Differentiated services

Modern data networks carry many different types of services, including voice, video, streaming music, web pages and email. Many of the proposed QoS mechanisms that allowed these services to co-exist were both complex and failed to scale to meet the demands of the public Internet. In December 1998, the IETF published RFC 2474 - Definition of the Differentiated services field (DS field) in the IPv4 and IPv6 headers, which replaced the IPv4 TOS field with the DS field. In the DS field, a range of eight values (Class Selectors) is used for backward compatibility with the IP precedence specification in the former TOS field. Today, DiffServ has largely supplanted TOS and other layer-3 QoS mechanisms, such as integrated services (IntServ), as the primary architecture routers use to provide QoS.

DiffServ is a coarse-grained, class-based mechanism for traffic management. In contrast, IntServ is a fine-grained, flow-based mechanism. DiffServ relies on a mechanism to classify and mark packets as belonging to a specific class. DiffServ-aware routers implement per-hop behaviors (PHBs), which define the packet-forwarding properties associated with a class of traffic. Different PHBs may be defined to offer, for example, low-loss or low-latency service.

Rather than differentiating network traffic based on the requirements of an individual flow, DiffServ operates on the principle of traffic classification, placing each data packet into one of a limited number of traffic classes. Each router on the network is then configured to differentiate traffic based on its class. Each traffic class can be managed differently, ensuring preferential treatment for higher-priority traffic on the network. The premise of Diffserv is that complicated functions such as packet classification and policing can be carried out at the edge of the network by edge routers. Since no classification and policing is required in the core router, functionality there can then be kept simple. Core routers simply apply PHB treatment to packets based on their markings. PHB treatment is achieved by core routers using a combination of scheduling policy and queue management policy.

A group of routers that implement common, administratively defined DiffServ policies are referred to as a DiffServ domain.[2]

While DiffServ does recommend a standardized set of traffic classes,[3] the DiffServ architecture does not incorporate predetermined judgments of what types of traffic should be given priority treatment. DiffServ simply provides a framework to allow classification and differentiated treatment. The standard traffic classes (discussed below) serve to simplify interoperability between different networks and different vendors' equipment.

Network traffic entering a DiffServ domain is subjected to classification and conditioning. A traffic classifier may inspect many different parameters in incoming packets, such as source address, destination address or traffic type and assign individual packets to a specific traffic class. Traffic classifiers may honor any DiffServ markings in received packets or may elect to ignore or override those markings. For tight control over volumes and type of traffic in a given class, a network operator may choose not to honor markings at the ingress to the DiffServ domain. Traffic in each class may be further conditioned by subjecting the traffic to rate limiters, traffic policers or shapers.[4]

The per-hop behavior is determined by the DS field in the IP header. The DS field contains the 6-bit DSCP value.[5] Explicit Congestion Notification (ECN) occupies the least-significant 2 bits of the IPv4 TOS field and IPv6 traffic class (TC) field .[6][7][8]

In theory, a network could have up to 64 different traffic classes using the 64 available DSCP values. The DiffServ RFCs recommend, but do not require, certain encodings. This gives a network operator great flexibility in defining traffic classes. In practice, however, most networks use the following commonly defined per-hop behaviors:

• Default Forwarding (DF) PHB — which is typically best-effort traffic
• Expedited Forwarding (EF) PHB — dedicated to low-loss, low-latency traffic
• Assured Forwarding (AF) PHB — gives assurance of delivery under prescribed conditions
• Class Selector PHBs — which maintain backward compatibility with the IP precedence field.

Under DiffServ, all the policing and classifying are done at the boundaries between DiffServ domains. This means that in the core of the Internet, routers are unhindered by the complexities of collecting payment or enforcing agreements. That is, in contrast to IntServ, DiffServ requires no advance setup, no reservation, and no time-consuming end-to-end negotiation for each flow.

The details of how individual routers deal with the DS field are configuration specific, therefore it is difficult to predict end-to-end behavior. This is complicated further if a packet crosses two or more DiffServ domains before reaching its destination. From a commercial viewpoint, this means that it is impossible to sell different classes of end-to-end connectivity to end users, as one provider's Gold packet may be another's Bronze. DiffServ or any other IP based QoS marking does not ensure the quality of the service or a specified service-level agreement (SLA). By marking the packets, the sender indicates that it wants the packets to be treated as a specific service, but there is no guarantee this happens. It is up to all the service providers and their routers in the path to ensure that their policies will take care of the packets in an appropriate fashion.

A Bandwidth Broker in the framework of DiffServ is an agent that has some knowledge of an organization's priorities and policies and allocates bandwidth with respect to those policies.[9] In order to achieve an end-to-end allocation of resources across separate domains, the Bandwidth Broker managing a domain will have to communicate with its adjacent peers, which allows end-to-end services to be constructed out of purely bilateral agreements.

• RFC 2474 — Definition of the differentiated services field (DS field) in the IPv4 and IPv6 headers.
• RFC 2475 — An architecture for differentiated services.
• RFC 2597 — Assured forwarding PHB group.
• RFC 2983 — Differentiated services and tunnels.
• RFC 3086 — Definition of differentiated services per-domain behaviors and rules for their specification.
• RFC 3140 — Per hop behavior identification codes. (Obsoletes RFC 2836.)
• RFC 3246 — An expedited forwarding PHB. (Obsoletes RFC 2598.)
• RFC 3247 — Supplemental information for the new definition of the EF PHB (expedited forwarding per-hop behavior).
• RFC 3260 — New Terminology and Clarifications for Diffserv. (Updates RFC 2474, RFC 2475 and RFC 2597.)
• RFC 4594 — Configuration Guidelines for DiffServ Service Classes.
• RFC 5865 — A differentiated services code point (DSCP) for capacity-admitted traffic. (Updates RFC 4542 and RFC 4594.)
• RFC 8622 — A Lower-Effort Per-Hop Behavior (LE PHB) for Differentiated Services. (Updates RFC 4594 and RFC 8325, obsoletes RFC 3662.)

• Class of service
• Teletraffic engineering

• John Evans; Clarence Filsfils (2007). Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice. Morgan Kaufmann. ISBN 0-12-370549-5.
• Kalevi Kilkki (1999). Differentiated services for the Internet. Macmillan Technical Publishing. ISBN 1-57870-132-5.

• IETF DiffServ Working Group page
• Cisco Whitepaper — DiffServ-The Scalable End-to-End Quality of Service Model
• ACM SIGCOMM'09 paper-Modeling and Understanding End-to-End Class of Service Policies in Operational Networks: proposes a practical model for extracting DiffServ policies
• Cisco: Implementing Quality of Service Policies with DSCP