How Precision Time Protocol is being deployed at Meta

How Precision Time Protocol is being deployed at Meta
How Precision Time Protocol is being deployed at Meta

Implementing Precision Time Protocol (PTP) at Meta permits us to synchronize the programs that drive our services and products all the way down to nanosecond precision. PTP’s predecessor, Community Time Protocol (NTP), offered us with millisecond precision, however as we scale to extra superior programs on our option to constructing the subsequent computing platform, the metaverse and AI, we have to be certain that our servers are preserving time as precisely and exactly as doable. With PTP in place, we’ll have the ability to improve Meta’s applied sciences and packages — from communications and productiveness to leisure, privateness, and safety — for everybody, throughout time zones and all over the world.

The journey to PTP has been years lengthy, as we’ve needed to rethink how each the timekeeping {hardware} and software program function inside our servers and information facilities. 

We’re sharing a deep technical dive into our PTP migration and our improvements which have made it doable

The case for PTP

Earlier than we dive into the PTP structure, let’s discover a easy use case for very correct timing, for the sake of illustration.

Think about a state of affairs during which a consumer writes information and instantly tries to learn it. In giant distributed programs, chances are high excessive that the write and the learn will land on completely different back-end nodes.

If the learn is hitting a distant reproduction that doesn’t but have the most recent replace, there’s a probability the person is not going to see their very own write:

Precision Time Protocol
Schematic illustration of learn returning outdated info

That is annoying on the very least, however extra vital is that that is violating a linearizability assure that enables for interplay with a distributed system in the identical manner as with a single server.

The everyday option to remedy that is to subject a number of reads to completely different replicas and watch for a quorum determination. This not solely consumes further assets but additionally considerably delays the learn due to the lengthy community round-trip delay.

Including exact and dependable timestamps on a again finish and replicas permits us to easily wait till the reproduction catches up with the learn timestamp:

Precision Time Protocol
Schematic illustration of a commit-wait guaranteeing consistency assure (linearizability).

This not solely hastens the learn but additionally saves tons of compute energy.

A vital situation for this design to work is that each one clocks be in sync or that the offset between a clock and the supply of time be identified. The offset, nonetheless, modifications due to fixed correction, drifting, or easy temperature variations. For that objective, we use the notion of a Window of Uncertainty (WOU), the place we will say with a excessive chance the place the offset is. On this specific instance, the learn must be blocked till the learn timestamp plus WOU.

One might argue that we don’t really want PTP for that. NTP will do exactly fantastic. Properly, we thought that too. However experiments we ran evaluating our state-of-the-art NTP implementation and an early model of PTP confirmed a roughly 100x efficiency distinction:

Precision Time Protocol
Commit-wait reads issued in opposition to PTP- and NTP-backed clusters.

There are a number of extra use instances, together with occasion tracing, cache invalidation, privateness violation detection enhancements, latency compensation within the metaverse, and simultaneous execution in AI, a lot of which is able to vastly scale back {hardware} capability necessities. This can hold us busy for years forward.

Now that we’re on the identical web page, let’s see how we deployed PTP at Meta scale.

The PTP structure

Precision Time Protocol
Regional PTP structure.

After a number of reliability and operational critiques, we landed on a design that may be break up into three foremost parts: the PTP rack, the community, and the consumer.

Buckle up — we’re going for a deep dive.

The PTP rack

This homes the {hardware} and software program that serves time to shoppers; the rack consists of a number of vital parts, every of which has been fastidiously chosen and examined.

The antenna

The GNSS antenna is well one of many least appreciated parts. However that is the place the place time originates, not less than on Earth. 

We’re striving for nanosecond accuracy. And if the GNSS receiver can’t precisely decide the place, it will be unable to calculate time. We’ve to strongly think about the signal-to-noise ratio (SNR). A low-quality antenna or obstruction to the open sky can lead to a excessive 3D location normal deviation error. For time to be decided extraordinarily precisely, GNSS receivers ought to enter a so-called time mode, which usually requires a <10m 3D error.

It’s completely important to make sure an open sky and set up a stable stationary antenna. We additionally get to get pleasure from some lovely views:

Precision Time Protocol
GNSS antenna in a Meta information middle location.

