IP QoS Principles
Theory and Practice
Dimitrios Kalogeras
A Bit of History
 The Internet, originally designed for U. S. government
use, offered only one service level: Best Effort.
– No guarantees of transit time or delivery
– Rudimentary prioritization was available, but it was rarely used.
 Commercialization began in early 1990’s
–
–
–
–
Private (intranet) networks using Internet technology appeared.
Commercial users began paying directly for Internet use.
Commerce sites tried to attract customers by using graphics.
Industry used the Internet and intranets for internal, shared
communications that combined previously-separate, specialized
networks -- each with its own specific technical requirements.
– New technologies (voice over the Internet, etc.) appeared,
designed to capitalize on inexpensive Internet technologies.
The Demands on Modern Networks
 Network flexibility is becoming central to enterprise
strategy
– Rapidly-changing business functions no longer carried out in
stable ways, in unchanging locations, or for long time-periods
– Network-enabled applications often crucial for meeting new
market opportunities, but there’s no time to custom-build a
network
 Traffic is bursty
 Interactive voice, video applications have stringent
bandwidth and latency demands
 Multiple application networks are being combined into
consolidated corporate utility networks
– Bandwidth contention as critical transaction traffic is squeezed by
web browsing, file transfers, or other low-priority or bulk traffic
– Latency problems as interactive voice and video are squeezed by
transaction, web browsing, file transfer, and bulk traffic
Definitions
 Quality of Service (QoS) classifies network traffic and
then ensures that some of it receives special handling.
– May track each individual dataflow (sender:receiver) separately.
– May include attempts to provide better error rates, lower
network transit time (latency), and decreased latency variation
(jitter).
 Differentiated Class of Service (CoS) is a simpler
alternative to QoS.
– Doesn't try to distinguish among individual dataflows; instead,
uses simpler methods to classify packets into one of a few
categories.
– All packets within a particular category are then handled in the
same way, with the same quality parameters.
 Policy-Based Networking provides end-to-end control.
– The rules for access and for management of network resources
are stored as policies and are managed by a policy server.
QoS Background
QoS development inspired by new types of applications
in IP environment:




5
Video Streaming Services
Video Conferencing
VoIP
Legacy SNA / DLSw
QoS Architecture Models
 Best Effort Service
 Integrated Service
 Differentiated Service
6
Best Effort Service
What exactly IP does:
 All packets treated equally
 Unpredictable bandwidth
 Unpredictable delay and jitter
7
IntServ (RFC1633)
8
DiffServ (RFC2474/2475)
9
QoS Architecture Components
 Classification
 Coloring
 Admission Control
 Traffic Shaping/Policing
 Congestion Management
 Congestion Avoidance
 Signaling
10
Statistical Behavior: Random
Arrival
 In random arrival, the time that each packet arrives is
completely independent of the time that any other packet
arrives.
– If the true situation is that arrivals tend to be evenly spaced, then
random arrival calculations will overestimate the queuing delay.
– If the true situation is that arrivals are bunched in groups (typical
of data flows, such as packets and acknowledgements), then
random arrival calculations will underestimate the queuing delay.
 Our intuition is usually misleading when we think of
random processes.
– We tend to assume that queue size increases linearly as the
number of customers increases.
– But, with random arrival, there is a drastic increase in queue size
as the customer arrival rate approaches 80% of the theoretical
server capacity. There’s no way to store the capacity that is
unused by late customers, but early customers increase the
queue.
Random Arrival and Intuition
Queue Length
 The surprising increase in queue length is best
shown by a graph:
Actual
Intuitive
20%
40%
60%
System Capacity
80%
Random Arrival vs. Self-Similar
 Although random arrival is very convenient
mathematically (it’s relatively simple to do random arrival
calculations), it has been shown that much data traffic is
self-similar.
– Ethernet and Internet traffic flows, in particular, are self-similar.
– The rate of initial connections is still random, however.
 Self-similar traffic shows the same pattern regardless of
changes in scale.
– Fractal geometry (e.g., a coastline) is an example.
 Self-similar traffic has a heavy tail.
– The probabilities of extremely large values (e.g., file lengths of a
gigabyte or more) don’t decrease as rapidly, as they would with
random distributions of file lengths.
– This matches real data traffic behaviors.
