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Lecture 14

High-speed TCP connectionsWraparoundKeeping the pipeline fullEstimating RTT

Fairness of TCP congestion control

Internet resource allocation and QoS

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Protection against wraparoundWhat is wraparound: A byte with a sequence number x may be sent at one time and then on the same connection a byte with the same sequence number x may be sent again.Wrap Around: controlled by the 32-bit SequenceNumThe maximum lifetime of an IP datagram is 120 sec thus we need to have a wraparound time at least 120 sec.For slow links OK but no longer sufficient for optical networks. Bandwidth & Time Until Wrap Around

BandwidthT1 (1.5Mbps)Ethernet (10Mbps)T3 (45Mbps)FDDI (100Mbps)STS-3 (155Mbps)STS-12 (622Mbps)STS-24 (1.2Gbps)

Time Until Wrap Around6.4 hours57 minutes13 minutes6 minutes4 minutes55 seconds28 seconds

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Keeping the pipe full

The SequenceNum, the sequence number space (32 bits) should be twice as large as the window size (16 bits). It is.

The window size (the number of bytes in transit) is given by the the AdvertisedWindow field (16 bits).

The higher the bandwidth the larger the window size to keep

the pipe full.Essentially we regard the network as a storage system and the amount of data is equal to: ( bandwidth x delay )

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Required window size for a 100 msec RTT.

BandwidthT1 (1.5Mbps)Ethernet (10Mbps)T3 (45Mbps)FDDI (100Mbps)STS-3 (155Mbps)STS-12 (622Mbps)STS-24 (1.2Gbps)

Delay x Bandwidth Product18KB122KB549KB1.2MB1.8MB7.4MB14.8MB

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Original Algorithm for Adaptive Retransmission

Measure SampleRTT for each segment/ACK pair Compute weighted average of RTTEstimatedRTT = x EstimatedRTT + (1- x SampleRTT

where 0.8 < 0.9

Set timeout based on EstimatedRTT TimeOut = 2 x EstimatedRTT

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Karn/Partridge Algorithm

Do not sample RTT when re-transmitting

Double timeout after each retransmission

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Karn/Partridge Algorithm

Sender SenderReceiver Receiver

Sample RTT

O rig ina l transm ission

R e-transm ission

A cknow ledgm ent

O rig ina l transm ission

A cknow ledgm ent

R e-transm ission

Sample RTT

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Jacobson/Karels Algorithm

New calculation for average RTTDiff = SampleRTT - EstimatedRTT EstimatedRTT = EstimatedRTT + ( x Deviation = Deviation + (|Diff|- Deviation) where is a fraction between 0 and 1

Consider variance when setting timeout value TimeOut = x EstimatedRTT + x Deviation where = 1 and = 4

Notes algorithm only as good as granularity of clock (500

microseconds on Unix) accurate timeout mechanism important to congestion

control (later)

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Congestion Control Mechanisms

The sender must perform retransmissions to compensate for lost packets due to buffer overflow.Unneeded retransmissions by the sender due to large delays causes a router to use link bandwidth to forward unneeded copies of a packet.

When a packet is dropped along a path the capacity used used at each upstream routers to forward packets to the point where it was dropped was wasted.

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Delay/Throughput Tradeoffs Q uality o f S ervice

(D e lay)

Q uantity o f S ervice(Throughput)

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1

2

3

4

5

6

7

8

9

10

11

12

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1 2 3 4 5 6 7 8 9 10 11 12

threshold

new threshold

time

timeout occurs

slowstart

window size

congestionavoidance

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Router with infinite buffer capacity

H ost A

H ost B

R outerinl

inl

outl

inl

outl

C

C/2

C/2

inlC/2

D elay

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Fairness of TCP congestion mechanism

Equal bandwidthline

Fu ll bandw idthutiliza tion line

Throughput o fconnection 1

Throughput o fconnection 2

R

R

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Flows and resource allocation

Flow: sequence of packets with a common characteristics

A layer-N flow the common attribute a layer-N attributeAll packets exchanged between two hosts

network layer flow All packets exchanged between two

processes transport layer flow

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who makes decisions

host-centric

router-centric

basis fordecisions

the needs of the flow

the state of network

how are decisionsenforced

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Min-max fair bandwidth allocation

Goal: fairness in a best-effort network.

