Quality-of-Service (QoS) mechanisms and architectures · QoS degradation factors and mechanisms...

Quality-of-Service (QoS) mechanisms and architectures

Rui Valadas

Network congestion causes QoS problems

©Rui Valadas, version 1.0, 22/11/2017 2

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Packets can

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End-to-end delay

• End-to-End delay = Σ propagation delay + Σ processing delay + Σ queuing delay

• Queuing delay is variable; propagation and processing delays are fixed per link and router respectively


Src Rcv

Propagation delay Queuing delay

Processing delay

QoS degradation factors and mechanisms

• QoS degradation factors

• Excessive packet loss, excessive packet delay, excessive packet delay variation, insufficient throughput (bandwidth), excessive session blocking

• QoS mechanisms

• Mechanisms that help improving the QoS of all users, or differentiating among to QoS of groups of users

• Routing, flow control, admission control, scheduling, dropping

• QoS performance metrics (packet level)

• Throughput (bandwidth), delay, delay-jitter, loss


QoS mechanismsService A

Service B

Service B

©Rui Valadas, version 1.0, 22/11/2017

FC

/AC

FC

/AC

FC/A

C

PACKET SCHEDULLING

&

PACKET DISCARD

link

ROUTING

switch

FLOW CONTROL

ADMISSION CONTROL

5

Routing

•Routing is the function that determines the path (or paths) taken by packets from source node to destination node

• IP routing is hop-by-hop, destination based, using shortest path routing principle

•How good is this?


Minimum hop routing


R1

R2

R4

R8

R5

R3

R7

Links have 150 Mb/s of capacity, except R6-R3

R6-R3 has 2 Gb/s of capacity

Flow R1-R3: 100 Mb/s


Minimum hop routing (e.g. RIP)

R6

congested links

Shortest path routing


R1

R2

R4

R8

R5

R6

R3

R7

Links have 150 Mb/s of capacity, except R6-R3

R6-R3 has 2 Gb/s of capacity



Shortest path routing with user-defined weights (e.g. OSPF)

congested links

Shortest path routing with equal cost splitting


R1

R2

R4

R8

R5

R6

R3

R7

Links have 620 Mb/s of capacity, except R6-R3 and R8-R6

R6-R3 has 2 Gb/s; R8-R6 has 150 Mb/s



Administratively, cost of R4-R5-R6 = cost R4-R7-R8

275

275

275

275

275

50

50

50

50

50

100100

550

550

congested link

Routing through LSPs (MPLS)


R1

R2

R4

R8

R5

R6

R3

R7

Links have 620 Mb/s of capacity, except R6-R3 and R8-R6

R6-R3 has 2 Gb/s; R8-R6 has 150 Mb/s



Each flow is transported in a different LSP

550

550

100

100

100

100100

550

550

no congested

linksLSP1

LSP2

Routing is no longer tied to the destination address!

QoS differentiation via packet scheduling


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Classification

• Process of categorizing an aggregate of traffic flows into a number of classes to which specific QoS mechanisms may be applied


Classifier

Flow 1

Flow 2

Flow 3

Flow 4

Flow 5

Flow 6

Class-of-Service 1

Class-of-Service 2

Class-of-Service 3

Classification• Many possible criteria for grouping packets:

• Place in the same Class-of-Service packets with same pair (IP source address, IP destination address)

• Place in the same Class-of-Service packets with same tuple (IP source address, IP destination address, source port number, destination port number, IP protocol type)

• Place in the same Class-of-Service packets that entered the router via the same interface

• Place in the same Class-of-Service packets with same TOS field


Classifier

Flow 1

Flow 2

Flow 3

Flow 4

Flow 5

Flow 6

Class-of-Service 1

Class-of-Service 2

Class-of-Service 3

Packet scheduling algorithms

• Packet scheduling algorithm – decides the order of packet transmission at a link

• With and without work conservation

• With work conservation – the transmission link is only inactive when no packets are waiting to be transmitted

