Upload
anji-dudigam
View
220
Download
0
Embed Size (px)
Citation preview
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 1/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.
Page 1 of 28
WHITE P APER
Cisco IOS(TM) Software
Quality of Service Solutions
Executive Summary
Toda y’s netwo rks are carrying more data in the form of b andw idth-intensive, real-time voice, video a nd da ta, w hich stretch netwo rk
capability a nd resources. Cisco IOS® softw are provides a toolbo x full of qua lity of service (Qo S) solutions to help you solve
problems caused by increasing traf fi c demands on t he netw ork.
The goal of Q oS is to provide better and mo re predictable netwo rk service by providing d edicated bandw idth, contro lled jitter
and latency, a nd improved loss characteristics. Qo S achieves these goals by providing too ls for ma naging netw ork congestion,
shaping netwo rk traf fi c, using expensive wide-area links more efficiently, a nd setting traf fic policies across the network.
Internet service providers, small and medium-sized business netw orks, as w ell as large enterprise netw orks can ta ke adva nta ge
of the solutions provided by the C isco IO S QoS softw are features.
This white paper presents the Cisco IOS QoS implementation in the following sections:
• Introduct ion
• QoS Framework
• The Cisco QoS Toolkit
• QoS Signaling
• Cisco Q oS Policy, M anagement, and Accounting Capabilities
• Network QoS Application Examples
• QoS Looking Forward
In ad dition, reference matrices and commo nly requested Q oS defi nitions are included in t he appendices.
Introduction
Cisco Systems, the w orldw ide leader in netw orking for the Internet, provides products and services that give people access to the
informat ion they need by co nnecting informa tion d evices through intelligent, secure, a nd reliable netwo rks. As the leader in global
netwo rking, Cisco’s breadth o f product co vers virtually every ma rket segment. Cisco is, therefore, in a unique position to coord inate
and deliver end-to-end Q oS across vary ing netw ork technologies and a rchitectures to a chieve end-to end Q oS solutions.
Netw orking users in general span three major ma rket segments: majo r enterprises, netw ork service providers, and the small
and m edium-sized business segment. Each segment has its ow n Qo S requirements, but they also have many o verlapping needs.
Netw ork mana gers in tod ay’s enterprise networks must contend w ith numerous and diverse system platforms, netw ork
architectures, and protocols. Providing end-to-end Q oS solutions across the various platfo rms often requires more than just linking
them together; it a lso requires a different approa ch for each technology. Enterprises are increasingly depending on their networks
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 2/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 2 of 28
to carry complex mission-critical a pplications a nd dat aba ses such as SAP, PeopleSoft, and Ora cle. These netw orks are a lso
experiencing increased traffi c from Web and multimedia applications. Q oS prioritizes this traf fi c to ensure that mission-critical
applica tions get the service they require, w hile simulta neously servicing these new er multimedia applica tions.
Internet service providers (ISPs) require assured scalability and performance. The ISP marketplace is also highly competitive
and characterized by phenomenal grow th. ISPs, who ha ve traditionally o ffered best-effort IP connectivity, are now planning
netwo rks to transport voice, video, a nd other real-time, critical application da ta. ISPs need a model that w ill allow them to offer
differentiated services to t heir customers, yet a llow them to remain profi table. Q oS provides the basis for a new business model by
allow ing ISPs to diff erentiate traffi c from various customers or applications.
In the small and medium-sized business segment, managers are experiencing first hand the rapid growth of business on the
Internet. No t so long a go, the “ global netw orked business” concept w as just tha t. Every day w e witness more and mo re businesses
participating in the reality. Besides the increased demands of Internet traffic, small and medium-sized business networks must also
handle increasingly complex business applications. Q oS lets the netw ork ha ndle the diffi cult task of utilizing a n expensive wide-area
netwo rk connection in the most efficient wa y fo r business applications.
With these increasing demands, it is important to fi nd w ays of utilizing and expanding upon existing network resources. Cisco
IOS softw are allow s the addition of Qo S capabilities to the network primarily through softw are upgrades, helping to preserve
valuable investments in netwo rk equipment, w hile meeting constantly gro w ing needs.
This white paper provides an overview o f the Qo S architectural framew ork that explains how Qo S applies in the netwo rk, and
it provides details on t echnologies that Cisco IO S softw are provides in each piece of the architecture. The paper concludes with
some examples of how these pieces wo rk togeth er to provide QoS services tha t help you get the most from scarce netw ork resources.
Tables summarizing feature capabilities and a vailability are conta ined in the appendix, and a glossary of Q oS terms is also provided.
Network QoS Defined
Q oS refers to the a bility of a netwo rk to provide better service to selected netwo rk traf fi c over various technologies, including Frame
Relay, Asynchrono us Transfer M ode (ATM ), Ethernet and 802.1 netw orks, SON ET, as w ell as IP-rout ed netw orks tha t ma y use
any or a ll of these underlying technologies. Primary goals of Q oS include dedicated band w idth, contro lled jitter and la tency,
(required by some real-time and interactive traffic), and improved loss characteristics. QoS technologies provide the elemental
building b locks that w ill be used for futur e business applica tion s in campus, WAN, an d service provider netwo rks.
The Cisco IOS Q oS softw are enables complex netw orks to control a nd predictably service a variety of netw orked applications
and t raffi c types. Almost any netw ork can take adva ntage of Q oS for optimum efficiency, w hether it is a small corporate netwo rk,
Internet service provid er, or enterprise netw ork. The C isco IOS Q oS softw are provid es these benefit s:
• Cont rol over resources —You ha ve control over w hich resources (band w idth, equipment, w ide-area f acilities, and so on) are being
used. As an example, you can limit the bandw idth consumed over a ba ckbone link by FTP tra nsfers or give priority to an
important database access.
• More efficient use of network resour ces —Using Cisco’s netw ork ana lysis mana gement and a ccounting tools, you will know w hat
your netw ork is being used for and t hat y ou are servicing the most important tra ffi c to your business.
• Tailo red servi ces —The control and visibility provided by QoS enables Internet service providers to offer carefully tailored grades
of service differentiation to their customers.
• Coexi stence of mission -crit ical appli cations— C isco’s Q oS technolog ies make certain t hat your WAN is used efficiently by mission-
critical applications tha t are most importa nt to your business; tha t ba ndw idth and minimum delays required by time-sensitive
multimedia and voice applications a re available; a nd t hat other applications using the link get their fa ir service without interfering
w ith mission-critical tra ffi c.
• Foundation for a fully integrated network in t he future —Implementing C isco Q oS technologies in your netwo rk now is a good
fi rst step tow ard the fully integrated multimedia netw ork needed in the near future. For example, you can implement w eighted
fair q ueuing toda y a nd get its immediate benefi t of increasing service predictability a nd IP Precedence signaling for traffi c
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 3/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 3 of 28
differentiation. You reap a dditiona l benefits in the future, because weighted f air q ueuing is resource ReSerVation Protoco l (RSVP)
enabled, thereby allow ing you to ta ke advanta ge of dyna mically signaled Q oS from the inevitable coming wa ve of RSVP-enabled
applications.
The follow ing sections more fully describe the Cisco Q oS architecture and the Qo S tools that are provided in the Cisco IOS Q oS
software.
QoS Framework
This section describes the basic framew ork fo r providing Q oS in the netw ork. The “ Basic Qo S Architecture” section describes the
three components necessary to deliver Qo S across a netw ork comprising heterogeneous technolo gy (IP, ATM , LAN sw itches, and
so on). The “ End-to-End QoS Service Levels” section d escribes the three basic levels of Q oS service tha t can b e provided a cross a
heterogeneous infrastructure.
Basic QoS Architecture
The basic architecture introduces the three fundamental pieces for QoS implementation (see Figure 1):
• Q oS within a single netwo rk element (for example, queuing, scheduling, and tra ffi c shaping tools)
• Q oS signaling techniques for coordina ting QoS from end to end between netwo rk elements
• Q oS policy, management, and accounting functions to control, and administer end-to-end traffi c across a netwo rk
Figure 1 Basic QoS Architecture
ConnectedNetwork
ConnectedNetwork
ClientNode
HostNode
1. QoS in the Node (Queuing, Shaping,
and so on
2. QoS Signaling
3. Policy, Management, Accounting
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 4/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 4 of 28
End-to-End QoS Service Levels
Service levels refer to the actual end-to-end QoS capabilities, meaning the ability of a network to deliver service needed by specific
netwo rk tra ffi c from end-to-end or edge-to-edge. The QoS services differ in t heir level of “ Qo S strictness,” w hich describes how
tightly the service can b e bound by specific b andw idth, d elay, jitter, a nd lo ss characteristics.
There are three basic levels of end-to-end QoS service that can be provided across a heterogeneous network, as shown in
Figure 2.• Best Ef for t Servi ce —Also know n a s lack of Qo S, best effort service is basic connectivity w ith no guarant ees.
• D if ferentiated Servi ce (also Called Soft Q oS) —Some traffi c is treated better than the rest (faster handling, more bandw idth on
average, low er loss rate on average). This is a statistical preference, not a hard and fast gua rantee.
• Guarant eed Service (also Called Hard Q oS) —An ab solute reservation of netw ork resources for specifi c traf fi c.
These terms are defined in more detail in Appendix 2.
Figure 2 End-to-End QoS Service Levels
D eciding on w hich type of service is appropriate to d eploy in the netwo rk depends on several factors:
• The application o r problem the customer is trying to solve. Each of the three types of service is appropriate for certain a pplications.
This does not imply that a customer must migrate to differentiated and then to gua ranteed service (although w e believe that many
eventually will). A differentiated service—or even best effort service—may be appropriate depending on the customer application
requirements.
