Congestion Control Supplementary

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Congestion Control - Supplementary

• Slides are adapted on Jean Walrands Slides

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TOC Congestion Control

• Congestion control for bandwidth sharing
• Cheating TCP
• Buffer Size
• ECN
• RED
• TCP Unfairness
• TCP Vegas

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Congestion Control Questions

• How to avoid network congestion?
• How to recover from congestion?
• Congestion occurs at access link and access
  network
• How to share the links bandwidth?
• If bandwidth is allocated and enforced properly,
  no congestion should occur.
• Can improve treatment of flows
• E.g., one flow should not get a much smaller
  fraction of bandwidth
• Some flows might need some guaranteed bandwidth
• Discovering available bandwidth
• What is fair?

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Questions Available bandwidth?

• Example

x, y, z throughput of flows
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Questions Available bandwidth?

• Example

• What is available bandwidth? (See later)
• How does A discover the available bandwidth to
  B?
• Some approaches
• Reservation e.g., Telephone
• Adapt to congestion TCP
• Pricing congestion research topic

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Available bandwidth Reservation

• Routers (or manager) keep track of reserved rates
• A requests a rate R to B from the network
• The network figures out if R is available
• If R is available, routers (or manager) update
  reservations and confirm to A
• Note complex, slow, requires connection setup
  and enforcement

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Available bandwidth Adapt

• Transmit and slow down if congestion occur
• Example
• Initially x 0, y 3, z 3
• Then A increases its rate C and E notice
  congestion and slow down
• Later, C stops A and E increase rates
• Notes
• No guarantees throughput may drop
• Key question how to adapt rates, e.g., TCP

8
Available bandwidth Pricing

• Example

• When they get saturated, routers mark packets
• If a flow with rate R uses saturated links, it
  gets marks with rate R
• Each mark costs one unit
• Source slows down if price becomes excessive
• x 1, y 2, z 2 ? pA 1 1 pC pE 2
• x 2, y 1, z 1? pA 2 2 pC pE 1

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Questions What is Fair?

• Example

• x y z 1.5 fair in max-min sense
• x 0, y z 3 maximizes x y z
• 5x 4y 4z equalizes resources flows use with
  x 1.33, y z 1.67
• What if A?B needs 2Mbps?(and is willing to pay
  for it)

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Cheating Increase Faster
y
C
x increases by 2 MSS/RTT per RTT y increases by 1
MSS/RTT per RTT
Limit rates x 2y
x
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Cheating Increase Faster
Rate
time
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Cheating Start SS with CongWin gt 1
x starts SS with CongWin 4 y starts SS with
CongWin 1
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Cheating Open Many Connections

• Assume
• A starts 10 connections to B
• D starts 1 connection to E
• Each connection gets about the same throughput
• Then A gets 10 times more throughput than D

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ECN Explicit Congestion Notification

• Standard TCP
• Losses needed to detect congestion
• Wasteful and unnecessary
• ECN
• When congested, routers mark (IP) packets instead
  of dropping them
• Destination marks ACKs of marked packets
• Source set CongWin CongWin/2 when it sees mark
• Advantages
• No time wasted to retransmit
• Link errors not confused with congestion
• Example without ECN, at wireless link, packets
  can get corrupted by noise and interference, and
  get dropped eventually. Sender will think
  congestion has occurred.

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Explicit Congestion Notification

• Illustration

A
B
A
B
CongWin CongWin/ 2
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Explicit Congestion Notification

• Backward Compatibility
• Unused bits from TCP and IP headers are used for
  ECN.
• One bit in IP header indicates if hosts
  implement ECN
• If it does, router marks packet
• If it does not, router drops packet

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RED

• Random Early DetectionAs queue builds up, drop
  or mark packets with increasing probability
  (before queue gets full)
• Advantages
• Avoids penalizing streams with large bursts
• This is done by keeping the queue size smaller
  than the buffer size
• Earlier marking reduces dropped packets
• De-synchronizes the source behaviors

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RED Illustration
C Mbps
PACKETS

• Calculate recent average of queue length Qav
  Qav(n1) (1 b) Qav(n) b Q(n)
• 0 lt b lt 1
• Determine drop or mark probability p(Qav)

1
k
Qav
0
H
L
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Router Buffers Rule of Thumb

• Imagine that all connections on input port with
  rate R burst for RTT seconds (until stopped by
  RED)
• Router must store RxRTT for each port
• Example
• 40 Gbps throughput (sum of port rates)
• RTT 200 ms (worst case?)
• Then storage 8 Gbits (about) 1 GByte
• Question Is this reasonable?

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Unfairness

• Fact
• TCP favors connections with short RTT
• Cause
• Increase rate is 1 MSS/RTT, so that it is faster
  for connections with small RTT
• Recall our discussion of Cheating RTT
• It is quite possible for a connection to get
  only a few percent of its fair share
• Solutions
• Modify TCP to increase in proportion to
  RTT?Problem Estimate of RTT is noisy
• TCP Vegas (see next)

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TCP Vegas

• Consider one queue

C
x
A
B
y
D
E
Assume both connections have the same
backlog Then they have the same throughput
Vegas Algorithm Estimate backlog by Q
Outstanding Rate ? Base_RTT Rate
Bytes_Sent_Successfully / Current_RTT If Q gt 3
MSS, then slow down otherwise, speed up Problem
Reno clobbers Vegas (unlike in real life)