Quality of Service (QoS)
An Overview
TAC-TOI-01
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1
What Is Quality of Service?
The ability of the network to
provide better or “special” service
to users/applications.
Voice - Video - Data
Consistent, Predictable
Performance
TAC-TOI-01
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Not All Traffic Is Equal
Voice
FTP
ERP and
Mission-Critical
Low to
Moderate
Moderate
to High
Low
Random Drop Sensitive
Low
High
Moderate
To High
Delay Sensitive
High
Low
Low to
Moderate
Jitter Sensitive
High
Low
Moderate
Bandwidth
Traffic Is Grouped into SLAs
TAC-TOI-01
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Step 1: Identify Traffic and
its Requirements
• Network audit
What is running and when?
• Business audit
How important is it for business?
• Application audit
What are it’s requirements
from network?
• Service levels required
TAC-TOI-01
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Step 2: Divide the Traffic into Classes
and Color It
Differentiated
IP Services
Voice
Platinum Class
Low Latency
Gold
Guaranteed: Latency
and Delivery
Silver
Guaranteed Delivery
Bronze
Best Effort Delivery
E-Commerce
Application
Traffic
Traffic
Classification
E-mail, Web
Browsing
Voice
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What Is a Class?
• Single user
MAC address, IP address…
• Department, customer
Sub net, interface…
• Application
Port numbers, URL…
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What Is Coloring?
IP Precedence & DiffServ
IP Packet
Version ToS 1
Len
Length Byte
ID
Offset TTL Proto FCS IP-SA IP-DA Data
Type of Service (ToS)
IP Precedence
• Use ToS field to signal
business QoS policies
IP Differentiated Service
• Differentiate network services
across any media or topology
Data, Voice, Video
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Coloring at Layer 2 and Layer 3
Three Bits Used for CoS
(User Priority)
Layer 2
802.1Q/p
PREAM. SFD
DA
Layer 3
IPv4
Version ToS
Len
Length 1 Byte
TAC-TOI-01
SA
TAG
4 Bytes
PT
DATA
FCS
Standard IPV4: Bits 0-2 Called IP Precedence (Three MSB)
(DiffServ Uses Six ToS bits…: Bits 0-5, with Two Reserved)
ID
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offset
TTL Proto FCS IP-SA IP-DA
Data
8
Color the Packets
Coloring Engine
VolP
HTTP
FTP
Separate “Conform” and
“Exceed” Actions
VolP
HTTP
FTP
VolP Platinum Class
HTTP Gold Class
FTP Bronze Class
• Color closer to the application
• Set the DSCP (DiffServ Code Point) at the edge of
network
• Avoid host application-based coloring
TAC-TOI-01
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9
!! Detour !!
Differentiated Services
(DiffServ)
The Formula for Scalable QoS
TAC-TOI-01
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The IETF DiffServ Model
(RFC-2474,2475,2597,2598)
• The idea is VERY simple—Offer service levels for
packets: Gold, Silver, Bronze, etc…
• What is a service?
“Some significant characteristics of packet transmission
in one direction across a set of one or more paths within
a network (e.g.: Bandwidth,Latency,etc..)”...RFC-2475
• Packets of a particular service are referred to as
packets of a particular “class”
• Meaningful services constructed using
Per-Hop Behaviors (PHB)
TAC-TOI-01
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The DiffServ
Traffic Conditioner Block (TCB)
• Classifier: Identifies packets for assignment to Classes
• Meter: Checks compliance to traffic parameters (Token Bucket) and passes
result to Marker and Shaper/Dropper to trigger particular action for in/outof-profile packets
• Marker: Writes/rewrites the DSCP value
• Shaper: Delays some packets for them to be compliant with the profile
• Dropper: Drops packets that exceed the profile (Bc or Be)
TAC-TOI-01
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The DiffServ Recipe for
Constructing Services
•
At the Ingress Network-Edge:
(Traffic Conditioning Block—TCB)
1) Classify the packets into ‘Classes’
2) Mark (Color) the packets for purposes
of classification in the core
3) Optionally meter a class
4) If performing (3), police or shape the class
(at network ingress and/or egress)
5) Queue and/or drop packets toward the core
•
In the network core: (implementing the PHB)
6) Queue and/or drop packets
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How DiffServ Works
Step 1: Classifying Packets into Classes
• The most popular techniques:
Incoming/outgoing interface
All/any IP traffic
Standard or extended access control list
IP RTP ports (real-time traffic)
Source/destination MAC address
DSCP or IP precedence value
(If trusted and marked appropriately)
MPLS EXP (experimental bits)
(If trusted and marked appropriately)
Network-Based Application Recognition (NBAR)
• E.g.: all VoIP (RTP) packets between UDP ports
16384 and 16484 belong to the “Premium Class”
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The Hook for Scalable IPv4
Packet-Marking and Classification
Packets are Marked @ the Edge, for Purposes of Classification in the Core
Version ToS
Len
Length 1 Byte
ID
offset
TTL Proto FCS IP-SA IP-DA
Data
The IPv4 Header and the Type of Service (ToS) Byte
TAC-TOI-01
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15
IPv4 ToS vs. DS-Field
(The ToS Byte Is Re-Defined)
Just Remember
TAC-TOI-01
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“DSCP”
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How DiffServ Works
Step 2: Marking Packets of the Defined Classes
• Remember that marking can also be in Layer2!