Whereas we had been testing completely different antenna options, a comparatively new GNSS-over-fiber expertise bought our consideration. It’s free from nearly all disadvantages — it doesn’t conduct electrical energy as a result of it’s powered by a laser through optical fiber, and the sign can journey a number of kilometers with out amplifiers. 

Contained in the constructing, it could use pre-existing structured fiber and LC patch panels, which considerably simplifies the distribution of the sign. As well as, the sign delays for optical fiber are effectively outlined at roughly 4.9ns per meter. The one factor left is the delay launched by the direct RF to laser modulation and the optical splitters, that are round 45ns per field.

PTP
Huber-Suhner GNSS-over-fiber expertise examined in Meta’s Dublin workplace.

By conducting exams, we confirmed that the end-to-end antenna delay is deterministic (sometimes about a number of hundred nanoseconds) and might simply be compensated on the Time Equipment facet.

Time Equipment

The Time Equipment is the center of the timing infrastructure. That is the place time originates from the info middle infrastructure viewpoint. In 2021, we revealed an article explaining why we developed a brand new Time Equipment and why present options wouldn’t reduce it.

However this was largely within the context of NTP. PTP, then again, brings even greater necessities and tighter constraints. Most significantly, we made a dedication to reliably assist as much as 1 million shoppers per equipment with out hurting accuracy and precision. To attain this, we took a vital have a look at lots of the conventional parts of the Time Equipment and thought actually laborious about their reliability and variety.

The Time Card

PTP Time Card
Time Card.

To guard our infrastructure from a vital bug or a malicious assault,we determined to begin diversification from the supply of time — the Time Card. Final time, we spoke rather a lot concerning the Time Card design and the benefits of an FPGA-based resolution. Below the Open Compute Undertaking (OCP), we’re collaborating with distributors akin to Orolia, Meinberg, Nvidia, Intel, Broadcom, and ADVA, that are all implementing their very own time playing cards, matching the OCP specification.

Oscillatord

The Time Card is a vital element that requires particular configuration and monitoring. For this objective, we labored with Orolia to develop a disciplining software, known as oscillatord, for various flavors of the Time Playing cards. This has turn out to be the default software for:

  • GNSS receiver configuration: setting the default config, and adjusting particular parameters like antenna delay compensation. It additionally permits the disabling of any variety of GNSS constellations to simulate a holdover situation.
  • GNSS receiver monitoring: reporting variety of satellites, GNSS high quality, availability of various constellations, antenna standing, leap second, and so on.
  • Atomic clock configuration: Totally different atomic clocks require completely different configuration and sequence of occasions. For instance, it helps SA53 TAU configuration for quick disciplining, and with mRO-50, it helps a temperature-to-frequency relation desk.
  • Atomic clock monitoring: Parameters akin to a laser temperature and lock need to be monitored completely, and quick choices should be made when the values are outdoors of operational vary.

Successfully, the info exported from oscillatord permits us to resolve whether or not the Time Equipment ought to take site visitors or must be drained.

Community card

Our final aim is to make protocols akin to PTP propagate over the packet community. And if the Time Card is the beating coronary heart of the Time Equipment, the community card is the face. Each time-sensitive PTP packet will get {hardware} timestamped by the NIC. This implies the PTP {Hardware} Clock (PHC) of the NIC should be precisely disciplined.

If we merely copy the clock values from Time Card to the NIC, utilizing the phc2sys or an analogous software, the accuracy is not going to be almost sufficient. In truth, our experiments present that we’d simply lose ~1–2 microseconds whereas going by PCIe, CPU, NUMA, and so on. The efficiency of synchronization over PCIe bus will dramatically enhance with the rising Precision Time Measurement (PTM) expertise, as the event and assist for numerous peripherals with this functionality is in progress.

For our utility, since we use NICs with PPS-in capabilities, we employed ts2phc, which copies clock values at first after which aligns the clock edges based mostly on a pulse per second (PPS) sign. This requires a further cable between the PPS output of the Time Card and the PPS enter of the NIC, as proven within the image beneath.

PTP
Brief cable between PPS-out of the Time Card and PPS-in of the NIC.

We consistently monitor offset and ensure it by no means goes out of a ±50ns window between the Time Card and the NIC:

Precision Time Protocol
Offset between the Time Card and the Community Card PHC.

