INF3190 / INF4190 - Data Communication
Congestion 2 problem areas
• Receiver capacity Approached by flow control
• Network capacity Approached by congestion control
Possible approach to avoid both bottlenecks• Receiver capacity: “actual window”, credit window• Network capacity: “congestion window”• Valid send window = min(actual window, congestion window)
Terms• Traffic
All packets from all sources• Traffic class
All packets from all sources with a common distinguishing property, e.g. priority
INF3190 / INF4190 - Data Communication
Congestion Persistent congestion
• Router stays congested for a long time• Excessive traffic offered
Transient congestion• Congestion occurs for a while• Router is temporarily overloaded• Often due to burstiness• Burstiness
Average rate r Burst size b (#packets that appear at the
same time)Token bucket model
INF3190 / INF4190 - Data Communication
Congestion Reasons for congestion,
among others• Incoming traffic overloads outgoing lines• Router too slow for routing algorithms• Too little buffer space in router
When too much traffic isoffered
• Congestion sets in• Performance degrades sharply
maximum transmissioncapacity ofthe subnet
perfect
desirable
congested
packets sent by application
pack
ets
deliv
ered
Congestion tends to amplify itselfNetwork layer: unreliable service
Router simply drops packet due to congestionTransport layer: reliable service
Packet is retransmitted
Congestion => More delays at end-systemsHigher delays => RetransmissionsRetransmissions => Additional traffic
INF3190 / INF4190 - Data Communication
Congestion Control General methods of resolution
• Increase capacity• Decrease traffic
Strategies• Repair
When congestion is noticed Explicit feedback (packets are sent from the point of congestion) Implicit feedback (source assumes that congestion occurred due to other
effects) Methods: drop packets, choke packets, hop-by-hop choke packets, fair
queuing,...• Avoid
Before congestion happens Initiate countermeasures at the sender Initiate countermeasures at the receiver Methods: leaky bucket, token bucket, isarithmic congestion control,
reservation, …
INF3190 / INF4190 - Data Communication
Repair Principle
• No resource reservation• Necessary steps
Congestion detected Introduce appropriate procedures for reduction
INF3190 / INF4190 - Data Communication
Repair by Packet dropping Principle
• At each intermediate system• Queue length is tested• Incoming packet is dropped if it cannot be buffered
We may not wait until the queue is entirely full
To provide• Connectionless service
No preparations necessary• Connection-oriented service
Buffer packet until reception has been acknowledged
INF3190 / INF4190 - Data Communication
Repair by Packet dropping
Assigning buffers to queues at output lines1. Maximum number of buffers per output line
Packet may be dropped although there are free lines
2. Minimal number of buffers per output line Sequences to same output line (“bursts”) lead to drops
3. Dynamic buffer assignment Unused lines are starved
Outputlines
Inputlines
INF3190 / INF4190 - Data Communication
Repair by Packet dropping4. Content-related dropping: relevance
Relevance of data connection as a wholeor every packet from one end system to another end system
• Examples Favor IPv6 packets with flow id 0x4b5 over all others Favor packets of TCP connection
(65.246.255.51,80,129.240.69.49,53051) over all others
Relevance of a traffic class• Examples
Favor ICMP packets over IP packets Favor HTTP traffic (all TCP packets with source port 80) over FTP
traffic Favor packets from 65.246.0.0/16 over all others
INF3190 / INF4190 - Data Communication
Repair by Packet dropping Properties
• Very simple
But• Retransmitted packets waste bandwidth• Packet has to be sent 1 / (1 - p) times before it is accepted
(p ... probability that packet will be dropped)
Optimization necessary to reduce the waste of bandwidth• Dropping packets that have not gotten that far yet⇒ e.g. Choke packets
INF3190 / INF4190 - Data Communication
Repair by Choke Packets Principle
• Reduce traffic during congestion by telling source to slow down
Procedure for router• Each outgoing line has one variable
Utilization u ( 0≤u≤1 )
• Calculating u: Router checks the line usage f periodically (f is 0 or 1) u = a * u + ( 1 - a ) * f 0 ≤ a ≤ 1 determines to what extent "history" is taken into account
• u > threshold: line changes to condition "warning“ Send choke packet to source (indicating destination) Tag packet (to avoid further choke packets from down stream router) & forward it
