1. Congestion Control Chapter 6
Outline
Resource Allocation Issues
Queuing Disciplines
FCFS (FIFO queues)
Priority Queuing
Fair Queuing (for flows)
TCP Congestion Control
Detection – Resolution approach (AIMD and Slow Start)
Alternatives: Fast Transmit / Fast Recovery
Congestion Avoidance
router-centric: DECbit and RED Gateways
host-centric: TCP Vegas
QoS

2. Congestion Control ISSUES:
How to fairly allocate resources (link bandwidths and switch buffers) among users.
Two sides of the same coin:
Resource allocation so as to avoid congestion (difficult with any precision)
Congestion control if (and when) it occurs
Resource allocation and congestion control involve both:
hosts at the edges of the network (transport protocols)
routers inside the network (queuing disciplines)
Underlying service model can be
best-effort (assume here – end-hosts
given no opportunity for QoS demands)
multiple qualities of service QoS (later)
Congestion in a packet-switched network
Destination
1.5-Mbps T1 link
Router
Source 2 1
100-Mbps FDDI
10-Mbps Ethernet

3. Framework
Connectionless flows assumed: What are they? Even tho datagrams from a source to a dest are switched independently, they typically flow thru the same path.
Routers maintain soft state info
Somewhere between the hard
state info of a VC switch (bandwidth,
cell-loss ratio, etc) and no state
info of pure connectionless.
Correct operation does not depend on
soft state info but is improved by it.
Implicitly defined: router watches
for what appears to be a flow – used in TCP Congestion Control.
Explicitly defined: source sends flow-setup (flow about to start) across network.
(a step down from a VC since explicit flow has no reliable, ordered delivery)
Taxonomy of Resource Allocation/Congestion Control mechanisms
Router-centric: address prob inside net (decide forwards/drops, inform hosts) versus Host-centric: address problem from outside the network)
Reservation-based: hosts request capacity when flow is established; versus Feedback-based: Explicit (e.g., congested router sends “slow-down message)
Implicit (eg, host adjust rate based on, eg, cell-loss rate)
Window-based (telling sender remaining buffer space – as in flow control) versus
Rate-based (telling sender the rate at which data can be absorbed)
Router
Source 2 1 3
Destination
Multiple flows passing thru a set of routers

4. Evaluation Criteria(of resource allocation effectiveness & fairness)
Optimal
load
Load
Throughput/delay
Effective Resource Allocation
(utilization issue – network-wide point of view)
measured by Power = ratio of thruput to delay.
Fair Resource Allocation (to individual senders)
Can assume Fair means Equal shares
E.g., Raj Jain proposed metric when Fair means Equal and all paths are equal length:
Jain’s Fairness Index: Given flow thruputs (units/sec) x1, x2, …, xn
f(x1, x2, …, xn) = ( ni=1 xi )2 / ( n ni=1 xi2 )
If all n flows have thruput of 1 unit/sec, f = n2 / n*n = 1.
However if k have thruput 1 and n-k have thruput 0, f = k2 / n*k = k/n (less fair)
Thrashing or
congestion collapse
Power

5. Queuing Discipline (Each router specifies a queuing discipline regardless of resource allocation mechanism. Algorithm can be thought of as allocating bandwidth (which packets get transmitted) and buffer space (which packets get dropped))
First-In-First-Out or FIFO (AKA: FCFS)
Packets transmitted in arrival order.
No discrimination between traffic sources.
Usually used with “tail drop” policy.
FIFO + tail-drop = bundle.
Widely used in Internet.
Variations include priority queuing.
Fair Queuing (FQ) for Flows
explicitly segregates traffic based on flows
(separate queue per flow)
Weighted Fair Queuing allows a
weight to be assigned to each flow.
Flow 1
Flow 2
Flow 3
Flow 4
Round-robin
service

6. Fair Queuing - FQ Algorithm
For simplicity, suppose clock ticks each time bit is transmitted (bit = tic)
Let Pi = length of packet i
Si = time when transmission of packet i starts
Fi = time when transmission of packet i finishes
Fi = Si + Pi
For a single flow, when does a router start transmitting packet i?
if it’s before router is finished with this flow’s packet i-1, right after last bit of i-1 (Fi-1)
if no current packets for this flow, then start transmitting when 1 arrives (at time Ai)
Thus: MAX (Fi - 1, Ai) and Fi = MAX (Fi - 1, Ai) + Pi
For multiple flows (Not perfect: can’t preempt current packet)
calculate Fi for each packet that arrives on each flow (treat as timestamps)
packet with lowest timestamp is next.
Flow 1
Flow 2
(a)
(b)
Output
F = 8
F = 10
F = 5
F = 2
(arriving)
(transmitting)
Queue discipline: Shortest packet first
Longer packet already in progress is completed first