• The most popular techniques:
IP DSCP—Layer 3
MPLS EXP bits—Layer 2.5
ATM CLP-bit—Layer 2
Frame-relay DE-bit—Layer 2
IEEE 802.1Q/p user-priority bits—Layer 2
• E.g.: The Premium Class (VoIP) packets get
marked with IP DSCP—‘101110’
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How DiffServ Works Optional
Step 3: Metering (The Token Bucket)
• Tokens keep pouring into the bucket at a
pre-defined average-rate
• If Token available, can transmit a packet
• Used by policer and shaper
• Explained in detail: Next talk
and Sess#: IPS-230
Tokens
B—Burst Size
p
p—Token Arrival Rate
Overflow
Tokens
B
Packets
Arriving
Conform
Exceed
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Traffic
Traffic
How DiffServ Works
Step 4: MeteringPolicing (Dropping)/Shaping
Traffic Rate
Traffic Rate
Policing
Time
Traffic Rate
Traffic
Traffic
Time
Traffic Rate
Shaping
Time
TAC-TOI-01
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Time
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On Policing…
• Policing is used not only to drop out-of-profile packets,
but also to re-mark them, and indicate to dropping
mechanisms downstream that they should be dropped
ahead of the in-profile packets!
Web
ERP
Other
Direction of Traffic Flow
TAC-TOI-01
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On Shaping…
• Shaping is commonly used where speed-mismatches exist (e.g.:
Going from a HQ site with a T1/E1 connection to a Frame-Relay
Network, down to a remote site with a 128Kbps connection)
• Shaping involves buffering, and various queuing/scheduling
techniques may be used when the shaped rate is reached!
FR/ATM WAN
T1/E1
128 Kbps
Bottleneck
Central
Site
Branch
Office
Shaping!
Direction of Traffic Flow
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How DiffServ Works
Steps 5&6: PHB by Queuing and/ Dropping
• Queuing refers to: (congestion management)
Buffering packets when interface is congested
Scheduling packets out of the buffer onto the link
(Algorithms: FIFO, CBQ, WRR, etc…)
Scheduler
Outbound Packets
Packets in
Various Queues
TAC-TOI-01
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How DiffServ Works
Steps 5&6: PHB by Queuing and/ Dropping…(Cont.)
• Dropping can happen:
At the edge when policing
In the edge/core when buffers are exhausted and signal
congestion to the end-nodes for back-off (Tail Drop)
In the edge/core to do congestion avoidance and signal
congestion to the end-nodes that can back-off
TAC-TOI-01
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23
The Various PHBs
(Using Queuing and Dropping)
• Expedited Forwarding (EF): RFC2598
Very low delay, low jitter, assured bandwidth
Compare to express mail, with overnight delivery
• Assured Forwarding (AF): RFC2597
Assured amount of bandwidth
IETF has defined four AF classes
Compare to registered mail—very safe, and assured
• Class Selector: Backwards compatible with
IP precedence for Forwarding Probability (FP)
FP(Precedence (x+1))  FP(Precedence (x))
Compare to FP(Express Mail)  FP(Priority Mail)
• Default: Best effort ~ normal mail
TAC-TOI-01
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The DiffServ AF PHB
(4 Classes, 3 Drop Preferences)
AF Class 1: 001dd0
AF Class 2: 010dd0
AF Class 3: 011dd0
AF Class 4: 100dd0
dd = drop preference
E.g. AF12 = Class 1, Drop 2,
thus “001100”
• Four independently forwarded/queued classes
• Within each AF class, 3 levels of drop preference
Used to increase the probability of dropping, especially
when traffic exceeds configured rate/CIR (Out-of-Profile)
TAC-TOI-01
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25
Cisco IOS DiffServ
• Cisco IOS 12.1(5)T,12.2(1)M and later versions are
fully compliant with all the core DiffServ RFCs
(RFCs: 2474,2475,2597,2598)
• Compliant platforms*:
C26xx, C36xx, C72xx, C75xx
Other platforms have most of the pieces
Full compliancy in the near future…
 ARM your network with DiffServ!
