TSN is designed to work through Virtual Local Area Networks or VLANs. VLANs were defined in IEEE 802.1Q, and the majority of TSN-related standards are amendments of this. This page describes how to configure VLANs/virtual network interfaces such that they have known traffic/priority mappings, a local IP address for that virtual interface, and the ability to be propagated into the network (thereby avoiding manual configuration for each port for each switch).
VLANs are simple in practice. In Ethernet, which we assume to be using, it encapsulates the regular ethernet frame with a few extra bytes to specify a VLAN ID (12 bits) and a priority (3 bits). In Linux, this is generally handled by creating a virtual interface with its own IP so that sending and receiving through it will add or strip this header automatically.
To handle priorities, Linux has 16 'traffic classes', which can be mapped to the 8 possible traffic classes. By default, these are not one-to-one; see Additional Info on VLANs for more.
Disclaimer: These directions are based on an Ubuntu 20.04 LTS Linux distribution, and may not work correctly on other distributions. Further, we assume the device contains an intel i225-LM. This appears to universally name the interface enp87s0 (although if there are multiple, the last number is likely an index).
This configuration, along with many other mechanisms used, requires root permissions. It uses the iproute2 tool inherent to modern Ubuntu distributions.
First, let's create a simple VLAN with default configurations by add a virtual interface based on the ethernet NIC. We'll give it a VLAN ID of '3'. The interface of the ethernet NIC is "enp87s0", and conventions suggest to append ".$VLAN_ID" to its name for the new virtual interface.
sudo ip link add link enp87s0 name enp87s0.3 type vlan id 3
Next, we'll set the egress (outgoing) mapping between traffic classes and VLAN priorities. Note that you may manipulate the traffic class at the socket level with setsockopt and SO_PRIORITY. There is a helper function, set_socket_priority in helpers.h that will do this.
sudo ip link set enp87s0.3 type vlan egres 0:2 1:1 2:0 3:3 4:4 5:5 6:6 7:7
This is the default configuration, to convert traffic_class to vlan_priority. To make this VLAN send data at a higher priority than best effort be default, map traffic class 0 to something higher than 2, e.g. 0:3 1:0 2:1 3:2 4:4 5:5 6:6 7:7.
You may similarly setup the ingress mapping, which tells the kernel how to prioritize incoming enqueued traffic. While not configured for our usecase, you can also set how the kernel handles queueing in hardware and/or software using the traffic class utility.
MVRP means Multiple VLAN Registration Protocol; it allows VLAN information to be propagated through a LAN so members of the same VLAN can reach each other through switches that are not necessarily on that VLAN. This is simply a binary option: on or off.
sudo ip link set enp87s0.3 type vlan mvrp on
The above three points can also be done in a single command for ease of use. For example, creating a VLAN with ID 5 that will use MVRP and a default priority of 3:
sudo ip link add link enp87s0 name enp87s0.5 type vlan id 5 mvrp on egress-qos-map 0:3 1:0 2:1 3:2 4:4 5:5 6:6 7:7
To communicate through the VLAN, the virtual interface should be given an IP address. The following gives an IP address of 10.0.5.101 to the interface for VLAN 5 as we configured above:
sudo ip addr add 10.0.5.101/24 dev enp87s0.5
Note that the netmask (here, the first 24 bits) is important. In this instance, it is configuring those first 24 bits (the first 3 octets of the IP address) to designate the subnet and the last 8 bits to specify the device itself.
Finally, we need to set the interface into the 'up' state, as it is not on by default. Otherwise, we cannot send or receive anything directly through this interface:
sudo ip link set enp87s0.5 up
Replacing up with down will similarly turn the interface off.
To test the VLAN, you can use the C files in latency_vlan_tests. The corresponding README describes what each file does. While these are running, you may analyze packets using tcpdump or wireshark.
If you are using tcpdump (which may require installation with apt), you can call it like so: sudo tcpdump -xevvv > dump.log. Let it run for a few seconds, and kill the program. When you read the new dump.log, it will show frame contents, ethernet header information, and other protocol-specific information.
Most importantly, you should see "vlan 5 p 3" within the frames sent using VLAN 5 as we configured here. The vlan and priority values will be different for other configurations of course; this is showing that a frame was sent using VLAN 5 with a priority of 3. You can see this in the sender or receiver.