So the top off service field in an IP header consists again of eight binary bits.
The most significant six bits are used for DCP and the remaining two bits are used to explicit congestion
notification which we won't discuss in the CCMA in the old days.
The precedents levels were set as follows from 0 to 7.
These are called Class selectors today so IP precedents one is closely linked to one IP precedence.
Two is class elected to three as three four is four five is known as express forwarding 6 stays the
same and is used for IP routing protocols 7 stays the same and is used for Lincoln layer and writing
protocol keeper lives.
Nothing is more important than keeping lives.
If your links go down or your running protocols can't communicate.
No traffic is going to be sent through your network.
So we need to prioritize those over other traffic times.
We have our assured forwarding classes.
We have class one class to class 3 and Class 4 once again.
So a F Class one has three drop probabilities or 3 drop values.
We have a f 1 1 A F 1 2 and a f 1 3 if 1 3 is high drop probability a f 1 2 is medium drop probability
under that class and a f 1 1 is low drop probability on that class you need to look at the binary.
So the first 3 binary but indicate to the class.
So this is Class 1 or f 1 the next two binary bits indicated the drop probability so this is an example
is three in decimal.
The last but is not used.
It's set to zero so f 1 1 means that this one is the class the first three binary bits.
This one indicates the drop probability the next two binary bits and the zero in binary is not used.
That equates to 10 in decimals.
If you look at that by itself it's 10 if you look at that by itself.
That's 12 because this is four plus eight.
If you look at this that's 14 because we've got two plus four plus eight.
So in decimal you'll see it written as 14 or twelve or ten.
It can also be denoted as a F eleven.
So in other words is it a tomato.
Is it a tomato.
This is the same as this which is the same as this.
Now in close to the first three binary bits indicate to the class.
So that's 2.
Notice the first three binary bits here are two drop probabilities the same as in zero one indicates
a 1 1 0 in binary indicates a 2 1 1 in binary indicates a 3.
So within class 2 we have a drop probability in Class 2 thus traffic will be dropped before this traffic
which will be dropped before this traffic.
Now you can determine when traffic is dropped but that's sort of the conventional or the rule that's
followed plus 2 is seen as more important than class 1.
But with inner class such as class 2 we can set when packets are dropped when there's congestion.
Class 3 and Class 4 also have dropped probabilities.
So notice that indicates the class that's 3 in binary.
This indicates the drop probability that's a one that's a 2 and that's a 3.
That equates to this.
This equates to that and if you look at that that equates to 26 and decimal.
That equates to 28 in decimal.
That equates to thirty in decimal Class Four is something similar so we have a F four.
In other words that's the class which is for drop probability heroes 1.
So those two bits indicate the drop probability class of 4 a probability of 2 class of 4 drop a probability
of 3 lost binary but is not used once again that equates to that in decimal.
So that's the assured forwarding classes typically in an assured forwarding class the higher the second
number the higher the drop a probability but an expedited forwarding it doesn't work that way.
Expedited forwarding is used for low loss low latency low chatter assured bandwidth end to end service
through a server domain.
In other words this is a premium service typically used for voice.
Notice how they've written this class is 5.
So that's very similar to IP precedents 5.
This however doesn't mean high drop probability it actually equates to low drop probability.
If you look at these values in decimals you convert the binary to decimal.
You get a value of 46 so a DCP of f equates to a decimal value of 46 which equates to a binary value
of 1 0 1 1 1 0 so an IP phone as an example will indicate it to the network that its traffic is very
important by marking the DCP to 1 0 1 1 1 0 and the costs to 5.