• Without work conservation – the transmission link can be inactive while packets are waiting to be transmitted


First-In First-Out (FIFO)

• Also know as First-Come First-Served (FCFS)

• Packets are served by order of arrival

• Default packet scheduling mechanism used in IP routers

• Service given to specific packet flows can not be differentiated

• No protection against misbehaved flows


Scheduler

3

4

2

5

1

Flow 1

Flow 2

Flow 3

Flow 4

Flow 5

Flow 6

1245 3

order of link transmission

Strict priority

• Each Class-of-Service is assigned a priority

• Packets with higher priority are always transmitted before packets with lower priority

• A higher priority packet flow can degrade the service of lower priority ones


Scheduler

Highest Priority Queue

Middle Priority Queue

Lowest Priority Queue order of link transmission

16

Rate-controlled strict priority

• Strict priority scheduling but queues are constrained to not exceed a certain percentage of the link capacity

• Is this scheme work-conserving?


Scheduler

Highest Priority Queue

Lowest Priority Queue

order of link transmission

TTTT

Time interval between successive transmissions not smaller than T

→ Transmission rate not larger than 1/T packets/s

Weighted Round-Robin

• Each Class-of-Service is assigned a queue and each queue is assigned a weight

• The queues are served in round-robin order

• The queue weight defines the guaranteed minimum percentage of the link capacity that a queue will get

• Fixed size packets:

• Every time a queue is visited, transmit a number of packets up to the queue weight before proceeding to the next queue

• If queue gets empty, proceed immediately to the next queue

• Variable packet lengths:

• Same procedure but needs to know a priori the average packet length of each flow in order to calculate the correct queue weights

• Can stay for too long serving a single flow


Weighted Round-Robin (fixed-size packets)


If link has capacity 𝐶 bits/s, the minimum guaranteed rate of queue 𝑖 , with weight 𝑤𝑖 , is:

𝑟𝑖 =𝑤𝑖

σ𝑖𝑤𝑖𝐶

Maximum achievable rate (any queue): 𝐶 bits/s

Scheduler

Queue 1, Weight = 2

Queue 2, Weight = 3

Queue 3, Weight = 4 order of link transmission

19

Weighted Round-Robin (variable-sized packets)


Problem with variable-sized packets: all queues get

the same percentage of the link capacity (in

bits/second), although the weights of queues 2

and 3 are the double of queue 1

Can determine appropriate weights, but needs to

know average packet size associated to each

queue

Scheduler

Queue 1, Weight = 1

Queue 2, Weight = 2

Queue 3, Weight = 2 order of link transmission

Deficit Round-Robin

• Handles variable packets sizes without knowing the average packet size of each flow in advance

• Each Class-of-Service is assigned a queue and each queue is assigned a quantum (in bytes); the queues are served in round-robin order; the quantum defines the guaranteed minimum percentage of the link capacity that a queue will get

• Operation:

• Every queue has a deficit counter initialized to 0

• Every time a visit to a queue is initiated add quantum bytes to the deficit counter; if the queue is found empty reset the deficit counter to 0

• While visiting a queue one or more packets can be transmitted; a packet can be transmitted if its packet size is equal or lower to the deficit counter; when a packet is transmitted subtract its size from the deficit counter; if no more packets can be transmitted proceed to the next queue immediately

• Can stay for too long serving a single flow


Deficit Round-Robin


Scheduler

1300

Queue 1, Quantum = 1000

500

1500200

Queue 2, Quantum = 2000

order of link transmission100900

1500

150020010013005009001500

1st cycle:▪ F1 not served, credit Q1 = 1000

▪ P1-Q2, credit Q2 = 2000 – 1500 = 500

▪ P2-Q2, credit Q2 = 500 – 200 = 300

▪ P3-Q2, credit Q2 = 300 – 100 = 200

2nd cycle:▪ P1-Q1, credit Q1 = 2000 – 1300 = 700

▪ P2-Q1, credit Q1 = 700 – 500 = 200

▪ P4-Q2, credit Q2 = 2200 – 900 = 1300

3rd cycle:▪ Q1 not served, credit Q1 = 1200

▪ Credit Q2 = 0

4th cycle:▪ P3-Q1, credit Q1 = 2200 – 1500 = 700

▪ Credit Q2 = 0

5th cycle:▪ Credit Q1 = 0

▪ Credit Q2 = 0

If link has capacity 𝐶 bits/s, the minimum guaranteed rate of queue 𝑖, with quantum 𝑞𝑖 , is:

