TCP Congestion Control

1
TCP Congestion Control

• NETS3303/3603
• Week 9

2
Outline

• What is congestion?
• Approaches to congestion control
• TCP congestion control
• TCP vs UDP

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Principles of Congestion Control

• Congestion
• informally too many sources sending too much
  data too fast for network to handle
• different from flow control!
• manifestations
• lost packets (buffer overflow at routers)
• long delays (queueing in router buffers)
• a top-10 problem!

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Causes/costs of congestion

• two senders, two receivers
• one router, infinite buffers
• no retransmission

• large delays when congested
• maximum achievable throughput

5
Costs of congestion

• Reality routers have finite buffers
• So packets can be lost
• Congestion causes
• sender retransmit of lost packet
• retransmission of delayed (not lost) packet
• when packets dropped, any transmission capacity
  used for that packet was wasted!

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Congestion Control

• routers may drop packets as its service is best
  effort
• Implies congestion
• routers dont have effective mechanism to
  indicate congestion to sender
• ICMP source quench is not it...
• assumption
• packet loss due to damage is small, therefore TCP
  assumes it means congestion since ACKs do not
  come back

7
Approaches towards congestion control
Two broad approaches towards congestion control

• Network-assisted congestion control
• routers provide feedback to end systems
• single bit indicating congestion (TCP/IP ECN,
  ATM)
• explicit rate sender should send at

• End-end congestion control
• no explicit feedback from network
• congestion inferred from end-system observed
  loss, delay
• approach taken by TCP

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TCP Congestion Control

• end-end control (no network assistance)
• Assumes long delays (packet loss) is due to
  congestion
• Uses successive retransmissions as measure of
  congestion
• Reduces effective window as retransmissions
  increase
• Effective window is minimum of receivers
  advertisement and computed quantity known as the
  congestion window (cwnd)

9
Congestion Control II

• TCP uses slow start and multiplicative decrease
  to deal with congestion
• Van Jacobson 1988 outlined these ideas
• slow-start roughly whenever starting traffic or
  recovering from congestion, start cwnd at the
  size of a single segment and increase it (up to a
  point) as ACKs show up

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Slow Start

• Used when starting traffic or when recovering
  from congestion
• Self-clocking startup to increase transmission
  rate rapidly as long as no packets are lost
• When starting traffic, initialize the cwnd to the
  size of a single MSS
• Increase cwnd by size of 1 segment each time an
  ACK arrives without retransmission till ssthresh

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Slow Start II
Host A
Host B

• When connection begins, increase rate
  exponentially until first loss event
• double cwnd every RTT
• done by incrementing cwnd for every ACK received
• Summary initial rate is slow but ramps up
  exponentially fast

one segment
RTT
two segments
four segments
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Congestion Avoidance

• Slow start is used until cwnd reaches ssthresh
• Exponential growth is stopped
• Above threshold, slow down and increase cwnd by
  1 segment per RTT

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TCP Congestion control
CW Size
ssthresh
Time
congestion avoidance
slow-start
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Retransmissions

• TCP uses both Go back-N and selective repeat for
  retransmissions
• If timeout (gt congestion)
• Go Back-N and go into slow start
• If an isolated error (more next slide)
• Selective repeat and go to ssthresh, enter
  congestion avoidance

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Fast Retransmit Rule

• RTO expires up to secs after segment dropped
• If an isolated error, may be can do better than
  being too conservative!
• Fast retransmission
• Sender uses three duplicate ACKs as trigger
• Sender retransmits early before timeout
• Sender reduces cwnd to half (instead of 1)

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CC Refinements

• After 3 dup ACKs
• cwnd is cut in half
• window then grows linearly
• But after timeout event
• cwnd instead set to 1 MSS
• window then grows exponentially
• to a threshold, then grows linearly

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Summary of TCP Congestion Control

• When cwin is below SSThresh, sender in slow-start
  phase, window grows exponentially.
• When cwin is above SSThresh, sender is in
  congestion-avoidance phase, window grows
  linearly.
• When a triple duplicate ACK occurs, SSThresh set
  to cwin/2 and cwin set to SSThresh.
• When timeout occurs, SSThresh set to cwin/2 and
  cwin is set to 1 MSS.

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Congestion and Routers with RED

• routers might use an obvious queue-drop mechanism
• too many packets drop packets at end of queue
  call this atail-drop policy
• on heavily multiplexed router TCP connections may
  lose many packets and be forced into slow-start
• routers may use Random Early Detection (or RED) -
  basically randomly discard packets in queue at
  certain thresholds
• thus avoid tail-drop policy

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Comparison Of UDP and TCP
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Summary Of TCP

• Major transport service in the Internet (85 of
  traffic)
• Connection oriented
• Provides end-to-end reliability
• Uses adaptive retransmission
• Includes facilities for flow control and
  congestion avoidance
• Uses 3-way handshake for connection startup and
  shutdown