7. TCP Congestion Control
Idea
assumes best-effort network (FIFO or FQ routers) each source determines network capacity for itself
uses implicit feedback (host adjusts rate based on its knowledge)
ACKs pace transmission (self-clocking) (I.e., only allow n outstanding un-Ack’ed packets.
Challenge
determining the available capacity in the first place
adjusting to changes in the available capacity
AIMD and Slow Start were the original solutions for TCP

8. Additive Increase/Multiplicative Decrease (AIMD)
Objective: adjust to changes in the available capacity
New state variable per connection: CongestionWindow
set by source to limit number of packets in transit
Recall, FlowCtrl AdvertisedWindow = # of packets destination can still buffer)
MaxWin = MIN( CongestionWindow, AdvertisedWindow )
EffWin = MaxWin - ( LastByteSent - LastByteAcked )
# of outstanding packets
Idea:
increase CongestionWindow when congestion goes down
decrease CongestionWindow when congestion goes up
Question: how does the source determine whether or not the network is congested?
Answer: a packet timeout occurs (I.e., an Ack is late)
Assumes timeout signals that a packet was dropped due to congestion
(packet loss is so seldom due to transmission error)
lost packet implies congestion

9. AIMD (cont)
Source
Destination …
Algorithm: Each time source successfully sends a CongestionWindow
of packets, increase CongestionWindow by 1 packet (additive incr).
Divide CongestionWindow by 2 each timeout (multiplicative decr)
(never below Min Seg Size – MSS is in bytes – usually 1 packet)
In practice however, TCP increments a little for each ACK, using:
Increment = MSS * (MSS/CongestionWindow)
CongestionWindow += Increment
Trace: CongestionWindow sawtooth behavior with AIMD
AIMD works well when
source is operating close
to the available capacity of
the network. But takes too
long to ramp up from scratch.
SLOW START (ironically name)
is intended to solve that using multiplicative increase. 60 20
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ime (seconds) 70 30 40 50 10
10.0

10. Slow Start (2nd mechanism provided by TCP)
Source
Destination …
Start with CongestionWindow (CW) = 1 packet
a slow start compared to a CongestionWindow=AdvertisedWindow start
Double CongestionWindow each RTT (multiplicative incr)
until it reaches CongestionThreshold (CT), then increment by 1 per RTT.
Used when first starting connection and if connection goes dead
waiting for timeout (another “start over” situation).
Slow Start Trace:
No increase; No Acks arriving – due to lost packets
Timeout; 17=CTCW/2; CW  0
Multiplicative increase until CT, then Additive increase
No increase; No Acks arriving
Hash marks =times when each packet is transmitted
timeouts 60 20
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time when retransmitted packets
were first transmitted
mult increase
Time in sec
Multiplicative increase until CT, then Additive increase
Timeout; 11=CTCW/2; CW  0

11. Fast Retransmit
Packet 1
Packet 2
Packet 3
Packet 4
Packet 5
Packet 6
Retransmit
packet 3
ACK 1
ACK 2
ACK 6
Sender
Receiver
Problem: Coarse-grain TCP timeouts lead to idle periods
Fast retransmit: use duplicate ACKs to trigger retrans.
Idea: every time a packet arrives, receiver sends ACK.
Thus, when a packet arrives out-of-order (and TCP can’t
ACK because earlier packets have not yet arrived)
TCP resends last legit cumm ACK (called duplicate ACK).
When sender sees 3 dups, retransmits next packet.
Trace of CongestionWindow with fast retransmit
Fast Recovery: Upon congestion, rather than drop back to 0 and use Slow Start, just cut window in half and resume additive increase.
Hash marks =times when each packet is transmitted
timeout 60 20
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time when retransmitted packets
were first transmitted
Time in sec
Eliminates many of the flat areas where no packets were transmitted

12. Congestion Avoidance
TCP’s strategy is to control congestion once it happens
(repeatedly increase load to find the point at which congestion occurs, and then back off)
Alternative strategy
predict when congestion is about to happen
reduce rate before packets start being discarded
call this congestion avoidance, instead of congestion control
Two possibilities
router-centric: DECbit and RED Gateways
host-centric: TCP Vegas

