmission Protocol) 的 TCP well known port 是 (1) 80 (2) 23 (3) 67 (4) 53
( ) 何者不包含在 TCP congestion control policy 之中 (1) quick shift (2) congestion avoidance (3) fast retransmission (4) slow start
( ) 哪一個版本的 TCP 開始加入 fast recovery 功能 (1) Tahoe (2) Vegas (3) New Reno (4) Reno
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選擇題 選擇題 (2/2)(2/2)
( ) 何者不屬於 Passive Queue Management 演算法 (1) drop-tail (2) drop-front (3) drop-middle (4) push out
( ) 何者不是 Passive Queue Management 的缺點 (1) lock out (2) global synchronization (3) empty queue (4) full queue
( ) 相較於 Passive Queue Management ,何者不是 Active Queue Management 的改進目標 (1) reduce dropped packets (2) support low-delay interactive services (3) increase link utilization (4) improve the fairness
( ) 何者不是 Random Early Detection (RED) 需要的參數 (1) MAXth (2) average queue length (3) total throughput (4) MAXdrop
( ) 在 Random Early Detection (RED) 的下列改進方案中何者不屬於aggregate control (1) SRED (2) FRED (3) DSRED (4) REM
( ) 何者為 admission control 的方法之ㄧ (1) leaky bucket (2) gum bucket (3) leaf bucket (4) toy bucket
( ) 何者不是網際網路壅塞控制可見未來的主要挑戰 (1) increasing traffic volume (2) varied QoS requirements (3) complicated networking environment (4) 以上皆是
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