 Long file downloads mixed with short acknowledgements
 Compressed video with action scenes mixed with static scenes
Traffic Classification
 Most fundamental QoS building block
 The component of a QoS feature that recognizes
and distinguishes between different traffic
streams
 Without classification, all packets are treated the
same
14
Traffic Classification/
Admission Control Issues
 Always performed at the network perimeter
 Makes traffic conform to the internal network
policy
 Marks packets with special flags (colors)
 Colors used afterwards inside the network for
QoS management
15
Classification/
Admission Control Scheme
Meter
Admitted
Classifier
Marker
Shaper/
Policer
Packet
Dropped
16
Classification Criteria
 IP header fields
 TCP/UDP header fields
 Routing information
 Packet Content (NBAR)
i.e. HTTP, HTTPS, FTP, Napster etc.
17
Traffic Coloring Options
 IP Precedence
 DSCP
 QoS Group
 802.1p CoS
 ATM CLP
 Frame Relay DE
18
Type-of-Service (RFC791)
Precedence
Version
Length
0
19
D
T
ToS Field
R
Unused
…
Total Length
8
15
31
0
1
D
Normal Delay
Low Delay
T
Normal Throughput
High Throughput
R
Normal Reliability
High Reliability
IP Precedence Values
20
111
Network Control
110
Internetwork Control
101
Critical
100
Flash Override
011
Flash
010
Immediate
001
Priority
000
Routine
DSCP
Diffserv Code Point
DSCP (6 bits)
Class 1
Class 2
Class 3
Class 4
001010
010010
011010
100010
Medium Drop
Precedence
001100
010100
011100
100100
High Drop
Precedence
001110
010110
011110
100110
Low Drop
Precedence
21
Unused
Classification mechanisms
 MQC ( Modular Qos Command Line Interface)
 CAR ( Commited Access Rate)
22
Modular QoS CLI
Modular QoS CLI (MQC)
Command syntax introduced in 12.0(5)T
Reduces configuration steps and time
Uniform CLI across all main Cisco IOS-based
platforms
Uniform CLI structure for all QoS features
23
Basic MQC Commands
router(config)#
class-map [match-any | match-all] class-name
• 1. Create Class Map - a traffic class ( match access list, input
interface, IP Prec, DSCP, protocol (NBAR) src/dst MAC address, mpls
exp).
router(config)#
policy-map policy-map-name
• 2. Create Policy Map (Service Policy) - Associate a
class map with one or more QoS policies (bandwidth, police, queuelimit, random detect, shape, set prec, set DSCP, set mpls exp).
router(config-if)#
service-policy {input | output} policy-map-name
• 3. Attach Service Policy - Associate the policy map with an
input or output interface.
24
Basic MQC Commands
 1. Create Class Map – defines traffic selection criteria
Router(config)# class-map class1
Router(config-cmap)# match ip precedence 5
Router(config-cmap)# exit
 2. Create Policy Map- associates classes with actions
Router(config)# policy-map policy1
Router(config-pmap)# class class1
Router(config-pmap-c)# set mpls experimental 5
Router(config-pmap-c)# bandwidth 3000
Router(config-pmap-c)# queue-limit 30
Router(config-pmap)# exit
 3. Attach Service Policy – enforces policy to interfaces
Router(config)# interface e1/1
Router(config-if)# service-policy output policy1
Router(config-if)# exit
25
Classification Configuring Sample
MQC based
IOS 12.1(5)T
class-map match-all premium
match access-group name premium
!
Traffic class definitions
class-map match-any trash
match protocol napster
match protocol fasttrack
!
policy-map classify
class premium
set ip precedence priority
QoS policy definition
class trash
police 64000 conform-action set-prec-transmit 1
excess-action drop
!
ip access-list extended premium
ACL definition
permit tcp host 10.0.0.1 any eq telnet
!
interface serial 2/1
QoS Policy attached
ip unnumbered loopback 0
to interface
service-policy input classify
26
Classification Configuring Sample
CAR based
ip cef
!
interface serial 2/1
ip unnumbered loopback 0
rate-limit input access-group 100 64000 8000 8000
conform-action set-prec-transmit 1 exceed-action
set-prec-transmit 0
!
access-list 100 permit tcp host 10.0.0.1 any eq http
CAR definition
ACL definition
27
Classification Configuring Sample
Route-map based
route-map classify permit 10
match ip address 100
set ip precedence flash
!
