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Differentiated Services or DiffServ is a computer networking architecture that specifies a simple, scalable and coarse-grained mechanism for classifying, managing network traffic and providing Quality of Service ( QoS ) guarantees on modern IP networks. DiffServ can, for example, be used to provide low-latency, guaranteed service ( GS ) to critical network traffic such as voice or video while providing simple best-effort traffic guarantees to non-critical services such as web traffic or file transfers.
Background
Since 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 1998, the IETF published RFC 2475 (An Architecture for Differentiated Services). Today, DiffServ has largely supplanted other Layer 3 QoS mechanisms (such as IntServ) as the primary protocol routers use to provide different levels of service.
Traffic Management Mechanisms
DiffServ is a coarse-grained , class-based mechanism for traffic management. In contrast, IntServ is a fine-grained , flow-based mechanism.
DiffServ operates on the principle of traffic classification , where each data packet is placed into a limited number of traffic classes, rather than differentiating network traffic based on the requirements of an individual flow. Each router on the network is 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 DiffServ model does not incorporate premade judgements of what types of traffic should be given priority treatment; that is left up to the network operator. DiffServ simply provides a framework to allow classification and differentiated treatment. DiffServ does recommend a standardized set of traffic classes (discussed below) to make interoperability between different networks and different vendors' equipment simpler.
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, low-latency forwarding properties or best-effort forwarding properties. All the traffic flowing through a router that belongs to the same class is referred to as a Behavior Aggregate (BA).
DiffServ Domain
A group of routers that implement common, administratively defined DiffServ policies are referred to as a DiffServ Domain .
Classification and Marking
Network traffic entering a DiffServ domain is subjected to classification and conditioning. Traffic may be classified by many different parameters, such as source address, destination address or traffic type and assigned to a specific traffic class. Traffic classifiers may honor any DiffServ markings in received packets or may elect to ignore or override those markings. Because network operators want tight control over volumes and type of traffic in a given class, it is very rare that the network honors 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.
Per-Hop Behavior
The Per-Hop Behavior (PHB) is indicated by encoding a 6-bit value—called the Differentiated Services Code Point (DSCP)—into the 8-bit Differentiated Services (DS) field of the IP packet header. The DS field is the same as the TOS field, and ECN occupies the upper 2 bits.
In theory, a network could have up to 64 (i.e. 2 6 ) different traffic classes using different markings in the DSCP. 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 PHB—which is typically best-effort traffic
- Expedited Forwarding (EF) PHB—dedicated to low-loss, low-latency traffic
- Assured Forwarding (AF) PHB— which gives assurance of delivery under conditions
- Class Selector PHBs—which are defined to maintain backward compatibility with the IP Precedence field.
Default PHB
A default PHB is the only required behavior. Essentially, any traffic that does not meet the requirements of any of the other defined classes is placed in the default PHB. Typically, the default PHB has best-effort forwarding characteristics. The recommended DSCP for the default PHB is '000000' (in binary).
Expedited Forwarding (EF) PHB - DSCP=(46 D or 101110 B , 2E H )
The IETF defines Expedited Forwarding behavior in RFC 3246. The EF PHB has the characteristics of low delay, low loss and low jitter. These characteristics are suitable for voice, video and other realtime services. EF traffic is often given strict priority queuing above all other traffic classes. Because an overload of EF traffic will cause queuing delays and affect the jitter and delay tolerances within the class, EF traffic is often strictly controlled through admission control, policing and other mechanisms. Typical networks will limit EF traffic to no more than 30%—and often much less—of the capacity of a link.
Assured Forwarding (AF) PHB Group
The IETF defines the Assured Forwarding behavior in RFC 2597. Assured forwarding allows the operator to provide assurance of delivery as long as the traffic does not exceed some subscribed rate. Traffic that exceeds the subscription rate faces a higher probability of being dropped if congestion occurs.
The AF behavior group defines four separate AF classes. Within each class, packets are given a drop precedence (high, medium or low). The combination of classes and drop precedence yields twelve separate DSCP encodings from AF11 through AF43 (see table)
Some measure of priority and proportional fairness is defined between traffic in different classes. Should congestion occur between classes, the traffic in the higher class is given priority. Rather than using strict priority queueing, more balanced queue servicing algorithms such as fair queueing or weighted fair queuing are likely to be used. If congestion occurs within a class, the packets with the higher drop precedence are discarded first. To prevent issues associated with tail drop, the random early detection (RED) or weighted random early detection (WRED) algorithms are often used to drop packets.
Usually, traffic policing is required to encode drop precedence. Typically, all traffic assigned to a class is initially given a low drop precedence. As the traffic rate exceeds subscription thresholds, the policer will increase the drop precedence of packets that exceed the threshold.
Class Selector PHB
Prior to DiffServ, IP networks could use the Precedence field in the Type of Service (TOS) byte of the IP header to mark priority traffic. The TOS byte and IP precedence was not widely used. The IETF agreed to reuse the TOS byte as the DS field for DiffServ networks. In order to maintain backward compatibility with network devices that still use the Precedence field, DiffServ defines the Class Selector PHB.
The Class Selector codepoints are of the form 'xxx000'. The first three bits are the IP precedence bits. Each IP precedence value can be mapped into a DiffServ class. If a packet is received from a non-DiffServ aware router that used IP precedence markings, the DiffServ router can still understand the encoding as a Class Selector codepoint.
Advantages of DiffServ
One advantage of DiffServ is that all the policing and classifying is done at the boundaries between DiffServ clouds. This means that in the core of the Internet, routers can get on with doing the job of routing, and not care about the complexities of collecting payment or enforcing agreements. That is, DiffServ requires no advance setup, no reservation, and no time-consuming end-to-end negotiation for each flow, as with integrated services. This leads DS to be relatively easy to implement.
Disadvantages of DiffServ
End-to-end and peering problems
One disadvantage is that the details of how individual routers deal with the type of service field is somewhat arbitrary, and it is difficult to predict end-to-end behaviour. This is complicated further if a packet crosses two or more DiffServ clouds before reaching its destination.
From a commercial viewpoint, this is a major flaw, as it 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. Internet operators could fix this, by enforcing standardised policies across networks, but are not keen on adding new levels of complexity to their already complex peering agreements. One of the reasons for this is set out below.
Diffserv operation only works if the boundary hosts honour the policy agreed upon. However, this assumption is naive. A host can always tag its own traffic with a
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