7.5.1.1 Objective

The objective of the JoBS algorithm is to provide absolute and relative (proportional) loss and delay differentiation independently at each node for classes of traffic. JoBS therefore provides service guarantees on a per-hop basis. The set of performance requirements are specified to the algorithm as a set of per-class Qualtiy of Service (QoS) constraints. As an example, for three classes, the QoS constraints could be of the form:

• $\mbox{Class-1 Delay} \approx 2 \cdot \mbox{Class-2 Delay}$,
• $\mbox{Class-2 Loss Rate} \approx 10^{-1} \cdot\mbox{Class-3 Loss Rate}$, or
• $\mbox{Class-3 Delay} \leq 5~ms$.

Here, the first two constraints are relative constraints and the last one is an absolute constraint. The set of constraints can be any mix of relative and absolute constraints. More specifically, JoBS supports the five following types of constraints:

• Relative delay constraints (RDC) specify a proportional delay differentiation between classes. As an example, for two classes $1$ and $2$, the RDC enforces a relationship
  
  $$\frac{\mbox{Delay of Class 2}}{\mbox{Delay of Class 1}}\approx \mbox{constant}\ .\nonumber$$
  
  

• Absolute delay constraints (ADC): An ADC on class $i$ requires that the delays of class $i$ satisfy a worst-case bound $d_i$.
• Relative loss constraints (RLC) specify a proportional loss differentiation between classes.
• Absolute loss constraints (ALC): An ALC on class $i$ requires that the loss rate of class $i$ be bounded by an upper bound $L_i$.
• Absolute rate constraints (ARC): An ARC on class $i$ means that the throughput of class $i$ is bounded by a lower bound $\mu_i$.

JoBS does not rely on admission control or traffic policing, nor does it make any assumption on traffic arrivals. Therefore, a system of constraints may become infeasible, and some constraints may need to be relaxed. QoS constraints are prioritized in the following order.

$$\mbox{ALC} > \mbox{ADC, ARC} > \mbox{Relative Constraints} \ .$$

That is, if JoBS is unable to satisfy both absolute and relative constraints, it will give preference to the absolute constraints.