TAC-TOI-01
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Back on Track
TAC-TOI-01
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Step 3: Define Policies
for the Classes
• Set minimum bandwidth guarantee
This is the minimum guaranteed bandwidth to
the class all the time
• Set maximum bandwidth limits
This is the maximum amount of bandwidth
class will ever get
• Assign priorities to each class
Class is treated in a strict priority manner
• Manage congestion
TAC-TOI-01
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Minimum Bandwidth Guarantee/
Priority for a Class
“
Policy required:
Make sure my platinum class gets a
priority treatment and gold class gets
a minimum bandwidth guarantee
”
TAC-TOI-01
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Scheduling
Gold
40%
Guaranteed: Latency, Delivery
Silver
25%
Guaranteed: Delivery
Bronze
10%
Best Effort
Step 1:
Define Buffering
•
•
•
•
•
Step 2:
Define Bandwidth
Weights guarantee minimum bandwidth
Buffering controls latency
Unused capacity is shared amongst the other classes
Each queue can be separately configured for QoS
Benefits:
Maximize transport of paying traffic
No loss of service class guarantees
No wasted bandwidth as with PVCs
TAC-TOI-01
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Low Latency Queuing
PQ—Voice
V V
1 1
Interface
WFQ—Data
2 2
PQ
WFQ—Data
3 3 3 3
WFQ—Data
4 4 4 4
TAC-TOI-01
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WFQ
Exhaustive
Queuing
V V
4
3
2
1
1
WAN
Circuit
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Large Packets “Freeze Out” Voice
Voice Packet
60 Bytes
Every 20 ms
Voice Packet
60 Bytes
Every >214 ms
Voice Packet
60 Bytes
Every >214 ms
~214 ms Serialization Delay
Voice
1500 Bytes of Data Voice
Voice
1500 Bytes of Data Voice
10 Mbps Ethernet
Voice
1500 Bytes of Data Voice
10 Mbps Ethernet
56 Kb WAN
• Large packets can cause playback buffer
underrun, resulting in slight voice degradation
• Jitter or playback buffer can accommodate
some delay/delay variation
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Large Packets “Freeze Out” Voice
Voice Packet
60 Bytes
Every 20 ms
Voice Packet
60 Bytes
Every >214 ms
Voice Packet
60 Bytes
Every >214 ms
~214 ms Serialization Delay
Voice
1500 Bytes of Data Voice
Voice
1500 Bytes of Data Voice
10 Mbps Ethernet
Voice
1500 Bytes of Data Voice
10 Mbps Ethernet
56 Kb WAN
• Large packets can cause playback buffer
underrun, resulting in slight voice degradation
• Jitter or playback buffer can accommodate
some delay/delay variation
TAC-TOI-01
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33
Fragmentation Recommendations
Assuming 10 ms Max Blocking Delay “Rules of Thumb”
10 ms/Time for 1
Byte at BW =
Fragment Size
Link
Speed
56kbps
64kbps
128kbps
256kbps
512kbps
768kbps
1536kbs
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Frag
Size
70
Bytes
80
Bytes
160
Bytes
320
Bytes
640
Bytes
1000
Bytes
2000
Bytes
X
Fragmentation Not Needed if
Max Frame Size Is 1500 Bytes
34
Link Fragmentation and
Interleaving (LFI)
Fragment
• Fragment large packets and interleave
with voice packets over WAN links
• Reassemble at other end of link
• Reduces voice delay and jitter
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Maximum Rate Limiting
“
Policy required:
Make sure my bronze traffic does
not get more than x kbps of
bandwidth at any time
TAC-TOI-01
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”
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Traffic Rate
Policing
Traffic
Traffic
Traffic Policing vs. Shaping
Traffic Rate
Time
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Time
Shaping
Traffic
Traffic
Time
Traffic Rate
Traffic Rate
Time
37
Policer
Web
ERP/SAP
Trash
Pointcast
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Shaper
Internet Service
128 Kbps Provider (ISP) Cloud
T1
Branch
Office
Bottleneck
I Need to Reduce
the Pace at Which I
Send Packets
Central
Site
• Reduces outbound traffic flow to avoid
congestion (via buffering)
• Eliminates bottlenecks in topologies with
data rate mismatch
• Provides mechanism to partition
interfaces to match far-end requirements
TAC-TOI-01
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39
Congestion Avoidance
“
Policy required:
Make sure my bronze or silver traffic
gets dropped when there is
congestion and not gold traffic
”
TAC-TOI-01
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40
Weighted Random
Early Detection
TAC-TOI-01
Gold
Silver
Bronze
High
Precedence
Medium
Precedence
Low
Precedence
Guarantee MissionCritical Apps, e.g.,
ERP, Customer Care,
Unified Messaging
E-Mail, Interactive
Video, Web
E-Fax, FTP
© 2001, Cisco Systems, Inc. All rights reserved.