Route-map definitions
route-map classify permit 20
match ip next-hop 1
set ip precedence priority
!
interface serial 2/1
ip unnumbered loopback 0
Route-map attached
ip policy route-map classify
to interface
!
access-list 1 permit 192.168.0.1
access-list 100 permit tcp host 10.0.0.1 any eq http
28
ACL definitions
Shaping/Policing
 Used to assign more predictive behavior to traffic
 Uses Token Bucket model
29
Token Bucket Model
Token Bucket characterizes traffic source
Tokens
v
Token Bucket main parameters:
 Token Arrival Rate - v
 Bucket Depth - Bc
 Time Interval – tc
Overflow Tokens
 Link Capacity - C
Bc
C
tc = Bc/v
30
Incoming
packets
Conform
Exceed
Token Bucket Model
 Bucket is being filled with tokens at a rate v token/sec.
 When bucket is full all the excess tokens are discarded.
 When packet of size L arrives, bucket is checked for
availability of corresponding amount of tokens.
 If several packets arrive back-to-back and there are
sufficient tokens to serve them all, they are accepted at
peak rate (usually physical link speed).
 If enough tokens available, packet is optionally colored
and accepted to the network and corresponding amount of
tokens is subtracted from the bucket.
 If not enough tokens, special action on packet is
performed.
31
Token Bucket Model
Actions performed on nonconforming packets:
 Dropped (Policing)
 Delayed in queue either FIFO or WFQ (Shaping)
 Colored/Recolored
32
Token Bucket Model
Bucket depth variation effect:
Bc = 0 Constant Bit Rate (CBR)
Bc No Regulation
Bucket depth is characteristic of traffic burstiness
Maximum number of bytes transmitted over period of time t:
A(t)max = Bc+v·t
33
Excess Burst (Be)
Cisco Implementation
GTS ( Generic Traffic Shaping)
If during previous tcn-1 interval bucket Bc was not depleted (there is
no congestion), in the next interval tcn Bc+Be bytes are available for
burst.
In frame relay implementations packets admitted via Be tokens are
marked with DE bit.
34
Excess Burst (Be)
Cisco Implementation
CBTS (Class Based Traffic Shaping)
allows higher throughput in uncongested environment up to peak
rate calculated as
vPeak = vCIR(1+Be/Bc)
Peak rate can be set up manually.
35
Excess Burst (Be)
Cisco Implementation
CAR
allows RED like behavior:
 traffic fitting into Bc always conforms
 traffic fitting into Be conforms with probability proportional to
amount of tokens left in the bucket
 traffic not fitting into Be always exceeds
CAR uses the following parameters:




36
t – time period since the last packet arrival
Current Debt (Dcur) – Amount of debt during current time interval
Compound Debt (Dcomp) – Sum of all Dcur since the last drop
Actual Debt (Dact) – Amount of tokens currently borrowed
Excess Burst (Be)
Cisco Implementation
Packet of length
L arrived
Bccur – L > 0
Y
CAR Algorithm
Conform
Action
Bccur = Bccur – L
N
Dcur = L - Bccur
Bccur = 0
Dcomp = Dcomp + Dcur
Dact = Dact + Dcur
+v·t
Y
Dact > Be
N
Y
Dcomp > Be
N
37
Exceed
Action
Dcomp = 0
Shaping Configuration Sample
GTS Based
interface serial 2/1
ip unnumbered loopback 0
traffic-shape rate 64000 8000 1000 256
!
Shaper Definitions
interface serial 2/2
ip unnumbered loopback 0
traffic-shape group 100 64000 8000 8000 512
!
access-list 100 permit tcp host 10.0.0.1 any eq http
ACL definition
Shaper can be only used to control egress traffic flow!
38
Policing Configuration Sample
CAR Based
IOS 12.0(5)T
ip cef
interface serial 2/1
ip unnumbered loopback 0
rate-limit output access-group 100 64000 8000 16000
conform-action transmit excess-action drop
CAR Definitions
!
interface serial 2/2
ip unnumbered loopback 0
rate-limit input 128000 16000 32000 conform-action
transmit excess-action drop
!
access-list 100 permit tcp host 10.0.0.1 any eq http
ACL definition
39
Policer can be used to control ingress traffic flow!