𝑟𝑖 =𝑞𝑖

σ𝑖 𝑞𝑖𝐶

Maximum achievable rate (any queue): 𝐶 bits/s

Case study – Packet scheduling


Router A

Router B

Router C

10.0.0.0 20.0.0.0

10.0.0.10 20.0.0.20

30.0.0.0

40.0.0.0

30.0.0.30 40.0.0.40 40.0.0.42 40.0.041

ligação série

S1

sentido do tráfego FTP

fonte: 10.0.0.10

fonte: 20.0.0.20

fonte: 30.0.0.30

destino: 40.0.0.40

destino: 40.0.0.42

destino: 40.0.0.41

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Case study - FIFO


Pacotes capturados em 40.0.0.0

Router A

Router B

Router C

10.0.0.0 20.0.0.0

10.0.0.10 20.0.0.20

30.0.0.0

40.0.0.0

30.0.0.30 40.0.0.40 40.0.0.42 40.0.041

ligação série

S1

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Case study – Strict Priority


Captura em 40.0.0.0, no momento em que

se iniciou a transferência de 20.0.0.20

Router A

Router B

Router C

10.0.0.0 20.0.0.0

10.0.0.10 20.0.0.20

30.0.0.0

40.0.0.0

30.0.0.30 40.0.0.40 40.0.0.42 40.0.041

ligação série

S1

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se iniciou a transferência de 10.0.0.10

Router A

Router B

Router C

10.0.0.0 20.0.0.0

10.0.0.10 20.0.0.20

30.0.0.0

40.0.0.0

30.0.0.30 40.0.0.40 40.0.0.42 40.0.041

ligação série

S1

26



Captura em 40.0.0.0, no momento em que se

cancelou a transferência de 10.0.0.10

Router A

Router B

Router C

10.0.0.0 20.0.0.0

10.0.0.10 20.0.0.20

30.0.0.0

40.0.0.0

30.0.0.30 40.0.0.40 40.0.0.42 40.0.041

ligação série

S1

27




se cancelou a transferência de 20.0.0.20

Router A

Router B

Router C

10.0.0.0 20.0.0.0

10.0.0.10 20.0.0.20

30.0.0.0

40.0.0.0

30.0.0.30 40.0.0.40 40.0.0.42 40.0.041

ligação série

S1

28

Case study – Deficit Round Robin


Pesos iguais em cada fila de espera:

queue-list 1 queue 1 byte-count 1000



Router A

Router B

Router C

10.0.0.0 20.0.0.0

10.0.0.10 20.0.0.20

30.0.0.0

40.0.0.0

30.0.0.30 40.0.0.40 40.0.0.42 40.0.041

ligação série

S1

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Case study – Deficit Round Robin


Fila que serve 10.0.0.10 com peso duplo:




Router A

Router B

Router C

10.0.0.0 20.0.0.0

10.0.0.10 20.0.0.20

30.0.0.0

40.0.0.0

30.0.0.30 40.0.0.40 40.0.0.42 40.0.041

ligação série

S1

30

Flow and admission control mechanisms

• Controls the amount of traffic injected in a network

• Admission control – Determines if a (complete) traffic session can or cannot be accepted by the network

• Like in telephony networks, where phone calls can be accepted or rejected

• Flow control – Allows each traffic source to adapt its transmission rate according to the needs of the receiver and/or the network

• As in TCP, which determines if a network is congested through the detection of packet losses

• Can be implemented at different network layers


Flow control

• Open-loop flow control

• Source regulates traffic injected in the network according to traffic contract previously established with the network

