Video Transcript
Quantum Networking (Part 5)
7m 32s
00:08
Welcome back.
00:09
Let’s remind ourselves of what happens in the OSI model.
00:14
Layer 1 makes our physical connections, and Layer 2 makes use of MAC addresses to transmit Ethernet ‘frames’.
00:22
And then layer 3 allows us to transmit packets, make use of IP addresses, routing and multicast protocols.
00:30
There’s one other aspect of layer 3 that comes into play with time-sensitive audio and video protocols:
00:36
giving packets a marking that indicates their priority on the network.
00:41
This is something we discuss a great deal in Q-SYS networking and troubleshooting.
00:46
The DSCP marking, which stands for Differentiated Services Code Point,
00:51
is a marking in each packet that can help the infrastructure prioritize delivery of certain packets over others.
00:58
Let’s say someone just sent a giant email with a cat video…
01:03
well, we wouldn’t want that email to stop or disrupt the audio to or from our videoconference.
01:10
The infrastructure can use the markings of PTPv2 and Q-LAN audio traffic from Q-SYS
01:16
as a way to know to prioritize those packets over others.
01:20
We ask that the packets be prioritized on the network using a ‘strict priority’ queuing model.
01:27
Note that DSCP values have a ‘name’ and a decimal value.
01:32
Different switches use different methods to configure prioritization.
01:36
A table like this can be very useful in making sure you know which name maps to the decimal value.
01:43
PTPv2 and Q-LAN are protocols that require both low latency and
01:48
jitter to be successfully implemented on a given network.
01:52
For example, Q-LAN packets must arrive in less than 280ms to be considered ontime.
01:59
We do recommend that PTPv2 and Q-LAN audio packets are the highest priorities on the network.
02:06
The feature set required on the switch to allow this prioritization is called Quality of Service, or QoS.
02:14
We’ve now covered the pertinent features of the media layers of the OSI model.
02:19
You can see that this is the real workhorse of the Ethernet model.
02:23
While the media layers determine how it gets there,
02:26
layers 4-7 determine how devices communicate on a higher level.
02:31
Layer 4, the transport layer, for example
02:34
determines if and how connections between devices are supervised
02:38
and how a device handles multiple services at once.
02:41
There are two major categories of traffic in the transport layer:
02:46
The user datagram protocol (or UDP) and the transmission control protocol (or TCP).
02:53
UDP is considered connectionless in the sense that there is no assurance
02:58
that the intended device received a given packet.
03:01
Sender, receiver and infrastructure are all on the honor system to deliver the communication.
03:07
On the other hand, when TCP is used, a supervised connection is negotiated before any data is even sent at all.
03:14
The transmitter tells the receiver how many packets they should receive
03:18
and how big each packet should be.
03:20
The receiver then confirms that each packet was received properly.
03:25
Although not supervised, UDP packets are good for rapid transmission (like Q-LAN, for example).
03:32
A Q-LAN stream consists of 3000 packets per second.
03:36
It would introduce a lot more processing overhead to have the sender tell the receiver
03:41
before sending every packet and the receiver confirming that every packet got there.
03:46
TCP is great for things like file transfers where you want to make sure every bit got there,
03:52
but you’re not terribly worried about how fast the transfer occurs.
03:56
Another important concept included in the transport layer is the idea of network ports.
04:03
Almost every networked device handles a number of different services and applications at once.
04:09
Think of a network port as a receiving dock in a large warehouse.
04:13
There might be twenty different ports
04:15
and each truck must be told where to drop off its merchandise so it gets to the right place.
04:20
This is the same for network traffic.
04:22
Every packet has a source port…the dock from which it left and a destination port,
04:28
the dock at which it should be dropped off. Ports are just numbers in this case.
04:33
Let’s look at an example.
04:35
Let’s say a computer wants to send a Q-SYS core a message.
04:39
Different services running on the Q-SYS core are ‘listening’ on different ports.
04:44
In this case the core is listening on port 8554.
04:49
If the computer sends a message to destination port 8551, that door is closed.
04:54
It’s as if the message wasn’t received as all, because it can’t be unloaded into the warehouse.
05:00
If the computer sends the message to a port that’s listening for the right service type,
05:04
we have a successful transmission.
05:07
The information in the packet goes into the right place and can be processed accordingly.
05:12
Many services, such as FTP, http, and other protocols have standard port numbers associated with them.
05:20
Here’s a list of many of the UDP and TCP ports used by various services supported by Q-SYS.
05:26
Feel free to pause here if you want to study these.
05:29
Next, let's take a look at a PCAP capture of an Ethernet packet that clearly illustrates
05:36
how all these layers fit together to form a cohesive package.
05:40
At layer one, we see how many bits and bytes are in the transmission. The ones and zeroes, if you will.
05:46
At layer two we see the source and destination MAC addresses.
05:51
At layer three, we see the source and destination IP addresses.
05:56
At layer four, we see the transport type, and the source/destination ports.
06:02
Finally, at layer five we have the data to be processed. This is called the ‘session’ layer.
06:09
When troubleshooting networking issues,
06:12
thinking in terms of along the OSI model is a good place to start.
06:16
We start at Layer 1 – is it plugged in? Right? Do we have normal LED activity on the switch?
06:22
Are we sure that the cabling is good and we have the expected connection speeds?
06:27
At layer 2, we might log into the switch and make sure that the switch is aware of the problem device.
06:33
A quick PCAP capture might show if there are any broadcast level packets from the device in question.
06:39
If you can ‘see’ the device in question or you can access it through other means,
06:43
well then check the layer 3 configuration of the device.
06:47
Does it have the right IP, subnet mask, gateway assignments?
06:50
Is this a case where IGMP is going to be required?
06:53
If so, is it enabled, is it working?
06:56
If we’re dealing with external control protocols, for instance,
07:00
we should confirm that the right type of socket is being opened.
07:03
Are the port numbers correct?
07:05
If TCP is being used, you should be able to tell if the connection is opened…if not,
07:10
well then you've got a problem.
07:12
If the connection is open,
07:14
well then maybe the formatting of the data that you’re sending is the issue rather than the network at all.
07:20
Alright, so... that is it.
07:23
That is a crash course in the basics of networking and network troubleshooting.
07:28
Thanks for watching!
Downloads and Links
Quantum Networking (Part 5)
7m 32s