QoS Markings: DSCP Class Selectors

In this last blog, we examined the IP Precedence usage of the ToS field in the IP header for QoS markings. The DSCP (differentiates services code point) field was also introduced. In this blog, we will examine the assured forwarding (AF) model to examine how the 64 possible markings of the 6-bit DSCP are used in a way that provides backward compatibility with the IP Precedence usage of the ToS byte. The three most significant bits of the ToS octet (byte) are shared between the IP Precedence and DSCP usages. There are 8 possible IP precedence binary patterns possible from this usage. The DSCP marking has provided backward compatibility to this model by indicating a default pattern and 7 class selectors. In the language of the DSCP model, the 8 DSCP markings that adapt to IP precedence (prec) usage are as follows: PHB keyword DSCP IP Prec binary Default 0 0 000 000 CS1 8 1 001 000 CS2 16 2 010 000 CS3 24 3 011 000 CS4 32 4 100 000 CS5 40 5 101 000 CS6 48 6 110 000 CS7 56 7 111 000 Each IP precedence value is indicated by a per-hop behavior (PHB) class selector (CS) in the differentiated services model. The PHB terminology is indicative of how the model achieves the stated QoS objective. Instead of sending a signaling protocol throughout the network like the Integrated Services model and RSVP (Resource Reservation Protocol), the DSCP model relies on independent queuing decisions made on a router by router basis. The router to router decision model is a per-hop behavior. There is no CS0. A marking of 0 indicates a best effort (BE) transmission that it’s coined “default” in the differentiated services model because this is what most traffic is marked as by default. I was very careful to include a space in between the three most significant bits and the next three bits in the above table. The three high order bits indicate the class to which the packet belongs, the next three bits in a class selector are always 000 because these bits were not used with IP Precedence based QoS. The next blog will examine the three drop preference levels in each assured forwarding classes.