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Quality of Service Design Overview

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Dec 17, 2004.

Chapter Description

This chapter provides an overview of the QoS design and deployment process. This process requires business-level objectives of the QoS implementation to be defined clearly and for the service-level requirements of applications to be assigned preferential or deferential treatment so that they can be analyzed.

QoS Requirements of Data

Hundreds of thousands of data applications exist on the Internet, in all shapes and sizes. Some are TCP, others are UDP; some are delay sensitive, others are not; some are bursty in nature, others are steady; some are lightweight, others are bandwidth hogs—the list goes on.

Data traffic characteristics vary from one application to another, as illustrated in Figure 2-4, which compares an enterprise resource planning (ERP) application (Oracle) with another (SAP).

Figure 4Figure 2-4 Data Application Differences

To make the matter even more complicated, it is crucial to recognize that, just as applications vary one from another, even the same application can vary significantly from one version to another.

A brief anecdote speaks to this point: After a southern California semiconductor company provisioned QoS for Voice and Mission-Critical Data (SAP R/3), everything went well for about six months. At that point, users began complaining of excessive delays in completing basic transactions. Operations that previously required a second or less to complete were taking significantly longer. The application teams blamed the networking teams, claiming that "QoS was broken." Further investigation produced the information in the following graph, shown in Figure 2-5.

Figure 5Figure 2-5 Data Application Version Differences

The Mission-Critical Data application—in this instance, SAP—had been upgraded from version 3.0F to 4.6C. As a result, a basic order-entry transaction required 35 times more traffic than the original version. Additional provisioning and policy tuning was required to accommodate the new version of the same application.

Given this reality, the question on how best to provision QoS for data is a daunting one. After wrestling with this question for several years, the authors of the QoS Baseline came up with four main classes of data traffic, according to their general networking characteristics and requirements. These classes are Best-Effort, Bulk Data, Transactional Data/Interactive Data and (Locally-Defined) Mission-Critical Data. Each of these classes is examined in more detail in the following sections.

Best-Effort Data

When addressing the QoS needs of Best-Effort traffic, the following guidelines are recommended:

  • Best-Effort traffic should be marked to DSCP 0.

  • Adequate bandwidth should be assigned to the Best-Effort class as a whole because the majority of applications default to this class. It is recommended to reserve at least 25 percent for Best-Effort traffic.

The Best-Effort class is the default class for all data traffic. Only if an application has been selected for preferential or deferential treatment is it removed from the default class.

In 2003, one Wall Street financial company did an extensive study to identify and categorize the number of different applications on its networks. It found more than 3000 discrete applications traversing its infrastructure. Further research has shown that this is not uncommon for larger enterprises. Therefore, because enterprises have several hundred—if not thousands of—data applications running over their networks (of which the majority default to the Best-Effort class), adequate bandwidth needs to be provisioned for this default class to handle the sheer volume of applications that are included in it. Otherwise, applications that default to this class easily are drowned out, typically resulting in an increased number of calls to the networking help desk from frustrated users. It is therefore recommended that at least 25 percent of a link's bandwidth be reserved for the default Best-Effort class.

Bulk Data

When addressing the QoS needs of Bulk Data traffic, the following guidelines are recommended:

  • Bulk Data traffic should be marked to DSCP AF11; excess Bulk Data traffic can be marked down by a policer to AF12 or AF13.

  • Bulk Data traffic should have a moderate bandwidth guarantee but should be constrained from dominating a link.

The Bulk Data class is intended for applications that are relatively noninteractive and not drop sensitive, and that typically span their operations over a long period of time as background occurrences. Such applications include FTP, e-mail, backup operations, database synchronizing or replicating operations, video content distribution, and any other type of application in which users typically cannot proceed because they are waiting for the completion of the operation (in other words, a background operation).

The advantage of provisioning moderate bandwidth guarantees to Bulk Data applications (instead of applying policers to them) is that Bulk Data applications dynamically can take advantage of unused bandwidth and thus can speed up their operations during nonpeak periods. This, in turn, reduces the likelihood that they will bleed into busy periods and absorb inordinate amounts of bandwidth for their non-time-sensitive operations.

Transactional Data/Interactive Data

When addressing the QoS needs of Transactional Data and Interactive Data traffic, the following guidelines are recommended:

  • Transactional Data traffic should be marked to DSCP AF21; excess Transactional Data traffic can be marked down by a policer to AF22 or AF23.

  • Transactional Data traffic should have an adequate bandwidth guarantee for the interactive, foreground operations that it supports.

The Transactional Data/Interactive Data class is a combination of two similar types of applications: Transactional Data client/server applications and interactive messaging applications. For the sake of simplicity, this class is referred to as Transactional Data only.

The response-time requirement separates Transactional Data client/server applications from generic client/server applications. For example, with Transactional Data client/server applications (such as SAP, PeopleSoft, and Oracle), the user waits for the operation to complete before proceeding (in other words, the transaction is a foreground operation). E-mail is not considered a Transactional Data client/server application because most e-mail operations happen in the background, and users usually do not notice even delays of several hundred milliseconds in mailspool operations.

Locally Defined Mission-Critical Data

When addressing the QoS needs of Locally-Defined Mission-Critical Data traffic, the following guidelines are recommended:

  • Locally-Defined Mission-Critical Data traffic should be marked to DSCP AF31; excess Mission-Critical Data traffic can be marked down by a policer to AF32 or AF33. However, Cisco IP Telephony equipment currently is using DSCP AF31 to mark Call-Signaling traffic; until all Cisco IPT products mark Call-Signaling to DSCP CS3, a temporary placeholder code point, DSCP 25, can be used to identify Locally-Defined Mission-Critical Data traffic.

