Lecture 22
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Reminders: Programming Project 4 due today
Homework 7 and Programming Project 5
(reassigned Homework Problem 5.48) posted.
Questions?
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Outline
Chapter 6 - Congestion Control and Resource
Allocation
6.1 Issues in Resource Allocation
6.2 Queuing Disciplines
6.3 TCP Congestion Control
6.4 Congestion-Avoidance Mechanisms
6.5 Quality of Service
6.6 Summary
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TCP Congestion Control
Congestion control was not added to TCP until
about 8 years after the birth of the Internet. At
that time the Internet was on the verge of collapse
due to congestion.
TCP uses feedback from the receiver to
determine how fast to transmit packets. TCP
congestion control will work with either FIFO or
fair queuing.
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TCP Congestion Control
TCP uses ACK arrival as a signal to transmit a
new packet. Since connections come-and-go
TCP congestion control must be adaptive.
TCP congestion control consists of three separate
mechanisms:
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Additive Increase/Multiplicative Decrease
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Slow Start
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Fast Retransmit and Fast Recovery
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Additive Increase /
Multiplicative Decrease (AIMD)
For congestion control TCP maintains a
CongestionWindow variable that is similar to the
AdvertisedWindow variable used in flow control.
The sender is limited to sending no more than
EffectiveWindow bytes:
MaxWindow = min(CongestionWindow,
AdvertisedWindow)
EffectiveWindow = MaxWindow –
(LastByteSent - LastByteAcked)
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Additive Increase /
Multiplicative Decrease (AIMD)
It is assumed that the network is congested when
a timeout occurs (a packet is not ACKed). Each
time there is a timeout CongestionWindow is
reduced by one-half (multiplicative decrease).
If CongestionWindow was equal to 8 packets,
successive timeouts would reduce it to 4, then 2,
and then 1 (it is not allowed to fall below 1 packet
or rather 1 MSS).
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Additive Increase /
Multiplicative Decrease (AIMD)
For each ACK that arrives, CongestionWindow is
incremented slightly (additive increase) via:
Increment = MSS x (MSS/CongestionWindow)
CongestionWindow += Increment
This doubles CongestionWindow after a
CongestionWindow of packets have been ACKed.
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Additive Increase /
Multiplicative Decrease (AIMD)
AIMD results in the sawtooth pattern shown below
for the congestion window size. Timeouts due to
dropped packets have occurred at the instances
in which there is a sharp decrease (by ½) of the
window size.
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Slow Start
When a connection is first
started we want to increase
CongestionWindow as
rapidly as possible to near
its steady-state value(s).
Instead of increasing it
linearly at start up, it is
doubled each time all the
packets sent in a RTT are
ACKed.
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Slow Start
Slow start increases the window more rapidly than
additive increase. It is called slow start because
TCP would originally send out a burst of data the
size of the AdvertisedWindow. This burst would
often overwhelm Internet routers.
Slow start is also used immediately after a timeout
until the window reaches ½ its size prior to the
timeout (with slow start the window is reset to 1
packet after a time out). The sizing algorithm then
switches to AIMD.
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Slow Start
Notice the slow start intervals after a timeout in
the graph of congestion window size below.
Bullets indicate timeout events. Horizontal lines
at top correspond to packet transmission times.
Verticals lines are times at which a packet(s)
that eventually times out is first transmitted.
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Fast Retransmit and Fast Recovery
Notice the gaps in the horizontal lines on the
graph on the previous slide. These represent
intervals in which the sender has sent a window of
data and is waiting for an ACK (a timeout
eventually occurs).
Fast retransmit allows a sender to retransmit a
lost packet before it times out. Fast retransmit
and fast recovery were not in the original TCP
congestion control algorithm, but were added
later.
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Fast Retransmit and Fast Recovery
When an out-of-order
packet is received the
receiver sends a
duplicate ACK of the
last in-order packet.
When the sender sees
three duplicate ACKs it
resends the (presumed)
dropped packet.
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Fast Retransmit and Fast Recovery
Fast retransmit results in about a 20% increase in
throughput. This is shown in the graph below.
The congestion window is now decreased when
duplicate ACKs are received (as well as when a
timeout occurs).
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Fast Retransmit and Fast Recovery
With fast recovery the window size is decreased
by ½ after a fast retransmit and the algorithm
enters AIMD mode.