INF3190 / INF4190 - Data Communication
Repair by Choke Packets Principle
• Reduce traffic during congestion by telling source to slow down
Procedure for source• Source receives the choke packet
Reduces the data traffic to the destination in question by x1%• Source recognizes 2 phases
(gate time so that the algorithm can take effect) Ignore: source ignores further Choke packets until timeout Listen: source listens if more Choke packets are arriving
• yes: further reduction by X2%;go to Ignore phase
• no: increase the data traffic
INF3190 / INF4190 - Data Communication
Repair by Choke Packets Hop-by-Hop Choke Packets Principle
• Reaction to Choke packets already at router (not only at end system)
Plain Choke packets
A heavy flow is establishedCongestion is noticed at DA Choke packet is sent to AThe flow is reduced at AThe flow is reduced at D
A
B C
D
E F
Hop-by-hop Choke packets
A heavy flow is establishedCongestion is noticed at DA Choke packet is sent to AThe flow is reduced at FThe flow is reduced at D
A
B C
D
E F
INF3190 / INF4190 - Data Communication
Repair by Choke Packets Variation
• u > threshold: line changes to condition "warning“ Procedure for router
• Do not send choke packet to source (indicating destination)• Tag packet (to avoid further choke packets from down stream router) & forward it
Procedure at receiver• Send choke packet to sender
Other variations• Varying choke packets depending on state of congestion
Warning Acute warning
• For u instead of utilization Queue length ....
INF3190 / INF4190 - Data Communication
Repair by Choke Packets Properties
• Effective procedure• But
Possibly many choke packets in the network• Even if Choke bits may be included in the data at the senders
to minimize reflux End systems can (but do not have to) adjust the traffic Choke packets take time to reach source
• Transient congestion may have passed when the source reacts Oscillations
• Several end systems reduce speed because of choke packets• Seeing no more choke packets, all increase speed again
INF3190 / INF4190 - Data Communication
Repair with Fair Queuing Background
• End-system adapting to traffic (e.g. by Choke-Packet algorithm) should not bedisadvantaged
Principle• On each outgoing line each end-system receives its own queue• Packet sending based on Round-Robin
(always one packet of each queue (sender))
Enhancement "Fair Queuing with Byte-by-Byte Round Robin“• Adapt Round-Robin to packet length• But weighting is not taken into account
Enhancement "Weighted Fair Queuing“• Favoring (statistically) certain traffic• Criteria variants
In relation to VPs (virtual paths) Service specific (individual quality of service) etc.
Congestion Avoidance
INF3190 / INF4190 - Data Communication
Avoidance Principle
• Appropriate communication system behavior and design
Policies at various layers can affect congestion• Data link layer
Flow control Acknowledgements Error treatment / retransmission / FEC
• Network layer Datagram (more complex) vs. virtual circuit (more procedures available) Packet queueing and scheduling in router Packet dropping in router (including packet lifetime) Selected route
• Transport layer Basically the same as for the data link layer But some issues are harder (determining timeout interval)
INF3190 / INF4190 - Data Communication
Avoidance by Traffic Shaping Motivation
• Congestion is often caused by bursts• Bursts are relieved by smoothing the traffic (at the price of a delay)
ProcedureNegotiate the traffic contract beforehand (e.g., flow specification)The traffic is shaped by sender
Average rate andBurstiness
AppliedIn ATMIn the Internet (“DiffServ” - Differentiated Services)
Smoothed streamPeak rate
Original packet arrival
time
INF3190 / INF4190 - Data Communication
Traffic Shaping with Leaky Bucket Principle
• Continuous outflow• Congestion corresponds to data loss
Described by• Packet rate• Queue length
Implementation• Easy if packet length stays constant
(like ATM cells)
Outputlines
Inputlines
Symbolic:bucket with
outflow per time
Implementationwith limited buffers
INF3190 / INF4190 - Data Communication
Traffic Shaping with Token Bucket Principle
• Permit a certain amount of data toflow off for a certain amount of time
• Controlled by "tokens“• Number of tokens limited• Number of queued packets limited
Implementation• Add tokens periodically
Until maximum has been reached)• Remove token
Depending on the length of the packet(byte counter)
Comparison• Leaky Bucket
Max. constant rate (at any point intime)
• Token Bucket Permits a limited burst
