Data Center PTP Profile

Abstract​

This document defines a PTP profile for time-sensitive applications within a data center environment. The document is developed within the Open Compute Project (OCP) Timing Appliances Project community [1]. The PTP profile is based on IEEE Std 1588 TM-2019 [2]. When applicable, the profile also references and reuses information from other PTP profiles or other industry specifications. The document provides a set of requirements for implementing, deploying and operating timing appliances within a data center. A timing appliance is an element that is PTP-aware such as a switch/router, time server, NIC card, software module, timing card, monitoring device, etc.

1. Introduction​

Time is a key element to get the highest efficiency in a distributed system. The performance of a distributed system depends in part on the level of synchronization between the elements. Several industries such as telecom, power, industrial, automotive, professional audio and video have embraced the need for highly accurate and reliable distribution and synchronization of time across packet networks. Although the use case scenario for each of the industries is different, they all share one common thing and that is, time synchronization. Each use case scenario defines a set of requirements and configurations that are specified in a 'PTP profile'. This document defines a PTP profile to serve the needs of data center time-sensitive applications, data center network infrastructure and the use of synchronized clocks [3]. The profile specifies the set of PTP features and attribute values applicable to a PTP instance that operates in a single device (eg., such as a switch, router, server) and within exactly one PTP domain. Additionally, this specification also addresses additional requirements and use cases that are outside the definition of a PTP profile.

2. Terminology​

The IEEE 1588 committee is working on a project to recommend alternative terminology that is more inclusive than some of terminology currently used in IEEE Std 1588-2019. The IEEE project has not yet decided on the alternative terminology as of August 2021.

This document uses the following translation of terms used by IEEE1588:

IEEE Std 1588-2019 termsOCP DC PTP Profile terms
SlaveFollower
GrandmasterOpen Time Server or GM

3. PTP Profile Definition​

A PTP profile is "a document, or a portion of a document, specifying the set of PTP features and attribute values applicable to a PTP instance, and written by an organization following the specification of IEEE Std IEEE1588-2019. The profile allows organizations to specify selections of attribute values and optional features of PTP for the purpose of meeting requirements of a particular application. A PTP profile applicable to data center is defined in this document.

A PTP profile is a set of required options, prohibited options, and the ranges and defaults of configurable attributes. Some example are:

• Path delay measurement option (delay request-response or peer delay)

• Range and default values of all configurable attributes and dataset members

• PTP Instances types

• Options required, permitted, prohibited

• Uncertainty specifications

• Transport mechanisms required, permitted, or prohibited

• If relevant, the value of the observation interval τ used for PTP Variance measurements.

4. Reference Model

The model referenced in this section is designated as Model 1. The model consists of three layers. The time reference layer consists primarily of sourcing a time reference (e.g, GNSS) and the PTP Open Time Server (GM) functionality [4]. The network fabric layer consists of a set of network elements that support PTP clocks such as the transparent clock (TC) or the boundary clock (BC). The server layer consists of a group of end-hosts that support PTP clocks such as the ordinary clock (OC), and where the time-sensitive applications typically reside.

In Model 1, the network fabric layer consists of a chain of TCs.

4.1. Model 1 – Transparent Clock Model​

The high-level characteristics of Model 1 shown in Figure 1 are:

• GM (or Open Time Server) has a single network physical port and always distribute time towards the network fabric layer and server layer. The GM defined in this PTP profile (and the term GM used in this document) is a leader-only OC with a single PTP port according to 9.2.2.2 of IEEE Std 1588-2019.
• TC can have multiple network physical ports (eg., 16, 48). The TC can have multiple capable PTP ports.
• OC has a single network physical port and always receives time from the network fabric layer and the GM. The OC defined in this PTP profile (and the term OC used in this document) is a follower-only OC according to 9.2.2.1 of IEEE Std 1588-2019.
• All network elements that provide transport of the PTP messages between a GM and an OC are PTP-aware (i.e., TC-capable).
• Hardware timestamping (as close to the medium as possible) is available at each PTP port.
• In normal operating mode, an OC has connectivity to more than 1 GM.
• There are a number of GMs that are either active or standby.
• An OC communicates with a GM based on the unicast discovery and unicast negotiation protocol.
• Communication between PTP clocks is primarily based on IPv6.
• The PTP clock discovery and selection algorithm do not rely on multicast or broadcast communication.
• The meanPathDelay computation is based on the end-to-end delay mechanism.
• The number of TCs between GM and OC is constant. For example, if the number of TC = 5, then there will be 7 clocks in total (i.e., including 1 GM and 1 OC) with 6 links interconnecting the clocks.
• The forward path direction (GM to OC) and reverse path direction (OC to GM) might not be congruent. That is, PTP packets in the forward direction might traverse a different set of TCs from PTP packets in the reverse direction. However, the number of TCs in both directions in a non-congruent scenario is expected to always be the same and the effect on delay asymmetry is expected to be negligible.
• Delay asymmetry due to fiber links is assumed to be negligible, but likely not zero, in comparison to the time error requirements.