• Closed-loop flow control

• Source adapts dynamically its transmission rate to the share of resources it may get from the network at each time

• Hybrid flow control

• Combines open-loop and closed-loop flow control

• Example: source reserves a minimum of resources during session set-up, but receive a large share if the network utilization is low


Open-loop flow control

• User and network establish a contract, called Service Level Agreement (SLA)

• User describes the traffic it needs to inject in the network using a set of traffic descriptors

• User regulates traffic to be in agreement with SLA. This is also called traffic shaping

• Network provider can measure the traffic, and apply marking and/or dropping policies if traffic is not conformant to contract. This is also called traffic policing

• Traffic descriptors/regulators/policers• Peak rate

• Average rate

• LBAP / Leaky bucket


Regulators (shapers) versus policers

• Regulators (shapers):

• Modify input traffic characteristics to be in agreement with traffic descriptors

• May have to queue packets

• Policers

• Measure the input traffic characteristics to verify if they are in agreement with traffic descriptors; if not, discard or mark packets (e.g. for high-priority discard)

• Do not queue packets


INPUT OUTPUT

TEST

DATA

BUFFER

INPUT OUTPUT

TEST

DISCARD

or MARK

Peak rate descriptor

• Peak rate – highest rate at which the source can transmit

• Descriptors:

• Fixed-size packets – inverse of the closest spacing between the starting times of consecutive packets

• If closest spacing is T, peak rate is 1/T packets/s

• Variable-sized packets – needs two parameters, peak rate and time window over which it is measured


time

packet

inter-arrival times closest spacing = T

packet packet packet

time

packet

measurement window


Peak rate descriptor

• Example:

• If all packets of a traffic session are 50 bytes long, and the closest spacing is 10 ms, what is the peak rate? (A: 100 packets/s leading to 5 Kbytes/s)

• If the peak rate is 8 Mb/s over all intervals of 15 ms, what is the largest amount of data that can be generated in 75 ms? And in 70 ms? (A: 600 Kb)


time

packet

inter-arrival times closest spacing = T


time

packet

measurement window


Peak rate regulator

• Regulator for fixed-size packets

• Requires buffer + timer

• Operation

• T – inverse of peak-rate

• Timer is set initially to zero

• When a packet starts being transmitted, the timer is set to T, and starts counting down

• Packets that arrive while the timer is counting down are stored in the buffer

• When the timer expires, the packet in the head of the buffer (if any) is transmitted immediately

• Packets that arrive when the timer value is zero and there are no packets waiting in the buffer are transmitted immediately

• Regulator for variable-sized packets

• Similar to the regulator for the average rate (later)


INPUT OUTPUT

TEST

DATA

BUFFER

Peak rate regulator


INPUT OUTPUT

TEST

DATA

BUFFER

0.2 0.3 1.01.

6

1.

7 time

time

Arrivals

Departures

T T

0.4

T T T

0.2 0.5 0.8 1.1 1.6 1.9

T=0.3

Peak rate policer


0.2 0.31.

7 time

time

Arrivals

Departures

T

0.4

T T T

0.2 0.7 1.0 1.7

T=0.3

0.7 0.9

T

1.0 1.3

1.3

1.5

INPUT OUTPUT

TEST

DISCARD

or MARK

Packets discarded

Peak rate policer


INPUT OUTPUT

TEST

DISCARD

or MARK

0.2 0.31.