Shaping/Policing Configuration
Sample
MQI Based
40
IOS 12.1(5)T
class-map match-all policed
match protocol http
Class definitions
class-map match-all shaped
match access-group name ftp-downloads
!
policy-map bad-boy
class policed
police 64000 8000 8000 conform-action transmit
exceed-action drop
class shaped
QoS policy definition
shape average 128000
!
interface serial 2/1
QoS Policy attached
ip unnumbered loopback 0
to interface
service-policy output bad-boy
!
ip access-list extended ftp-downloads
ACL definition
permit tcp any eq ftp-data any
CAR Policing Problem
Why cannot my traffic reach CIR value?
Cause: Improper setting of Bc and Be values
CAR is aggressive, as drops excessive packets and the lost data needs to
be retransmitted by upper layers (mainly TCP) after timeout. This also
causes TCP to shrink its window reducing flow throughput.
Cisco Systems recommends the following settings:
Bc = 1.5xCIR/8
Be = 2xBc
41
Congestion Management
42
Queuing
 Traffic burst may temporarily exceed
interface capacity
 Without queuing this excess traffic will
be lost
 Queuing allows bursty traffic to be
transmitted without drops
 Queuing strategy defines order in
which packets are transmitted through
egress interface
 Queuing introduced additional delay
which signals to adaptive flows (like
TCP) to back off their throughput
43
Queuing Algorithms
 FIFO
 Priority (Absolute)
 Weighted Round Robin (WRR)
 Fair
44
FIFO
 Simplest queuing method with the least CPU
overhead
 No congestion control
 Transmits packets in the order of arrival
 High volume traffic can suppress interactive flows
 Default queuing for interfaces > 2Mbps (i.e. Ethernet)
45
FIFO
FIFO average queue depth dependence on load
46
Absolute Priority Queuing
 Generic Priority Queuing
 Custom Queuing
 RTP Priority Queuing
 Low Latency Queuing (LLQ)
47
Simplest QoS Algorithm: Priority
Queuing
Stated requirement:
–“If <application> has traffic waiting,
send it next”
Commonly implemented
–Defined behavior of IP precedence
48
Priority Queuing Implementation
Approach
Identify interesting traffic
–Access lists
Place traffic in various queues
Dequeue in order of queue precedence
49
Priority Queuing (PQ)
High
Traffic
Destined
for Interface
Medium
Classify
Interface Hardware
• Ethernet
• Frame Relay
• ATM
• Serial Link
• Etc.
Normal
Low
Transmit
Queue
Output
Line
Q Length Defined
by Q Limit
Classification by:
• Protocol (IP, IPX, AppleTalk,
SNA, DecNet, Bridge, etc.)
• Incoming Interface
(EO, SO, S1, etc.)
50
Interface Buffer
Resources
Absolute Priority
Scheduling
Priority Queuing Scheme
Y
High Empty?
N
Send packet
from High
51
Y
Medium Empty?
N
Send Packet
from Medium
Y
Normal Empty?
N
Send Packet
from Normal
Y
Low Empty?
N
Send Packet
from Low
Generic PQ Drawbacks
 Needs thorough admission control
 No upper limit for each priority level
 High risk of low priority queues` starvation effect
52
Generic PQ Configuration Sample
priority-list 1 protocol ip high tcp telnet
priority-list 1 protocol ip high list 100
PQ Definition
priority-list 1 protocol ip medium lt 1000
priority-list 1 interface ethernet 0/0 medium
priority-list 1 default low
!
interface serial 2/1
ip unnumbered loopback 0
PQ Attached
priority-group 1
to Interface
!
access-list 100 permit tcp host 10.0.0.1 any eq http
ACL definition
53
Custom Queuing (CQ)
(Weighted Round Robin)
1/10
Interface Hardware
• Ethernet
• Frame Relay
• ATM
• Serial Link
• Etc.
2/10
Traffic
Destined
for Interface
3/10
Classify
2/10
Transmit
Queue
Output
Line
3/10
Up to 16
Q Length
Deferred by
Queue Limit
54
Classification by:
• Protocol (IP, IPX, AppleTalk,
SNA, DecNet, Bridge, etc.)
• Incoming interface
(EO, SO, S1, etc.)
Interface
Buffer
Resources
Link
Utilization Weighted Round
Robin Scheduling
Ratio
(byte count)
Allocate
Proportion of
Link Bandwidth)
WRR Drawbacks
 Unpredictable jitter
 Fairness significantly depends on MTU and TCP
window size
 Complex calculations to achieve desired traffic
proportions
55
CQ Byte-count Calculus
Distribute bandwidth to 3 queues with proportion x:y:z and packet sizes qx, qy, qz.