• The rate at w hich customers can realistically upgrade their infrastructures. There is a natura l upgrade path f rom the technology
needed to provide differentiated services to that needed to provide guarant eed services, w hich is a superset of that needed fo r
differentiated services.
• The cost-implementing and deploying guarant eed service is likely to be more expensive than do ing so for a d ifferentiated service.
The next three sections describe the tools tha t C isco IO S provides in each section of the architecture, w hich, w hen combined, can
create end-to-end Q oS or simply solve specific prob lems at various points in the netwo rk.
The Cisco QoS Toolkit
Cisco IO S softw are provides a variety of Qo S tools to provide the service levels described a bove. These tools a re typically usedw ithin a single netw ork element, as show n in the basic architecture depicted in Figure 1. Typically, these too ls are turned on at a n
interface to provide the right Q oS characteristics for a specific netw ork application. The Cisco IO S Qo S tools provide three major
functions—congestion ma nagement (queuing and scheduling), congestion avoida nce, and tra ffi c shaping and po licy making. In
add ition, C isco IO S tools provides link efficiency mechanisms that integrate with t he other three functions to provide add itional
improved QoS service.
“The Network”
Solved—IPInternet
UbiquitousConnectivity
Some Traffic isMore Importantthan the Rest
CertainApplications
Require SpecificNetwork
Resources
Best Effort(IP, IPX,
AppleTalk)
Differentiated(First, Business,Coach Class)
Guaranteed(Bandwidth,Delay, J itter)
Best Effort
Differentiated
Guaranteed
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 5/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 5 of 28
Congestion Management Tools
One w ay tha t netwo rk elements handle an overflo w o f arriving traffi c is to use a q ueuing algorithm to sort the traffi c, then determine
some method o f prioritizing it onto an o utput link. Cisco IOS softw are includes the follow ing queuing tools:
• First In, First Out (FIFO) Queuing
• Priority Queuing (PQ)
• Custom Queuing (CQ )
• Weighted Fair Queuing (WFQ)
Each queuing algorithm was designed to solve a specific network traffic problem and has a particular effect on network
performance, as described in the follow ing sections.
FIFO Provides Basic Store-and-Forward Capability
In its simplest fo rm, FIFO q ueuing involves storing pa ckets w hen the network is congested a nd fo rw arding them in order of arrival
w hen the network is no longer congested. FIFO is the default q ueuing algorithm in some instances, thus requiring no confi guration,
but it ha s several shortcomings. M ost important ly, FIFO q ueuing makes no decision ab out packet priority; the order of arrival
determines bandw idth, promptness, and buffer allocat ion. N or d oes it provide protection aga inst ill-behaved applications (sources).
Bursty sources can ca use high delays in d elivering time-sensitive application tra ffi c, and potentially to netwo rk control and signaling
messages. FIFO queuing w as a necessary fi rst step in controlling netwo rk traffi c, but to day ’s intelligent netw orks need more
sophisticated a lgorithms. Cisco IOS softw are implements queuing algorithms tha t avo id the shortcomings of FIFO q ueuing.
PQ Prioritizes Traffic
PQ ensures that important traffi c gets the fastest handling at each point w here it is used. It w as designed to give strict priority to
important traffi c. Priority q ueuing can fl exibly prioritize according to netwo rk protocol (for example IP, IPX , or AppleTalk),
incoming int erface, packet size, source/destinat ion a ddress, and so on.
In PQ , each packet is placed in one of four q ueues—High, M edium, Normal, o r Low —based on a n assigned priority. Packets
that are not classified by this priority-list mechanism fall into the Norma l q ueue; see Figure 3. During t ransmission, the a lgorithm
gives higher-priorit y queues ab solute preferentia l treatment over low -priority queues. This is a simple and intuit ive approa ch but
can cause queuing delays that the higher-priority tra ffi c might ha ve experienced to be ra ndomly transferred t o the low er-priority
traffi c, increasing jitter on the low er-priority traf fi c. H igher-priority traf fi c can be rat e limited to a void this problem.
Figure 3 Priority Queuing
*For information on specific interface support, see Appendix 1.
“Class” Queues:Length Defined by Queue Limit
Absolute PriorityScheduling
Classify
Traffic Destinedfor Interface
Classification by:•Protocol (IP, IPX, AppleTalk,
SNA, DecNet, Bridge, and so on)•Incoming, Interface
(EO, SO,S1, and so on)
Interface Hardware•Ethernet
•Frame Relay•ATM•Serial Link
Allocate Link Bandwidthby Strict Priority
Manage InterfaceBuffer Resources
TransmitQueue
Output Hardware
High
Medium
Normal
Low
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 6/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 6 of 28
PQ is useful for making sure that mission-critical traffic traversing various WAN links gets priority treatment. For example,
Cisco uses PQ to ensure that important Ora cle-based sales reporting data gets to its destination a head of other less critical tra ffi c.
PQ currently uses static configuration and thus does not automatically adapt to changing network requirements.
CQ Guarantees Bandwidth
Custom queuing (CQ) was designed to allow various applications or organizations to share the network among applications with
specific minimum band w idth or latency requirements. In these environments, bandw idth must be shared proportionally betw eenapplications and users. You can use the Cisco CQ feature to provide guaranteed bandw idth at a potential congestion point, assuring
the specified traffic a fixed portion of available bandwidth and leaving the remaining bandwidth to other traffic. Custom queuing
handles traffi c by a ssigning a specified a mount o f q ueue space to each class of pa ckets and then servicing the q ueues in a round-robin
fashion; see Figure 4.
Figure 4 Custom Queuing
As an example, encapsulated SNA requires a guaranteed minimum level of service. You could reserve half of available
bandwidth for SNA data, allowing the remaining half to be used by other protocols such as IP and IPX.
The queuing a lgorit hm pla ces the messages in one of 17 queues (queue 0 holds system messages such a s keep-alives, signaling,
and so on), and is emptied w ith w eighted priorit y. The router services queues 1 through 16 in round-rob in order, dequeuing a
confi gured byte count fro m each q ueue in each cycle. This feature ensures that no a pplication (or specified gro up of a pplications)
achieves more than a predetermined proport ion of overall capacity w hen the line is under stress. Like PQ, CQ is statically confi gured
and does not automatically adapt to changing network conditions.
“Class” Queues:Length Defined by Queue Limit
WeightedRound Robin(Byte Count)Link
UtilizationRatio
Classification by:•Protocol (IP, IPX, AppleTalk,
SNA, DecNet, Bridge, and so on)•Incoming, Interface
(EO, SO,S1, and so on)
Interface Hardware•Ethernet•Frame Relay•ATM•Serial Link
Allocate ConfiguredProportion of Link Bandwidth
Manage InterfaceBuffer Resources
TransmitQueue
Output HardwareClassify
1/10
1/10
3/10
2/10
3/10
Up to 16
Traffic Destinedfor Interface
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 7/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 7 of 28
WFQ: Cisco’s Intelligent Queuing Tool for Today’s Networks
For situations in w hich it is desirable to provide consistent response time to heavy and light netwo rk users alike without add ing
excessive bandw idth, the solution is WFQ. WFQ is one of Cisco’s premier q ueuing techniques. It is a fl ow -based q ueuing algorithm
that does tw o things simultaneously: It schedules interactive traffi c to the front of t he queue to reduce response time, and it fa irly
shares the remaining bandwidth between high bandwidth flows.
WFQ ensures that q ueues do not sta rve for bandw idth, a nd tha t traf fic gets predictable service. Low -volume traffi c
streams—which comprise the majority of traffic—receive preferential service, transmitting their entire offered loads in a timely
fashion. H igh-volume traffi c streams share the remaining capa city proportiona lly between them, as shown in Figure 5.
WFQ is designed to minimize configuration effort and adapts automatically to changing network traffic conditions. In fact,
WFQ does such a go od job f or most a pplications tha t it ha s been mad e the default q ueuing mode on mo st serial interfaces confi gured
to run at or below E1 speeds (2.048 Mbps).
WFQ is efficient in that it w ill use w hatever bandw idth is available to forw ard traffi c from lower priority fl ow s if no traffi c
from higher priority flows is present. This is different from Time Division Multiplexing (TDM) which simply carves up the
bandwidth and lets it go unused if no traffic is present for a particular traffic type. WFQ works with both of Cisco’s primary QoS
signaling techniques, IP Precedence and RSVP, described later in this white paper, to help provide differentiated QoS as well as
guaranteed QoS services.
Figure 5 Weighted Fair Queuing
Configurable Numberof “Flow” Queues
WeightedFair Scheduling
Flow-BasedClassification by:•Source and destination
address
•Protocol•Session identifier(port/socket)
Weight determined by:•Requested QoS (IP Precedence, RSVP)•Frame Relay FECN, BECN, DE
(for FR Traffic)•Flow throughput (weighted-fair)
Allocate “fair”Proportion of Link Bandwidth
Manage InterfaceBuffer Resources
TransmitQueue
Output Hardware
Classify
Traffic Destinedfor Interface
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 8/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 8 of 28
The WFQ algorithm also addresses the problem of round-trip delay variability. If multiple high-volume conversations are
active, their transfer rates and interarrival periods a re made much mo re predictab le. WFQ greatly enhances algorithms such as the
SNA Logical Link C ontrol (LLC) and the Transmission C ontrol P rotocol (TCP ) congestion control a nd slow -start feat ures. The
result is more predictable throughput a nd response time for each a ctive flow, a s shown in Figure 6.