INF3190 / INF4190 - Data Communication
Traffic Shaping with Token Bucket Principle
• Permit a certain amount of data toflow off for a certain amount of time
• Controlled by "tokens“• Number of tokens limited• Number of queued packets limited
Implementation• Add tokens periodically
Until maximum has been reached)• Remove token
Depending on the length of the packet(byte counter)
Comparison• Leaky Bucket
Max. constant rate (at any point intime)
• Token Bucket Permits a limited burst
packet burst
INF3190 / INF4190 - Data Communication
Traffic Shaping with Token Bucket Principle
• Permit a certain amount of data toflow off for a certain amount of time
• Controlled by "tokens“• Number of tokens limited• Number of queued packets limited
Implementation• Add tokens periodically
Until maximum has been reached)• Remove token
Depending on the length of the packet(byte counter)
Comparison• Leaky Bucket
Max. constant rate (at any point intime)
• Token Bucket Permits a limited burst
INF3190 / INF4190 - Data Communication
Avoidance by Reservation: Admission Control
Principle• Prerequisite: virtual circuits• Reserving the necessary resources (incl. buffers) during connect• If buffer or other resources not available
Alternative path Desired connection refused
Example• Network layer may adjust routing based on congestion• When the actual connect occurs
A
B
A
B
INF3190 / INF4190 - Data Communication
Avoidance by Reservation: Admission Control
Sender oriented• Sender (initiates reservation)
Must know target addresses (participants) Not scalable Good security
1. reserve
2. reserve
3. reserve
receiver
sender
data flow
INF3190 / INF4190 - Data Communication
Avoidance by Reservation: Admission Control
Receiver oriented• Receive (initiates reservation)
Needs advertisement before reservation Must know “flow” addresses
• Sender Need not to know receivers More scalable Insecure
1. reserve
2. reserve
3. reserve
receiver
sender
data flow
INF3190 / INF4190 - Data Communication
Avoidance by Reservation: Admission Control
Combination?
• Start sender oriented reservation
receiver
sender
data flow
reserve from nearest router
1. reserve
2. reserve
3. reserve
INF3190 / INF4190 - Data Communication
Avoidance by Buffer Reservation Principle
• Buffer reservation Implementation variant: Stop-and-Wait
protocol• One buffer per router and connection
(simplex, VC=virtual circuit) Implementation variant: Sliding Window
protocol• m buffers per router and (simplex-)
connection Properties
• Congestion not possible• Buffers remain reserved,
Even if there is no data transmission forsome periods
• Usually only with applications that requirelow delay & high bandwidth
1
2
3
unreservedbuffers
rsvd forconn 1
1
2
3
INF3190 / INF4190 - Data Communication
Avoidance by Isarithmic Congestion Control Principle
• Limiting the number of packets in the network by assigning "permits“ Amount of "permits" in the network A "permit" is required for sending
• When sending: "permit" is destroyed• When receiving: "permit" is generated
Problems• Parts of the network may be overloaded• Equal distribution of the "permits" is difficult• Additional bandwidth for the transfer of "permits" necessary• Bad for transmitting large data amounts (e.g. file transfer)• Loss of "permits" hard to detect
INF3190 / INF4190 - Data Communication
Avoidance: combined approaches Controlled load
• Traffic in the controlled load class experiences the network as empty
Approach• Allocate few buffers for this class on each router• Use admission control for these few buffers
Reservation is in packets/second (or Token Bucket specification) Router knows its transmission speed Router knows the number of packets it can store
• Strictly prioritize traffic in a controlled load class
Effect• Controlled load traffic is hardly ever dropped• Overtakes other traffic
INF3190 / INF4190 - Data Communication
Avoidance: combined approaches Expedited forwarding
• Very similar to controlled load• A differentiated services PHB (per-hop-behavior)
Approach• Set aside few buffers for this class on each router• Police the traffic
Shape or mark the traffic Only at senders, or at some routers
• Strictly prioritize traffic in a controlled load class
Version IHL DSIdentification
Total lengthD M Fragment offset
Time to live Protocol
Destination AddressSource address
Header checksum
0 0001 1 1 1
EffectShapers drop excessive trafficEF traffic is hardly everdroppedOvertakes other traffic
Internet Congestion Control
TCP Congestion Control
INF3190 / INF4190 - Data Communication
