1
00:00:01,230 --> 00:00:07,400
So the top off service field in an IP header consists again of eight binary bits.

2
00:00:07,560 --> 00:00:15,000
The most significant six bits are used for DCP and the remaining two bits are used to explicit congestion

3
00:00:15,000 --> 00:00:20,870
notification which we won't discuss in the CCMA in the old days.

4
00:00:20,890 --> 00:00:26,040
The precedents levels were set as follows from 0 to 7.

5
00:00:26,060 --> 00:00:32,430
These are called Class selectors today so IP precedents one is closely linked to one IP precedence.

6
00:00:32,440 --> 00:00:40,220
Two is class elected to three as three four is four five is known as express forwarding 6 stays the

7
00:00:40,220 --> 00:00:48,110
same and is used for IP routing protocols 7 stays the same and is used for Lincoln layer and writing

8
00:00:48,110 --> 00:00:49,840
protocol keeper lives.

9
00:00:49,940 --> 00:00:52,370
Nothing is more important than keeping lives.

10
00:00:52,610 --> 00:00:56,950
If your links go down or your running protocols can't communicate.

11
00:00:57,170 --> 00:01:00,170
No traffic is going to be sent through your network.

12
00:01:00,800 --> 00:01:05,410
So we need to prioritize those over other traffic times.

13
00:01:05,470 --> 00:01:08,230
We have our assured forwarding classes.

14
00:01:08,440 --> 00:01:12,580
We have class one class to class 3 and Class 4 once again.

15
00:01:12,580 --> 00:01:18,640
So a F Class one has three drop probabilities or 3 drop values.

16
00:01:18,640 --> 00:01:30,250
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

17
00:01:30,610 --> 00:01:37,770
under that class and a f 1 1 is low drop probability on that class you need to look at the binary.

18
00:01:37,780 --> 00:01:42,350
So the first 3 binary but indicate to the class.

19
00:01:42,610 --> 00:01:51,970
So this is Class 1 or f 1 the next two binary bits indicated the drop probability so this is an example

20
00:01:51,970 --> 00:01:54,130
is three in decimal.

21
00:01:54,130 --> 00:01:56,520
The last but is not used.

22
00:01:56,560 --> 00:02:05,780
It's set to zero so f 1 1 means that this one is the class the first three binary bits.

23
00:02:06,010 --> 00:02:14,210
This one indicates the drop probability the next two binary bits and the zero in binary is not used.

24
00:02:14,290 --> 00:02:16,570
That equates to 10 in decimals.

25
00:02:16,570 --> 00:02:21,590
If you look at that by itself it's 10 if you look at that by itself.

26
00:02:21,590 --> 00:02:24,720
That's 12 because this is four plus eight.

27
00:02:24,860 --> 00:02:30,720
If you look at this that's 14 because we've got two plus four plus eight.

28
00:02:30,740 --> 00:02:35,650
So in decimal you'll see it written as 14 or twelve or ten.

29
00:02:35,660 --> 00:02:38,480
It can also be denoted as a F eleven.

30
00:02:39,380 --> 00:02:41,410
So in other words is it a tomato.

31
00:02:41,420 --> 00:02:43,740
Is it a tomato.

32
00:02:43,760 --> 00:02:48,150
This is the same as this which is the same as this.

33
00:02:48,380 --> 00:02:53,190
Now in close to the first three binary bits indicate to the class.

34
00:02:53,210 --> 00:02:54,960
So that's 2.

35
00:02:55,010 --> 00:03:02,090
Notice the first three binary bits here are two drop probabilities the same as in zero one indicates

36
00:03:02,090 --> 00:03:10,040
a 1 1 0 in binary indicates a 2 1 1 in binary indicates a 3.

37
00:03:10,130 --> 00:03:19,550
So within class 2 we have a drop probability in Class 2 thus traffic will be dropped before this traffic

38
00:03:19,970 --> 00:03:22,760
which will be dropped before this traffic.

39
00:03:22,760 --> 00:03:28,520
Now you can determine when traffic is dropped but that's sort of the conventional or the rule that's

40
00:03:28,520 --> 00:03:33,450
followed plus 2 is seen as more important than class 1.

41
00:03:33,470 --> 00:03:39,680
But with inner class such as class 2 we can set when packets are dropped when there's congestion.

42
00:03:39,740 --> 00:03:43,820
Class 3 and Class 4 also have dropped probabilities.

43
00:03:43,820 --> 00:03:49,810
So notice that indicates the class that's 3 in binary.

44
00:03:49,820 --> 00:03:55,530
This indicates the drop probability that's a one that's a 2 and that's a 3.

45
00:03:55,530 --> 00:03:57,020
That equates to this.

46
00:03:57,020 --> 00:04:04,350
This equates to that and if you look at that that equates to 26 and decimal.

47
00:04:04,440 --> 00:04:06,870
That equates to 28 in decimal.

48
00:04:06,870 --> 00:04:14,060
That equates to thirty in decimal Class Four is something similar so we have a F four.

49
00:04:14,100 --> 00:04:19,190
In other words that's the class which is for drop probability heroes 1.

50
00:04:19,200 --> 00:04:28,080
So those two bits indicate the drop probability class of 4 a probability of 2 class of 4 drop a probability

51
00:04:28,080 --> 00:04:36,390
of 3 lost binary but is not used once again that equates to that in decimal.

52
00:04:36,390 --> 00:04:43,020
So that's the assured forwarding classes typically in an assured forwarding class the higher the second

53
00:04:43,020 --> 00:04:50,200
number the higher the drop a probability but an expedited forwarding it doesn't work that way.

54
00:04:50,520 --> 00:04:57,780
Expedited forwarding is used for low loss low latency low chatter assured bandwidth end to end service

55
00:04:57,780 --> 00:04:59,860
through a server domain.

56
00:04:59,910 --> 00:05:06,060
In other words this is a premium service typically used for voice.

57
00:05:06,090 --> 00:05:08,920
Notice how they've written this class is 5.

58
00:05:08,970 --> 00:05:12,220
So that's very similar to IP precedents 5.

59
00:05:12,240 --> 00:05:18,690
This however doesn't mean high drop probability it actually equates to low drop probability.

60
00:05:18,690 --> 00:05:23,420
If you look at these values in decimals you convert the binary to decimal.

61
00:05:23,460 --> 00:05:34,470
You get a value of 46 so a DCP of f equates to a decimal value of 46 which equates to a binary value

62
00:05:34,950 --> 00:05:43,920
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

63
00:05:43,920 --> 00:05:50,940
important by marking the DCP to 1 0 1 1 1 0 and the costs to 5.