Figure 6 Interactive Traffic Delay (128-Kbps Frame Relay WAN Link)
Cooperation between WFQ and QoS Signaling TechnologiesWFQ is IP Precedence-aw are, tha t is, it is able to detect higher priority packets marked w ith precedence by the IP Forw arder a nd
can schedule them faster, providing superior response time for this tra ffi c. The IP Precedence fi eld has values betw een 0 (the default)
and 7. As the precedence value increases, the algorithm a llocates more bandw idth to that conversation t o ma ke sure that it gets
served more quickly w hen congestion o ccurs. WFQ a ssigns a w eight to each fl ow, w hich determines the transmit o rder for q ueued
packets. In t his scheme, low er w eights are served fi rst. IP Precedence serves as a divisor to this w eighting fa ctor. For instance, tra ffi c
w ith an IP Precedence field value of 7 gets a low er weight than tra ffi c w ith an IP Precedence field value of 3, a nd thus has priority
in the transmit order.
An example: If you ha ve one flow at each precedence level on an interface, each flow w ill get precedence+ 1 parts of the link,
as follows:
1+2+3+4+5+6+7+8 = 36
and the flo w s will get 8/36, 7/36, 6/36, an d 5/36 of t he link, and so on. H ow ever, if you ha ve 18 precedence-1 flo w s and one of each
of the others, the formula loo ks like this:
1+18*2+3+4+5+6+7+8 = 36-2+18*2 = 70
and t he flo w s will get 8/70, 7/70, 6/70, 5/70, 4/70, 3/70, 2/70, a nd 1/70 of the link, an d 18 of th e flow s will get a ppro ximat ely 2/
70 of the link.
See “ IP Precedence Signals Differentiated Q oS” on page 20 for more on this.
WFQ is also RSVP aw are; RSVP (see “ RSVP G uarant ees QoS” on page 21) uses WFQ to allocate buffer space and schedule
packets, and guarantees bandwidth for reserved flows.
Additionally, in a Frame Relay netwo rk, the presence of congestion is flagged by the forw ard explicit congestion notifi cation
(FECN ) and ba ckwa rd explicit congestion notifica tion (BECN) bits. WFQ w eights are aff ected by Frame Relay discard eligible (D E),
FECN, a nd BECN bits when the traffi c is switched by the Frame Relay sw itching module. O nce congestion is fla gged, the weights
used b y the a lgorithm a re altered so tha t t he conversation encountering the congestion transmits less frequently.
0
500
1000
1500
2000
2500
3000
3500
0 50 100 150 200 250 300 350 400 450 500 550 600
Without WFQ
Time(Seconds)
Round Trip Delay(Milliseconds)
Mean RTT for Sample
Round Trip Delay(Milliseconds)
0
500
1000
1500
2000
2500
3000
3500
0 50 100 150 200 250 300 350 400 450 500 550 600
With WFQ
Time(Seconds)
Mean RTT for Sample
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 9/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 9 of 28
D-WFQ-A High-Speed Version for the 7500 Platform
C isco IOS softw are also provid es distribut ed weight ed fair q ueuing (D-WFQ), a special high-speed version of WFQ d esigned
initially for IP-only netw orks. D -WFQ is currently ava ilable only on VIP processors and only in C isco IO S release 11.1cc, a special
version for 7500 VIP processors. The 11.1cc functionality was initially distributed to a select set of ISP customers, but will be
released w ith an upcoming version of C isco IO S softw are for enterprise customers as well (see the “ Cisco IO S Qo S Ca pabilities
Ma trix” in the appendices for more details).
Congestion Avoidance Tools
Co ngestion avoida nce techniques monitor netwo rk traffi c loads in an effort to anticipate and avoid congestion at common network
bottlenecks, as opposed to congestion ma nagement techniques that operate to control congestion once it o ccurs. The primary C isco
IOS congestion avoida nce tool is Weighted R ando m Early D etection (WRED ), which is described next.
WRED Avoids Congestion
The Rand om Ea rly D etection (RED) class of a lgorithms are designed to avoid congestion in internetwo rks before it becomes a
problem. RED works by monitoring traffic load at points in the network and stochastically discarding packets if the congestion
begins to increase. The result of the drop is that the source detects the dropped traffi c and slows its tra nsmission. RED is primarily
designed to w ork w ith TCP in IP internetw ork environments.
WRED is Cisco’s implementation. In early d ocumentation it w as simply called RED , but the name ha s been changed to WRED
to better reflect its capabilities, as described below.
Figure 7 Weighted Random Early Detection
WRED Cooperation with QoS Signaling Technologies
WRED comb ines the capabilities of the RED algorithm w ith IP Precedence. This combination provides for preferential tra ffi c
handling f or higher-priority packets. It ca n selectively discard low er-priority tra ffi c w hen the interface starts to get congested and
provide differentiated performance characteristics for different classes of service. See Figure 7.
WRED is a lso RSVP-aw are, a nd ca n provide an integrated services controlled-load Qo S service.
Discard Text Based on:
•Average queue depth•IP Precedence•RSVP session
Avoid Congestion
on Link
Manage Interface
Buffer Resources
TransmitQueue
Output Hardware
BitBucket
FIFOScheduling
Fail
Pass
DiscardTest
Traffic Destined
for Interface
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 10/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 10 of 28
D-WRED Delivers High-Speed Differentiated Traffic on the 7500 Platform
Cisco IO S softw are also provides Distributed Weighted R ando m Early D etection (D-WRED ), a high-speed version of WRED tha t
runs on VIP-distributed processors. The D-WRED algorithm provides functionality beyond w hat WRED pro vides, such as
minimum and maximum q ueue depth thresholds and drop ca pabilities for each class of service. D -WRED is currently available only
in the C isco IO S 11.1cc release, a special image fo r IP-only applications. The 11.1cc functionality w as initially d istributed to a select
set of ISP customers, but w ill released w ith an upcoming version of C isco IO S softw are for enterprise customers as w ell (see the
“ Cisco IOS Qo S Capa bilities Matrix” in the appendices for more details).
Traffic Shaping and Policing Tools
Cisco’s QoS softw are solutions include two traf fic shaping to ols—G eneric Traffi c Shaping (G TS) and Frame Relay Traffi c Shaping
(FRTS)—to mana ge traffi c and congestion on the network.
GTS Controls Outbound Traffic Flow
G eneric Traffi c Shaping (G TS) provides a mechanism to contro l the traffi c flow on a particular interface. It reduces outbound traffi c
flo w to a void congestion by constraining specified traf fi c to a part icular bit rat e (also know n as the token bucket approa ch), w hile
queuing bursts of the specified traf fic. Thus, traf fi c adhering to a particular profi le can be shaped to meet dow nstream requirements,
eliminating bo ttlenecks in topo logies with da ta -rat e misma tches. Figure 8 illustrates G TS.
Figure 8 Generic Traffic Shaping
G TS applies on a per-interface basis, can use access lists to select the tra ffi c to shape, and w orks w ith a variety of Layer 2
technologies, including Frame Relay, ATM, Switched M ultimegabit D ata Service (SMD S), a nd Ethernet.
On a Frame Relay subinterface, G TS can be set up to ada pt dynamically to a vailable bandw idth by integrating BECN signals,
or set up simply to sha pe to a prespecifi ed rat e. G TS can also be confi gured on a n ATM /AIP interface card t o respond to RSVP
signaled over statically configured ATM permanent virtual circuits (PVCs).
FRTS Manages Frame Relay Traffic
Frame Relay Traffi c Shaping (FRTS) provides parameters that are useful for ma naging netw ork tra ffi c congestion. These include
committed informat ion rate (CIR), forw ard a nd ba ckwa rd explicit congestion notifi cation (FECN /BECN ), and the discard eligibility
(DE) bit. For some time, Cisco has provided support for FECN for DECnet and OSI, BECN for SNA traffic using direct LLC2
encapsulation via R FC 1490, and D E bit support. The FRTS feature builds upon this Frame Relay support w ith ad ditional
capabilities that improve the scalability a nd performance of a Frame Relay netwo rk increasing the density of virtual circuits and
improving response time.
ConfiguredQueuing
(e.g. WFQ, CQ)
Classification by:•Extended Access List Functionality
“Token Bucket”Shaping
GTS Can be Configured withAny Output Queuing
TransmitQueue
Output HardwareClassify
No Match
Match
ConfiguredRate
Traffic Destinedfor Interface
TokenBucket
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 11/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 11 of 28
For example, you can confi gure rate enforcement—a peak rate confi gured to limit outbound tra ffi c—to either the CIR or some
other defined value, such as the excess information rate (EIR), on a per-virtual-circuit (VC) basis.
You can also defi ne priority and custom q ueuing at the VC or subinterface level. This allows fo r fi ner granularity in the
prioritization and queuing of traffi c and provides more control over the traffic fl ow on an individual VC. If you combine CQ with
the per-VC q ueuing and ra te enforcement capab ilities, you enab le Frame Relay VCs to ca rry multiple traf fi c types such as IP, Systems
Netw ork Architecture (SNA), and Internetw ork Packet Exchange (IPX), w ith band w idth guara nteed fo r each traffi c type.
FRTS can eliminate bot tlenecks in Frame Relay netw orks w ith high-speed connections at the central site and low -speed
connections at t he branch sites. You can confi gure rate enforcement to limit the rate at w hich data is sent on the VC a t the central
site. You can also use rate enforcement w ith the existing D LCI Prioritization f eature to further improve performance in this
situation.
FRTS applies only to Frame Relay Permanent Virtual Connections (PVCs) and Switched Virtual Connections (SVCs).
Using information cont ained in BECN -tagged pa ckets received from the netw ork, FRTS can also dy namically throt tle traffi c.
With BECN -based thrott ling, packets are held in the router’s buffers to reduce the data flo w from t he router into the Frame Relay
netwo rk. The throttling is done on a per-VC b asis and t he transmission rat e is adjusted ba sed o n the number of BECN -tagged
packets r eceived.