We additionally monitor the PPS-out interface of the NIC to behave as a fail-safe and be certain that we really know what’s happening with the PHC on the NIC.

ptp4u

Whereas evaluating completely different preexisting PTP server implementations, we skilled scalability points with each open supply and closed proprietary options, together with the FPGA-accelerated PTP servers we evaluated. At greatest, we might get round 50K shoppers per server. At our scale, this implies we must deploy many racks full of those units.

Since PTP’s secret sauce is the usage of {hardware} timestamps, the server implementation doesn’t need to be a extremely optimized C program and even an FPGA-accelerated equipment.

We carried out a scalable PTPv2 unicast PTP server in Go, which we named ptp4u, and open-sourced it on GitHub. With some minor optimizations, we had been in a position to assist over 1 million concurrent shoppers per system, which was independently verified by an IEEE 1588v2 licensed system.

This was doable by the straightforward however elegant use of channels in Go that allowed us to go subscriptions round between a number of highly effective employees.

As a result of ptp4u runs as a course of on a Linux machine, we routinely get all the advantages, like IPv6 assist, firewall, and so on., totally free.

c4u

The ptp4u server has many configuration choices, permitting it to go dynamically altering parameters akin to PTP Clock Accuracy, PTP Clock Class, and a UTC offset — that’s at the moment set to 37 seconds (we’re  trying ahead this changing into a relentless) — all the way down to shoppers.

So as to incessantly generate these parameters, we carried out a separate service known as c4u, which consistently screens a number of sources of data and compiles the energetic config for ptp4u:

Precision Time Protocol
Schematic illustration of the c4u structure.

This offers us flexibility and reactivity if the atmosphere modifications. For instance, if we lose the GNSS sign on one of many Time Home equipment, we’ll change the ClockClass to HOLDOVER and shoppers will instantly migrate away from it. It is usually calculating ClockAccuracy from many alternative sources, akin to ts2phc synchronization high quality, atomic clock standing, and so forth.

We calculate the UTC offset worth based mostly on the content material of the tzdata bundle as a result of we go Worldwide Atomic Time (TAI) all the way down to the shoppers.

Calnex Sentinel

We wished to ensure our Time Home equipment are consistently and independently assessed by a well-established licensed monitoring system. Fortunately, we’ve already made plenty of progress within the NTP house with Calnex, and we had been ready to use an analogous strategy to  PTP.

We collaborated with Calnex to take their subject system and repurpose it for information middle use, which concerned altering the bodily kind issue and including assist for options akin to IPv6.

PTP
Calnex Sentinel 2.0 put in within the PTP rack.

We join the Time Equipment NIC PPS-out to the Calnex Sentinel, which permits us to observe the PHC of the NIC with nanosecond accuracy.

We are going to discover monitoring in nice element in “How we monitor the PTP structure,” beneath. 

The PTP community

PTP protocol

The PTP protocol helps the usage of each unicast and multicast modes for the transmission of PTP messages. For giant information middle deployments, unicast is most popular over multicast as a result of it considerably simplifies community design and software program necessities.

Let’s check out a typical PTP unicast movement:

A consumer begins the negotiation (requesting unicast transmission). Due to this fact, it should ship: 

  • A Sync Grant Request (“Hey server, please ship me N Sync and Comply with-Up messages per second with the present time for the subsequent M minutes”)
  • An Announce Grant Request (“Hey server, please ship me X Announce messages per second together with your standing for the subsequent Y minutes”)
  • A Delay Response Grant Request (“Hey server, I’m going to ship you Delay Requests — please reply with Delay Response packets for the subsequent Z minutes”)
  1. The server must grant these requests and ship grant responses. 
  2. Then the server wants to begin executing subscriptions and sending PTP messages. 
  • All subscriptions are unbiased of each other.
  • It’s on the server to obey the ship interval and terminate the subscription when it expires. (PTP was initially multicast solely, and one can clearly see the multicast origin on this design).
  • In two-step configuration, when the server sends Sync messages, it has to learn the TX {hardware} timestamp and ship a Comply with-Up message containing that timestamp.
  1. The consumer will ship Delay Requests inside the agreed-upon interval to find out the trail delay. The server must learn the RX {hardware} timestamp and return it to the consumer.
  2. The consumer must periodically refresh the grant, and the method repeats.

Schematically (only for the illustration), it’ll seem like this:

Precision Time Protocol
Schematic illustration of the two-step PTP trade.

