February 5, 2023

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Technology Room

How Precision Time Protocol is being deployed at Meta

4 min read

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

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 potential

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 scenario by which a consumer writes information and instantly tries to learn it. In giant distributed programs, likelihood is excessive that the write and the learn will land on completely different back-end nodes.

If the learn is hitting a distant duplicate that doesn’t but have the newest replace, there’s a likelihood the consumer is not going to see their very own write:

Precision Time Protocol
Schematic illustration of learn returning outdated data

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

The standard solution to clear up that is to concern a number of reads to completely different replicas and await a quorum choice. This not solely consumes further assets but in addition 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 duplicate catches up with the learn timestamp:

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

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

An important 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, nevertheless, 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 are able to 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 may argue that we don’t actually need PTP for that. NTP will just do superb. Effectively, 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 towards PTP- and NTP-backed clusters.

There are a number of extra use circumstances, 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 cut back {hardware} capability necessities. It will 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 evaluations, we landed on a design that may be cut up into three primary elements: 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 purchasers; the rack consists of a number of important elements, every of which has been rigorously chosen and examined.

The antenna

The GNSS antenna is definitely one of many least appreciated elements. However that is the place the place time originates, no 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. Now we have 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 customary 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 stunning views:

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

Whereas we had been testing completely different antenna options, a comparatively new GNSS-over-fiber expertise bought our consideration. It’s free from virtually 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 possibly 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 checks, we confirmed that the end-to-end antenna delay is deterministic (usually about a couple of hundred nanoseconds) and might simply be compensated on the Time Equipment aspect.

Time Equipment

The Time Equipment is the guts of the timing infrastructure. That is the place time originates from the information heart infrastructure viewpoint. In 2021, we printed an article explaining why we developed a brand new Time Equipment and why current options wouldn’t reduce it.

However this was largely within the context of NTP. PTP, however, brings even increased necessities and tighter constraints. Most significantly, we made a dedication to reliably help as much as 1 million purchasers per equipment with out hurting accuracy and precision. To realize this, we took a important have a look at lots of the conventional elements 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 important bug or a malicious assault,we determined to begin diversification from the supply of time — the Time Card. Final time, we spoke loads concerning the Time Card design and the benefits of an FPGA-based answer. Below the Open Compute Venture (OCP), we’re collaborating with distributors equivalent 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 important 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 state of affairs.
  • 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 equivalent to a laser temperature and lock must be monitored completely, and quick selections have to be made when the values are exterior of operational vary.

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

Community card

Our final aim is to make protocols equivalent 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 have to be precisely disciplined.

If we merely copy the clock values from Time Card to the NIC, utilizing the phc2sys or the same software, the accuracy is not going to be practically sufficient. Actually, our experiments present that we’d simply lose ~1–2 microseconds whereas going by way of 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 help for varied 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 an extra cable between the PPS output of the Time Card and the PPS enter of the NIC, as proven within the image beneath.

PTP
Quick 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 make sure that we truly know what’s occurring 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 finest, we may get round 50K purchasers 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 must 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 capable of help over 1 million concurrent purchasers per system, which was independently verified by an IEEE 1588v2 licensed system.

This was potential by way of the easy however elegant use of channels in Go that allowed us to cross subscriptions round between a number of highly effective staff.

As a result of ptp4u runs as a course of on a Linux machine, we robotically get all the advantages, like IPv6 help, firewall, and so on., without spending a dime.

c4u

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

With a view to continuously generate these parameters, we carried out a separate service known as c4u, which consistently displays a number of sources of knowledge and compiles the lively config for ptp4u:

Precision Time Protocol
Schematic illustration of the c4u structure.

This provides us flexibility and reactivity if the surroundings modifications. For instance, if we lose the GNSS sign on one of many Time Home equipment, we are going to change the ClockClass to HOLDOVER and purchasers will instantly migrate away from it. It’s also calculating ClockAccuracy from many alternative sources, equivalent 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 cross Worldwide Atomic Time (TAI) all the way down to the purchasers.