FRTS also provides a mechanism for sharing media by multiple VCs. Rate enforcement allows the transmission speed used by
the router to b e controlled by criteria other tha n line speed, such as the C IR o r EIR. The rate enforcement feature can a lso be used
to preallocate band w idth to each VC , creating a virtual time division multiplexing netwo rk.
And fi nally, with the Cisco’s FRTS feature, you can integrate StrataC om ATM Foresight closed loo p congestion control to
actively adapt to downstream congestion conditions.
Link Efficiency Mechanisms
Currently, C isco IO S softw are off ers two link effi ciency mechanisms—Real Time Protocol H eader Co mpression (RTP-H C) and Link
Fragmentation a nd Interleaving (LFI)—which w ork w ith queuing and tra ffi c shaping to improve the efficiency and predictability of
the a pplicatio n service levels.
LFI Fragments and Interleaves IP Traffic
Interact ive traffi c (Telnet, voice on IP, an d the like) is susceptible to increased latency a nd jitter w hen the netw ork pro cesses large
packets, (LAN-to-LAN FTP tra nsfers traversing a WAN link, fo r exa mple), especially a s they are q ueued on slow er links. The Cisco
IOS Link Fragmentation and Interleaving (LFI) feature reduces delay and jitter on slower-speed links by breaking up large
dat agra ms and interleaving low delay tra ffi c packets with t he resulting smaller packets; see Figure 9.
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 12/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 12 of 28
Figure 9 Link Fragmentation and Interleaving
LFI was designed especially for lower-speed links where serialization delay is significant. LFI requires that multilink PPP be
confi gured on the interface w ith interleaving turned o n. A related IETF Dra ft, ca lled M ulticlass Extensions to M ultilink PPP
(MC ML) implements almost the same function as LFI.
RTP Header Compression Increases Efficiency of Real-time Traffic
Real-time Transport Pro tocol (RTP) is a ho st-to-host prot ocol used for carrying new er multimedia application tra ffi c, including
packetized a udio a nd video, over an IP netwo rk. RTP provides end-to-end netwo rk transport functions intended for a pplications
transmitting real-time requirements, such as aud io, video, or simulation dat a over multicast or unicast netwo rk services. RTP
H eader Co mpression increases effi ciency for many of t he newer voice-over-IP or multimedia applications that take ad vanta ge of
RTP, especially on slow links. Figure 10 illustrates RTP header co mpression.
For compressed-payloa d a udio a pplications, the RTP packet has a 40-byte header and typically a 20- to 150-byte payload .
G iven the size of t he IP/UD P/RTP header combin at ion, it is ineffi cient to tr ansmit a n uncompr essed header. RTP head er compression
helps RTP run more effi ciently—especially o ver low er-speed links—by compressing the RTP/UD P/IP hea der fro m 40 b ytes to tw o
to fi ve bytes. This is especially benefi cial fo r smaller packets (such as IP voice traffi c) on slow er links (385 kbps and b elow ), where
RTP head er compression can reduce overhead and transmission d elay significant ly.
RTP header compression reduces line overhead for multimedia RTP traffic with a corresponding reduction in delay, especially
for t raffi c that uses short packets relative to header length.
Weighted FairScheduling
Traffic Destinedfor Interface
PacketFragmentation
J umbogram
IP Voice
TransmitQueue
Output Hardware
Classify
LFI WFQ
Flow-BasedClassification by:•Source and destination
address•Protocol•Session identifier
(port/socket)
Multilink PPPwith LFI OnLarge PacketFragmentation:Fragment sizebased on requireddelay
Fragmented FramesInterleaved with Time-Sensitive Traffic
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 13/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 13 of 28
Figure 10 RTP Header Compression
RTP header compression is supported o n serial lines using Frame Relay, H D LC, or P PP encapsulation. It is also supported o ver
ISDN interfaces. A related IETF Draft, called Compressed RTP (CRTP), defines essentially the same functionality.
QoS Signaling
Think of QoS signaling as a form of network communication. It provides a way for an end station or network element to signal
certain req uests to a neighbor. For example, an IP netwo rk can use part of the IP packet header to request special hand ling of priority
or t ime-sensitive traffi c. Q oS signaling is useful for coordina ting the tra ffi c hand ling techniques described earlier in this paper and
has a key role in confi guring successful end-to-end Qo S service acro ss your netw ork.
True end-to-end Q oS requires that every element in the netw ork pat h—switch, ro uter, fi rew all, ho st, client, and so on—deliver
its part of Q oS, and it a ll must be coordinat ed with Q oS signaling. How ever, the challenge is fi nding a ro bust QoS signaling solution
that can o perate end-to-end o ver heterogeneous netw ork infra structures. Although many viable Qo S signaling solutions provide
Qo S at some places in the infrastructure, they of ten have limited scope across the netw ork, a s shown in Figure 11.
Identify RTP Traffic Compress
Traffic Destinedfor Interface
TransmitQueue
Output HardwareNon-RTP Traffic
RTPCompression
RTP Traffic(Video,Audio,etc.)
VOIP
Efficiencies
*Also ~5ms Reduction inSerialization Delay at 64 Kbps
PayloadPacket SizeReduction*
SQL
FTP
20 Byte
256 Byte
1,500 Byte ~2.3%
~13%
~240%IP
20
UDP
8
RTP
12
5
IP Data
IP Data
ConfiguredQueuing
(WFQ, PQ, CQ, etc.)
Classify
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 14/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 14 of 28
Figure 11 QoS Signaling Solutions
Cisco IO S softw are takes adva ntage of the end-to-end na ture of IP to meet this challenge by overlaying Lay er 2
technology-specific Q oS signaling solutions with the La yer 3 IP Q oS signaling methods of RSVP and IP Precedence.
This paper focuses on IP Precedence and RSVP, because both of these methods take advantage of the end-to-end nature ofthe IP protocol. As the majority of applications converge on the use of IP as the primary netw orking protocol, IP Precedence and
RSVP provide a pow erful combinatio n for Q oS signaling—IP Precedence signals for D ifferentiated Q oS, and RSVP for G uarant eed
QoS.
In ad dition to these mechanisms, Cisco leads the industry in Q oS signaling integration, a s shown in Figure 11. To a chieve the
end-to-end b enefi ts of IP Precedence and RSVP signaling, C isco IO S softw are of fers ATM user to netwo rk interface (UNI) signaling
and Frame Relay local mana gement interface (LMI) to provide signaling into their ATM and Frame Relay ba ckbone technologies.
C isco also provides similar priority signa ling in its implementatio n of the IETF’s multipro toco l label switching (M PLS), called Tag
Switching.
With the Remote Sw itch Mod ule (RSM) now running Cisco IOS softw are on the Cat alyst® switch platforms, C isco w ill soon
support IEEE 802.1p for differentiated QoS, and Subnet Bandwidth Manager (SBM) for RSVP signaling of guaranteed QoS on
sw itched internetw orks. Using a feat ure called SNA type of service (ToS), Cisco IOS soft w are also integra tes na tive SNA class of
service to provide the QoS required by mission-critical mainframe applications across IP-routed networks. All of these are
standa rds-based mechanisms to integrate Q oS functiona lity a cross heterogeneous netw orks; how ever, a s previously mentioned, IP
Precedence and RSVP are the tw o primary Qo S signaling methods for t he future.
IP Precedence Signals Differentiated QoS
IP Pr ecedence utilizes the three precedence bits in the IPv4 head er’s ToS fi eld to specify cla ss of service for each pa cket, as sho w n in
Figure 12. You can partition traffi c in up to six classes of service using IP P recedence (tw o o thers are reserved f or internal netw ork
use). The queuing technologies throughout the netwo rk can then use this signal to provide the appropriate expedited hand ling.
Figure 12 IP Precedence ToS Field
Users
Users
Applications
Applications
Client, Server
802.1p/ISL
Client, Server
Edge
Edge
Core
Core
Backbone
Access, Campus
Access, Campus
RSVP
ATM
Frame Relay
SNA/APPN Tag
IP Precedence
3Bits
IP Precedence
IPv4Packet
Data
ToS Byte
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 15/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 15 of 28
Features such as policy-based ro uting a nd C AR can b e used to set precedence ba sed on extended a ccess-list classifica tion. This
allow s considerable fl exibility f or precedence assignment, including assignment b y a pplication or user, or by d estination and source
subnet, a nd so on. Typically this functionality is deployed a s close to the edge of the netwo rk (or ad ministrat ive domain) as possible,
so that each subsequent network element can provide service based on the determined policy.
IP Precedence can also be set in the host or netwo rk client, a nd this signaling can be used optiona lly; how ever, this can b e
overridden by policy w ithin the netw ork.
IP Precedence enables service classes to be established using existing network queuing mechanisms (for example, WFQ or
WRED) with no changes to existing applications or complicated network req uirements. No te that this same approa ch is easily
extended to IPv6 using its priority fi eld.
RSVP Guarantees QoS
RSVP is an IETF Internet Standard (RFC 2205) protocol fo r allow ing an application to dyna mically reserve netw ork band w idth.
RSVP enables applications to req uest a specifi c Qo S for a d ata flo w, as show n in Figure 13. C isco’s implementation a lso allow s
RSVP to be initiated w ithin the netw ork using confi gured proxy RSVP. Using this capab ility, netwo rk managers can take ad vanta ge
of the benefits of RSVP in the network, even for no n-RSVP enabled applications and ho sts.
Figure 13 Resource ReSerVation Protocol
The press has written extensively abo ut RSVP as a solution for guara nteeing bandw idth fo r new m ultimedia a pplications;
how ever, RSVP’s applicability is much broa der than multimedia, as it is currently the only standa rd signaling protoco l designed to
guarantee network bandwidth from end to end for IP networks.