Figure 1. Model 1 – Chain of Transparent Clocks

5. Model 1 - Additional Requirements

• The higher layer applications require UTC traceability. The PTP protocol transports the PTP Timescale (i.e., TAI) plus all information to derive the UTC timescale from the TAI timescale. It is up to the application to perform timescale conversion.
• The maximum time error between any two OCs must be within ± 5 microseconds, i.e., |TOC,j – TOC,k| <= 5 μs for kj.
• The maximum time error between a GM and any OCs must be within ± 2.5 microseconds. i.e., |TGM – TOC| <= 2.5 μs.
• The maximum time error between any two GMs must be within ± 100 nanoseconds, i.e., |TGM,j – TGM,k | <= 100 ns for kj.
• The maximum time error generated by a TC must be within ± 100 nanoseconds, i.e., |TTC,j | <= 200 ns.
• In normal operating conditions, each OC has connectivity into multiple GMs. Under failure of a GM, an OC must be capable of having connectivity to at least another GM.

6. PTP Profile

The PTP profile is based on IEEE Std 1588-2019.

6.1. Profile Identifier​

The information below identifies the PTP profile. The profile is defined by OCP.

profileName: PTP profile for data center application (DC-PTP Profile 1)profileNumber: 1primaryVersion: 1revisionNumber: 0profileIdentifier: 7A-4D-2F-01-01-00organizationName: Open Compute Project (OCP)sourceIdentification: This profile is specified by OCP and can be downloaded from [https://www.opencompute.org]( https://www.opencompute.org/contributions?refinementList%5Bcontributor%5D%5B0%5D=Facebook&refinementList%5Baccept_year%5D%5B0%5D=2021&refinementList%5Bproject%5D%5B0%5D=Time%20Appliances%20Project%20%28TAP%29&refinementList%5Bfamily%5D%5B0%5D=Information&configure%5BfacetFilters%5D%5B0%5D=archived%3Afalse&page=1)

6.2. Clock Types​

The profile allows for the following clocks to be used. See clause 3 of IEEE Std 1588-2019 for the full definitions.

ClockDefinition
GMThe PTP clock that is the source of time for all clocks in the PTP domain.
TCA PTP clock that measures the time of a PTP event message transiting the PTP clock, and that provides this information to PTP clocks receiving this PTP event message. The PTP clock in this profile supports the delay request – response mechanism (i.e., end-to-end Transparent Clock).
OCA PTP clock that has a single PTP port in the PTP domain and maintains the timescale used in the domain.

Some additional requirements that pertain to the GM and that are outside the PTP Profile are defined in the OCP-TAP Open Time Server project [4].

6.3. Message Types​

The profile allows for the following messages:

1. Announce
2. Sync
3. Follow_Up
4. Delay_Req
5. Delay_Resp
6. Signaling
7. Management

6.4. Transport mechanisms required, permitted, or prohibited​

The transport mechanism UDP over IPv6 per Annex D of IEEE Std 1588-2019 must be supported.

The transport mechanism UDP over IPv4 per Annex C should be supported.

The UDP checksum must be computed when a PTP message is retransmitted (see 3.1.65 of IEEE Std 1588-2019) by a TC.

The UDP destination port numbers per Annex C.2 of IEEE Std 1588-2019 must be supported. The UDP source port number of a unicast PTP message can be any ephemeral port number and should be preserved throughout the lifetime of a PTP connection that has been established using the unicast negotiation mechanism.

