IP Precedence and DSCP Values

Hello Muhammad

The rest of the lesson deals with Per Hop Behaviour. Essentially, this just refers to the way or the behaviour with which each router (per hop) will deal with a packet when it receives it based on the code point values found within the DS field. So when we talk about PHB it really means how the router will handle the particular packet compared to all the rest it receives whenever there is congestion.

The default PHB has a DSCP value of zero or 000000. Such packets are treated as best effort, meaning, first come first serve. No special treatment or priority is given to theses.

The next three sections simply talk about ways in which the DS field is used and interpreted.

Class-Selector PHB is a way in which the new structure of the field can be compatible with the old IP Precedence structure described earlier in the article. Class selector PHB separates the DS field into two 3-bit sections. The first three bits are only used, just like in IP Precedence, thus making it compatible.

Assured Forwarding PHB is a set of predefined DSCP values that are placed into four queues (or classes) with three levels of drop precedence. This just means that there are three different levels that packets can be defined with that will either make the eligible to be dropped (or not) if congestion occurs. These are simply another set of defined parameters.

Expedited forwarding PHB is just an additional construct that is used to mark packets that are sent into a priority queue, this is a queue in addition to the other four mentioned in Assured Forwarding PHB. Packets marked with EF (or 101110 in binary) will be placed into this priority queue to be sent before all other queues.

These are just definitions of how the specific field in the IP packet is being used and interpreted. They are predefined methodologies of applying QoS to packets that correspond to specific values in the DSCP field.

I hope this has been helpful!

Laz

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