QoS Service Level Agreements SLAs
An SLA is a contractual agreement between an enterprise (customer) and a service provider regarding data, voice, and other service or a group of services. Internet access, leased line, Frame Relay, and ATM are examples of such services. After the SLA is negotiated, it is important that it is monitored for compliance of the parties involved with the terms of the agreement. The service provider must deliver services as per the qualities assured in the SLA, and the customer must submit traffic at the rates agreed upon, to receive the QoS level assured by the SLA. Some of the QoS parameters that are often explicitly negotiated and measured are delay, jitter, packet loss, throughput, and service availability. The vast popularity of IP Telephony, IP conferencing, e-learning, and other real-time applications has made QoS and SLA negotiation and compliance more important than ever.
Traditionally, enterprises obtained Layer 2 service from service providers. Virtual circuits (VC), such as permanent VCs, switched VCs, and soft PVCs, provided connectivity between remote customer sites, offering a variety of possible topologies such as hub and spoke, partial mesh, and full mesh. Point-to-point VCs have been the most popular type of circuits with point-to-point SLA assurances from the service provider. With Layer 2 services, the provider does not offer IP QoS guarantees; the SLA is focused on Layer 1 and Layer 2 measured parameters such as availability, committed information rate (CIR), committed burst (Bc), excess burst (Be), and peak information rate. WAN links sometimes become congested; therefore, to provide the required IP QoS for voice, video, and data applications, the enterprise (customer) must configure its equipment (WAN routers) with proper QoS mechanisms. Examples of such configurations include Frame Relay traffic shaping, Frame Relay fragmentation and interleaving, TCP/RTP header compression, LLQ, and class-based policing.
In recent years, especially due to the invention of technologies such as IPsec VPN and MPLS VPN, most providers have been offering Layer 3 services instead of, or at least in addition to, the traditional Layer 2 services (circuits). In summary, the advantages of Layer 3 services are scalability, ease of provisioning, and service flexibility. Unlike Layer 2 services where each circuit has a single QoS specification and assurance, Layer 3 services offer a variety of QoS service levels on a common circuit/connection based on type or class of data (marking). For example, the provider and the enterprise customer might have an SLA based on three traffic classes called controlled latency, controlled load, and best effort. For each class (except best effort) the SLA states that if it is submitted at or below a certain rate, the amount of data drop/loss, delay, and jitter will be within a certain limit; if the traffic exceeds the rate, it will be either dropped or re-marked (lower).
It is important that the SLA offered by the service provider is understood. Typical service provider IP QoS SLAs include three to five traffic classes: one class for real-time, one class for mission-critical, one or two data traffic classes, and a best-effort traffic class. The real-time traffic is treated as a high-priority class with a minimum, but policed, bandwidth guarantee. Other data classes are also provided a minimum bandwidth guarantee. The bandwidth guarantee is typically specified as a percentage of the link bandwidth (at the local loop). Other parameters specified by the SLA for each traffic class are average delay, jitter, and packet loss. If the interface on the PE device serves a single customer only, it is usually a high-speed interface, but a subrate configuration offers the customer only the bandwidth (peak rate) that is subscribed to. If the interface on a PE device serves multiple customers, multiple VLANs or VCs are configured, each serving a different customer. In that case, the VC or subinterface that is dedicated to each customer is provisioned with the subrate configuration based on the SLA.
Figure 6-4 displays a service provider core network in the middle offering Layer 3 services with IP QoS SLA to its customer with an enterprise campus headquarters and an enterprise remote branch. The customer in this example runs IP Telephony applications such as VoIP calls between those sites.
To meet the QoS requirements of the typical telephony (VoIP) applications, the enterprise must not only negotiate an adequate SLA with the provider, but it also must make proper QoS configurations on its own premise devices so that the end-to-end QoS becomes appropriate for the applications. In the example depicted in Figure 6-4, the SLA guaranteed a delay (latency) <= 60 ms, jitter <= 20 ms, and packet loss <= 0.5 percent. Because the typical end-to-end objectives for delay, jitter, and packet loss for VoIP are <= 150 ms, <= 30 ms, and <= 1 percent, respectively, the enterprise must make sure that the delay, jitter, and loss within its premises will not exceed 90 ms, 10 ms, and 0.5 percent, respectively.
Figure 6-4 Example for IP QoS SLA for VoIP
Maximum One-Way End-to-End QoS Budget for Delay (Latency), Jitter, and Packet Loss: Latency <= 150 ms/Jitter <= 30 ms/Loss <= 1%
Maximum One-Way End-to-End QoS Budget for Delay (Latency), Jitter, and Packet Loss: Latency <= 150 ms/Jitter <= 30 ms/Loss <= 1%
Figure 6-4 Example for IP QoS SLA for VoIP
Continue reading here: WAN Edge QoS Implementations
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