Table Of Contents
Classifying VoIP Signaling and Media with DSCP for QoS
Supported Standards, MIBs, and RFCs
Verifying IP DSCP Configuration
Classifying VoIP Signaling and Media with DSCP for QoS
Feature History
12.2(2)XB
This feature was introduced.
12.2(2)T
This feature was integrated into Cisco IOS Release 12.2(2)T.
This document describes the Classifying VoIP Signaling and Media with DSCP for QoS feature in Cisco IOS Release 12.2(2)T. It includes the following sections:
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Supported Standards, MIBs, and RFCs
Feature Overview
The ip precedence command in dial-peer configuration mode, was originally designed to allow the prioritizing of H.323 traffic and the priority used, typically higher than that of IP data traffic. There was no means, however, for the end user to configure prioritization of H.245, H.225, and SIP signaling packets, which resulted in a delay when a call was set up over a congested network.
In order to provide finer tuning of priorities, the ip precedence command has been replaced by the ip qos dscp command. If a non zero value is specified for a particular type of traffic stream, this value is stored in the DSCP (Differentiated Services Code Point) before the gateway sends the packet out its WAN interface.
About Differentiated Services
Differentiated Services (DiffServ) describes a set of end-to-end QoS (Quality of Service) capabilities. End-to-end QoS is the ability of the network to deliver service required by specific network traffic from one end of the network to another. Cisco IOS QoS software supports three types of service models: best-effort services, Integrated Services (IntServ), and Differentiated Services.
Differentiated Services is a multiple service model that can satisfy differing QoS requirements. With Differentiated Services, the network tries to deliver a particular kind of service based on the QoS specified by each packet. This specification can occur in different ways, for example, using the 6-bit DSCP setting in IP packets or source and destination addresses. The network uses the QoS specification to classify, mark, shape, and police traffic, and to perform intelligent queueing.
Differentiated Services is used for several mission-critical applications and for providing end-to-end QoS. Typically, Differentiated Services is appropriate for aggregate flows because it performs a relatively coarse level of traffic classification.
DS Field Definition
A replacement header field, called the DS field, is defined by Differentiated Services. The DS field supersedes the existing definitions of the IPv4 type of service (ToS) octet (RFC 791) and the IPv6 traffic class octet. Six bits of the DS field are used as the DSCP to select the Per Hop Behavior (PHB) at each interface. A currently unused (CU) 2-bit field is reserved for explicit congestion notification (ECN). The value of the CU bits is ignored by DS-compliant interfaces when determining the PHB to apply to a received packet.
Per-Hop Behaviors
RFC 2475 defines PHB as the externally observable forwarding behavior applied at a DiffServ-compliant node to a DiffServ Behavior Aggregate (BA).
With the ability of the system to mark packets according to DSCP setting, collections of packets with the same DSCP setting and sent in a particular direction can be grouped into a BA. Packets from multiple sources or applications can belong to the same BA.
In other words, a PHB refers to the packet scheduling, queueing, policing, or shaping behavior of a node on any given packet belonging to a BA, as configured by a service level agreement (SLA) or a policy map.
The following sections describe the four available standard PHBs:
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Default PHB
The default PHB essentially specifies that a packet marked with a DSCP value of 000000 (recommended) receives the traditional best-effort service from a DS-compliant node (that is, a network node that complies with all of the core DiffServ requirements). Also, if a packet arrives at a DS-compliant node, and the DSCP value is not mapped to any other PHB, the packet will get mapped to the default PHB.
Class-Selector PHB
To preserve backward-compatibility with any IP Precedence scheme currently in use on the network, DiffServ has defined a DSCP value in the form xxx000, where x is either 0 or 1. These DSCP values are called Class-Selector Code Points. (The DSCP value for a packet with default PHB 000000 is also called the Class-Selector Code Point.)
The PHB associated with a Class-Selector Code Point is a Class-Selector PHB. These Class-Selector PHBs retain most of the forwarding behavior as nodes that implement IP Precedence-based classification and forwarding.
For example, packets with a DSCP value of 110000 (the equivalent of the IP Precedence-based value of 110) have preferential forwarding treatment (for scheduling, queueing, and so on), as compared to packets with a DSCP value of 100000 (the equivalent of the IP Precedence-based value of 100). These Class-Selector PHBs ensure that DS-compliant nodes can coexist with IP Precedence-based nodes.