H osts and routers use RSVP to deliver QoS requests to the routers along the paths of the dat a stream and to ma intain router
and ho st state to provide the requested service, usually band w idth and latency. RSVP uses a mean da ta ra te, the largest amo unt of
dat a the router w ill keep in queue, and minimum Qo S to determine band w idth reservation.
WFQ or WRED acts as t he wo rkhorse for RSVP, setting up the packet classifica tion a nd scheduling required for the reserved
flo w s. Using WFQ, R SVP can deliver an Integrated Services G uarant eed Service. Using WRED, it ca n deliver a Co ntrolled Loadservice. WFQ continues to provide its adva ntageous handling of non reserved traffi c by expediting interactive traffi c and fairly
sharing the remaining bandw idth betw een high-band w idth flo w s, and WRED provides its commensurate adva ntages for non-RSVP
flow traffic. RSVP can be deployed in existing networks with a software upgrade.
HostClientHost
RSVP(Guaranteed Service)
End-to-End
High BW Flows
Interactive Flows
Reserved Flows W F Q u e u i n g
BestEffort/“Fair”
Guaranteed
Intel,Microsoft,
Sun,HP,SGI
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 16/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 16 of 28
Tag Switching Allows Flexible Traffic Engineering
Cisco’s Tag Sw itching feature conta ins the mechanisms to interoperate w ith and take ad vanta ge of bot h RSVP and IP Precedence
signaling. The Tag Switching header conta ins a three-bit fi eld t hat can be used as a traf fic prioritization signal. It can a lso be used
to ma p particular flo w s and classes of traf fi c along engineered Tag Sw itching paths to obta in the required Q oS through the Tag
Switching portion o f a netwo rk. The QoS capa bilities provided by Tag Sw itching, along w ith its general operation, is covered in
Tag Switching w hite papers and technical do cumentatio n.
Cisco’s QoS Policy, Management, and Accounting Capabilities
Cisco IO S softw are provides technologies that enable policy control, ma nagement, and a ccounting of the Q oS techniques described
in this document. The follo w ing sections provide an overview of th ese technolo gies.
QoS Policy Control
The QoS policy control architecture is being developed as a key initial piece of the CiscoAssure policy networking initiative. This
initiative leverages standa rds-based Q oS policy control prot ocols and mechanisms to implement Q oS policy from a single console
interfa ce. The CiscoAssure arch itecture is covered in deta il in CiscoAssure specifi c documenta tion . The focus in the follow ing
sections is on packet-level services required in the infrastructure for QoS policy to be implemented.
At the infrastructure level, packet classifi cation is a key capability fo r each policy technique that allow s the appropriate packets
traversing a network element or particular interface to be selected for QoS service. These packets can then be marked for the
appropriate IP Precedence in some cases, or identified a s an R SVP. Po licy control also req uires integration w ith underlying link layer
netwo rk technologies, or no n-IP proto cols.
SNA ToS
SNA ToS in conjunction w ith D ata Link Switching+ (DLSw+ ), allows ma pping of tra ditional SNA C lass-of-Service (Co S) into IP
D ifferentiated service. This feature takes advanta ge of bo th Q oS signaling and pieces of t he architecture. DLSW+ opens four TCP
sessions and maps each SNA ToS tra ffi c into a d ifferent session. Each session is mar ked by IP Precedence. Cisco’s congestion cont rol
technologies (custom queuing, priority q ueuing, and w eighted fa ir queuing) acts on these sessions to provide a band w idth guara ntee
or o ther improved ha ndling across an intra net, as show n in Figure 14. This provides a migrat ion path for t raditiona l SNA customers
onto an IP-based intranet, while preserving the performance characteristics expected of SNA.
Figure 14 SNA ToS
SNA Interact
Telnet
SNA Batch
FTP
25%
25%
25%
25%
NN
Data Center
Map SNA CoS
to TCP ToS
Queuing Technology in the Network
Provides QoS Service(Custom Queuing Shown Here)
NN
DLSw+
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 17/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 17 of 28
DLSW+ supports the following applications:
• LAN Network Manager (LNM)
• Native Service Point (NSP)
• Dow n Stream Physical Unit (DSPU)
• Advanced Peer-to-Peer Netw orking (APPN)
• Source-route-bridged FDDI LANs
Thus, traditiona l mainfra me-based, mission-critical applications can ta ke advant age of evolving IP intra nets and extra nets without
sacrificing the Qo S capabilities historically provided by SNA netw orking.
QoS Policy Setting with Policy-Based Routing (PBR)
Cisco IO S policy-based routing (PBR) allow s you t o classify traf fi c ba sed o n extended access list criteria, set IP P recedence bits, and
even route to specifi c traffi c-engineered pat hs that ma y be required to a llow a specifi c Qo S service through the netwo rk. By setting
precedence levels on incoming tra ffi c a nd using them in com bination w ith the q ueuing too ls described earlier in this paper, yo u can
create differentiated service. These tools provide you with powerful, simple, and flexible options for implementing QoS policies in
your network.
You can also set up PBR as a w ay t o route packets based on confi gured policies. Some applications or tra ffi c can benefit from
Qo S-specific ro uting-transferring stock records to a corporate o ffi ce (for example-on a higher-bandw idth, higher-cost link for a
short time), w hile transmitting routine application dat a such as e-mail over a low er-band w idth, low er-cost link. PBR can be used
to d irect packets to ta ke different pat hs than the path derived fro m the routing proto cols. It provides a mo re flexible mechanism for
routing pa ckets, complementing the existing mechanisms provided by routing prot ocols.
CAR Manages Access Bandwidth Policy and Performs Policing
Similar in some wa ys to PBR , the Co mmitted Access Rate (CAR) feature allow s you to classify and police traffi c on an incoming
interface. It also allow s specifica tion of policies for handling traffi c that exceeds a certain band w idth allocatio n. CAR looks at tra ffi c
received o n an interface, or a subset of t hat traf fic selected by access list criteria, compares its rate to a co nfi gured token bucket, a nd
then takes action ba sed o n the result (for example, drop or rew rite IP Precedence). CAR is currently available only in Cisco IO S
11.1cc, a special version designed initially for IP-only netw orks on t he 7200, and in distributed mode on VIP processors on the 7500
(see “ Cisco IOS Q oS Ca pabilities Ma trix” in the appendices for more details).
QoS Management
A variety of mechanisms described below, help control a nd ma nage Qo S in the netwo rk. In add ition, a number of the accounting
tools described in the next section play a key role in proactively managing and designing Q oS services in the network.
Netsys Network Connectivity and Performance Policies
Cisco Netsys Service-Level Management Suite 4.0 provides a policy-based service-level management solution that allows you to
define, monitor, and assess network connectivity, security, and performance policies, and to troubleshoot problems quickly. The
Cisco Netsys Service-Level Management Suite consists of three products: Cisco Netsys Connectivity Service Manager, Cisco Netsys
Perfor man ce Service M ana ger, and C isco Netsys LAN Service Ma nager. The Netsys Co nnectivity Too ls, the first in a series of
simulation-based planning and problem-solving prod ucts, assist netw ork ma nagers and ana lysts. They a lso assist w ith problem-
solving, design, and planning a ctivities focusing on netw ork connectivity, route, and fl ow ana lysis.
The Netsys Performa nce Baseliner and Perform ance Solver developed by Netsys Technologies, Inc., a re simulat ion-based
netwo rk modeling tools that assist netw ork ma nagers, analysts, and designers w ith performance-related problem solving and
planning functions.
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 18/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 18 of 28
Building upon the actual netw ork confi guration ca ptured and mo deled by the Netsys Connectivity Tools, the Performance
Baseliner and Solver a dd netwo rk application tra ffi c and performance analysis functions. With the Performance Tools, users can
create a network baseline from configuration and performance data, then analyze the interactions between traffic flow, topology,
routing para meters, and Cisco IO S features. They can also dia gnose and solve operational pro blems, test scenarios, tune the
network configuration for improved performance, and plan for incremental network changes.
QoS MIB SupportCisco IOS provides QoS MIB support through a queuing MIB, and through support of standard MIBs for RSVP and the IETF
Integrated Services MIB definitions.
QoS Accounting Tools
The Cisco IO S softw are offers NetFlow and Cisco Enterprise Accounting (CEA) to pro vide accounting capabilities.
Netflow Network Accounting and Statistics Application
NetFlow softw are identifies IP packet flo w s, performs effi cient stat istics collection, a ccelerates security fi ltering, a nd exports the
collected statistics to dow nstream collectors, all w hile minimizing router performa nce impact. The Cisco NetFlow family a lso
includes a set of mana gement utilities, the FlowC ollector a nd the Flow Analyzer, and Cisco a pplications, N etsys and Cisco Enterprise
Accounting, a ll designed to help your netwo rk operate more cost effectively through fl exible netw ork billing, planning, and
monitoring.
Cisco is also w orking w ith a number of pa rtners to deliver comprehensive solutions for NetFlow -based billing, planning and
monitoring. NetFlow provides a key building block for delivering ad vanced, Q oS-based services by providing comprehensive data
collection and export.
Cisco Enterprise Accounting
Solutions that help identify, monitor, and contro l network ba ndw idth costs and proa ctively mana ge netw ork usage are key to
toda y’s successful informa tion systems (IS) organiza tions. C isco Enterprise Accounting (CEA), a new family of netwo rk
mana gement softw are, delivers powerful, easy-to-use solutions that help mana ge and contro l your netw ork costs. CEA is an
indispensable tool for ma nagers who w ant to gain a b etter understanding of their network costs and ensure that their netw orks
perform at optimum levels.
A member of the C isco netw ork mana gement fa mily, C EA is designed to low er the overall cost of netw ork ow nership, detect
equipment problems, and provide valuab le information to IS staf f. It a llow s an enterprise to make informed decisions about thecosts of owning and operating a network.