Clear clocks

We initially thought of leveraging boundary clocks in our design. Nevertheless, boundary clocks include a number of disadvantages and issues:

  • You want community gear or some particular servers to behave as a boundary clock. 
  • A boundary clock acts as a time server, creating better demand for short-term stability and holdover efficiency.
  • Because the info has to go by the boundary clocks from the time servers all the way down to the shoppers, we must implement particular assist for this. 

To keep away from this extra complexity, we determined to rely solely on PTP clear clocks.

Clear clocks (TCs) allow shoppers to account for variations in community latency, guaranteeing a way more exact estimation of clock offset. Every information middle change within the path between consumer and time server reviews the time every PTP packet spends transiting the change by updating a subject within the packet payload, the aptly named Correction Subject (CF).

PTP shoppers (additionally known as extraordinary clocks, or OCs) calculate community imply path delay and clock offsets to the time servers (grandmaster clocks, or GMs) utilizing 4 timestamps (T1, T2, T3, and T4) and two correction subject values (CFa and CFb), as proven within the diagram beneath:

Precision Time Protocol
Schematic illustration of the clear clock and correction subject.
  • T1 is the {hardware} timestamp when the SYNC packet is distributed by the Time Server.
  • T2 is the {hardware} timestamp when the OC receives the SYNC packet.
  • CFa is the sum of the change delays recorded by every change (TC) within the path from time server to the consumer (for SYNC packet).
  • T3 is the {hardware} timestamp the delay request is distributed by the Consumer.
  • T4 is the {hardware} timestamp when the time server receives the delay request.
  • CFb is the sum of the change delays recorded by every change within the path from the Consumer to the time server (for Delay Request packet).

To grasp the impression of only one disabled clear clock on the best way between consumer and a server, we will look at the logs:

We are able to see the trail delay explodes, typically even changing into unfavourable which shouldn’t occur throughout regular operations. This has a dramatic impression on the offset, shifting it from ±100 nanoseconds to -400 microseconds (over 4000 occasions distinction). And the worst factor of all, this offset is not going to even be correct, as a result of the imply path delay calculations are incorrect.

In line with our experiments, trendy switches with giant buffers can delay packets for as much as a few milliseconds which is able to end in lots of of microseconds of a path delay calculation error. This can drive the offset spikes and will likely be clearly seen on the graphs:

The underside line is that operating PTP in datacenters within the absence of TCs results in unpredictable and unaccountable asymmetry within the roundtrip time. And the worst of all – there will likely be no easy option to detect this. 500 microseconds could not sound like rather a lot, however when clients count on a WOU to be a number of microseconds, this will likely result in an SLA violation.

The PTP Consumer

Timestamps

Timestamping the incoming packet is a comparatively outdated function supported by the Linux kernel for many years. For instance software program (kernel) timestamps have been utilized by NTP daemons for years. It’s vital to know that timestamps should not included into the packet payload by default and if required, should be positioned there by the person utility.

Studying RX timestamp from the person house is a comparatively easy operation. When packet arrives, the community card (or a kernel) will timestamp this occasion and embrace the timestamp into the socket control message, which is simple to get together with the packet itself by calling a recvmsg syscall with MSG_ERRQUEUE flag set.

A really tough illustration of a socket management message containing timestamps.
128 bits 64 bits 64 bits 64 bits
Socket management message header Software program Timestamp Legacy Timestamp {Hardware} Timestamp

For the TX {Hardware} timestamp it’s slightly extra sophisticated. When sendto syscall is executed it doesn’t result in a right away packet departure and neither to a TX timestamp technology. On this case the person has to poll the socket till the timestamp is precisely positioned by the kernel. Typically we’ve to attend for a number of milliseconds which naturally limits the ship charge.

{Hardware} timestamps are the key sauce that makes PTP so exact. A lot of the trendy NICs have already got {hardware} timestamps assist the place the community card driver populates the corresponding part. 

It’s very straightforward to confirm the assist by operating the ethtool command:

$ ethtool -T eth0
Time stamping parameters for eth0:
Capabilities:
	hardware-transmit
	hardware-receive
	hardware-raw-clock
PTP {Hardware} Clock: 0
{Hardware} Transmit Timestamp Modes:
	off
	on
{Hardware} Obtain Filter Modes:
	none
	All

It’s nonetheless doable to make use of PTP with software program (kernel) timestamps, however there received’t be any robust ensures on their high quality, precision, and accuracy.

