So the top of service field in an IP header consists again of eight binary bits.
The most significant six bits are used for DHCP and the remaining two bits are used for explicit congestion
notification, which we won't discuss in the Q&A.
In the old days, the precedence levels were set as follows from 0 to 7.
These are called class selectors today so IP precedence one is close select one IP precedence two is
close.
Selector to three is three.
Four is four, five is known as express forwarding.
Six stays the same and is used for IP routing protocols.
Seven stays the same and is used for linker layer and routing protocol.
Keep allies.
Nothing is more important than keeping lives.
If your links go down or you're writing protocols can't communicate, no traffic is going to be sent
through your network.
So we need to prioritize those over other traffic types.
We have our assured forwarding classes.
We have class one, class two, class three and class four once again.
So if class one has three drop probabilities or three drop of values, we have af one, one, af one,
two and af one, two, three af one three is high drop probability.
If one two is medium drop probability under that class and AF one one is a low drop probability on that
class.
You need to look at the binary.
So the first three binary bits indicate the class.
So this is class one or AF one.
The next two binary bits indicate the drop probability.
So this is an example is three in decimal the lost but is not used.
It's set to zero.
So if one one 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 ten in decimals.
If you look at that by itself, it's ten.
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 12 or ten.
It can also be denoted as an 11.
So in other words, is it a tomato or 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 to notice the first three binary bits.
Here are two drop probability is the same as in zero.
One indicates a110 in binary indicates a211 in binary indicates a three.
So within class two we have a drop probability.
In class two, this 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 convention or the rule that's
followed.
Clause two is seen as more important than Class one, but within a class such as close to we can set
when packets are dropped, when there's congestion, class three and class four also have dropped probabilities.
So notice that indicates the class that's three in binary.
This indicates the drop probability.
That's a one, that's a two and that's a three.
So equates to this.
This equates to that.
And if you look at that, that equates to 26 in decimal, that equates to 28 in decimal, that equates
to three in decimal.
Close for is something similar.
So we have a four.
In other words, that's the class which is full drop probability.
Here is one.
So those two bits indicate the drop probability.
Class of four, drop probability of two.
Class of four drop probability of three.
Last 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 probability.
But in expedited forwarding, it doesn't work that way.
Expedited forwarding is used for low loss, low latency, low jitter, assured bandwidth, end to end
service through a serve domain.
In other words, this is a premium service typically used for voice.
Notice how they've written this class is five, so that's very similar to IP precedence five.
This, however, doesn't mean high drop probability.
It actually equates to low drop probability.
If you look at these values in decimal, so 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 101110.
So an IP phone, as an example, will indicate to the network that it's traffic is very important by
marking the DCP 2101110 and the costs to five.