class should be treated independently and should have allocated bandwidth that is based on the QoS policy. There are three DSCP values assigned to each of the four AF classes as illustrated in Figure and the “DSCP Values Assigned to AF Classes” table. DSCP Values Assigned to AF Classes
AF Class Drop Probability DSCP Value AF Class 1 AF11 (low) 001 01 0 AF12 (medium) 001 10 0 AF13 (high) 001 11 0 AF Class 2 AF21 (low) 010 01 0 AF22 (medium) 010 10 0 AF23 (high) 010 11 0 AF Class 3 AF31 (low) 011 01 0 AF32 (medium) 011 10 0 AF33 (high) 011 11 0 AF Class 4 AF41 (low) 100 01 0 AF42 (medium) 100 10 0 AF43 (high) 100 11 0
Content 4.1 Introducing Classification and Marking 4.1.7 Standard PHB Groups Recall that a PHB is 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—each 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. The IETF defines a PBH group as a set of one or more PHBs that can only be meaningfully specified and implemented simultaneously, due to a common constraint applying to all PHBs in the set such as a queue servicing or queue management policy. In other words, a PHB group refers to the packet scheduling, queuing, 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 three standard PHBs are as follows: The default PHB specifies that a packet marked with a DSCP value of 000000 (recommended) receives best-effort service from a DiffServ-compliant node. If a packet arrives at a DiffServ-compliant node and the DSCP value is not mapped to any other PHB, the packet is mapped to the default PHB. The AF PHB defines four AF classes (n = 1–4: AF1, AF2, AF3, and AF4). Each class is assigned a specific amount of buffer space and interface bandwidth. It is allowed to obtain bandwidth from other AF classes, if bandwidth is available. IP packets using the AF PHB group are assigned by the customer or the provider DS domain into one or more of these AF classes according to the services to which the customer has subscribed. The EF PHB defines one class, which assigns a fixed amount of bandwidth only for that class.
Content 4.1 Introducing Classification and Marking 4.1.8 Mapping CoS to Network Layer QoS Figure shows how CoS is mapped to Layer 3 QoS. IP headers are preserved end to end when IP packets are transported across a network; data link layer headers are not preserved. This means that the IP layer is the most logical place to mark packets for end-to-end QoS. However, there are edge devices that can mark frames only at the data link layer, and there are many other network devices that operate only at the network layer. To provide true end-to-end QoS, the ability to map QoS markings between the data link layer and the network layer is essential. Enterprise networks typically consist of a number of remote sites connected to the headquarters campus via a WAN. Remote sites typically consist of a switched LAN, and the headquarters campus network is both routed and switched. Providing end-to-end QoS through such an environment requires that CoS markings that are set at the LAN edge are mapped into QoS markings (such as IP precedence or DSCP) for transit through Campus or WAN routers. Campus and WAN routers can also map the QoS markings to new data-link headers for transit across the LAN. With the mapping, QoS can be preserved and uniformly applied across the enterprise. Service providers offering IP services have a requirement to provide robust QoS solutions to their customers. The ability to map network layer QoS to link layer CoS allows these service providers to offer a complete end-to-end QoS solution that does not depend on any specific link layer technology. Compatibility between an MPLS transport layer and network layer QoS is also achieved by mapping between MPLS EXP bits and the IP precedence or DSCP bits. A service provider can map the customer network layer QoS marking as is or change it to fit an agreed-upon SLA. The information in the MPLS EXP bits can be carried end to end in the MPLS network, independent of the transport media. In addition, the network layer marking can remain unchanged so that when the packet leaves the service provider MPLS network, the original QoS markings remain intact. Thus, a service provider with an MPLS network can help provide a true end-to-end QoS solution.
Content 4.1 Introducing Classification and Marking 4.1.9 QoS Service Class Defined QoS service class is defined in Figure . When you create an administrative policy requiring QoS, you must determine how network traffic is to be treated. As part of that policy definition, network traffic must be associated with a specific service class. QoS classification mechanisms are used to separate traffic and identify packets as belonging to a specific service class. QoS marking mechanisms are used to tag each packet as belonging to the assigned service class. After the packets are identified as belonging to a specific service class, QoS mechanisms, such as policing, shaping, and queuing techniques, can be applied to each service class to meet the specifications of the administrative policy. Packets belonging to the same service class are given the same treatment for QoS. A QoS service class, being a logical grouping, can be defined in many ways: Figure is an example of how to define QoS service class. A network administrator wants to apply QoS to the corporate network to better control bandwidth allocation of different network applications. Before QoS can be applied, an administrative QoS policy is first defined as follows: As a result of this policy, three QoS service classes have been defined: The remaining 15 percent is for management, signaling, and routing. These percentages are not necessarily recommended values, but are simply representative of this particular example.
Content 4.1 Introducing Classification and Marking 4.1.10 Implementing QoS Policy Using a QoS Service Class Specifying an administrative policy for QoS requires that a specific set of service classes be defined. QoS mechanisms are uniformly applied to these individual service classes to meet the requirements of the administrative policy. Because the application of QoS mechanisms is applied to different service classes and used to differentiate among applications, users,