  • Locally-Defined Mission-Critical Data traffic should have an adequate bandwidth guarantee for the interactive, foreground operations that it supports.

The Locally-Defined Mission-Critical class is probably the most misunderstood class specified in the QoS Baseline. Under the QoS Baseline model, all traffic classes (with the exclusion of Scavenger and Best-Effort) are considered "critical" to the enterprise. The term locally defined is used to underscore the purpose of this class: for each enterprise to have a premium class of service for a select subset of its Transactional Data applications that have the highest business priority for it.

For example, an enterprise might have properly provisioned Oracle, SAP, BEA, and Siebel within its Transactional Data class. However, the majority of its revenue might come from SAP, so it might want to give this Transactional Data application an even higher level of preference by assigning it to a dedicated class (such as the Locally-Defined Mission-Critical class).

Because the admission criteria for this class is nontechnical (being determined by business relevance and organizational objectives), the decision about which application(s) should be assigned to this special class easily can become an organizationally and politically charged debate. It is recommended to assign as few applications to this class (from the Transactional Data class) as possible. In addition, it is recommended that executive endorsement for application assignments to the Locally-Defined Mission-Critical class be obtained: The potential for QoS deployment derailment exists without such an endorsement.

For the sake of simplicity, this class is referred to simply as Mission-Critical Data.

Based on these definitions, Table 2-3 shows some applications and their generic networking characteristics, which determine what data application class they are best suited to.

Table 2-3 Data Applications by Class

Application Class

Example Applications

Application/Traffic Properties

Packet/Message Sizes



Telnet, Citrix, Oracle Thin-Clients, AOL Instant Messenger, Yahoo! Instant Messenger, PlaceWare (Conference), Netmeeting Whiteboard.

Highly interactive applications with tight user-feedback requirements.

Average message size < 100 bytes.

Max message size < 1 KB.



SAP, PeopleSoft—Vantive, Oracle—Financials, Internet Procurement, B2B, Supply Chain Management, Application Server, Oracle 8i Database, Ariba Buyer, I2, Siebel, E.piphany, Broadvision, IBM Bus 2 Bus, Microsoft SQL, BEA Systems, DLSw+.

Transactional applications typically use a client/server protocol model.

User-initiated, client-based queries are followed by server response. The query response can consist of many messages between client and server.

The query response can consist of many TCP and FTP sessions running simultaneously (for example, HTTP-based applications).

Depends on application; could be anywhere from 1 KB to 50 MB.



Database syncs, network-based backups, Lotus Notes, Microsoft Outlook, e-mail download (SMTP, POP3, IMAP, Exchange), video content distribution, large FTP file transfers.

Long file transfers.

Always invokes TCP congestion management.

Average message size 64 KB or greater.


PHB: Default

All noncritical traffic, HTTP web browsing, other miscellaneous traffic.



DLSw+ Considerations

Some enterprises support legacy IBM equipment that requires data-link switching plus (DLSw+) to operate across an enterprise environment.

In such cases, it is important to recognize that DLSw+ traffic, by default, is marked to IP Precedence 5 (DSCP CS5). This default marking could interfere with VoIP provisioning because both DSCP EF and DSCP CS5 share the same IP Precedence, 802.1Q/p CoS, and MPLS EXP value (of 5). Therefore, it is recommended to re-mark DLSw+ traffic away from this default value of IP Precedence 5.

Unfortunately, at the time of writing, Cisco IOS does not support marking DSCP values within the DLSw+ peering statements; it supports only the marking of type of service (ToS) values using the dlsw tos map command.


To explain this behavior from a historical perspective, when DLSw+ was developed, ToS was a term loosely used to refer to the first 3 bits (the IP Precedence bits) of the IP ToS byte, as defined in RFC 791. For many years, these were typically the only bits of the ToS byte in use (the others almost always were set to 0), so it seemed to make sense at the time.

However, with the development of newer standards defining the use of the first 6 bits of the IP ToS byte for DiffServ markings (RFC 2474) and the last 2 bits for IP explicit congestion notification (RFC 3168), using the term ToS to refer to only the first 3 bits (the IP Precedence bits) of the IP ToS byte has become increasingly inaccurate.

However, marking DLSw+ to an IP Precedence/class selector value could interfere with other QoS Baseline recommended markings. For example, if DLSw+ is marked to IPP 1, it would be treated as Scavenger traffic; if it is marked to IPP 2, it could interfere with Network-Management traffic; if it is marked to IPP 3, it could interfere with Call-Signaling; if it is marked to IPP 4, it would be treated as Streaming-Video traffic; and if it is marked to IPP 6 or 7, it could interfere with routing or Network Control traffic.

Therefore, a two-step workaround is recommended for marking DLSw+ traffic:

Step 1

Disable native DLSw+ ToS markings with the dlsw tos disable command.

Step 2

Identify DLSw+ traffic either with access lists (matching the DLSw+ TCP ports 1981 to 1983 or 2065) or with the match protocol dlsw command.

When DLSw+ traffic is identified, it can be marked as either Transactional (AF21) or Mission-Critical Data (DSCP 25), depending on the organization's preference.

4. QoS Requirements of the Control Plane | Next Section Previous Section

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