6.5. Clock identity​

The clockIdentity must be an EUI-64 as specified in 7.5.2.2 of IEEE Std 1588-2019. The EUI-64 must be globally unique. If the EUI-64 is formed from an existing EUI-48, it must be done by appending two octets after the final six octets of the EUI-48 such that the 64 bits of the clockIdentity are not the same as the bits of any EUI-64 that has previously been assigned or may be assigned in the future by an authorized assignee of the MA-L, MA-M, or MA-S from which the EUI-48 was assigned. This means that either the entity that forms the EUI-64 owns the MA-L, MA-M, or MA-S from which the EUI-48 was formed, or the owner of that MA-L, MA-M, or MA-S has given the entity that forms the EUI-64 the sole right to the clockIdentity being formed.

Note: When using the MAC address, the clock identity is created by appending two octets after the final six octets of the MAC address. Note that in IEEE Std 1588-2008 the clock identity was formed by adding the two octets 'FFFE' between the 3rd octet and 4th octet of the MAC address, however, that mapping has been deprecated by the IEEE.

6.6. Path delay Measurement Mechanism​

The path delay measurement mechanism must be the delay request-response mechanism. The value of the data set member portDS.delayMechanism must be E2E.

6.7. Class of Service​

PTP event messages should set the DSCP field of the IPv6 Traffic Class field to the highest class of service possible. This should minimize latency and delay variation as PTP packets traverse a set of transparent clocks. In Model 1, the GM and OC should set the traffic class value.

6.8. PTP Security​

PTP security is out of scope given the network will be a single trusted domain managed by a single entity.

6.9. Profile Isolation and Domain Number​

All PTP instances must communicate using a single domain number, and the domainNumber value must be zero.

The sdoId is a new parameter in IEEE Std 1588-2019. A recognized standards organization, industry trade association, regulatory or government organization, or other organization as described in 20.3.2 of IEEE Std 1588-2019, can obtain an sdoId from the IEEE Registration Authority (RA). The sdoId is used to ensure that a PTP profile is isolated from any other PTP profiles running on the same network that are developed by other organizations.

An organization can obtain only one sdoId. If the organization develops multiple PTP profiles and requires that they be isolated, the isolation is further done using domainNumber. If an organization does not obtain an sdoId, the PTP profile will use the sdoId 0x000.

This PTP profile does not require an sdoId since it is assumed it will be the only profile within the data center network. If the assumption is not correct, another profile running on the network will conflict with this profile if the sdoId and domainNumber of the other profile are both zero.

Note – The sdoId is backward compatible with IEEE Std 1588-2008. The first nibble of the sdoId, i.e., the majorSdoId, corresponds to the transportSpecific field of IEEE Std 1588-2008. The final 8 bits of the sdoId, i.e., the minorSdoId, was reserved in IEEE Std 1588-2008 and was specified as 0x00.

6.10. One-step and two-step operation​

A GM defined in this profile must support one-step or two-step operation on transmit, or can support both on transmit.

A TC defined in this profile must support one-step operation on transmit (i.e., egress) on all of its ports, or must support two-step operation on transmit on all of its ports. A TC defined in this profile should support one-step operation on all of its ports (see 6.11).

All PTP clocks must support both one-step and two-step operation on receive (i.e., ingress).

A PTP port can transmit a Sync message as one-step or two-step. If the transmission of the Sync message is one-step, the twoStepFlag of the PTP common header is set to FALSE, otherwise it is set to TRUE. For PTP messages other than Sync, the twoStepFlag must always be set to FALSE. All PTP Ports must be capable of receiving and processing one-step and two-step Sync messages.

Note: one-step operation reduces the number of PTP messages transmitted by a PTP port. This may be applicable when considering scalability of unicast communication that a GM can serve. A one-step operation might ease meeting the requirements regarding the transmission of Sync messages specified in 9.5.9 of IEEE Std 1588-2019.

Note: IEEE Std 1588-2019 allows one-step versus two-step operation to be configured on a PTP port basis. This profile requires that all PTP ports on a per clock basis be the same.

6.11. End-to-End TC with two-step operation​

This section applies to the scenario where two-step TC operation may be used.