We evaluated this chance as effectively and even thought of implementing a change within the kernel for “faking” the {hardware} timestamps with software program the place {hardware} timestamps are unavailable. Nevertheless, on a really busy host we noticed the precision of software program timestamps jumped to lots of of microseconds and we needed to abandon this concept.

ptp4l

ptp4l is an open supply software program able to appearing as each a PTP consumer and a PTP server. Whereas we needed to implement our personal PTP server resolution for efficiency causes, we determined to stay with ptp4l for the consumer use case.

Preliminary exams within the lab revealed that ptp4l can present glorious synchronization high quality out of the field and align time on the PHCs within the native community all the way down to tens of nanoseconds.

Nevertheless, as we began to scale up our setup some points began to come up.

Edge instances

In a single specific instance we began to note occasional “spikes” within the offset. After a deep dive we recognized elementary {hardware} limitations of one of the standard NICs in the marketplace:

  • The NIC has solely a timestamp buffer for 128 packets.
  • The NIC is unable to differentiate between PTP packets (which want a {hardware} timestamp) and different packets which don’t.

This in the end led to the reliable timestamps being displaced by timestamps coming from different packets. However what made issues rather a lot worse – the NIC driver tried to be overly intelligent and positioned the software program timestamps within the {hardware} timestamp part of the socket management message with out telling anybody.

It’s a elementary {hardware} limitation affecting a big portion of the fleet which is inconceivable to repair.

We needed to implement an offset outliers filter, which modified the habits of PI servo and made it stateful. It resulted in occasional outliers being discarded and the imply frequency set throughout the micro-holdover:

If not for this filter, ptp4l would have steered PHC frequency actually excessive, which might end in a number of seconds of oscillation and dangerous high quality within the Window of Uncertainty we generate from it.

One other subject arose from the design of BMCA. The aim of this algorithm is to pick the perfect Time Equipment when there  are a number of to select from within the ptp4l.conf. It does by evaluating  a number of attributes provided by Time Servers in Announce messages:

  1. Precedence 1
  2. Clock Class
  3. Clock Accuracy
  4. Clock Variance
  5. Precedence 2
  6. MAC Deal with

The issue manifests itself when all aforementioned attributes are the identical. BMCA makes use of Time ApplianceMAC handle because the tiebreaker which implies underneath regular working circumstances one Time Server will appeal to all consumer site visitors.

To fight this, we launched a so-called “sharding” with completely different PTP shoppers being allotted to completely different sub-groups of Time Home equipment from the complete pool.

Precision Time Protocol
Schematic illustration of sharding.

This solely partially addressed the difficulty with one server in every subgroup taking the complete load for that grouping. The answer was to allow shoppers to specific a choice, and so we launched Priority3 into the choice standards simply above the MAC handle tiebreaker.  Which means that shoppers configured to make use of the identical Time Home equipment can want completely different servers.

Consumer 1:

[unicast_master_table]

UDPv6 time_server1 1

UDPv6 time_server2 2

UDPv6 time_server3 3

Consumer 2:

[unicast_master_table]

UDPv6 time_server2 1

UDPv6 time_server3 2

UDPv6 time_server1 3

This ensures we will distribute load evenly throughout all Time Home equipment underneath regular working circumstances.

One other main problem we confronted was guaranteeing PTP labored with multi-host NICs – a number of hosts sharing the identical bodily community interface and due to this fact a single PHC. Nevertheless, ptp4l has no information of this and tries to self-discipline the PHC like there are not any different neighbors.

Some NIC producers developed a so-called “free operating” mode the place ptp4l is simply  disciplining the method contained in the kernel driver. The precise PHC is just not affected and retains operating free. This mode leads to a barely worse precision, nevertheless it’s utterly clear to ptp4l

Different NIC producers solely assist a “actual time clock” mode, when the primary host to seize the lock really disciplines the PHC. The benefit here’s a extra exact calibration and better high quality holdover, nevertheless it results in a separate subject with ptp4l operating on the opposite hosts utilizing the identical NIC as makes an attempt to tune PHC frequency don’t have any impression, resulting in inaccurate clock offset and frequency calculations.

PTP profile

To explain the datacenter configuration, we’ve developed and published a PTP profile, which displays the aforementioned edge instances and plenty of extra.