TCP Congestion Control
TCP limits sending rate as a function of perceivednetwork congestion• Little traffic – increase sending rate• Much traffic – reduce sending rate
TCP’s congestion algorithm has four major“components”:• Additive-increase• Multiplicative-decrease (together AIMD algorithm)• Slow-start• Reaction to timeout events
INF3190 / INF4190 - Data Communication
TCP Congestion Control
sender receiver Initially, the CONGESTION WINDOWis 1 MSS (message segment size)
roun
d 1
roun
d 2
roun
d 3
roun
d 4
sent packetsper round(congestion window)
time
16
8
4
2
1
Then, the size increases by 1 for eachreceived ACKuntila threshold is reachedoran ACK is missing
INF3190 / INF4190 - Data Communication
TCP Congestion Control
16
8
4
2
1
Normally, the threshold is 64K
sent packetsper round
(congestion window)
time
40
20
10
5
80
15
30
25
35
75
55
45
50
65
60
70
Loosing a single packet (TCP Tahoe): threshold drops to half CONGESTION WINDOW CONGESTION WINDOW back to 1
Loosing a single packet (TCP Reno): if notified by timeout: like TCP Tahoe if notified by fast retransmit: threshold drops to half CONGESTION WINDOW CONGESTION WINDOW back to new threshold
INF3190 / INF4190 - Data Communication
AIMD Threshold
• Adaptive• Parameter in addition to the actual and the congestion window
Assumption• Threshold, i.e. adaptation to the network: “sensible window size”
Use: on missing acknowledgements• Threshold is set to half of current congestion window• Congestion window is reduced
Implementation- and situation-dependant: to 1 or to new threshold• Use slow start of congestion window is below threshold
Use: on timeout• Threshold is set to half of current congestion window• Congestion window is reset to one maximum segment• Use slow start to determine what the network can handle
Exponential growth stops when threshold is hit From there congestion window grows linearly (1 segment) on successful transmission
INF3190 / INF4190 - Data Communication
TCP Congestion Control Some parameters
• 65.536 byte max. per segment• IP recommended value TTL interval 2 min
Optimization for low throughput rate• Problem
1 byte data requires 162 byte incl. ACK(if, at any given time, it shows up just by itself)
• Algorithm Acknowledgment delayed by 500 msec because of window adaptation
• Comment Often part of TCP implementation
INF3190 / INF4190 - Data Communication
TCP Congestion Control TCP assumes that every loss is an indication for congestion
• Not always true Packets may be discarded because of bit errors Low bit error rates
• Optical fiber• Copper cable under normal conditions• Mobile phone channels (link layer retransmission)
High bit errors rates• Modem cables• Copper cable in settings with high background noise• HAM radio (IP over radio)
• TCP variations exist
INF3190 / INF4190 - Data Communication
TCP Congestion Control TCP congestion control is based on the notion that the
network is a “black box”• Congestion indicated by a loss
Sufficient for best-effort applications, but losses mightseverely hurt traffic like audio and video streams congestion indication better enabling features like
quality adaptation
Approaches• Use ACK rate rather than losses for bandwidth estimation
Example: TCP Westwood
• Use active queue management to detect congestion
Internet Congestion Control
TCP Congestion Avoidance
INF3190 / INF4190 - Data Communication
Random Early Detection (RED) Random Early Detection (discard/drop) (RED) uses active queue management
Drops packet in an intermediate node based on average queue length exceeding athreshold• TCP receiver reports loss in ACK• Sender applies multiple decrease
Idea• Congestion should be attacked as early as possible• Some transport protocols (e.g., TCP) react to lost packets by rate reduction
INF3190 / INF4190 - Data Communication
Random Early Detection (RED) Router drops some packet before congestion significant (i.e., early)
• Gives time to react
Dropping starts when moving avg. of queue length exceeds threshold• Small bursts pass through unharmed• Only affects sustained overloads• Packet drop probability is a function of mean queue length
Prevents severe reaction to mild overload
RED improves performance of a network of cooperating TCP sources
No bias against bursty sources
Controls queue length regardless of endpoint cooperation
INF3190 / INF4190 - Data Communication
Early Congestion Notification (ECN) Early Congestion Notification (ECN) - RFC 2481
• an end-to-end congestion avoidance mechanism• Implemented in routers and supported by end-systems• Not multimedia-specific, but very TCP-specific• Two IP header bits used