Network QoS Application Examples
Cisco IO S provides QoS services for a w ide range of applications, from mission-critical to new b andw idth-intensive multimedia.
Cisco’s goal for QoS service is simple to deliver the right end-to-end QoS solutions for every important application, including
integrated voice, data , and video networking. C isco ha s already d elivered much of the technology required to achieve this vision.
As previously described, Q oS service spans enterprise, Internet-service-provider, a nd sma ll-business netw orks, a nd much of the
technolo gy to deliver QoS service is common betw een them. The examples below span all thr ee segments.
QoS for Mission-Critical Applications: Oracle Sales Database
One of the primary business requirements for QoS service is to provide priority service for mission-critical traffic. The following
example illustrates a possible application of Cisco IO S softw are Q oS technology for t his purpose.
In this example, a fi eld sales force for a pharmaceutical company needs timely access to t he sales data base. This is an
Oracle-based sales application primarily provided at sales office locations; see Figure 15. The sales application has been deployed
for several years; how ever, the grow th in traf fi c from Web applications on t he corporate intranet has started to reduce application
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 19/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 19 of 28
performa nce on the WAN links because of increasing congestion. C orpo rat e policy dictat es tha t the sales applica tion receive priority
treatment, as this a pplication directly impacts the corporate bott om line. It is also necessary to make sure that intra net users also
get service from on t he WAN, but t hey have a low er importance.
Figure 15 Sales Database Application
To implement this policy, the compa ny uses WFQ w ith IP Precedence signaling t o ensure tha t da ta t o the sales applicatio n
receives the netw ork r esource it needs, w hile ensuring t hat oth er WAN users also receive service, albeit slightly less timely if t he sales
application is being used heavily.
Figure 15 illustrates the bra nch router confi gured to recognize high-priority tra ffi c destined for the sales application server over
a Frame Relay netw ork. At each branch, PBR is confi gured to recognize traffi c destined for the sales application. At headqua rters,
the campus router has PBR confi gured to recognize the return traffi c. All routers use WFQ to provide needed bandw idth for this
traffi c and a llow it to reach its destination ahead of other traffic on the network.
HQ
Policy-BasedRouting: Sets
IP Precedencefor SQL-Net
Traffic
WFQ:Gives Critical Traffic Expedited
Handling onto WANBranchOffice
4700
BranchOffice
4700 7505Sales
ApplicationServer:Oracle
Database
CorporateFrame Relay
Network
128-kbpsFR-Link
128-kbpsFR-Link
T1FR-Link
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 20/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 20 of 28
QoS for Mission-Critical Applications: IBM SNA Order-Entry Application
Ma inframe-based order-entry applications are w idely used in many industries. One key requirement fo r ma ny of these
applica tions is timely, predicab le user response time. Trad itiona lly, end-to-end SNA services have been used t o crea te such netw orks;
how ever, the same level of priority service can now be provided for these traditiona l applications on IP-based netwo rks.
Using SNA ToS w ith q ueuing features in C isco IO S softw are, da ta center applications that have trad itionally run on SNA can
now be migrated to IP netw orks, w hile still preserving the reliability a nd service levels that are ava ilable through nat ive SNA.
Figure 16 SNA ToS for an Intranet
In Figure 16, C Q is used to ensure that appropriate netw ork resources are allocat ed for the reservation system, which ha s
transitioned from a leased-line network t o a new IP-based netw ork. The new netwo rk also allow s other applications to share the
same infrastructure without impacting the reservation system.
SNA ToS leveraging D LSw+ is configured on the C isco routers, so different types of SNA tra ffi c such as ba tch or interactive
can be kept strictly separate through the IP netwo rk. CQ is confi gured to provided 60 percent of the link bandw idth for t he
important reservation tra ffi c, leaving the remaining 40 percent fo r other uses. In a ddition, FRTS can be used at the mainfra me site
to constrain tra ffi c on va rious DLC s to remote locat ions. In this w ay, far-end access links, (in this case 64-kbps links), a re not
overloaded by a b urst from some other application tra nsmitting on the T1 link from the mainfra me site.
2505
2505
64-kbps FR-Links
2505
7513
HQ
IBM 39XX
ReservationAgents
CQ Allocates BandwidthBased on Configured
Requirements
SNA ToS MapsSNA CoS to Separate
TCP Sessions throughDLSw+Software
Frame Relay Traffic Shaping:Avoids Violating FR ServiceContract, Prevents Far-End
FR Congestion
SNA-BasedReservation Entry
System
T1 FR-Link
Frame RelayService
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 21/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 21 of 28
QoS for Packetized Voice
One of the most promising uses for IP netw orks is to a llow sharing of vo ice traffi c with the trad itional da ta a nd LAN-to-LAN tra ffi c.
Typically, this can help reduce tran smission costs by reducing the number of netw ork conn ections, sharing existing conn ections and
infrastructure, and so on.
Cisco ha s a w ide range of voice netw orking products a nd t echnologies, including a number of Voice on IP (VoIP) solutions. To
provide the required voice quality, how ever, Q oS capability must be added to the trad itional da ta-only network. C isco IO S Softw are
Qo S features give VoIP tra ffi c the service it needs, w hile providing the trad itional da ta t raffi c w ith the service it needs as w ell.
Figure 17 QoS VoIP Solution
Figure 17 shows a business that ha s chosen to reduce some of its voice costs by combining voice traffi c onto its existing IP
netwo rk. Voice traffi c at each offi ce is digitized on voice modules on 3600 processors. This traffi c is then routed via H .323
G atekeeper, w hich also requests specific Qo S for the voice traffi c. In this case, IP Precedence is set to H igh for the voice traffi c. WFQ
is enabled on a ll the router interfaces for this netw ork. WFQ autom atically expedites the forw arding o f high precedence voice traffi c
out each interface, reducing delay and jitter for this traffi c.
Since the IP netwo rk wa s originally handling LAN-to-LAN traffi c, many da tagra ms traversing the netw ork are large 1500 byte
packets. O n slow links (below T1/E1 speeds), voice packets may be fo rced to w ait b ehind one of th ese large packets, a dd ing tens
or even hundreds of milliseconds to the delay. LFI is used in conjunction w ith WFQ to b reak up these “ jumbograms” and interleave
the voice traffi c to reduce this delay as w ell as jitter.
3620
KeySystem
3620
3640
PBX
Link Fragmentationand Interleavingbreakes up large
LAN-LAN datagramsto reduce voice jitter
WFQ withPrecedence
signaling used toprovide lower latency
and jitter for voiceservice
Basic RateISDN Primary Rate
ISDN
PSTN
KeySystem
Shared DataVoice Network
Site B
Site C
Site A
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 22/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 22 of 28
QoS for Streaming Video
O ne of the mo st signifi cant cha llenges for IP-based netw orks, w hich have trad itionally provided o nly best-effort service, has been
to provide some type of service guarantees for d ifferent types of t raffi c. This has been a particular challenge for streaming video
applications that o ften require a significant amount of reserved bandw idth to b e useful.
In the netw ork show n below, RSVP is used in conjunction w ith ATM PVCs to provide guara nteed ba ndw idth to a mesh of
locations. RSVP is config ured from w ithin Cisco IOS to provide paths from the router netw orks, at t he edges, and t hrough the ATM
core. Simulation traffi c then uses these guaranteed paths t o meet the constraints o f geogra phically distributed real-time simulation.
Video-enabled ma chines at t he various sites also use this netw ork to d o live video conferencing.
Figure 18
In this instance, OC-3 ATM links are configured with multiple 3 Mbps PVCs connecting to various remote sites. RSVP ensures
that Qo S from this PVC is extended to the appropriate application a cross the local routed netw ork. In the future, C isco IO S will
extend this RSVP capability to dyna mically set up ATM SVCs. This will reduce confi guration complexity a nd a dd a g reat degree of
automatic configuration.
QoS Looking Forward
In a continued evolution to w ards end-to-end Qo S services, Cisco is expanding Q oS interwo rking to o perate more seamlessly across
heterogeneous link layer technologies, a nd w orking very closely w ith our ho st platform partners to ensure interoperation between
netwo rks and end systems.
7513
4707
4707
ATM PVCMesh
OC-3Access
Site B
Site C
Site A
LightStream
1010
OC-3Access
RSVP used tocreate 3Mbps
guaranteed pathsto other sites
Guaranteed Pathscreated over both
routed and switched fabrics
OC-3Access
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 23/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 23 of 28
ATM QoS Interworking
ATM is a perva sive ba ckbone technolo gy, both in la rge-scale service provider netw orks and in campus LAN ba ckbones. Thus, one
major a rea for Q oS interworking is at the IP-to-ATM bounda ry.
IP-to-ATM QoS Interworking for Differentiated Service
Cisco IO S softw are is being upgraded to ensure that IP D ifferentiated services, using IP Precedence signaling, are a utomat ically
preserved across ATM PVCs in backbone networks. These enhancements will take advantage of new capabilities in the next
generation of ATM interface cards and will provide contiguous classes of service across both routed and ATM infrastructures.
RSVP-to-ATM Interworking for Guaranteed Service
Ca pabilities are also being add ed so that RSVP signaling can set up ATM SVCs of the appropriat e parameters to dyna mically reserve
bandwidth across routed networks with ATM backbones.
Switched LAN QoS Interworking
Ca mpus netw orks are increasingly migrating to switched LAN technology, and increasingly the underlying technology is based on
Ethernet, w hether tra ditional switched 10 M pbs, Fast-Ethernet o r G ig-Ethernet. Although these netw orks are of ten lightly load ed,
Qo S will be an increasingly important capa bility, especially on heavily loaded uplinks and other key congestion points. Q oS w ill
become even more important a s new netw orked multimedia applications get deployed for normal business activities.
