Table Of Contents
Cisco IOS Software:
The Differentiated Services Model (DiffServ)
The Challenge: Organizations delivering network-based services need powerful end-to-end solutions to effectively and predictably deliver the differing Quality-of-Service (QoS) requirements of voice, video, and data applications. Voice, for example, requires a small but assured amount of bandwidth, low delay, low jitter and low packet loss. A data application such as file transfer protocol (FTP) needs more bandwidth, but can tolerate the delay & jitter.
The Solution: Cisco IOS® software's DiffServ offers application-level QoS and traffic management in an architecture that incorporates mechanisms to control bandwidth, delay, jitter and packet loss. Cisco's Diffserv complements Cisco's IntServ offering by providing a more scalable architecture for end-to-end QoS. This scalability is achieved by the mechanisms controlling QoS at an aggregate level. Application traffic can be categorized into multiple classes (aggregates), with QoS parameters defined for each class. A typical arrangement would be to categorize traffic into premium, gold, silver, bronze, and best-effort classes.
Standards-based: Cisco IOS software's DiffServ is fully compliant with the Internet Engineering Task Force (IETF) standards defined in RFC 2474, RFC 2475, RFC 2597 and RFC 2598. Cisco's solution leverages the new IETF definition of the IPv4 Type of Service (ToS) octet in the IP packet-header by utilizing the Differentiated Services Code Point (DSCP) field to classify packets into any of the 64 possible classes. Once the packets are classified IETF-defined per-hop behaviors (PHBs) including assured forwarding (AF) and expedited forwarding (EF) are implemented using Cisco's QoS tool chest. Traffic that is characterized as EF will receive the lowest latency, jitter and assured bandwidth services which is suitable for applications such as VoIP. AF allows carving out the bandwidth between multiple classes in a network according to desired policies. As a value-add, Cisco's implementation also allows you construction of user-defined PHBs, beyond the scope of AF & EF. Thus, DSCP code points other than the ones reserved for AF, EF, and best effort service can be associated with an arbitrary PHB.
Cisco IOS Software: Quality-of-Service Applications
Service providers want to provide value-added services to their customers by providing blanket Service Level Agreements (SLAs), as well as application-specific assurances (aSLAs). They could, for example divide a customer's traffic at the network edge into Gold, Silver, and Bronze classes (also referred to as Olympic Service) and provide relative / absolute assurances to each. Cisco also provides for mapping the per-class IP QoS requirements into ATM CoS parameters, thus providing mechanisms both at the edge and the core. Within the Service Provider network, Cisco enables end-to-end QoS, via MPLS-Diffserv. MPLS could also be used as a reference mechanism to translate the IP QoS to MPLS QoS.
(b) Packet Marking. The IPv4 ToS (Type of Service), octet has been re-defined from the 3-bit IP-Precedence to a 6-bit DSCP field (Figure 1). Packets can be marked with an arbitrary DSCP value / standard values, corresponding to the appropriate AF (Figure 2), EF or user define class. For example, EF is designated by the code-point "101110". Cisco IOS also supports class-selector codepoints, which is a way of marking the 6 DSCP bits, that is compatible with systems that only support the IP-precedence scheme. These codepoints are of the form "xyz000", where x, y, and z can represent a 1 or 0. Last but not least, the codepoint for best-effort traffic will be set to "000000". Cisco's implementation brings additional value-add by also allowing you to mark packets with an arbitrary DSCP, and mapping them to a locally significant (non-AF/EF/default) PHB. This allows for construction of new, and previously un-thought of services.
Figure 1 DiffServ Codepoint Field
Figure 2 The Original IPv4 ToS Byte
Table 1 DiffServ AF Codepoint Table
(c) Traffic Conditioning. At the edge of the network, this component is logically responsible for classifying, marking, metering, and shaping or policing the packets entering the network. In the Cisco IOS Diffserv model, classification and marking are done using the MQC (Modular QoS CLI). Metering is done using a token bucket algorithm, shaping is done using GTS (Generic Traffic Shaping) or FRTS (Frame Relay Traffic Shaping), and policing is done using class-based CAR (Committed Access Rate). On the value add side, Cisco also provides for the Per-Class Accounting MIB, wherein statistics for each class (regardless of congestion) can be gleaned for management purposes.
(d) Policy/PHB Enforcing. As the packet leaves the Ingress router, and into the network core, PHBs are enforced, depending on the packet marking with the appropriate DSCP. EF can be implemented using LLQ (Low Latency Queueing). AFxy PHBs can be implemented using CBWFQ (Class Based Weighted Fair Queuing) and WRED (Weighted Random Early Detect), CAR, or GTS. Locally defined PHBs can also be constructed using the same tools—GTS, CAR, CBWFQ, and WRED.
Key Cisco IOS Diffserv Features and Benefits
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