1
00:00:00,540 --> 00:00:08,340
Now, something to point out in the ICMP message, NPM is inserting data, so the NPM server 10.1

2
00:00:08,340 --> 00:00:14,670
100.100 is sending an echo request, but is including data in the message.

3
00:00:15,090 --> 00:00:16,670
NPM does this by default

4
00:00:17,430 --> 00:00:23,700
and one of the reasons for doing this is that some devices between your NMS and the device that

5
00:00:23,700 --> 00:00:30,090
you're pinging may block ICMP packets with certain criteria in the data fields.

6
00:00:30,660 --> 00:00:37,320
Some firewalls, as an example, may block packets with a zero size data field, and some firewalls

7
00:00:37,320 --> 00:00:39,600
will block packets with large payloads.

8
00:00:40,320 --> 00:00:45,120
Some devices even block ICMP packets that are even or odd in size.

9
00:00:45,810 --> 00:00:52,980
In this capture, we can see that the response packet was sent from router 2 to the NMS.

10
00:00:53,550 --> 00:01:00,810
So it's an ICMP echo reply message and the NMS will calculate the difference between the time that

11
00:01:00,810 --> 00:01:04,230
the request was sent and the reply received, 

12
00:01:04,830 --> 00:01:10,890
and it uses that to calculate the round trip, response time or latency of the network.