Calnex Sentinel

We needed to verify our Time Home equipment are consistently and independently assessed by a well-established licensed monitoring system. Fortunately, we’ve already made quite a lot of progress within the NTP area with Calnex, and we had been able to use the same method to  PTP.

We collaborated with Calnex to take their area system and repurpose it for information heart use, which concerned altering the bodily kind issue and including help for options equivalent 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 big information heart deployments, unicast is most well-liked over multicast as a result of it considerably simplifies community design and software program necessities.

Let’s check out a typical PTP unicast move:

A consumer begins the negotiation (requesting unicast transmission). Subsequently, 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 impartial 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 throughout 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’s going to appear like this:

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

Clear clocks

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

  • You want community tools 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 data has to cross by way of the boundary clocks from the time servers all the way down to the purchasers, we must implement particular help for this. 

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

Clear clocks (TCs) allow purchasers to account for variations in community latency, making certain a way more exact estimation of clock offset. Every information heart change within the path between consumer and time server studies the time every PTP packet spends transiting the change by updating a area within the packet payload, the aptly named Correction Discipline (CF).

PTP purchasers (additionally known as odd 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 area values (CFa and CFb), as proven within the diagram beneath:

Precision Time Protocol
Schematic illustration of the clear clock and correction area.
  • T1 is the {hardware} timestamp when the SYNC packet is shipped 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 shipped 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 know the influence of only one disabled clear clock on the way in which between consumer and a server, we are able to look at the logs:

We are able to see the trail delay explodes, generally even turning into unfavorable which shouldn’t occur throughout regular operations. This has a dramatic influence 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. It will 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 solution to detect this. 500 microseconds might not sound like loads, however when clients anticipate 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 characteristic supported by the Linux kernel for many years. For instance software program (kernel) timestamps have been utilized by NTP daemons for years. It’s necessary to know that timestamps should not included into the packet payload by default and if required, have to be positioned there by the consumer utility.

Studying RX timestamp from the consumer area 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 a little bit extra difficult. 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 consumer has to poll the socket till the timestamp is precisely positioned by the kernel. Usually we now have to attend for a number of milliseconds which naturally limits the ship charge.

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

It’s very simple to confirm the help 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 potential to make use of PTP with software program (kernel) timestamps, however there gained’t be any sturdy ensures on their high quality, precision, and accuracy.

We evaluated this risk as effectively and even thought-about 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 performing as each a PTP consumer and a PTP server. Whereas we needed to implement our personal PTP server answer for efficiency causes, we determined to stay with ptp4l for the consumer use case.

Preliminary checks within the lab revealed that ptp4l can present wonderful 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 circumstances

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 crucial standard NICs in the marketplace:

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

This finally led to the professional timestamps being displaced by timestamps coming from different packets. However what made issues loads 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 not possible to repair.

We needed to implement an offset outliers filter, which modified the conduct of PI servo and made it stateful. It resulted in occasional outliers being discarded and the imply frequency set through 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 concern 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 equipped by Time Servers in Announce messages:

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

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

To fight this, we launched a so-called “sharding” with completely different PTP purchasers 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 problem with one server in every subgroup taking the complete load for that grouping. The answer was to allow purchasers to precise a desire, and so we launched Priority3 into the choice standards simply above the MAC tackle tiebreaker.  Because of this purchasers configured to make use of the identical Time Home equipment can choose 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 are able to distribute load evenly throughout all Time Home equipment underneath regular working circumstances.

One other main problem we confronted was making certain PTP labored with multi-host NICs – a number of hosts sharing the identical bodily community interface and subsequently a single PHC. Nevertheless, ptp4l has no data 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 components contained in the kernel driver. The precise PHC is just not affected and retains operating free. This mode leads to a barely worse precision, however it’s utterly clear to ptp4l

Different NIC producers solely help a “actual time clock” mode, when the primary host to seize the lock truly disciplines the PHC. The benefit here’s a extra exact calibration and better high quality holdover, however it results in a separate concern with ptp4l operating on the opposite hosts utilizing the identical NIC as makes an attempt to tune PHC frequency haven’t any influence, 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 circumstances and lots of extra.