Consider:Unidirectional flowsRouters with infinite buffer space

Link capacity is the only limiting factor.

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AlgorithmStart with an allocation of zero Mbps for each flow.Increment equally the allocation for each flow until one of the links of the network becomes saturated. Now all the flows passing through the saturated link get an equal fraction of the link capacity.Increment equally the allocation for each flow that does not pass through the first saturated link until a second link becomes saturated. Now all the flows passing through the saturated link get an equal fraction of the link capacity.Continue by incrementing equally the allocations of all flows that do not use a saturated link until all flows use at least one saturated link.

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QoS in a datagram network?

Buffer acceptance algorithms.

Explicit Congestion Notification.

Packet Classification.

Flow measurements

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Buffer acceptance algorithms

Tail Drop.

RED – Random Early Detection

RIO – Random Early Detection with In and Out packet dropping strategies.

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1.0

maxDropProb

minThr_out maxThr_out

minThr_in maxThr_in

avgQue

minThr_inmaxThr_in

sampleQueueLength

dropProb

low load

maxThr_out minThr_out

low load

medium load

medium load

high load

high load

out

in

(a)

(b)sampleQueueLength

inout

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Explicit Congestion Notification (ECN) The TCP congestion control mechanism discussed earlier has a major flow; it detects congestion after the routers have already started dropping packets. Network resources are wasted because packets are dropped at some point along their path, after using link bandwidth as well as router buffers and CPU cycles up to the point where they are discharged.The question that comes to mind is: Could routers prevent congestion by informing the source of the packets when they become lightly congested, but before they start dropping packets? This strategy is called source quench.

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Source quench

Send explicit notifications to the source, e.g., use the ICMP. Yet, sending more packets in a network that shows signs of congestion may not be the best idea.

Modify a congestion notification flag in the IP header to inform the destination; then have the destination inform the source by setting a flag in the TCP header of segments carrying acknowledgments.

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Problems with ECN

(1) TCP must be modified to support the new flag.

(2) Routers must be modified to distinguish between ECN-capable flows and those who do not support ECN.

(3) IP must be modified to support the congestion notification flag.

(4) TCP should allow the sender to confirm the congestion notification to the receiver, because acknowledgments could be lost.

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Maximum and minimum bandwidth guarantees

A. Packet classification. Identify the flow the packet belongs to. At what layer should be done? Network layer? At each router too expensive. The edge routers may be able to do that. At application layer? Difficult. MPLS – multi protocol label switch. Add an extra header

in front of the IP header. Now a router decides the output link based upon the input link and the MPLS header.

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Maximum and minimum bandwidth guarantees

B. Flow measurementsHow to choose the measurement interval to

accommodate bursty traffic?Token bucket

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The token bucket filter

Characterized by : (1) A token rate R, and (2) The depth of the bucket, B

Basic idea the sender is allocated tokens at a given rate and can accumulate tokens in the bucket until the bucket is filled. To send a byte the sender must have a token. The maximum burst can be of size B because at most B token can be accumulated.

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Example

Flow A: generates data at a constant rate of 1 Mbps. Its filter will support a rate of 1 Mbps and a bucket depth of 1 byte,

Flow B: alternates between 0.5 and 2.0 Mbps. Its filter will support a rate of 1 Mbps and a bucket depth of 1 Mbps

Note: a single flow can be described by many token buckets.

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Example

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B: maximum bucket capacity

C: current number oftokens in bucket

Token Bucket

Router

Incoming packets Accepted packets

Droppedpackets

Initially: C = B1/r: filling rate of the bucket

During a period of T seconds thenumber of tokens accepted is:T x r.

r: the average rate of incoming flow

The number of packets acceptedduring a period of T seconds is atmost: B + T x r

A packet of length L: - is dropped if L > C - is accepted if L <= C andconsumes L tokens, C = C - L.

buffer

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Token bucket

L = packet lengthC = # of tokens in the bucket---------------------------------------------------if ( L <= C ) { accept the packet; C = C - L; }else drop the packet;

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A shaping buffer delays packets that do not confirm to the traffic shape

if ( L <= C ) {

accept the packet;

C = C - L;}

else { /* the packet arrived early, delay it */

while ( C < L ) {

wait; }

transmit the packet;

C = C - L;}

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A QoS Capable Router

Shaper

Policer

Classifier

Outputlink

Inputflows

Dispatcherand Buffer

Acceptance