7 time

time

Arrivals

Departures

T

0.4

T T T

0.2 0.7 1.0 1.7

T=0.3

0.7 0.9

T

1.0 1.3

1.3

1.5

Packets marked as out-of-profile

Average rate descriptor

• The average rate is described by two parameters:• T = time window over which the rate is measured

• A = number of bits that can be sent in the above time window

• … or equivalently by average rate and measurement window

• Also applies to peak rate with variable packet sizes

• Jumping-window: no more than A bits are transmitted over consecutive windows of length T

• Moving-window: no more than A bits are transmitted over all windows of length T

• Regulator requires buffer + timer + counter


Jumping-window regulator

• Operation

• Counter is set to 0 at t = 0

• When a packet is transmitted the counter is incremented by the packet size

• Every T time units the counter is reset to 0

• When a packet arrives, if counter value + packet size not greater than A, the packet is transmitted immediately; otherwise it is stored in the queue

• When the counter is reset to 0, queued packets are transmitted immediately until the counter reaches A


INPUT OUTPUT

TEST

DATA

BUFFER

Jumping-window regulator


INPUT OUTPUT

TEST

DATA

BUFFER

0.2 0.3 0.5 0.6 0.8 1.0 1.7 1.9

20K

0K

60K

80K

100K

40K

70K

100K

time

time

Arrivals

Departures

Counter

A = 100 K, T = 1

Moving-window regulator

• Operation

• Regulator saves the departure time and the size of each transmitted packet

• The test to transmit or store a packet at the buffer is identical to the jumping-window regulator

• Moreover, T seconds after a packet was transmitted the counter is decremented by its size

• NOTE: If the regulator decides to store a packet in the buffer, it can determine its departure time by examining the departure times of the other waiting packets


INPUT OUTPUT

TEST

DATA

BUFFER

Moving-window regulator


INPUT OUTPUT

TEST

DATA

BUFFER

0.2 0.3 0.5 0.6 0.8 1.0 1.7 1.9

20K

40K

20K

20K

10K

30K

30K

30K

20K

1.2 1.3

0K

60K

80K

100K90K

80K

60K

40K

70K

100K

1.5 1.6 time

time

Arrivals

Departures

Counter

A = 100 K, T = 1

Linear Bounded Arrival Process (LBAP)

• An LBAP-constrained source bounds the number of bits transmitted in any interval of length t by a linear function of t, A(t), characterized by two parameters, r and s :

A(t) ≤ rt + s

• r is the long-term average rate and s is the longest burst a source may send


s

time

actual transmitted bits

bound A(t) ≤ rt + s

Leaky bucket regulator

• Regulates an LBAP descriptor

• Includes buffer, bucket, and timer

• Operation

• Token bucket is a special type of counter: collects tokens of fixed size (i.e., a number of bytes) at a steady rate of r tokens/second; bucket has only capacity for s tokens; tokens will be discarded when arriving at a full bucket

• When a packet arrives, it is transmitted immediately if the number of bytes in the bucket is greater or equal to its packet size; otherwise it is stored in the data buffer

• When a packet is transmitted the regulator removes from the bucket a number of bytes equal to its packet size

• A packet waiting at the head of the queue is transmitted as soon as the number of bytes in the bucket reaches its packet size


INPUT OUTPUT

TEST

DATA

BUFFER

TOKEN

BUCKETs

r

Leaky bucket regulator

• Example: Tokens with a size of 100 bytes are added to a leaky bucket regulator of capacity 500 bytes at a rate of 2 tokens/second• What is the average rate, peak rate, and largest burst size of the regulated

traffic?

• Can this regulator handle a packet of size 700 bytes?

• If a packet of size 400 bytes arrives when the bucket contains tokens worth 200 bytes, and there are no other packets awaiting service, what is the least and most delay it could have before transmission?


INPUT OUTPUT

TEST

DATA

BUFFER

TOKEN

BUCKETs

r

Packet discard mechanisms• When packets arrive at a full buffer, how are packets discarded?

• Drop tail – drop the incoming packet

• Drop head – drop packet at the head of queue

• Drop random – drop packet at a random position in the queue

• Drop priorities

• Network may drop first packets that have been marked by source or by policer

• When different classes-of-service share same buffer, different drop priorities may be applied to each one

• Preventive discard: dropping packets may start before queue gets full (early drop)


Drop headDrop tailDrop

random

to tx link

Date post:	06-Mar-2021
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Quality-of-Service (QoS) mechanisms and architectures · QoS degradation factors and mechanisms...

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