Assured Forwarding

Assured Forwarding PHB is suggested for applications that require a better reliability than the best-effort service. There are four classes of service, and within each class, there are three drop precedences. Assured Service can be implemented as follows. First, classification and policing are done at the ingress routers of the ISP networks. If the Assured Service traffic does not exceed the bit-rate specified by the SLA, they are considered as in profile. Otherwise, the excess packets are considered as out of profile. Second, all packets, in and out, are put into an Assured Queue to avoid out of order delivery. Third, the queue is managed by a queue management scheme called RED with In and Out, or RIO.

RIO is a more advanced RED scheme, it basically maintains two RED algorithms, one for in packets and one for out packets. There are two thresholds for each queue. When the queue size is below the first threshold, no packets are dropped. When the queue size is between the two thresholds, only out packets are randomly dropped. When the queue size exceeds the second threshold, indicating possible network congestion, both in and out packets are randomly dropped, but out packets are dropped more aggressively.

In linux, there is a special kind of queue called the GRED - generalised RED which allows multiple drop precedences within an AF class.