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Services
The DS type of service is provided within a DS domain, which is defined as a contiguous portion of the Internet over which a consistent set of DS policies are administered. Typically, a DS domain would be under the control of one administrative entity. The services provided across a DS domain are defined in a service level agreement (SLA), which is a service contract between a customer and the service provider that specifies the forwarding service that the customer should receive for various classes of packets. A customer may be a user organization or another DS domain. Once the SLA is established, the customer submits packets with the DS octet marked to indicate the packet class. The service provider must ensure that the customer gets at least the agreed QoS for each packet class. To provide that QoS, the service provider must configure the appropriate forwarding policies at each router (based on DS octet value) and must measure the performance being provided each class on an ongoing basis.
If a customer submits packets intended for destinations within the DS domain, the DS domain is expected to provide the agreed service. If the destination is beyond the customer's DS domain, the DS domain will attempt to forward the packets through other domains, requesting the most appropriate service to match the requested service.
A draft DS framework document lists the following detailed performance parameters that might be included in an SLA:
Detailed service performance parameters such as expected throughput, drop probability, latency
Constraints on the ingress and egress points at which the service is provided, indicating the scope of the service
Traffic profiles that must be adhered to for the requested service to be provided, such as token bucket parameters
Disposition of traffic submitted in excess of the specified profile
The framework document also gives some examples of services that might be provided:
Traffic offered at service level A will be delivered with low latency.
Traffic offered at service level B will be delivered with low loss.
90% of in-profile traffic delivered at service level C will experience no more than 50 msec latency.
95% of in-profile traffic delivered at service level D will be delivered.
Traffic offered at service level E will be allotted twice the bandwidth of traffic delivered at service level F.
Traffic with drop precedence X has a higher probability of delivery than traffic with drop precedence Y.
Examples 1–2 are qualitative and are valid only in comparison to other traffic, such as default traffic that gets a best-effort service. Examples 3–4 are quantitative and provide a specific guarantee that can be verified by measurement of the actual service without comparison to any other services offered at the same time. Examples 5–6 are a mixture of quantitative and qualitative.