Service level agreement
A service level agreement measures compliance to a service contract between an application service provider (ASP) and customer. Measuring SLAs is important to customers as they often pay a premium for better than best-effort service and want to justify the extra expense. SLAs are important to providers to help them define what customers can expect.
An important part of defining SLAs is how they are measured; often SLAs are not application-oriented but are usually CoS aware and are measured POP-to-POP using a UDP echo or similar (if it’s an IP-based service) between CPE devices. The consequences of breaching an SLA are often limited to refunds on the monthly charges.
A typical SLA encompasses the metrics below.
- Availability: This metric measures how often a service is available for use. Typically it includes a mean time to repair (MTTR) and specification of a maintenance window. The availability can be measured POP-to-POP or CPE-to-CPE or even provide different metrics for either.
- Delay: Delay measures the time it takes for a packet to traverse the network one way. A large delay can significantly affect the usability of time-sensitive applications like real-time voice and video, but it can also affect the throughput of high bandwidth applications due to the effect delay has on TCP window size. Delay again can be measured POP-to-POP or CPE-to-CPE. Delay can also encompass a measure of jitter,; buffers can be very effective in providing a usable service (regarding voice and video).
- Loss: Loss can be measured as packets offered vs. those delivered or packets received in error.
Only traffic within the agreed volume is subject to an SLA. So, part of the SLA includes defining what classification of traffic and what volume of traffic will receive the premium service. From the customer’s point of view, not all traffic needs a premium service; it would be too expensive. From a service provider’s point of view there is a revenue opportunity in providing premium services for a subset of customer traffic.