Slow start is used only after a timeout. The
window size is reset to 1 packet. Slow start is
used until the window size reaches ½ its size
before the timeout. At all other times the
congestion window follows a pure AIMD pattern.
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Congestion-Avoidance Mechanisms
TCP uses congestion control. It relies on
timeouts and duplicate ACKs to detect when
congestion is occurring and then decreases the
window size.
An alternative strategy would be to use
congestion avoidance in which the sender
predicts when congestion will occur and reduces
the rate before packets are discarded. Ideally
packets would never be lost due to congestion.
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DECbit
In the Digital Network Architecture (DNA) network,
DNA routers would set a bit (the DECbit) when
they were busy (the average queue length
exceeded a threshold). The receiver would copy
the bit into its ACK.
At the sender, if more than 50% of the packets in
the previous congestion window had the DECbit
set the window would be decreased by 0.875. If
not, the window would be increased by one
packet.
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Random Early Detection (RED)
RED also requires special routers, but can be
used with TCP. RED routers monitor their queue
length and implicitly notify the sender of
congestion by dropping packets before
congestion actually occurs.
If the average router queue length is below
MinThreshold no packets are dropped. If the
length is above MaxThreshold all incoming
packets are dropped. If the length is between the
threshold values a packet is dropped with
probability P.
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Random Early Detection (RED)
The probability P is
a function of the
queue length as
shown at right.
It is also a function
of the time since
the last packet
was dropped. P increases with time to prevent
clusters of drops by distributing the drops over
time.
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Source-Based Congestion Control
There are a collection of congestion avoidance
algorithms that do not require special routers.
They look for signs that some router's queue is
building up and then throttle back to avoid
congestion.
One algorithm looks for an increase in RTT as a
signal. AIMD is normally used but every two
round-trip times it compares the current RTT to
the average of the min and max RTTs seen so
far. If the RTT is greater, the window is reduced
by 1/8th.
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Source-Based Congestion Control
A second algorithm looks for changes in both the
RTT and the window size. If
(CurrentWindow – OldWindow)
x
(CurrentRTT – OldRTT)
is positive the window is reduced by 1/8th.
Otherwise, it is increased by 1/8th.
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Source-Based Congestion Control
A third algorithm looks for a flattening in
throughput as a signal that congestion is
occurring.
Every RTT the congestion window is increased by
one packet. If the difference in throughput falls
below a threshold the window size is decreased
by one packet. (This algorithm effectively looks
for a change in the slope of the throughput.)
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Source-Based Congestion Control
A fourth algorithm, known as TCP Vegas,
compares the measured throughput to an
expected value.
The design of TCP Vegas is based on the
observation (illustrated in the graphs on the
following slide) that, prior to congestion, the
window size increases while the sending rate
remains flat.
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Source-Based Congestion Control
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Source-Based Congestion Control
TCP Vegas calculates an expected sending rate
from:
ExpectedRate = CongestionWindow/BaseRTT
where BaseRTT is the minimum of all round-trip
times.
The difference (Diff) between the ExpectedRate
and the ActualRate is compared to α and β
thresholds (α < β). If Diff < α the window is
increased linearly. If Diff > β the window is
decreased linearly.
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Source-Based Congestion Control
TCP Vegas congestion control. Top – congestion window.
Bottom – expected (black) and actual throughputs.
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Source-Based Congestion Control
TCP Tahoe is the name of the original TCP
congestion control algorithm (AIMD, slow start).
TCP Reno added fast retransmit and recovery.
TCP Reno is currently the most widely used TCP
congestion control algorithm. Linux allows the
system administrator to choose which algorithm to
use. TCP Reno is currently the default, but TCP
Vegas can be selected. (See the tcp man page.)
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Explicit Congestion Notification
(ECN)
Explicit Congestion Notification (ECN) is a
congestion avoidance mechanism that is also an
IETF standard.
Bits 6 and 7 of the IP TOS field are used. Bit 6 is
set by the sender to indicate that it supports ECN.
Bit 7 is set by a router to indicate congestion. Bit
7 is echoed back to the sender in the ACK. TCP
responds to the setting of the ECN bit the same
way it responds to a dropped packet.
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In-class Exercises
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Does Linux support ECN? If so, is it enabled
by default? If it is supported and not enabled
by default, how is it enabled?
What is the currently used congestion control
algorithm on csserver?
Does Linux support TCP Vegas congestion
avoidance? If so, how is it enabled?
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