DiffServ service classes for network QoS

Service Quality

Service Classes for Network Control

Network control traffic refers to packet streams that are necessary for stable operation of the network and exchanging information between neighboring networks. Traffic between adjacent networks is transmitted via a shared peering point and is assessed against service-level agreements (SLAs). Network control traffic is usually different from the control data of the user application and is broken down into the Network Control and OAM service classes.

The Network Control service class is used to transfer packets between routers that need to exchange control (routing) information between nodes within the administrative domain and, via a peering point, between different administrative domains. The traffic transmitted in this service class must be forwarded in a timely manner, because this is fundamental to network operation.

You need to configure this class of service so that a minimum bandwidth is guaranteed for the traffic to ensure timely packet forwarding. The traffic characteristics of packet streams in this class of service are primarily the transmission of messages between routers and network servers, the predominant transmission of packets of variable size, and that normal traffic between users does not use this class of service.

The basic service of this class is based on an extended best-effort service with high-bandwidth assurance. Network Control can be used to transport both elastic and non-elastic data streams. You need to design the service so that AQM uses only CS6-tagged packets according to RFC 4594. If you define RED (according to RFC 2309) as the AQM algorithm, the minth threshold value specifies a target queue depth and the maxth threshold value defines the queue depth from which all data traffic is interrupted or Explicit Congestion Notification (ECN) marked.

The OAM class of service is for the following protocols: Simple Network Management (SNMP), Trivial File Transfer (TFTP), FTP, and Common Open Policy Service (COPS). These applications need low packet loss on the network but are relatively insensitive to delays. OAM should be configured so that the traffic has a minimum guaranteed bandwidth to ensure that packets are forwarded in a timely manner.

The OAM service class should be used for provisioning and configuring network elements, performance monitoring on the network, and all operational alerts on the network. Applications or IP endpoints need to tag their packets with the CS2 DSCP value up front. If the endpoint is not capable of doing this, the router that is topologically closest to the endpoint should handle Multifield (MF) classification (according to RFC 2475).

OAM is also based on an extended best-effort service with controlled data rates. The service needs to be designed such that CS2-tagged packet streams have sufficient bandwidth on the network, thus ensuring a high level of delivery reliability. If RED (according to RFC 2309) is used as the AQM algorithm, the minth threshold indicates a target queue depth and the maxth threshold indicates the queue depth above which all traffic is interrupted or ECN tagged.

Service Classes for Telephony and Video

User traffic is defined as the packet stream between different users and refers to the data traffic that is transmitted to or from terminal equipment. This is characterized by a very wide variety of applications and services. The first of these is the Telephony service class, which is intended for applications that transmit data in real time. It is also characterized by a very low delay, very low jitter, and very low packet loss for relatively constant data streams (Figure 3).

Figure 3: Effects of FIFO queuing: The queue delay causes jitter – for example, in VoIP data streams.

The Telephony service class needs to use the Expedited Forwarding (EF) PHB (according to RFC 3246) and should be configured to provide guaranteed forwarding resources so that all packets are forwarded quickly. Applications using this class of service include VoIP, data over IP such as modem or fax, T.38 fax over IP, connection emulation over IP, virtual lines, and IP virtual private network (VPN) services.

The Signaling service class supports delay-sensitive client-server applications (telephony) and peer-to-peer applications. This service class is designed for session control. Applications in this service class require a relatively fast response, because typically multiple messages are transmitted to control the sessions. You need to configure the signaling to respond quickly to intermittent and brief disturbances and to maintain the real-time nature of the data streams. Applications in this class of services include peer-to-peer IP telephony (e.g., SIP or H.323), peer-to-peer signaling for multimedia applications, the control function for peer-to-peer real-time streams, signaling for client-server IP telephony, control of IP television (IPTV) applications with protocols such as IGMP, and signaling of data flows between telephony call servers or soft switches with protocols such as SIP-T.