Different PTP shoppers

We’re evaluating the potential of utilizing an alternate PTP consumer. Our foremost standards are:

  • Assist our PTP profile
  • Meets our synchronization high quality necessities
  • Open supply

We’re evaluating the Timebeat PTP consumer and, to date, it appears to be like very promising.

Constantly incrementing counter

Within the PTP protocol, it doesn’t actually matter what time we propagate so long as we go a UTC offset all the way down to the shoppers. In our case, it’s Worldwide Atomic Time (TAI), however some folks could select UTC. We like to consider the time we offer as a constantly incrementing counter.

At this level we’re not disciplining the system clock and ptp4l is solely used to self-discipline the NIC’s PHC.

fbclock

Synchronizing PHCs throughout the fleet of servers is nice, nevertheless it’s of no profit except there’s a option to learn and manipulate these numbers on the consumer.

For this objective, we developed a easy and light-weight API known as fbclock that gathers info from PHC and ptp4l and exposes straightforward digestible Window Of Uncertainty info:

Fbclock structure

 

By means of a really environment friendly ioctl PTP_SYS_OFFSET_EXTENDED, fbclock will get a present timestamps from the PHC, newest information from ptp4l after which applies math method to calculate the Window Of Uncertainty (WOU):

$ ptpcheck fbclock
"earliest_ns":1654191885711023134,"latest_ns":1654191885711023828,"wou_ns":694

As you may even see, the API doesn’t return the present time (aka time.Now()). As an alternative, it returns a window of time which comprises the precise time with a really excessive diploma of chance On this specific instance, we all know our Window Of Uncertainty is 694 nanoseconds and the time is between (TAI) Thursday June 02 2022 17:44:08:711023134 and Thursday June 02 2022 17:44:08:711023828.

This strategy permits clients to attend till the interval is handed to make sure precise transaction ordering.

Error sure measurement

Measuring the precision of the time or (Window Of Uncertainty) signifies that alongside the delivered time worth, a window (a plus/minus worth) is introduced that’s assured to incorporate the true time to a excessive stage of certainty. 

How sure we should be is set by how vital it’s that the time be right and that is pushed by the precise utility.

In our case, this certainty must be higher than 99.9999% (6-9s). At this stage of reliability you’ll be able to count on lower than 1 error in 1,000,000 measurements.

The error charge estimation makes use of remark of the historical past of the info (histogram) to suit a chance distribution perform (PDF). From the chance distribution perform one can calculate the variance (take a root sq. and get the usual deviation) and from there it is going to be easy multiplication to get to the estimation of the distribution based mostly on its worth.

Precision Time Protocol

Under is a histogram taken from the offset measurement from ptp4l operating on the extraordinary clock.

To estimate the overall variance (E2E) it’s essential to know the variance of the time error collected by the point server all the best way to the tip node NIC. This contains GNSS, atomic clock, and Time Card PHC to NIC PHC (ts2phc). The producer offers the GNSS error variance. Within the case of the UBX-F9T it’s about 12 nanoseconds. For the atomic clock the worth will depend on the disciplining threshold that we’ve set. The tighter the disciplining threshold, the smaller offset variance however decrease holdover efficiency. On the time of operating this experiment, the error variance of the atomic clock has been measured to 43ns (normal deviation, std). Lastly, the software ts2phc will increase the variance by 30ns (std) leading to a complete variance of 52ns.

The noticed outcomes matches the calculated variance obtained by the “Sum of Variance Regulation.”

In line with the sum of variance legislation, all we have to do is so as to add all of the variance. In our case, we all know that the overall observer E2E error (measured through the Calnex Sentinel) is about 92ns.

On the opposite fingers for our estimation, we will have the next:

Estimated E2E Variance = [GNSS Variance + MAC Variance + ts2phc Variance] + [PTP4L Offset Variance] = [Time Server Variance] + [Ordinary Clock Variance]

Plugging within the values:

Estimated E2E Variance = (12ns 2) + (43ns2) + (52ns2) + (61ns2) = 8418, which corresponds to 91.7ns

These outcomes present that by propagating the error variance down the clock tree, the E2E error variance might be estimated with a superb accuracy. The E2E error variance can be utilized to calculate the Window Of Uncertainty (WOU) based mostly on the next desk.

