# Downstream Linux PTP Clients

This directory contains the reusable Linux configuration for servers downstream from NEXUS switch.

The intended path is:

```text
RPi5 Grandmaster
      │
      \/
Linux Boundary Clock
      │
      \/
Nexus Boundary Clock
      │
      \/
Linux client NIC PHC
      │
   phc2sys
      │
      \/
CLOCK_REALTIME (UTC)
```

Each direct client normally needs only:

1. `ptp` enabled on its physical NEXUS switch port;
2. `ptp4l` to synchronize the NIC PHC from NEXUS; and
3. `phc2sys` to synchronize Linux `CLOCK_REALTIME` from that PHC.

## Files in this directory

| File | Install as | Purpose |
|---|---|---|
| `ptp4l-client.conf` | `/etc/linuxptp/ptp4l-client.conf` | Ordinary-clock/client-only PTP configuration |
| `ptp4l-client.service` | `/etc/systemd/system/ptp4l-client.service` | Runs `ptp4l` on the client NIC |
| `phc2sys-client.service` | `/etc/systemd/system/phc2sys-client.service` | Synchronizes `CLOCK_REALTIME` from the client NIC PHC |

The bundled files currently use `enp4s0` as the client interface. Change that name everywhere if the target server uses another interface.

## 1. Enable PTP on the corresponding Nexus port

On NEXUS:

```text
configure terminal

interface Ethernet1/<clientPort>
  ptp

end
copy running-config startup-config
```

Example:

```text
interface Ethernet1/10
  ptp
```

Once the client is online, the Nexus port should normally become `Master` in:

```text
show ptp brief
```

## 2. Install LinuxPTP

On the client:

```bash
sudo apt update
sudo apt install linuxptp ethtool
```

## 3. Identify the PTP NIC

Find the interface connected to NEXUS:

```bash
ip -br link
```

The examples below assume:

```text
enp4s0
```

If your interface has a different name, replace `enp4s0` in all three files before installing them.

For example, search the bundled files with:

```bash
grep -Rni 'enp4s0' .
```

## 4. Verify hardware timestamping

```bash
ethtool -T enp4s0
```

The NIC should expose hardware transmit, receive, and raw hardware timestamping plus a PTP Hardware Clock.

You can map it to the PHC device with:

```bash
readlink -f /sys/class/net/enp4s0/device/ptp/ptp*
```

If the NIC does not support hardware timestamping, do not use the bundled `time_stamping hardware` configuration unchanged.

## 5. Disable competing system-clock synchronization

`phc2sys-client.service` will discipline `CLOCK_REALTIME`. Disable services that would compete for the same system clock:

```bash
sudo systemctl disable --now chrony 2>/dev/null || true
sudo systemctl disable --now systemd-timesyncd 2>/dev/null || true
sudo systemctl disable --now ntp 2>/dev/null || true
sudo systemctl disable --now ntpd 2>/dev/null || true
```

## 6. Install the client files

If the interface really is `enp4s0`, install the files directly:

```bash
sudo install -D -m 0644 ptp4l-client.conf /etc/linuxptp/ptp4l-client.conf
sudo install -D -m 0644 ptp4l-client.service /etc/systemd/system/ptp4l-client.service
sudo install -D -m 0644 phc2sys-client.service /etc/systemd/system/phc2sys-client.service
```

If the interface name is different, edit the local copies first and replace `enp4s0` in:

```text
ptp4l-client.conf
ptp4l-client.service
phc2sys-client.service
```

The PTP client config intentionally uses:

```text
domainNumber      0
network_transport UDPv4
delay_mechanism   E2E
clientOnly        1
```

`clientOnly 1` prevents a downstream server from ever trying to become the grandmaster.

## 7. Enable the services

```bash
sudo systemctl daemon-reload
sudo systemctl enable --now ptp4l-client.service
sudo systemctl enable --now phc2sys-client.service
```

The data flow is:

```text
ptp4l:    NEXUS -> NIC PHC
phc2sys:  NIC PHC -> CLOCK_REALTIME
```

The bundled `phc2sys` service uses:

```bash
phc2sys -s enp4s0 -c CLOCK_REALTIME -w -m
```

`-w` waits for `ptp4l` and obtains the PTP/UTC offset from it, so the client does not need a hard-coded `-37` offset.

## 8. Verification

Check service state:

```bash
systemctl status ptp4l-client
systemctl status phc2sys-client
```

Follow the logs:

```bash
journalctl -fu ptp4l-client
journalctl -fu phc2sys-client
```

`ptp4l` should progress into the synchronized client/slave state and its master offsets should settle near zero.

Query PTP datasets:

```bash
sudo pmc -u -b 0 'GET PORT_DATA_SET'
sudo pmc -u -b 0 'GET CURRENT_DATA_SET'
sudo pmc -u -b 0 'GET PARENT_DATA_SET'
sudo pmc -u -b 0 'GET TIME_STATUS_NP'
```

For a client directly downstream from NEXUS, expect approximately:

```text
portState             CLIENT/SLAVE
stepsRemoved          3
grandmasterIdentity   <same RPi5 grandmaster identity>
```

In the current topology the RPi5 grandmaster identity observed on NEXUS is:

```text
88a29efffe826ce6
```

If the RPi5 NIC or grandmaster identity changes later, use the currently advertised identity instead of treating that value as permanent.

Also verify the Nexus side:

```text
show ptp brief
```

The client-facing Nexus port should normally be `Master`.

## 9. Quick deployment checklist for another server

```text
NEXUS:
  interface Ethernet1/X
    ptp

Linux server:
  1. install linuxptp + ethtool
  2. verify `ethtool -T <NIC>`
  3. replace `enp4s0` in the three bundled files if needed
  4. install the config and two systemd units
  5. disable chrony/timesyncd/ntpd if they discipline CLOCK_REALTIME
  6. enable ptp4l-client and phc2sys-client
  7. verify logs + pmc output
```

## 10. Troubleshooting

If `ptp4l` stays in LISTENING:

```bash
sudo tcpdump -ni enp4s0 'udp port 319 or udp port 320'
journalctl -u ptp4l-client -n 100 --no-pager
```

Then check NEXUS:

```text
show ptp brief
show ptp packet
```

If the NIC PHC synchronizes but Linux system time does not, inspect:

```bash
journalctl -u phc2sys-client -n 100 --no-pager
systemctl status chrony systemd-timesyncd ntp ntpd
```

Only one service should be responsible for disciplining `CLOCK_REALTIME` on the client.