The Multimedia Conferencing service provides real-time services for rate-adaptive applications. Senders (sources) in this service class have the option of dynamically changing their transmission rates according to the feedback from the receiver. Typical video conferencing solutions negotiate the establishment of multimedia sessions with the SIP/H.323 protocol. When a user or endpoint starts a multimedia session, it is essential to check that the data rate of the new connection matches the available transmission resources.

Multimedia Conferencing needs to use the Assured Forwarding (AF) PHB and be configured to provide sufficient bandwidth for AF41, AF42, and AF43 packets to be forwarded securely. Typically interactive, time-sensitive, and business-critical applications require this class of service, including video conferencing applications, non-bursty data transfers between two application servers that require very low delay, and an IP VPN service that requires two data rates and a medium network delay that is slightly longer than the network propagation delay.

The Real-Time Interactive service class is available for variable data rate applications that require low loss and jitter and very low delay. Such services include, for example, a video conferencing application that is unable to change encoding rates or tag packets with different DSCP values. Typically, applications in this class of service are configured to negotiate the connection with a real-time transport (RTP)/User Datagram Protocol (UDP) control session. When a user or endpoint starts a real-time interactive session, it is important to check that the data rate of the new connection matches the transmission resources provided.

The service class uses the Class Selector (CS) PHB; it needs to provide a minimum bandwidth for CS4-tagged packets and guarantee that they are forwarded. Typical uses for this class of service are interactive games; video conferencing applications that do not require rate control or traffic content marking; IP VPN services that require a constant data rate and average network delay; and non-elastic, interactive, time-sensitive, and mission-critical applications that require very low delay.

The next class, Multimedia Streaming, enables near-real-time packet forwarding of elastic traffic sources that require a variable data rate and are not delay-sensitive. In general, multimedia streaming assumes that traffic is buffered at the source and destination. Therefore, this class of service is less sensitive to delay and jitter; it uses the Assured Forwarding (AF) PHB defined in RFC 2597. Applications that benefit from this class of service include buffered streaming audio and video (unicast), webcasts, and IP VPN services that support two data rates and are not sensitive to delay and jitter.

The Broadcast Video service class is recommended for applications requiring near-real-time packet forwarding with very low packet loss at a constant rate and non-elastic data sources at a variable rate. This service class assumes that the target endpoints have a de-jitter buffer, which is typically a two- to eight-video-frame buffer for video applications, which are therefore less sensitive to delay and jitter. This class of service uses the CS PHP and needs to be set up to provide high bandwidth for CS3-tagged packets and guarantee that they are forwarded. Applications that benefit from this class include video surveillance (unicast), video-on-demand (unicast), and streaming of live audio events (both unicast and multicast).

Service Classes for Data Streams

The Low-Latency Data service class is suitable for responsive, elastic data streams. It is typically used for client-server applications that have relatively fast response times and asymmetric bandwidth requirements. This communication mechanism is used, for example, when a user clicks on a hyperlink on a web page and thus loads a new web page.

This class of service is configured for good results with short-lived, real-time streams transmitted over TCP at variable data rates. The service class ensures that AF21, AF22, and AF23-tagged packet streams have a minimum network bandwidth to ensure high delivery reliability. Examples of applications that use low-latency data include client-server applications, Systems Network Architecture (SNA) terminals for hosting transactions, web-based transactions in e-commerce, or enterprise resource planning (ERP) applications.

The High-Throughput Data class is designed for elastic applications that require real-time packet transmission from the data source at a variable rate. These applications must be configured to provide good throughput for TCP data streams over a long period of time. This service class ensures the timely forwarding of AF11, AF12, and AF13-tagged packets. Typical applications in this environment are store and forward, file transfer, email, and VPN services that support two data rates (a fixed information rate and a higher data rate).

The Standard class of service is used for data streams that cannot be assigned to any other class of service and offers only best-effort forwarding behavior and a minimum bandwidth guarantee. The service class uses default forwarding PHB and is used by network services such as DNS, DHCP, BootP, and any undifferentiated packet streams that travel over the DiffServ-enabled network.

Last but not least, the Low-Priority Data service class serves applications that transmit according to the TCP protocol. This service class is specified in RFC 3662 and provides only a best-effort service without any bandwidth guarantees.

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