Wi-Fi PTP (Wi-PTP)

Brief

This project implements Precision Time Protocol (PTP) for Wi-Fi. Likely to be used in time-sensitive WLAN, UAV, etc., Original PTP is designed for LAN but it is possible to port it to Wi-Fi. The simplest way is to timestamp PTP packets in the NIC driver but is subject to several software uncertainties. This project makes use of the hardware TSF counter to emulate the hardware PTP clock like Ethernet NICs. A standard PTP clock interface is implemented in ath9k. Its performance is at 1 μs level. More details can be found in the ATC'21 paper.

The above figure illustrates how Wi-PTP works.

Usage

The ath9k directory contains the modified ath9k driver based on Linux Kernel 4.19.37. The ptp directory contains the adjusted linuxptp.

Build & Replace ath9k Driver

1. Patch drivers/net/wireless/ath in the Kernel source tree with this repo's ath9k.diff. We recommend patch to do this.

   cd <kernel_source>/drivers/net/wireless/ath
   patch -sf -p1 < <repo_path>/wifi-ptp/ath9k.diff

2. Build ath9k driver.

   cd <kernel source>
   make M=drivers/net/wireless/ath

3. Replace the ath9k driver.

   WLAN_DEV=wlan0
   
   ifdown ${WLAN_DEV}
   
   rmmod ath9k
   rmmod ath9k_common
   rmmod ath9k_hw
   rmmod ath
   
   cd <kernel source>/drivers/net/wireless/ath
   insmod ./ath.ko
   insmod ./ath9k/ath9k_hw.ko
   insmod ./ath9k/ath9k_common.ko
   insmod ./ath9k/ath9k.ko
   
   sleep 1
   
   ifup ${WLAN_DEV}

   Note: please replace the wlan0 above with the actual ath9k interface.

4. Check for the PTP device.

   ls /sys/class/net/${WLAN_DEV}/device/ptp

   The above command should output a ptp<X>.

Build linuxptp

The ptp version in this repo is modified to disable PTP capability checking and implement the PHC function in a relatively simple but improper way. The modified version is based on the commit dd30b3a. We also provide a file linuxptp.patch, which is the diff information between the commit dd30b3a and our modified linuxptp.

cd <repo path>/ptp
make

After building the linuxptp, the ar9300+ wireless cards can be used to perform PTP as ethernet cards.

Running with ptp4l

Master: ./ptp4l -i wlan0 -p /dev/ptp<X> -m Slave: ./ptp4l -i wlan0 -p /dev/ptp<X> -m -s

Citation

@inproceedings {273763,
author = {Paizhuo Chen and Zhice Yang},
title = {Understanding Precision Time Protocol in Today{\textquoteright}s Wi-Fi Networks: A Measurement Study},
booktitle = {2021 {USENIX} Annual Technical Conference ({USENIX} {ATC} 21)},
year = {2021},
isbn = {978-1-939133-23-6},
pages = {597--610},
url = {https://www.usenix.org/conference/atc21/presentation/chen},
publisher = {{USENIX} Association},
month = jul,
}

Contact

If you have any questions, please don't hesitate to contact yangzhc@shanghaitech.edu.cn or chenpzh@shanghaitech.edu.cn.