1.Calculate ax=x/qx, ay=y/qy, az=z/qz.
2.Normalize and round ax, ay, az.
ax’= round(ax/min(ax, ay, az)); ay’= round(ay/min(ax, ay, az)); az’= round(az/min(ax, ay, az)).
3.Convert obtained packet proportion into byte count
bcx = ax’·qx; bcy = ay’·qy; bcz = az’·qz.
4.Actual bandwidth share of i-th queue can be calculated with the following formula:
sharei 
bci
C
n
 bc
j 1
j
5.For better approximation obtained byte-counts can be multiplied by some positive whole
number.
Starting with IOS 12.1 CQ employs Deficit Round Robin
algorithm and there is no need in such byte-count tuning.
56
CQ Configuration Sample
queue-list 1 protocol ip 1 tcp telnet
queue-list 1 protocol ip 2 list 100
queue-list 1 protocol ip 3 udp 53
queue-list 1 interface ethernet 0/0 4
queue-list 1 queue 1 byte-count 3000
CQ List Definition
queue-list 1 queue 2 byte-count 4500
queue-list 1 queue 3 byte-count 3000
queue-list 1 queue 4 byte-count 1500
queue-list 1 default 4
!
interface serial 2/1
CQ Attached
ip unnumbered loopback 0
to Interface
custom-queue-list 1
!
access-list 100 permit tcp host 10.0.0.1 any eq http
ACL Definition
57
“Bitwise Round Robin” Fair Queuing
TDM Model
Time Division
Multiplexer
Keshav, Demers, Shenker, and Zhang
Simulates a TDM
One flow per channel
58
TDM Message Arrival Sequence
6
4
5
1
2
3
59
Time Division
Multiplexer
TDM Message Delivery Sequence
5
Time Division
Multiplexer
60
6
4
1
3
2
Fair Queuing Algorithm
Employs virtual bit-by-bit round robin model (BRR)
R


BRR dynamics are described by the equation:
t N ac (t )
i-th packet from flow  arriving at time t0 is services at
time t :
R(ti )  R(ti 0 )  Pi
Servicing of i-th packet from flow  will start at Si and finish at Fi :
Si  MAX ( Fi1 , R(ti ))
Fi  Si  Pi
Additional  parameter is added for priority assignment to inactive flows :
Bi  MAX ( Fi1 , R(ti )   )
Packets are ordered for transmission according to Bi values.
61
Fair Queuing Approach
Enqueue traffic in the sequence
the TDM would deliver it
As a result, be as fair as the TDM
62
Effects of Fair Queuing
Low-bandwidth flows get
–As much bandwidth as they can use
–Timely service
High-bandwidth flows
–Interleave traffic
–Cooperatively share bandwidth
–Absorb latency
63
What Weighting Does
In TDM
–Channel speed determines message “duration”
In WFQ
–Multiplier on message length changes
simulated message “duration”
Result:
–Flow’s “fair” share predictably unfair
64
Weighted Fair Queuing (WFQ)
Traffic
Destined
for Interface
Transmit
Queue
Classify
Configurable
Number of
Queues
65
Flow-Based Classification by:
• Source and destination address
• Protocol
• Session identifier (port/socket)
Output
Line
Weighted Fair
Scheduling
Interface Weight Determined by:
Buffer
• Requested QoS (IP Procedure, RSVP)
Resources • Frame Relay FECN, BECN, DE
(For FR Traffic)
• Flow throughput (weighted-fair)
Weighted Fair Queuing (WFQ)
 Fair bandwidth per flow allocation
 Low delay for interactive applications
 Protection from ill-behaved sources
66
Weighted Fair Queuing (WFQ)
Flow classified by the following fields:





Source address
Source port
Destination address
Destination port
ToS
Weight of each flow (queue) depends on ToS:
weight = 1/(precedence+1)
Bandwidth distributed in 1/weight proportions
67
Weighted Fair Queuing (WFQ)
 Packets are ordered according to the expected virtual departure time
of their last bit.
 Low volume flows have preference over high volume transfers.
 Low volume flow is identified as using less than its share of
bandwidth.
 The special queue length threshold value is established, after which
only low volume flows can enqueue. All the packets, that belong to
high volume flows are dropped.