Assured Forwarding PHB
Assured Forwarding PHB is nearly equivalent to Controlled Load Service available in the integrated services model. AFny PHB defines a method by which BAs can be given different forwarding assurances.
For example, network traffic can be divided into the following classes:
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Further, the AFny PHB defines four AF classes: AF1, AF2, AF3, and AF4. Each class is assigned a specific amount of buffer space and interface bandwidth, according to the SLA with the service provider or policy map.
Within each AF class, you can specify three drop precedence (dP) values: 1, 2, and 3. Assured Forwarding PHB can be expressed as shown in the following example: AFny. In this example, n represents the AF class number (1, 2, or 3) and y represents the dP value (1, 2, or 3) within the AFn class.
In instances of network traffic congestion, if packets in a particular AF class (for example, AF1) need to be dropped, packets in the AF1 class will be dropped according to the following guideline:
dP(AFny) >= dP(AFnz) >= dP(AFnx)
where dP (AFny) is the probability that packets of the AFny class will be dropped. In other words, y denotes the dP within an AFn class.
In the following example, packets in the AF13 class will be dropped before packets in the AF12 class, which in turn will be dropped before packets in the AF11 class:
dP(AF13) >= dP (AF12) >= dP(AF11)
The dP method penalizes traffic flows within a particular BA that exceed the assigned bandwidth. Packets on these offending flows could be re-marked by a policer to a higher drop precedence.
An AFx class can be denoted by the DSCP value, xyzab0, where xyz can be 001, 010, 011, or 100, and ab represents the dP value.
Table 1 lists the DSCP value and corresponding dP value for each AF PHB class.
Expedited Forwarding PHB
Resource Reservation Protocol (RSVP), a component of the integrated services model, provides a Guaranteed Bandwidth Service. Applications such as Voice over IP (VoIP), video, and online trading programs require this kind of robust service. The EF PHB, a key ingredient of DiffServ, supplies this kind of robust service by providing low loss, low latency, low jitter, and assured bandwidth service.
EF can be implemented using priority queueing (PQ), along with rate-limiting on the class (or BA). When implemented in a DiffServ network, EF PHB provides a virtual leased line, or premium service. For optimal efficiency, however, EF PHB should be reserved for only the most critical applications because, in instances of traffic congestion, it is not feasible to treat all or most traffic as high priority.
EF PHB is ideally suited for applications such as VoIP that require low bandwidth, guaranteed bandwidth, low delay, and low jitter.
Benefits
This DS-compliant CLI provides a means by which you can specify different priority levels for voice payload traffic and signaling traffic at the originating gateway.
Restrictions
The default DSCP for voice is ef codepoint 101110 (RFC 2598) and the default DSCP for signaling is af31 codepoint 011010 (RFC 2597). Preferred settings are required for each type of traffic stream.
Related Documents
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Supported Platforms
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Determining Platform Support Through Cisco Feature Navigator
Cisco IOS software is packaged in feature sets that support specific platforms. To get updated information regarding platform support for this feature, access Cisco Feature Navigator. Cisco Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature.
Cisco Feature Navigator is a web-based tool that enables you to determine which Cisco IOS software images support a specific set of features and which features are supported in a specific Cisco IOS image. You can search by feature or release. Under the release section, you can compare releases side by side to display both the features unique to each software release and the features in common.
To access Cisco Feature Navigator, you must have an account on Cisco.com. If you have forgotten or lost your account information, send a blank e-mail to cco-locksmith@cisco.com. An automatic check will verify that your e-mail address is registered with Cisco.com. If the check is successful, account details with a new random password will be e-mailed to you. Qualified users can establish an account on Cisco.com by following the directions at http://www.cisco.com/register.
Cisco Feature Navigator is updated regularly when major Cisco IOS software releases and technology releases occur. For the most current information, go to the Cisco Feature Navigator home page at the following URL:
Availability of Cisco IOS Software Images
Platform support for particular Cisco IOS software releases is dependent on the availability of the software images for those platforms. Software images for some platforms may be deferred, delayed, or changed without prior notice. For updated information about platform support and availability of software images for each Cisco IOS software release, refer to the online release notes or, if supported, Cisco Feature Navigator.
Supported Standards, MIBs, and RFCs
Standards
None
MIBs
None
To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:
http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml
RFCs
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Prerequisites
Cisco IOS Release 12.2(2)XB or 12.2(2)T or a later release must be running.