Different PTP purchasers

We’re evaluating the potential for utilizing another PTP consumer. Our primary standards are:

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

We’re evaluating the Timebeat PTP consumer and, up to now, it appears very promising.

Constantly incrementing counter

Within the PTP protocol, it doesn’t actually matter what time we propagate so long as we cross a UTC offset all the way down to the purchasers. In our case, it’s Worldwide Atomic Time (TAI), however some individuals might 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, however it’s of no profit until there’s a solution 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 data from PHC and ptp4l and exposes simple digestible Window Of Uncertainty data:

Fbclock structure

 

Via 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 components to calculate the Window Of Uncertainty (WOU):

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

As you might even see, the API doesn’t return the present time (aka time.Now()). As an alternative, it returns a window of time which incorporates 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 method permits clients to attend till the interval is handed to make sure actual transaction ordering.

Error certain 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 have to be is decided by how important it’s that the time be appropriate and that is pushed by the particular utility.

In our case, this certainty must be higher than 99.9999% (6-9s). At this stage of reliability you possibly can anticipate lower than 1 error in 1,000,000 measurements.

The error charge estimation makes use of commentary of the historical past of the information (histogram) to suit a chance distribution operate (PDF). From the chance distribution operate 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 odd clock.

To estimate the full variance (E2E) it’s essential to know the variance of the time error amassed by the point server all the way in which to the tip node NIC. This consists of 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 depends upon 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 (customary 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 Legislation.”

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

On the opposite arms for our estimation, we are able to 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 could be estimated with a great 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 are able to 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

Because of this 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 logical and nice, there’s a large assumption there. The belief is that the connection to the open time server (OTS) is out there, and all the things is in regular operation mode. Numerous issues can go improper such because the OTS taking place, change taking place, Sync messages not behaving as they’re purported to, one thing in between decides to get up the on-calls and so on. In such a scenario the error certain calculation ought to enter the holdover mode. The identical issues apply to the OTS when GNSS is down. In such a scenario the system will improve the Window Of Uncertainty based mostly on a compound charge. The speed will likely be estimated based mostly on the steadiness 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 honest quantity of labor by way of calibrating coefficients right here and attending to the perfect end result and we’re nonetheless engaged on these superb tunings. 

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

Subsequently, it’s essential to first account for synchronization loss intervals and use the perfect estimation of frequency correction (often, the scrolling common of earlier correction values) and second, account for the error certain improve by wanting on the final correction worth and evaluating it with the scrolling common of earlier correction values.

How we monitor the PTP structure

Monitoring is likely one of the most necessary components of the PTP structure. As a result of nature and influence of the service, we’ve spent fairly a little bit of time engaged on the tooling.

Calnex

We labored with the Calnex staff 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 capable of monitor three primary metrics per time equipment — NTP, PTP, and PPS.

Precision Time Protocol

This permits us to inform engineers about any concern 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

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

diag

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

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

fbclock

Consumer subcommand that permits 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 are able to 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 need to go even additional. We need 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 permits us to obviously see when the consumer experiences a problem — and infrequently earlier than the consumer even notices it.

Linearizability checks

PTP protocol (and ptp4l specifically) don’t have a quorum choice course of (not like NTP and chrony). This implies the consumer picks and trusts the Time Server based mostly on the knowledge supplied through Announce messages. That is true even when the Time Server itself is improper.

For such conditions, we now have carried out a final line of protection known as a linearizability test.

Think about a scenario by 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 scenario, the PTP consumer will assume all the things is okay, however the data it offers to the applying consuming time will likely be incorrect, because the Window of Uncertainty will likely be shifted and subsequently 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 test.

PTP is for as we speak and the longer term

We consider PTP will turn out to be the usual for conserving 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 important have a look at our complete infrastructure stack — from the GNSS antenna all the way down to the consumer API — and in lots of circumstances we’ve even rebuilt issues from scratch.

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

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

This journey is only one p.c completed.

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