68
Drawbacks of Weighted Fair
Queuing
Requires more sorting
than other approaches
69
Weighted Fair Queuing (WFQ)
Delay
FTP
Telnet
t
70
Weighted Fair Queuing (WFQ)
FTP
Delay
Telnet
t
71
WFQ Configuration Sample
interface serial 2/1
ip unnumbered loopback 0
fair-queue 32 128 0
Queue Threshold
(packets)
Number of
reservable queues
Maximal number
of queues
72
RTP Priority Queuing
 Classifies only by UDP port range
 Only even ports from the range are classified
 Establishes upper limit via integrated policer
 Excess traffic dropped during congestion periods
 RTP PQ has priority over LLQ
73
RTP PQ Configuration Sample
interface serial 2/1
ip unnumbered loopback 0
ip rtp priority 16384 16383 256
Starting UDP port
Bandwidth Limit
(kbps)
Range length
74
Low Latency Queuing (LLQ)




75
Implemented using MQI
Very rich classification criteria (class-map)
Establishes upper limit via integrated policer
Excess traffic dropped during congestion periods
LLQ Configuration Sample
IOS 12.0(5)T
class-map match-all voice
match access-group name voip
!
policy-map llq
class voip
priority 30
class class-default
fair-queue 64
!
interface serial 2/1
ip unnumbered loopback 0
service-policy output llq
!
ip access-list extended voip
permit ip host 10.0.0.1 any
76
Class definitions
LLQ policy definition
LLQ Policy attached
to interface
ACL definition
Class Based WFQ (CBWFQ)
 Based on the same algorithm as WFQ
 Weights can be manually configured
 Allows to easily specify guaranteed bandwidth
for a class
 Configuration based on Cisco MQI
77
CBWFQ Configuration Sample
IOS 12.0(5)T
78
class-map match-all premium
match access-group name premium-cust
class-map match-all low-priority
match protocol napster
!
policy-map cbwfq-sample
class premium
bandwidth 512
class low-priority
shape average 128
shape peak 512
class class-default
fair-queue 64
!
interface serial 2/1
ip unnumbered loopback 0
max-reserved-bandwidth 85
service-policy output cbwfq-sample
!
ip access-list extended premium-cust
permit ip host 10.0.0.1 any
Class definitions
Qos policy definition
QoS Policy attached
to interface
ACL definition
CBWFQ Configuration Sample
Hierarchical Design
class-map match-all premium
match access-group name premium-cust
class-map match-all voice
match ip precedence flash
!
policy-map total-shaper
class class-default
shape average 1536
service-policy class-policy
policy-map class-policy
class premium
bandwidth 512
class voice
priority 64
class class-default
fair-queue 128
79
IOS 12.1(5)T
interface fastethernet 1/0
ip unnumbered loopback 0
max-reserved-bandwidth 85
service-policy output total-shaper
!
ip access-list extended premium-cust
permit ip host 10.0.0.1 any
Hierarchical CBWFQ Limitations
 Only two levels of hierarchy are supported
 set command not supported in child policy
 Shaping allows only in parent policy
 LLQ can be configured only either in child or
parent policies but not in both
 FQ allowed only in child policy
80
Congestion Avoidance
81
Load
Global Synchronization Effect
Link Capacity
Avg. Throughput
t
82
Tail Drop and TCP Flow Control
 Packet drops from all TCP sessions
simultaneously
 High probability of multiple drops from the same
TCP session
 Uniformly distributed drops from high volume and
interactive flows
Result: Low average throughput!
83
Random Early Detection (RED)
Developed by Van Jacobson in 1993
 Starts randomly dropping packets before actual
congestion occurs
 Keeps average queue depth low
 Increases average throughput
84
Load
Global Synchronization Removed
Link Capacity
Avg. Throughput
t
85
Random Early Detection (RED)
p
p
Tail Drop
RED
1
1

Adjustable
0
86
0
qmax
qavg
 min
 max
qavg
Random Early Detection (RED)
RED Parameters:
 min – Minimal threshold after which RED starts packet drops.
Minimal recommended value is 5 packets.
 max – Maximal threshold after which all packets are dropped.
Recommended value is 2-3 times min.
  - Mark probability denominator denotes packet drop probability
at max average queue depth. Optimal value – 0.1 .
  - Exponential weighting factor determines the level of
backward value-dependence in average queue depth
calculation:
qavg = (qold · (1 - 2-)) + (qcur · 2-)
General recommendation  = 9.