Configuration Tasks
See the following sections for configuration tasks for the Classifying VoIP Signaling and Media with Differentiated Services Code Point for QoS feature. Each task in the list is identified as either required or optional.
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Configuring IP DSCP
To configure IP DSCP precedence, use the following commands:
Verifying IP DSCP Configuration
To verify that IP DSCP is configured correctly, enter the show running-config privileged EXEC command to display the command settings for the router.
Configuration Examples
The following displays the screen output using the show running-config command.
Router# show running-configBuilding configuration...Current configuration :1494 bytes!version 12.2no service padservice timestamps debug uptimeservice timestamps log uptimeno service password-encryption!hostname!no logging bufferedenable password!spe 1/0 1/7firmware location bootflash:mica-modem-pw.2.7.1.0.bin!!resource-pool disable!ip subnet-zeroip domain-name cisco.comip name-server 172.16.0.0 ip name-server 172.16.1.1!!!!!!!!fax interface-type modemmta receive maximum-recipients 0!controller T1 0framing sfclock source line primarylinecode ami!controller T1 1framing sfclock source line secondary 1linecode ami!controller T1 2framing sflinecode ami!controller T1 3framing sflinecode ami!!!interface Ethernet0ip address 172.16.1.1 255.255.255.192no ip route-cacheno ip mroute-cache!interface FastEthernet0no ip addressno ip route-cacheno ip mroute-cacheshutdownduplex autospeed auto!interface Async1no ip addressno ip route-cacheno ip mroute-cache!interface Dialer0no ip addressno ip route-cacheno ip mroute-cache!interface Dialer2no ip addressno ip route-cacheno ip mroute-cache!ip classlessip route 0.0.0.0 172.19.169.65no ip http server!!!call rsvp-sync!mgcp!mgcp profile default!dial-peer voice 1 voipip qos dscp cs1 media!dial-peer voice 2 voip!!line con 0password #1writerline 1 48transport output lat pad telnet rlogin udptn v120 lapb-taline aux 0line vty 0 4password #login!endCommand Reference
This section documents the new ip qos dscp command. All other commands used with this feature are documented in the Cisco IOS Release 12.2 command reference publications.
ip qos dscp
To set the DSCP for the quality of service, use the ip qos dscp command in dial-peer configuration mode. To disable DSCP, use the no form of this command.
ip qos dscp [number | set-af | set-cs | default | ef] [media | signaling]
no ip qos dscp [number | set-af | set-cs | default | ef] [media | signaling]
Syntax Description
Defaults
DSCP is set to bit pattern 000000.
Command Modes
Dial-peer configuration
Command History
12.2(2)XB
This command was introduced. It replaced the ip precedence (dial-peer) command.
12.2(2)T
This command was integrated into Cisco IOS Release 12.2(2)T.
Usage Guidelines
To configure voice and signaling traffic priorities, use the ip qos dscp command.
Recommended values are ip qos dscp ef media and ip qos dscp af31 signaling.
Examples
The following example specifies that DSCP is set to precedence 1 and is applied to media payload packets.
dial-peer voice 1 voipip qos dscp cs1 mediaRelated Commands
Glossary
CLI—command-line interface. An interface that allows the user to interact with the operating system by entering commands and optional arguments.
DSCP—differentiated services code point. A 6-bit setting in IP packets or source and destination addresses.
IP—Internet Protocol. Network layer protocol in the TCP/IP stack offering a connectionless internetwork service. IP provides features for addressing, type-of-service specification, fragmentation and reassembly, and security. Defined in RFC 791.
QoS—quality of service. Measure of performance for a transmission system that reflects its transmission quality and service availability.
ToS—type of service. An indication of how an upper-layer protocol requires a lower-layer protocol to treat its messages.
VoIP—Voice over IP. The capability to carry normal telephony-style voice over an IP-based internet with POTS-like functionality, reliability, and voice quality. VoIP enables a router to carry voice traffic (for example, telephone calls and faxes) over an IP network. In VoIP, the DSP segments the voice signal into frames, which then are coupled in groups of two and stored in voice packets. These voice packets are transported using IP in compliance with ITU-T specification H.323.
WAN—wide-area network. Data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers. Frame Relay, SMDS, and X.25 are examples of WANs.