13. Random Early Dectection (RED)
Queue length
Current
time T
ime
cycle
Previous A
veraging
interval
DECbit
Queue length
Add congestion bit to packet header.
Router
monitors average queue length over
last busy-idle cycle + current busy cycle,
set congestion bit if average queue length > 1
End Host
Destination echoes bit back to source
Source records how many packets resulted in set bit
If less than 50% of last CongestionWindow’s worth had bit set
increase CongestionWindow by 1 packet
If 50% or more of last window’s worth had bit set
decrease CongestionWindow to 7/8th of its value.
Random Early Dectection (RED)
Notification is implicit
Router just drops the packet when congested (TCP will timeout)
Early random drop
rather than wait for queue to become completely full, drop each arriving packet with some drop probability whenever the queue length exceeds some drop level

14. RED Details Compute average queue length
AvgLen = (1-Weight)*AvgLen+Weight*SampleLen
0 < Weight < 1 (usually 0.002)
SampleLen = queue length each time packet arrives
Two queue length thresholds
if AvgLen  MinThreshold then enqueue packet
if MinThreshold < AvgLen < MaxThreshold
then calculate probability P
drop arriving packet with probability P
if MaxThreshold  AvgLen, then drop arriving packet
Computing probability P
TempP = MaxP * (AvgLen - MinThreshold)
(MaxThreshold - MinThreshold)
Count = # packets (denom of AvgLen)
P = TempP/(1 - count * TempP)
Weighted runnng avg queue length
MaxThreshold
MinThreshold A
vgLen
Drop probability curve
P(drop)
1.0
MaxP
MinThresh
MaxThresh A
vgLen

15. TCP Vegas (host-centric congestion avoidance)
Idea: source watches for some sign router’s queue is building
(eg, RTT grows; sending rate flattens)
ExpectedRate =CW/BaseRTT
Diff = ExpectedRate – ActualRate
if Diff < α increase CW linearly
else if Diff > β decrease CW linearly
else leave CW unchanged
( when α < Diff < β )
min of all measured RTTs,
Typically RTT of 1st packet
Source calculates current sending rate as the # bytes divided by the RTT for a distinguished packet
roughly corresponds to too little data in the network
roughly corresponds to too much data in the network

16. TCP Vegas (trace of congestion avoidance mechanism)
Parameters
a = 1 packet
b = 3 packets
Congestion Window Trace for TCP Vegas
Actual throughput Expccted throughput
Shaded area is region between a and b units away
From the Expected throughput (the goal to keep actual in
this region. Note the actual gets drug along by shaded.)

17. QoS Real-time App Playback Buffer Require “deliver on time” assurances
Microphone
Speaker
Sampler ,
A D
converter
Buffer
D A
QoS Real-time App
Require “deliver on time” assurances
must come from inside the network (hosts cannot make such guarantees alone)
Example application (audio)
sample voice once every 125us
each sample has a playback time
packets experience variable delay in network
add constant factor to playback time: playback point
Playback Buffer
Packet generation
Packet arrival
Buffer
Sequence number
Network delay
Playback
Time

18. Integrated Services
Refers to the body of work by IETF working group on Integrated Services.
Integrated Services allocates resources to individual flows
whereas Differentiated Services allocates resources by “classes of traffic”
Integrated Service Service Classes
E.g., Guaranteed service (packets are never late – guaranteed max delay time)
Flowspecs (Set of info we provide to the network to specify needs.)
Tspec
describes flow’s Traffic characteristics (e.g., average bandwidth, token issues..)
Rspec
describes the services Requested from the network
E.g., guarantees, such as, delay target

19. RSVP Resource reSerVation Protocol
While connection-oriented networks have setup protocols,
best-effort connectionless networks don’t – they need some sort of reservation protocl in order to offer QoS.
Internet resource reservation corresponds to signaling in ATM
Proposed Internet standard is called RSVP
Receiver-oriented
2 messages: PATH and RESV
Source transmits PATH messages
every 30 seconds to make requests.
Destination responds with
RESV message to ack.

20. RSVP versus ATM (Q.2931) RSVP ATM receiver generates reservation
soft state info used in routeers (it is refreshed/timedout)
separate from route establishment
QoS can change dynamically
ATM
sender generates connection request
hard state info (requires explicit delete at teardown)
concurrent with route establishment
QoS is static for life of connection

21. Differentiated Services (also IETF)
Problem with Integrated Services: scalability
Idea of Differentiated Serivces: support 2 classes of packets
DS adds new Premium Service to best effort traffic class)
Which packets are premium?
Use premium-bit in header.