87
TCP Rate Control - 1
 In TCP, the spacing of ACKs and the window size in the
ACKs controls the transmitter’s rate.
 Rate Control manipulates the ACKs as they pass through
the rate control device by:
– Adjusting the size of TCP ACK window
– Inserting new ACKs
– Re-spacing existing ACKs
 Rate Control works only with TCP; other methods, such
as Token Bucket, must be used with UDP.
 Rate Control violates the protocol layering design, as it
allows network devices to manipulate a higher-layer
protocol’s operation. Nevertheless, it usually functions
well and provides fine-grained control.
TCP Rate Control - 2
 Example:
Transmitter
Rate-control device
w: 80
o
d
n
i
w
w: 20
o
d
n
i
w
w:
windo
00
2000
w: 20
o
d
n
i
w
00
: 20
w
o
d
n
wi
00
Receiver
00
Weighted Random Early Detection
(WRED)
 Modified version of RED
 Weights determine the set of parameters: min ,
max and  .
 Weight depends on ToS field value
 Interactive flows are preserved
90
WRED Configuration Sample
Interface based
interface serial
ip unnumbered
random-detect
random-detect
random-detect
random-detect
random-detect
…
91
2/1
loopback 0
0
1
2
3
32
32
32
32
64
64
64
64
20
20
20
20
min
max

WRED Configuration Sample
MQI based
policy-map red
class class-default
random-detect
random-detect 0 32 64
random-detect 1 32 64
random-detect 2 32 64
random-detect 3 32 64
…
interface Serial2/1
ip unnumbered loopback 0
service-policy output red
min
20
20
20
20
max

WRED is incompatible with LLQ feature!
92
Link Optimization
93
Link Fragmentation and
Interleaving (LFI)
For links < 128kbps
Jumbogram
Voice
Packet
64 kbps
1500 bytes  190ms
94
Link Fragmentation and
Interleaving (LFI)
64 kbps
Supported interfaces:
 Multilink PPP
 Frame Relay DLCI
 ATM VC
95
LFI Configuration Sample
MLP version
interface virtual-template 1
ip unnumbered loopback 0
ppp multilink
ppp multilink interleave
ppp multilink fragment-delay 30
ip rtp interleave 16384 1024 512
…
96
Signaling
97
Resource Reservation Protocol
(RSVP)
 End-to-end QoS signaling protocol
 Used to establish dynamic reservations over the
network
 Always establishes simplex reservation
 Supports unicast and multicast traffic
 Actually uses WFQ and WRED mechanisms
98
Resource Reservation Protocol
(RSVP)
99
Resource Reservation Protocol
(RSVP)
10
0
Resource Reservation Protocol
(RSVP)
Reservation Types:
 Guaranteed Rate (uses WFQ and LLQ)
 Controlled Load (uses WRED)
10
1
Distinct
Shared
Explicit
Fixed Filter (FF)
Shared Explicit (SE)
Wildcard
X
Wildcard Filter (WF)
Resource Reservation Protocol
(RSVP)
10
2
QoS Policy Propagation over BGP
 QoS policy can be shared inside single AS or
among different ASs.
 Community attribute is usually used for color
assignments
 Prevents manual policy changes in network
devices
10
3
QoS Policy Propagation over BGP
10
4
QPPB Configuration Sample
Router A
ip bgp-community new-format
!
router bgp 10
neighbor 10.0.0.1 remote-as 20
neighbor 10.0.0.1 send-community
neighbor 10.0.0.1 route-map cout out
!
route-map cout permit 10
match ip address 20
set community 60:9
!
access-list 20 permit 192.168.0.0
0.0.0.255
10
5
Router B
ip bgp-community new-format
!
router bgp 20
neighbor 10.0.0.2 remote-as 10
table-map mark-pol
!
route-map mark-pol permit 10
match community 1
set ip precedence flash
!
ip community-list 1 permit 60:9
!
interface Serial 0/1
ip unnumbered loopback 0
bgp-policy source ip-prec-map
Topics not Covered
 Multiprotocol Label Switching (MPLS)
 Frame Relay QoS
 ATM QoS
 Distributed Queuing Algorithms
 Multicast
10
6
Conclusion
 QoS is not an exotic feature any more
 QoS allows specific applications (VoIP, VC) to
share network infrastructure with best-effort
traffic
 QoS in IP networks simplifies their
functionality avoiding Frame Relay and ATM
usage
10
7
10
8
Questions???