1
00:00:01,030 --> 00:00:06,939
So the top of service field in an IP header consists again of eight binary bits.

2
00:00:07,390 --> 00:00:14,980
The most significant six bits are used for DHCP and the remaining two bits are used for explicit congestion

3
00:00:14,980 --> 00:00:18,460
notification, which we won't discuss in the Q&A.

4
00:00:19,530 --> 00:00:25,020
In the old days, the precedence levels were set as follows from 0 to 7.

5
00:00:25,950 --> 00:00:32,729
These are called class selectors today so IP precedence one is close select one IP precedence two is

6
00:00:32,729 --> 00:00:32,940
close.

7
00:00:32,940 --> 00:00:35,010
Selector to three is three.

8
00:00:35,010 --> 00:00:38,790
Four is four, five is known as express forwarding.

9
00:00:39,120 --> 00:00:42,900
Six stays the same and is used for IP routing protocols.

10
00:00:43,260 --> 00:00:48,540
Seven stays the same and is used for linker layer and routing protocol.

11
00:00:48,540 --> 00:00:49,460
Keep allies.

12
00:00:49,740 --> 00:00:52,170
Nothing is more important than keeping lives.

13
00:00:52,500 --> 00:00:59,220
If your links go down or you're writing protocols can't communicate, no traffic is going to be sent

14
00:00:59,220 --> 00:01:00,330
through your network.

15
00:01:00,600 --> 00:01:04,019
So we need to prioritize those over other traffic types.

16
00:01:05,290 --> 00:01:08,020
We have our assured forwarding classes.

17
00:01:08,230 --> 00:01:11,980
We have class one, class two, class three and class four once again.

18
00:01:12,430 --> 00:01:21,130
So if class one has three drop probabilities or three drop of values, we have af one, one, af one,

19
00:01:21,130 --> 00:01:26,320
two and af one, two, three af one three is high drop probability.

20
00:01:27,140 --> 00:01:35,390
If one two is medium drop probability under that class and AF one one is a low drop probability on that

21
00:01:35,390 --> 00:01:36,140
class.

22
00:01:36,440 --> 00:01:37,760
You need to look at the binary.

23
00:01:37,760 --> 00:01:41,900
So the first three binary bits indicate the class.

24
00:01:42,440 --> 00:01:45,680
So this is class one or AF one.

25
00:01:46,310 --> 00:01:50,150
The next two binary bits indicate the drop probability.

26
00:01:50,750 --> 00:01:56,450
So this is an example is three in decimal the lost but is not used.

27
00:01:56,450 --> 00:01:58,010
It's set to zero.

28
00:01:58,400 --> 00:02:07,370
So if one one means that this one is the class, the first three binary bits, this one indicates the

29
00:02:07,370 --> 00:02:08,740
drop probability.

30
00:02:08,750 --> 00:02:13,670
The next two binary bits and the zero in binary is not used.

31
00:02:14,090 --> 00:02:16,550
That equates to ten in decimals.

32
00:02:16,550 --> 00:02:18,860
If you look at that by itself, it's ten.

33
00:02:19,850 --> 00:02:24,230
If you look at that by itself, that's 12 because this is four plus eight.

34
00:02:24,740 --> 00:02:30,110
If you look at this, that's 14 because we've got two plus four plus eight.

35
00:02:30,590 --> 00:02:35,150
So in decimal you'll see it written as 14 or 12 or ten.

36
00:02:35,570 --> 00:02:38,630
It can also be denoted as an 11.

37
00:02:39,170 --> 00:02:43,400
So in other words, is it a tomato or is it a tomato?

38
00:02:43,550 --> 00:02:47,780
This is the same as this, which is the same as this.

39
00:02:48,180 --> 00:02:53,120
Now, in close to the first three binary bits indicate to the class.

40
00:02:53,120 --> 00:02:56,450
So that's to notice the first three binary bits.

41
00:02:56,450 --> 00:03:01,220
Here are two drop probability is the same as in zero.

42
00:03:01,220 --> 00:03:09,710
One indicates a110 in binary indicates a211 in binary indicates a three.

43
00:03:09,920 --> 00:03:14,240
So within class two we have a drop probability.

44
00:03:14,480 --> 00:03:22,250
In class two, this traffic will be dropped before this traffic which will be dropped before this traffic.

45
00:03:22,670 --> 00:03:28,520
Now you can determine when traffic is dropped, but that's sort of the convention or the rule that's

46
00:03:28,520 --> 00:03:29,270
followed.

47
00:03:29,870 --> 00:03:36,620
Clause two is seen as more important than Class one, but within a class such as close to we can set

48
00:03:36,620 --> 00:03:43,400
when packets are dropped, when there's congestion, class three and class four also have dropped probabilities.

49
00:03:43,760 --> 00:03:49,460
So notice that indicates the class that's three in binary.

50
00:03:49,640 --> 00:03:51,650
This indicates the drop probability.

51
00:03:51,650 --> 00:03:54,950
That's a one, that's a two and that's a three.

52
00:03:55,490 --> 00:03:56,930
So equates to this.

53
00:03:56,930 --> 00:03:59,150
This equates to that.

54
00:04:00,460 --> 00:04:07,480
And if you look at that, that equates to 26 in decimal, that equates to 28 in decimal, that equates

55
00:04:07,480 --> 00:04:09,760
to three in decimal.

56
00:04:10,360 --> 00:04:11,800
Close for is something similar.

57
00:04:11,800 --> 00:04:13,420
So we have a four.

58
00:04:13,930 --> 00:04:18,339
In other words, that's the class which is full drop probability.

59
00:04:18,339 --> 00:04:19,180
Here is one.

60
00:04:19,180 --> 00:04:21,730
So those two bits indicate the drop probability.

61
00:04:22,360 --> 00:04:25,270
Class of four, drop probability of two.

62
00:04:25,690 --> 00:04:28,900
Class of four drop probability of three.

63
00:04:29,020 --> 00:04:35,740
Last binary but is not used once again that equates to that in decimal.

64
00:04:36,220 --> 00:04:38,530
So that's the assured forwarding classes.

65
00:04:39,010 --> 00:04:46,110
Typically in an assured forwarding class, the higher the second number, the higher the drop probability.

66
00:04:46,120 --> 00:04:50,080
But in expedited forwarding, it doesn't work that way.

67
00:04:50,410 --> 00:04:57,280
Expedited forwarding is used for low loss, low latency, low jitter, assured bandwidth, end to end

68
00:04:57,280 --> 00:04:59,670
service through a serve domain.

69
00:04:59,680 --> 00:05:05,590
In other words, this is a premium service typically used for voice.

70
00:05:05,920 --> 00:05:11,770
Notice how they've written this class is five, so that's very similar to IP precedence five.

71
00:05:12,040 --> 00:05:15,340
This, however, doesn't mean high drop probability.

72
00:05:15,340 --> 00:05:18,250
It actually equates to low drop probability.

73
00:05:18,490 --> 00:05:25,750
If you look at these values in decimal, so you convert the binary to decimal, you get a value of 46.

74
00:05:25,990 --> 00:05:37,840
So a DCP of F equates to a decimal value of 46, which equates to a binary value of 101110.

75
00:05:38,020 --> 00:05:44,800
So an IP phone, as an example, will indicate to the network that it's traffic is very important by

76
00:05:44,800 --> 00:05:51,130
marking the DCP 2101110 and the costs to five.