LAN QoS Interworking for Differentiated ServiceCisco IO S softw are w ill support IP Diff erentiated services by providing a mapping from IP Precedence signaling to the IEEE’s 802.1p
frame prioritization standa rd. This will allow differentiated services to b e mapped seamlessly a cross the data-link layer technology
on the cam pus and o nto t he WAN netwo rk to provide end-end Q oS services.
LAN QoS Interworking for Guaranteed Service
Cisco IOS softw are w ill also support the IETF’s Subnet Bandw idth M ana ger (SBM), currently an Internet Dra ft. SBM w ill extend
RSVP capabilities to campus sw itched netw orks by enabling campus switches with na tive RSVP capa bilities. This will allow RSVP
reservations to be extended from the host, through the campus network, and over the WAN to provide true end-to-end reservations
for a pplications tha t require reserved bandw idth.
Expanded Host/Client Support for RSVP
The major host and server providers in the industry have developed client-side RSVP support for their platforms: Microsoft for NT
version 5.0, Sun for Solaris, and H ewlett Packa rd for H P-UX . Cisco has performed interoperability testing w ith these
implementations, and Cisco netw orks support host-initiated R SVP requests from Q oS-aw are applications. In ad dition, M icrosoft
is shipping RSVP as part of the Winsock API, a nd f uture releases of a pplications such as M icrosoft’s NetShow and NetM eeting w ill
be RSVP enabled.
QoS Policy Networking
Cisco has lead the IETF effort t o standa rdize the first Qo S policy protocol, currently in Internet Dra ft status and called Commo n
Open Policy Service (COPS). Cisco IOS software will add COPS support as this work moves toward standardization. In addition,
as part of the CiscoAssure initiative, QoS po licy will be coord inated w ith security policy, na me services, and so on, in coordina ted
netwo rk policy a dministration services.
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 24/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 24 of 28
Appendix 1: QoS Features Matrices
Table 1 Cisco IOS QoS Capabilities Matrix
1. FIFO is the default queuing mode when an interface does not support fancy queuing techniques (e.g. X.25, GRE Tunnel, LAPB, or PPP Compressed). FIFO can be configured usingthe no-fair-queue command.
2. For interfaces that support fancy queuing, WFQ is on by default on WAN interfaces at E1 or lower speeds, including PPP, HDLC, FR, SMDS, and so on, and is inactive on LANsand high-speed WAN lines.
3. Distributed WFQ and WRED are performance-enhanced versions that run on VIP processors on the 7500 platform.
4. G TS supports subinterfaces on FR links and softw are interfaces on LAN or PPP links. It do es not support point-multipoint FR D LCIs, but do es support multipointlinks in general. GTS works w ith any o utput q ueuing.
5. Uses CQ, PQ, WFQ, WRED, or FIFO as output queuing method.
6. Extended Access Lists allow a more flexible way to characterize and classify traffic than was previously available for PQ and CQ (for example, by source and destination address,source and destination port, and so on).
7. FRTS supports multipoint FR subinterfaces and can work with FIFO, PQ, or CQ, but not WFQ.
Feature Function Classification by: QoS Signaling Protocol Support
Interfaces
Supported
VC / SubInterface
SupportFIFO Queuing1 Congestion
Management— — Multiprotocol All All
Priority Queuing(PQ)
CongestionManagement
• Protocol
• Source interface
— Multiprotocol Output – Ethernet, FR,ATM, SMDS, Serial
Per VC orSubinterface in 11.2
Custom Queuing(CQ)
CongestionManagement
• Protocol
• Source interface
— Multiprotocol Output – Ethernet, FR,ATM, SMDS, Serial
Per VC orSubinterface in 11.2
Weighted FairQueuing (WFQ)2 andDWFQ3
CongestionManagement
• Flow IP Precedence, RSVP Multiprotocol Output – Ethernet, FR,ATM, SMDS, Serial
Per VC orSubinterface in 11.2
— • Flow or Class IP Precedence IP VIP w/ SONET –Output
Per Interface in 11.1CC
Weighted RandomEarly Detection(WRED) andDWRED3
Congestion Avoidance • Class IP Precedence, RSVP Multiprotocol, butbuilt for TCP/IP
Output – Ethernet, FR,ATM, Serial
—
— • Class IP Precedence IP Output – VIP w/
SONET
Per Interface in 11.1CC
Generic TrafficShaping (GTS)4,5
CongestionManagement/Shaping
Extended Access List6 IP Precedence, RSVP Multiprotocol Output – Ethernet, FR,ATM, Serial
Per Subinterface, butnot per VC or DLCI
Frame Relay TrafficShaping (FRTS)7
CongestionManagement/Shaping
Extended Access List6 — Multiprotocol Output – Frame Relay Per DLCI, PQ and CQonly; Not onSubinterface
Link Fragmentationand Interleaving(LFI)
Link Efficiency Packet/Frame Size — IP, Multilink PPP Output – (requiresWFQ)
—
RTP HeaderCompression(RTP-HC)5
Link Efficiency Multimedia/RTPTraffic
— Ignores Non-RTPPackets
FR, HDLC or PPP —
SNA Type of Service5 Classification/PolicySetting
SNA CoS IP Precedence IP -DLSw+ Output–Ethernet, FR,ATM, Serial
—
Policy Based Routing(PBR)
Classification / PolicySetting
Extended Access List6 Sets IP Precedence orToS
IP Input/Output AnyInterface, Output, Input
Per Subinterface.
Committed AccessRate (CAR)
Classification/ CongestionManagement/PolicySetting
Extended Access List6 Sets IP Precedence IP Input/Output VIP w/ SONET–Output, Input
BGP PolicyPropagation
Policy IP Precedence Sets IP Prec. in ReverseFlow Direction
IP, Requires BGPConnectivity
All —
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 25/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 25 of 28
Table 2 IOS QoS Availability Matrix.
1. Switching Mode: P=Process, F=Fast, N=NetFlow, O=Optimum, CEF=Cisco Express Forwarding, d=Distributed (VIP), dCEF=Distributed CEF.
2. FIFO is the default queuing mode when an interface does not support fancy queuing techniques (e.g. X.25, GRE Tunnel, LAPB, or PPP Compressed). FIFO can be configuredusing the no-fair-queue command.
3. Fancy queuing modes are not supported for X.25 in 11.2.
4. For interfaces that support fancy queuing, WFQ is on by default on WAN interfaces at E1 or lower speeds, including PPP, HDLC, FR, SMDS, and so on, and is inactive on LANsand high-speed WAN lines.
5. Distributed WFQ and WRED are performance-enhanced versions that run on VIP processors on the 7500 platform.
6. GTS supports subinterfaces on FR links and software interfaces on LAN or PPP links. It does not support point-multipoint FR DLCIs, but does support multipoint links in general.GTS works with any output queuing.
7. Uses CQ, PQ, WFQ, WRED, or FIFO as output queuing method.
8. FRTS supports multipoint FR subinterfaces and can work with FIFO, PQ, or CQ, but not WFQ.
Feature Cisco IOS Version and Switching Mode1
Platform
Support
Rule of ThumbMaxAggregateThruput on
720010.3 11.0 11.1 11.2 11.1CC 11.3 — —
FIFO Queuing2,
3P,F,N,O P, F, N, O P, F, N, O P, F, N, O P, F, N, O, dCEF P, F, N, O All Cisco IOS
PlatformsInterface Speed
PriorityQueuing (PQ)
P P, F, N, O P, F, N, O P, F, N, O P, F, N, O P, F, N, O All Cisco IOSPlatforms
10 Mbps
CustomQueuing (CQ)
P P, F, N, O P, F, N, O P, F, N, O P, F, N, O P, F, N, O All Cisco IOSPlatforms
10 Mbps
Weighted FairQueuing (WFQ)4
— P, F, N, O P, F, N, O P, F, N, O P, F, N, O P, F, N, O All Cisco IOSPlatforms
10 Mbps
DWFQ5 — — — — dCEF — 7500 VIP T3-VIP2/40OC3-VIP2/504
WeightedRandom EarlyDetection(WRED)
— — — P, F, N, O — P, F, N, O All Cisco IOSPlatforms
20 Mbps
DWRED5 — — — — dCEF — 7500 VIP T3-VIP2/40OC3-VIP2/50
Generic TrafficShaping (GTS)6,
7
— — — P, F — P, F All Cisco IOSPlatforms
10 Mbps
Frame RelayTraffic Shaping(FRTS)8
— — — P, F 11.2(9) — P, F All Cisco IOSPlatforms
10 Mbps
LinkFragmentationand Interleaving(LFI)
— — — — — P All Cisco IOSPlatforms
Intended forSub- T1 speeds
RTP HeaderCompression(RTP-HC)7
— — — P — P All Cisco IOSPlatforms
10 Mbps
SNA Type of Service7
— — — — — P, F All Cisco IOSPlatforms
—
Policy BasedRouting (PBR)
— — — P, F11.2(9) P P, F All Cisco IOSPlatforms
10 Mbps
CommittedAccess Rate(CAR)
— — — — dCEF, F, CEF — 7500 VIP or RSP,7200
T3/E3 perVIP2-40 OC3 perVIP2-50
BGP PolicyPropagation
— — — — dCEF, CEF — 7500 VIP or RSP,7200
N/A
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 26/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 26 of 28
Appendix 2: QoS Terminology
Quality of Service Definitions
The term Q oS is an umbrella for all related technology a nd off erings in this area. Thus, all of the definitions that follow below a re
subsets of Q oS. In the past, Qo S has sometimes had more specific conno tations fo r particular t echnologies, such as ATM, but the
term is now used more bro adly to refer to a netwo rk’s ability to provide better service to selected network t raffi c for va rious
technolo gies, including IP rout ed netw orks, Frame-Relay, ATM , Ethernet, and 802.1 netw orks, SON ET, and so on .
