DSCP (Differentiated Services Code Point) is a packet header value used to request high priority or best effort delivery of traffic. Quality of Service (QoS) DSCP Marking can be used to determine the traffic classification for the data in a network.
In Windows, the default setting for the DSCP markings will be 0. Windows will ignore the marking settings on the client device.
Changing the DSCP Marking using the Group Policy Edit
The image below shows the initial test and highlights the DSCP value of “CS0”. To improve the QoS we need to raise this value.
Step 1 - Access the “Local Group Policy” Editor to do this select Search > “Local Group Policy”. Select “Local Computer Policy” > “Computer Configuration” > “Windows Settings” Right Click on the “Policy-based QoS” and select “Create new Policy”.
Step 2 – Name the policy in the example below we have called it “Quicklink Studio”. You then need to set a DSCP decimal value, for this example we have used 46. Select “Next” to continue.
Step 3 – Select “Only applications with this executable name”. Enter “peer.exe”. Select next to continue.
Step 4 – Select Any source and Any destination IP address, then select next to continue.
Step 5 – Select UDP from the “Select the protocol this QoS policy applies to”, drop box. Select “From any source port”. Next, select “To any destination port”. Then select “Finish”
Testing using Wireshark
We now need to use Wireshark and Quicklink Studio to do a test call, we can then capture the required data.
Put Quicklink Studio into a call with a device, (Someone you have sent a link too).
Open Wireshark, first we need to add DSCP to the columns. So right click on the column and select “Column Preferences”
Click on the “+” button at the bottom and this creates a new field. Add the following
- Double click the name filed and call DSCP
- Change the Type to “Custom”
- Double click the name field and type ip.dsfield.dscp
- Field occurrence = 0
Make sure you can see the “DSCP” in the columns. Then run Wireshark press stop and look for “Expedited Forwarding” under DSCP. Double click one of these and expand the Internet Protocol Version 4 drop arrow. Next expand the “Differentiated Services Codepoint” arrow, you should then see it has been forwarded to your value, 46 as in our case.
You can then setup your router to look for this information and give priority to these packets.
The picture below shows the policy we created using the “Group Policy Editor” with a DSCP value of 46.