Command Line Configuration of Frame Preemption (IEEE 802.1Qbu)
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Command Line Configuration of Frame Preemption (IEEE 802.1Qbu) Application Note Security Document number Version Date Status TTTech Industrial Automation AG Schonbrunner Str. 7, 1040 Vienna, Austria, Tel. +43 1 585 34 34–0, Fax +43 1 585 34 34–90, ̈ support@tttech-industrial.com © 2021 TTTech Industrial Automation AG. All rights reserved. Subject to changes and correcti

History of Changes Version 1.0 Modification initial version © 2021 TTTech Industrial Automation AG. All rights reserved.

1 Test Setup The configuration example presented in this document shows how IEEE 802.1Qbu frame preemption can be enabled on an Ethernet link between two DE-IP Evaluation Boards (#13122). As hardware two Evaluation Boards, two PCs and a separate engineering PC are used. The engineering PC is kept as separate device for simplicity. In this setup two independent VLANs are configured between the different devices, and the Evaluation Boards embedded switches are configured to perform frame preemption. Frame preemption is done by evaluating the priority (PCP) field of the 802.1Q VLAN headers....

Edge 2.2.0. It is assumed that all devices are already configured as members of a common test network 192.168.0.0/24. Please make sure that all your devices (Evaluation Boards and PCs) have assigned a unique IP addresses and are accessible via SSH before continuing. The VLANs will be configured below. As preparation please connect your PCs and Evaluation Boards as shown in figure 1.2. The locations of the Ethernet ports sw0p2 to sw0p5 are the following: Figure 1.2: Ethernet port locations © 2021 TTTech Industrial Automation AG. All rights reserved.

2 Configuration Details As shown in figure 1.1, VLAN2 is used for communication between two PCs, each connected to an Evaluation Boards external Ethernet port (sw0p5). For reducing the number of needed devices, the second VLAN (VLAN1) is setup between the Evaluation Boards embedded Linux systems. The engineering PC is only used to login to the Evaluation Boards via SSH and kept as separate device for simplicity. The Ethernet packets transmitted within VLAN1 will be configured as “Express” and therefore be allowed to preempt the packets of VLAN2 on the shared link between the two boards....

2.1 Step 1: Configure VLAN 1 between the Evaluation boards Embedded Linux Systems 2.1.1 Evaluation Board 1 From your engineering PC login to your Evaluation Board 1 (192.168.0.1) using SSH. Use following commands to create, configure and enable VLAN1 (192.168.1.1): >> i p l i n k add l i n k sw0ep name v l a n 1 t y p e v l a n i d 88 >> i p a d d r add 1 9 2 . 1 6 8 . 1 . 1 /24 dev v l a n 1 We want all outgoing traffic to VLAN 1 to use the same priority in the PCP field. So we map all Linux internal priorities to PCP 5 by executing: >> >> >> >> >> >> >> >> link link link link link link...

2.2 Step 2: Configure VLAN 2 between the two PCs This is the VLAN connecting PC1 and PC2, having low priority on the shared Ethernet line between the two Evaluation Boards. Therefore, Ethernet packets transmitted through VLAN2 can be preempted. As defined in table 2.1, for this VLAN the VID=99 and PCP = 4 is used. The following commands are tested on Ubuntu 20.04, but should also be available on different Linux distributions. Except changing the Ethernet interface names, VLAN configuration and IP address, the setup procedure is the same as on the Evaluation Boards: 2.2.1 PC 1 On an PC a...

2.3 Step 3: Enable Preemption on Ethernet link between Evaluation boards With the VLAN configuration from step 1 and step 2, it is now possible to enable the preemption on the trunk line connecting the two evaluation boards. Preemption needs to be enabled on both boards (192.168.0.1, 192.168.0.2). VLAN1 and VLAN2 are configured to use different priorities (PCPs). Now we will allow the VLAN1 (VID=88, PCP=5) to be “express” by executing the following commands: Evaluation Board 1 + Evaluation Board 2: Login on both boards and execute: >> t s n t o o l preempt s e t q 4 preempt sw0p5 >> t s n t...

Without applying network traffic to VLAN1 and VLAN2 all counters will stay 0 or count slowly due to some background traffic (SSH, PTP, etc. . . ). 2.4 Step 4: Generate traffic and observe effects of preemption To be able to demonstrate the effect of preemption in this hardware setup, we will switch the trunk line to 100Mbit operation mode. With 100Mbit/s a 1500 Byte Ethernet frame has a transmission time of about 120µs. This time scale allows to make the impact through preemption visible without detailed real-time considerations of the overall system by interpreting average end-to-end...

Figure 2.2: typical iperf3 server output 2.4.2 generate express traffic on VLAN1 To trigger preemption logic, login to Evaluation Board 1 and start a flood ping to Evaluation Board 2: >> sudo p i n g −f 1 9 2 . 1 6 8 . 1 . 2 This now leads to the wanted situation, that small ping packets from VLAN1 collide with the full-length packets within VLAN2 in the sw0p5 output port of Evaluation Board 1. As a result of this Ethernet frames belonging to VLAN2 (iperf3) are now preempted during transmission. When executing: >> t s n t o o l preempt show sw0p5 it now can be observed that the...

Figure 2.3: preemption status counters 2.4.3 Latency timing improvement through preemption As already mentioned, the transmission time of a 1500 Byte Ethernet frame is 120µs. Under this artificial generated full-load scenario, an Express packet in VLAN1 is expected to arrive in average 60µs earlier (half full-frame transmission time). This difference can be observed when comparing the outputs of the latest ping command, when preemption is turned on or off (see figure 2.4 and figure 2.5). Please be aware, that this may depend on the PC hardware you use. © 2021 TTTech Industrial Automation...

Figure 2.4: typical end-to-end latency without preemption Figure 2.5: typical end-to end latency with enabled preemption © 2021 TTTech Industrial Automation AG. All rights reserved.