Merely, by multiplying the estimated E2E error variance in 4.745 we will estimate the Window Of Uncertainty for the chance of 6-9s.

For our given system the chance of 6-9s is about 92ns x 4.745 = 436ns

Which means that given a reported time by PTP, contemplating a window dimension of 436ns round worth ensures to incorporate the true time by a confidence of over 99.9999%.

Compensation for holdover

Whereas all of the above appears to be like logical and nice, there’s a huge assumption there. The idea is that the connection to the open time server (OTS) is obtainable, and all the things is in regular operation mode. Numerous issues can go unsuitable such because the OTS happening, change happening, Sync messages not behaving as they’re alleged to, one thing in between decides to get up the on-calls and so on. In such a state of affairs the error sure calculation ought to enter the holdover mode. The identical issues apply to the OTS when GNSS is down. In such a state of affairs the system will improve the Window Of Uncertainty based mostly on a compound charge. The speed will likely be estimated based mostly on the soundness of the oscillator (scrolling variance) throughout regular operations. On the OTS the compound charge will get adjusted by the correct telemetry monitoring of the system (Temperature, Vibration, and so on). There’s a truthful quantity of labor by way of calibrating coefficients right here and attending to the perfect final result and we’re nonetheless engaged on these fantastic tunings. 

Throughout the intervals of community synchronization availability, the servo is consistently adjusting the frequency of the native clock on the consumer facet (assuming the preliminary stepping resulted in convergence). A break within the community synchronization (from dropping connection to the time server or the time server itself happening) will depart the servo with a final frequency correction worth. Consequently, such worth is just not aimed to be an estimation of precision of the native clock however as a substitute a brief frequency adjustment to scale back the time error (offset) measured between the cline and the time server.

Due to this fact, it’s essential to first account for synchronization loss intervals and use the perfect estimation of frequency correction (normally, the scrolling common of earlier correction values) and second, account for the error sure improve by trying on the final correction worth and evaluating it with the scrolling common of earlier correction values.

How we monitor the PTP structure

Monitoring is among the most vital components of the PTP structure. As a result of nature and impression of the service, we’ve spent fairly a little bit of time engaged on the tooling.

Calnex

We labored with the Calnex group to create the Sentinel HTTP API, which permits us to handle, configure, and export information from the system. At Meta, we created and open-sourced an API command line software permitting human and script pleasant interactions.

Utilizing Calnex Sentinel 2.0 we’re in a position to monitor three foremost metrics per time equipment — NTP, PTP, and PPS.

Precision Time Protocol

This enables us to inform engineers about any subject with the home equipment and exactly detect the place the issue is. 

For instance, on this case each PTP and PPS monitoring resorts in a roughly lower than 100 nanosecond variation over 24 hours when NTP stays inside 8 microseconds.

ptpcheck

So as to monitor our setup, we carried out and open-sourced a software known as ptpcheck. It has many alternative subcommands, however probably the most attention-grabbing are the next:

diag

Consumer subcommand offers an total standing of a ptp consumer. It reviews the time of receipt of final Sync message, clock offset to the chosen time server, imply path delay, and different useful info:

$ ptpcheck diag
[ OK ] GM is current
[ OK ] Interval since final ingress is 972.752664ms, we count on it to be inside 1s
[ OK ] GM offset is 67ns, we count on it to be inside 250µs
[ OK ] GM imply path delay is 3.495µs, we count on it to be inside 100ms
[ OK ] Sync timeout depend is 1, we count on it to be inside 100
[ OK ] Announce timeout depend is 0, we count on it to be inside 100
[ OK ] Sync mismatch depend is 0, we count on it to be inside 100
[ OK ] FollowUp mismatch depend is 0, we count on it to be inside 100

fbclock

Consumer subcommand that enables querying of an fbclock API and getting a present Window of Uncertainty:

$ ptpcheck fbclock
"earliest_ns":1654191885711023134,"latest_ns":1654191885711023828,"wou_ns":694

sources

Chrony-style consumer monitoring, permits to see all Time Servers configured within the consumer configuration file, their standing, and high quality of time.