ECT - ECN Capable Transport, set by sender CE - Congestion Experienced, may be set by router
Extends RED• if packet has ECT bit set
ECN node sets CE bit TCP receiver sets ECN bit in ACK sender applies multiple decrease (AIMD)
• else Act like RED
INF3190 / INF4190 - Data Communication
Early Congestion Notification (ECN)
Effects• Congestion is not oscillating - RED & ECN• ECN-packets are never lost on uncongested links• Receiving an ECN mark means
TCP window decrease No packet loss No retransmission
Tail drop
RED
ECN
INF3190 / INF4190 - Data Communication
Endpoint Admission Control Motivation
• Let end-systems test whether a desired throughput can be supported• In case of success, start transmission
Applicability• Only for some kinds of traffic (traffic classes)• Inelastic flows• Requires exclusive use of some resources for this traffic• Assumes short queues in that traffic class
Send probes at desired rate• Routers can mark or drop probes• Probe packets can have separate queues or use main queue
INF3190 / INF4190 - Data Communication
Endpoint Admission Control Thrashing and Slow Start Probing
• Thrashing Many endpoints probe concurrently Probes interfere with each other and all deduce insufficient bandwidth Bandwidth is underutilized
• Slow start probing Probe for small bandwidth Probe for twice the amount of bandwidth … Until desired speed is reached Start sending
INF3190 / INF4190 - Data Communication
XCP: eXplicit Control Protocol
IP header TCP headerXCP Payload
Round-trip-time
Desired congestion window
Feedback
initialize
update
Provide feedback
INF3190 / INF4190 - Data Communication
XCP: eXplicit Control Protocol Congestion Controller
• Goal: Match input traffic tolink capacity
• Compute an average RTT forall connections
• Looks at queue Combined traffic changes byΔ
Δ ~ Spare Bandwidth Δ ~ - Queue Size sendable
per RTT So, Δ = α Spare - β Queue
Fairness Controller• Goal: Divide Δ between flows
to converge to fairness• Looks at a state in XCP header
If Δ > 0 ⇒ Divide Δ equallybetween flows
If Δ < 0 ⇒ Divide Δ betweenflows proportionally to theircurrent rates
TCP Friendliness
INF3190 / INF4190 - Data Communication
TCP Friendliness - TCP Compatible A TCP connection’s throughput is bounded
• wmax - maximum retransmission window size• RTT - round-trip time
Congestion windows size changes• AIMD (additive increase, multiple decrease) algorithm
TCP is said to be fair• Streams that share a path will reach an equal share
A protocol is TCP-friendly if• Colloquial
It long-term average throughput is not bigger than TCP’s• Formal
Its arrival rate is at most some constant over the square root of the packet loss rate
INF3190 / INF4190 - Data Communication
TCP Friendliness
RTT
wRs
max=
21, =!= "" ww
In case of at least one loss in anRTT
In case of no loss ,
1, =+= !!ww
Congestion windows size changes AIMD algorithm additive increase, multiple decrease
TCP is said to be fair Streams that share a path will
reach an equal share
That’s not generally true Bigger RTT
higher loss probability per RTT slower recovery
Disadvantage for long-distance traffic
A TCP connection’s throughput isbounded
wmax - maximum retransmission window size
RTT - round-trip time
The TCP send rate limit is
INF3190 / INF4190 - Data Communication
TCP Friendliness A protocol is TCP-friendly if
• Colloquial: It long-term averagethroughput is not bigger thanTCP’s
• Formal: Its arrival rate is at mostsome constant over the squareroot of the packet loss rate
The AIMD algorithm with α≠1 /2 and β≠1 is still TCP-friendly,if the rule is not violated
TCP-friendly protocols may - if the rule is not violated -Probe for available bandwidth faster than TCPAdapt to bandwidth changes more slowly than TCPUse different equations or statistics, i.e., not AIMDNot use slow start (i.e., don’t start with w=0)
CpRr +!
P – packet loss rate
C – constant value
Rr – packet arrival rate
INF3190 / INF4190 - Data Communication
Datagram Congestion Control Protocol (DCCP) Datagram Congestion Control Protocol
• Under development• http://www.ietf.org/html.charters/dccp-charter.html
Transport Protocol• Offers unreliable delivery• Low overhead like UDP• Applications using UDP can easily change to this new protocol
Accommodates different congestion control mechanisms• Congestion Control IDs (CCIDs)
Add congestion control schemes on the fly Choose a congestion control scheme TCP-friendly Rate Control (TFRC) is included
• Half-Connection Data Packets sent in one direction