End-to-End (or Edge-to-Edge) QoS Service Levels
Service levels refer to t he actual Q oS capab ilities, meaning the ability o f a netwo rk to deliver service needed by a specific netw ork
application f rom end-to-end. This can a lso include edge-to-edge, a s in the case of a netwo rk tha t connects other netw orks rather
than hosts or end systems, (the typical service provider netwo rk, for example), w ith some level of control over band w idth, jitter,
delay, and loss, provided by t he netw ork.
QoS Strictness
The “ strictness” of the Qo S service describes how tightly the service can be bound by specifi c band w idth, d elay, jitter, a nd loss
characteristics. For example, the delay, loss, and jitter chara cteristics can be o ffered to w ithin tight to lerances on a terrestrial TD M
circuit, or for an ATM Variable Bit Ra te Real-Time (VBR-rt), or Co nstant Bit Ra te (CBR) service; w hereas they a re much harder to
bound on a typical Internet IP connection. Essentially, QoS can provide three levels of strictness from end-to-end or edge-to-edge:best effort, d ifferentiated, a nd guara nteed.
Best-Effort Service
Also know n a s lack of Qo S, best-effort service is basic connectivity w ith no guarant ees. The Internet to day is a go od example of
best-effort service. Altho ugh best effort service is the lack of Q oS, it provid es us with a reference point o n the nonstrict end of t he
spectrum (see Strictness of Q oS Service Level G uara ntee, abo ve). Also, best effort is suita ble for a w ide range of netw orked
applications such as general file transfers or e-mail.
Differentiated Service (also called “Soft” QoS)
Some traffi c is treated better tha n the rest (faster handling, more band w idth on average, low er loss rate on a verage). This is a
statistical preference, no t a hard and fast guarant ee. With proper engineering, including edge policing, differentiated service can
provide expedited hand ling appropriate for a w ide class of applications, including lower delay for mission-critical applications,packet voice applicat ions, and so o n. Typically, differentia ted service is associa ted w ith a course level of tr af fi c classificat ion (see
Packet Cla ssifica tion), which means that t raffi c gets grouped or a ggregated into a small number of classes, with each class receiving
a pa rticular Qo S in the netw ork. H ow ever, this does not have to be the case, as classification g ranularity is an independent issue.
The Differentiated Services (DiffServ) working group in the IETF is working on specific standards and definitions for services
that fall under the broad umbrella of D ifferentiated or Soft Qo S as defined abo ve. This effort is largely focused on t he use of the
ToS field in IPv4 header as a Qo S signaling mechanism, and it a ims to provide defi nitions appropriate for aggregated fl ow s for any
level of a ggregation. At least tw o services have initially been defined under this effort: The “ Assured Service” and the “ Preferred
Service,” each w ith slightly different d efinitions of service that from a t echnical perspective might be called “ engineered best effort.”
Technologies that ca n provide dif ferentiat ed service (by our previous defi nition ) for po rtion s of the end-to-end connection
include:
• IP-WRED, WFQ , combined with IP Precedence signaling or PQ on a single link• ATM-Unspecified Bit R ate (UBR) and Available Bit Ra te (ABR), especially if no Minimum C ell Rate (MCR ) can be specifi ed in
the implementation
• Frame prioritization in campus sw itches in conjunction w ith 802.1p signaling
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 27/28
Copyright © 1998 Cisco Systems, Inc. All Rights Reserved.Page 27 of 28
Guaranteed Service (also called “Hard” QoS or “reserved bandwidth”)
G uarant eed service is an a bsolute reservation of netwo rk resources, ty pically bandw idth, w hich implies reservation of b uffer space
along w ith the appropriat e queuing disciplines, a nd so o n, to ensure that specifi c traf fi c gets a specific service level. Ba ndw idth is
typically used a s a prox y fo r the other Q oS att ributes (delay, jitter, a nd so on), as the w idest a udience most easily understand s it.
Typically, guara nteed service is associated w ith a fi ne level of traffi c classifica tion (see Packet C lassifi cation below ), often dow n to
the level of individual tra ffi c flow s, which means that pa rticular flow s have netw ork resources reserved fo r them so that required
guarant ees can be met. H ow ever, this does not have to be the case, as aggregated fl ow s may receive guaranteed service.
The Integrat ed Services (IntServ) w orking gr oup in the IETF has developed specific standard s and defin itions fo r services that
fall under an umbrella definition o f G uarant eed or H ard Q oS provided above. This effort a ttempted to provide an a rchitecturally
sound w ay to specify fl ow s in the Internet w ith varying requirements. The primary services in use toda y a re Contro lled Loa d Service
and G uarant eed Service, each ha ving precise definitions in the context of t his work. RSVP w as developed as a Qo S signaling
mechanism to pro vide these types of fl ow -ba sed services.
Technologies tha t can pro vide guara nteed service for port ions of th e end-to-end con nection include:
• IP-WFQ combined with RSVP signaling or C Q on a single link.
• Ethernet-SBM (when used with a compliant switch)
• ATM-VBR and CBR
• FR -C IR
Packet Classification
Packet Cla ssifica tion orga nizes a packet into a group useful for QoS or o ther handling such as security or fi rewa lling, on the
network. Classification may be done over a range of granularities, from groups of aggregated flows to individual flows or even
subflows.
Typically, classification is done in a w ay similar to defining access lists, tha t is, ba sed on some contents of the packet header. In
this case, a packet ma y be classified by inf orma tion in th e L2, L3, or L4 headers (source/destinat ion ad dresses, port numb ers,
subarea ad dress, applications, user, a s well as various Lay er 2 attributes, such as D ata Link Connection Identifi er (DLC I) or ATM
Virtual Path Connection (VPC), and so on. Classification can also be done based on information within the packet payload, such
as is done with H .323 Proxy service, or CBAC Firew all functionality. Classificatio ns can broa d for aggregated fl ow s such as “ traffi c
destined for a subnetwo rk X,” or as narrow as a single flow or even subflow.
Policing and Shaping
Policing means limiting the amount of t raffi c flow ing into or out of a particular interface to a chieve a specifi c policy goal. Policing
typically refers to actions taken by the network to monitor and control traffic to protect network resources such as bandwidth
aga inst unintended o r ma licious behavior.
Traffi c shaping may be used t o a chieve policing goa ls as described herein, or t o d o congestion mana gement a s described below.
These activities are typically done at the edge of the network, w here it is used to cont rol the amount of t raffi c entering the
netwo rk from a particular ingress point (an administratively separat e netw ork or a ho st system).
Congestion Management (including Scheduling and Queuing)
Typically, this means dealing w ith congestion at multiplexing points in the network, including how to o rder or schedule traffi c,
leaving a congested interface to provide appropriate QoS for a flo w or set of aggregated fl ow s. Typically, some type of queuing
mechanism is used for congestion ma nagement. Traffi c shaping can also be considered a congestion ma nagement mechanism,depending on its part icular a pplication in t he netw ork.
8/9/2019 Cisco Press - Cisco IOS Quality of Service Implementation
http://slidepdf.com/reader/full/cisco-press-cisco-ios-quality-of-service-implementation 28/28
Cisco Systems has more than 200 offices in the following countries. Addresses, phone numbers, and fax numbers are listed on the
C i s c o C o n n e c t i o n O n l i n e W e b s i t e a t h t t p : / / w w w . c i s c o . c o m .
Argentina • Australia • Austria • Belgium • Brazil • Canada • Chile • China (PRC) • Colombia • Costa Rica • Czech Republic • Denmark
E l d F G G H I d I d I l d I l I l J K L b M l
Corporate Headquarters
Cisco Systems, Inc.170 West Tasm an D riveSan Jose, CA 95134-1706USAhtt p://w w w.cisco. comTel: 408 526-4000
800 553-NETS (6387)Fax: 408 526-4100
European Headquarters
Cisco Systems Europe s.a. r.l.Pa rc Evolic, Ba timent L1/L216 Avenue du QuebecVillebon, B P 70691961 Courtaboeuf CedexFrancehtt p://w w w -europ e.cisco.co mTel: 33 1 6918 61 00Fax: 33 1 6928 83 26
Americas
HeadquartersCisco Systems, Inc.170 West Tasma n D riveSan Jose, CA 95134-1706USAhtt p://w w w.cisco. comTel: 408 526-7660Fax: 408 527-0883
Asia Headquarters
Nihon Cisco Systems K.K.Fuji Building, 9t h Floor3-2-3 M aruno uchiCh iyoda -ku, Tokyo 100Japanhtt p://w w w.cisco. comTel: 81 3 5219 6250Fax: 81 3 5219 6001
Congestion Avoidance (also called Drop/Flow Control)
Co ngestion avo idance is the action a netw ork takes to avoid circumstances in w hich flow s or aggregated fl ow s no longer receive
their associated service levels due to excessive traffi c load s at points in the netw ork. This action could be a chieved through va rious
means, including constructive application of drop policy to provide implicit feedba ck to host systems to reduce network traffi c
during co ngestion.
QoS PolicyQo S policy is a set of a ctions a netwo rk takes to confi gure and signal for a pa rticular QoS service to be provided to a particular
traffi c classification.
QoS Signaling
QoS signaling is the means to deliver a QoS service requirement across the network. Either in-band signaling (for example, IP
Precedence or 802.1p) or out-of-band signaling (RSVP) is used to indicate that a particular QoS service is desired for a particular
traffi c classifica tion. IP Precedence and RSVP are the tw o most useful signaling mechanisms going forw ard b ecause they bot h take
advantage of the end-to-end nature of Layer 3 protocol and the growing ubiquity of IP as the network protocol of choice.