41
Putting it All Together
LLQ
IF
Policer/Marker
DSCP Written
IP Traffic
VolP HTTP FTP
VolP HTTP FTP
CBWFQ
W-RED Queues
or WFQ
Scheduler
• Colored (DSCP set) at ingress
• Classified and potentially discarded
by WRED (congestion management)
• Assigned to the appropriate outgoing queue
• Packets are:
• Scheduled for transmission by CBWFQ
TAC-TOI-01
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42
Policy
Packet Classified to
Code Point X on Client
or Router/Switch
RESV
PATH
Directory
RESV
PATH
RESV
PATH
Policy Server Response:
“Admit the Call and Use the
DiffServ Code Point X for
Data Flow”
Policy Server
RSVP (Quantative) Is Used for the Control Path Flow;
Data Path Uses an Aggregate as Identified by the DSCP;
RSVP Is Used to Signal the Data Path Aggregate
TAC-TOI-01
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43
Complete QoS Management
Network Wide
CONFIGURE
TRENDING
MONITORING
QPM
IPM
XML
XML
Device
SLAM
XML
QDM
TAC-TOI-01
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QDM
44
Cisco IOS IP Network Services
Quality of Service
Quality of Service
TAC-TOI-01
© 2001, Cisco Systems, Inc. All rights reserved.
…
Directory
Policy
Management
•Introduced in Release 12.1T
•Introduced in Release 12.1
Internet Application Technologies
Address Mgmt
Caching
Load Balancing
SNA Evolution
VPN
Multicast
Video
Voice
Security
QoS
•Class-based Marking
•Class-based Shaping
•QoS for 1750 Platform
•RSVP Support for LLQ
•Express Compressed Resource
Transport Protocol and TCP
Header Compression (CRTP)
•Common Open Policy Service for
Resource Reservation Protocol
(COPS for RSVP)
•DOCSIS 1.0+ Quality of Service
Enhancements
•DOCSIS 1.0+ features for uBR924
Cisco IP Fabric
Platforms
• IP QoS to ATM CoS integration
• CEF-Switched Compressed RTP
• Class-Based Weighted Fair Queuing
(CBWFQ)
• Low Latency Priority Queue with
CBWFQ
• IP RTP Priority for MLPPP
• Frame Relay Fragmentation
(FRF.12)
45
For More Information
http://www.cisco.com/warp/public/732/
http://www.cisco.com/warp/public/732/Tech/
http://www.cisco.com/warp/public/732/Tech/
quality.shtml
Overview: Network-Based
Application Recognition
TAC-TOI-01
© 2001, Cisco Systems, Inc. All rights reserved.
http://wwwin.cisco.com/cmc/cc/so/neso/
ienesv/cxne/nbar_ov.htm
46
Why QoS??
Congestion Scenario #1—Speed Mismatch
100Mbps
2Mbps
WAN
1000Mbps
100Mbps
Direction of Data-Flow
• The #1 Reason for Congestion!
• Possibly Persistent when going from LAN to WAN
• Usually Transient when going from LAN to LAN!
TAC-TOI-01
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47
Why QoS??
Congestion Scenario #2—Aggregation
HQ
2Mbps
Hubi
512Kbps
FR/ATM
N*56Kbps
Choke
Points
1000Mbps
S1
S2
Remotej
1000Mbps
Choke
Point
Direction of Data-Flow
• Transient Congestion fairly typical!
TAC-TOI-01
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48
Why QoS??
Congestion Scenario #3—Confluence
Net-1
Core1
Net-2
Core2
STM-64/OC-192c
STM-16/OC-48c
Net-n
• Always need mechanisms to provide guarantees!
• Transient Congestion occurs!
TAC-TOI-01
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