sptp/

SPTP

Simplified Unicast PTP client

Overview

SPTP was designed to greatly simplify the PTP unicast packet exchange, while still preserving the original PTP packet payload formats.

Motivation

PTP was initially designed to operate in networks that support multicast. Support for unicast was added later on. There are currently several issues that could be optimized:

• Protocol (as defined in IEEE 1588-2019) is too complex. In the context of unicast, the protocol requires a number of additional handshakes, subscriptions, timers, etc that might not be necessary (unicast negotiation, unicast discovery, duration field).
• Protocol makes any implementation fragile (multiple state machines)

Design

Packet exchange sequence:

1. Client sends DELAY_REQ (with ProfileSpecific1 flag set) effectively initiating an exchange with the Server. The Client records timestamp T3
2. Server records CF_2 from DELAY_REQ
3. Server records the RX timestamp T4
4. Server sends SYNC. The server adds timestamp T4 in the originTimestamp field and records the TX timestamp T1
5. Server sends ANNOUNCE with a TX timestamp T1 of the SYNC in originTimestamp field and CF_2 from DELAY_REQ in a correctionField
6. Client records T2 of the received SYNC packet, and also CF_1
7. Client records data from ANNOUNCE packet, and also CF_2

As a result of this exahange the client has access to T1, T2, T3, T4, CF_1, CF_2 to calculate mean path delay and offset metrics. And ANNOUNCE message content allows traditional BMC to be used for best master selection.

This sequence is repeated based on configured interval.

As you can see, there is no state on the server, no subscription to maintain. Client doesn't need to keep a complex state machine - all it needs it to send one packet and wait for two packets in response with some timeout.

By keeping the packets unchanged from original PTP spec we continue to enjoy PTP NICs timestamping support and network switches acting as Transparent Clocks. The only consideration here is that one-step Transparent Clocks are supported.

Messages

Only 3 message types are used:

• DELAY_REQ
• SYNC
• ANNOUNCE

DELAY_REQ

Packet with format described in IEEE 1588-2019 section 13.6, sent to port 319.

For server to recognise this as a start of SPTP exchange, flagField must have both Unicast and Profile Specific 1 bits set to 1.

SYNC

Packet with format described in IEEE 1588-2019 section 13.6, sent to port 319 in response to DELAY_REQ, with sequenceId copied from it.

Additionally, originTimestamp field contains T4 (time when server received DELAY_REQ)

ANNOUNCE

Packet with format described in IEEE 1588-2019 section 13.5, sent to port 320.

Additionally, originTimestamp field contains T1 (time when server sent SYNC), and correctionField contains CF_2 from received DELAY_REQ packet.

Quick Installation

go install github.com/facebook/time/cmd/sptp@latest

Requirements

Linux with a NIC that either has PHC and supports hardware timestamping or at very least supports SOF_TIMESTAMPING_TX_SOFTWARE to be used in 'software' ts mode (see Linux Kernel timestamping docs for details).

Configuration

Example config:

$ cat /etc/sptp.yaml
iface: eth0
interval: 1s
exchangetimeout: 100ms
timestamping: hardware
freerunning: false
monitoringport: 4269
dscp: 35
firststepthreshold: 1s
metricsaggregationwindow: 60s
maxclockclass: 7
maxclockaccuracy: 37
attemptstxts: 100
timeouttxts: 1ms
servers:
  "192.168.0.10": 1
  "192.168.0.11": 2
measurement:
  path_delay_filter_length: 59
  path_delay_filter: "median"
  path_delay_discard_filter_enabled: true
  path_delay_discard_below: 2us
  path_delay_discard_above: 1ms
backoff:
  mode: "linear"
  step: 10
  maxvalue: 60

Server

Currently the only server implementation is the latest ptp4u.