$ ptpcheck sources
+----------+----------------------+--------------------------+-----------+--------+----------+---------+------------+-----------+--------------+
| SELECTED |       IDENTITY       |         ADDRESS          |   STATE   | CLOCK  | VARIANCE |  P1:P2  | OFFSET(NS) | DELAY(NS) |  LAST SYNC   |
+----------+----------------------+--------------------------+-----------+--------+----------+---------+------------+-----------+--------------+
| true     | abcdef.fffe.111111-1 | time01.instance.com.      | HAVE_SYDY | 6:0x22 | 0x59e0   | 128:128 |         27 |      3341 | 868.729197ms |
| false    | abcdef.fffe.222222-1 | time02.instance.com.      | HAVE_ANN  | 6:0x22 | 0x59e0   | 128:128 |            |           |              |
| false    | abcdef.fffe.333333-1 | time03.instance.com.      | HAVE_ANN  | 6:0x22 | 0x59e0   | 128:128 |            |           |              |
+----------+----------------------+--------------------------+-----------+--------+----------+---------+------------+-----------+--------------+

oscillatord

Server subcommand, permits to learn a abstract from the Time Card.

$ ptpcheck oscillatord
Oscillator:
	mannequin: sa5x
	fine_ctrl: 328
	coarse_ctrl: 10000
	lock: true
	temperature: 45.33C
GNSS:
	repair: Time (3)
	fixOk: true
	antenna_power: ON (1)
	antenna_status: OK (2)
	leap_second_change: NO WARNING (0)
	leap_seconds: 18
	satellites_count: 28
	survey_in_position_error: 1
Clock:
	class: Lock (6)
	offset: 1

For instance, we will see that the final correction on the Time Card was simply 1 nanosecond.

phcdiff

This subcommand permits us to get a distinction between any two PHCs:

$ ptpcheck phcdiff -a /dev/ptp0 -b /dev/ptp2
PHC offset: -15ns
Delay for PHC1: 358ns
Delay for PHC2: 2.588µs

On this specific case the distinction between Time Card and a NIC on a server is -15 nanoseconds.

Consumer API

It’s good to set off monitoring periodically or on-demand, however we wish to go even additional. We wish to know what the consumer is definitely experiencing. To this finish, we embedded a number of buckets proper inside the fbclock API based mostly on atomic counters, which increment each time the consumer makes a name to an API:

Precision Time Protocol
Schematic illustration of fbclock API monitoring.

This enables us to obviously see when the consumer experiences a difficulty — and infrequently earlier than the consumer even notices it.

Linearizability checks

PTP protocol (and ptp4l specifically) don’t have a quorum choice course of (in contrast to NTP and chrony). This implies the consumer picks and trusts the Time Server based mostly on the data offered through Announce messages. That is true even when the Time Server itself is unsuitable.

For such conditions, we’ve carried out a final line of protection known as a linearizability examine.

Think about a state of affairs during which a consumer is configured to make use of three time servers and the consumer is subscribed to a defective Time Server (e.g., Time Server 2):

Precision Time Protocol
Consumer following Time Server 2.

On this state of affairs, the PTP consumer will assume all the things is ok, however the info it offers to the appliance consuming time will likely be incorrect, because the Window of Uncertainty will likely be shifted and due to this fact inaccurate. 

To fight this, in parallel, the fbclock establishes communication with the remaining time servers and compares the outcomes. If nearly all of the offsets are excessive, this implies the server our consumer follows is the outlier and the consumer is just not linearizable, even when synchronization between Time Server 2 and the consumer is ideal.

Precision Time Protocol
Schematic illustration of linearizability examine.

PTP is for right this moment and the long run

We imagine PTP will turn out to be the usual for preserving time in laptop networks within the coming a long time. That’s why we’re deploying it on an unprecedented scale. We’ve needed to take a vital have a look at our whole infrastructure stack — from the GNSS antenna all the way down to the consumer API — and in lots of instances we’ve even rebuilt issues from scratch.

As we proceed our rollout of PTP, we hope extra distributors who produce networking gear will benefit from our work to assist convey new gear that helps PTP to the trade. We’ve open-sourced most of our work, from our supply code to our {hardware}, and we hope the trade will be part of us in bringing PTP to the world. All this has all been achieved within the identify of boosting the efficiency and reliability of the present options, but additionally with an eye fixed towards opening up new merchandise, providers, and options sooner or later. 

We wish to thank everybody concerned on this endeavor, from Meta’s inner groups to distributors and producers collaborating with us. Particular thanks goes to Andrei Lukovenko, who related time fanatics.

This journey is only one % completed.