If an end-to-end TC uses two-step operation, each Delay_Req and corresponding Delay_Resp message must traverse the same end-to-end TC. This is because the end-to-end TC timestamps the Delay_Req message on ingress and egress and computes the residence time of the Delay_Req message. However, in the two-step case the TC updates the residence time of the corresponding Delay_Resp message. This is described in detail in 10.2.2.2.2 and 10.2.2.2.3 of IEEE Std 1588-2019. The former subclause describes the one-step case and specifies that "the <residenceTime> of the Delay_Req message must be added to the correctionField of the Delay_Req message by the egress PTP Port of the TC prior to the retransmission of the Delay_Req message." In this case, it is the Delay_Req message that is altered by the TC, and not the Delay_Resp message. However, the latter subclause describes the two-step case and specifies that the "<residenceTime> must be added to the correctionField of the Delay_Resp message associated with the Delay_Req message prior to transmission of the Delay_Resp message on the egress PTP Port, which is the ingress PTP Port for the Delay_Req message."

If all the TCs are two-step, the Delay_Req and Delay_Resp must traverse the same set of transparent clocks (links and network elements) between the GM and OC in order to meet the two-step subclause requirement. This property might not always hold true when using for example packet spraying, load balancing and equal cost multipath techniques. This is particularly applicable to data center environments and a reason for recommending the use of one-step operation TCs as noted in section 6.10.

If all the TCs are one-step, the Delay_Req and Delay_Resp need not traverse the same set of TCs (links and network elements) between the GM and OC.

6.12. PTP message rate​

Table 1 defines the range of message rates for Announce, Sync, Delay_Req, and Delay_Resp messages. A GM must support the full range. An OC should support the full range but can support a subset of the range. The message rate selected by an OC relates to the performance expected. A TC is agnostic to the PTP message rate.

MessageUpper end of logMessageInterval rangeMean rate corresponding to upper end of range (pps)Lower end of logMessageInterval rangeMean rate corresponding to lower end of range (pps)Default value of logMessageInterval
Announce01-380
Sync+30.125 (1 per 8 s)-71280
Delay_Req & Delay_Resp01-71280

Table 1. Range of logMessageInterval for a PTP Port

6.13. PTP inter-message interval​

The requirements for the actual inter-message intervals for unicast Announce, Sync, Delay_Req, and Delay_Resp messages are specified in 16.1 of IEEE Std 1588-2019. There are requirements for:

• (a) the arithmetic mean of the successive inter-message interval computed over a suitable number of successive intervals
• (b) the distribution of the inter-message intervals

For Announce and Sync messages, the arithmetic mean of the inter-message intervals must be within ± 30% of the granted inter-message period. For Delay_Req and Del_Resp messages, the arithmetic mean of the Delay_Req inter-message intervals must not be less than 90% of the granted inter-message interval for the Delay_Resp messages. The purpose of this requirement is to ensure that the GM port receives Delay_Req messages at rates that it is able to handle. If the mean inter-message interval of the Delay_Req messages is less than 90% of the granted inter-message interval for the Delay_Resp messages, the grantor port may ignore any Delay_Req messages in excess of the granted interval.

For the distribution of the inter-message intervals, at least 90% of the inter-message intervals must be within ± 30% of the granted mean inter-message interval. This requirement applies to Announce, Sync, and Delay_Req.

Consider N successive inter-message intervals deltat_i, i = 1, 2, ...,N, where deltat_i = (t_i - t_i-1) is as shown in the figure. The arithmetic mean of the inter-message intervals, t_av, is = (1/N)xsum(deltat_i from 1 to N).

For example, if the grantor port grants Sync or Announce messages with logMessageInterval equal to 0, the mean inter-message interval is 1 s. This means that (a) the average of the durations of a suitable number of successive inter-message intervals t av must be between 0.7 s and 1.3 s, and (b) 90% of the actual inter-message intervals must have durations that are between 0.7 s and 1.3 s. In addition, if the GM port grants Delay_Resp messages with logMessageInterval equal to 0, then (a) the average of the durations of a suitable number of successive Delay_Req inter-message intervals must be greater than or equal to 0.9 s, and (b) 90% of the actual Delay_Req inter-message intervals must have durations that are between 0.7 s and 1.3 s.

In principle, the mean Sync rate and the mean Delay_Req/Delay_Resp rate need not be the same. If the actual delay on the PTP communication path is changing sufficiently slowly (after the OC has processed any correction field), then infrequent delay measurements compared to the mean Sync interval might give acceptable performance. In this case, the mean Delay_Req/Delay_Resp rate can be chosen to be smaller than the mean Sync rate. The Sync rate that is chosen depends on the implementation of the OC filter and how much noise the oscillator at the OC generates. If the oscillator has a large noise generation, then the Sync rate would likely be larger. In this case, the OC would use new Sync information more frequently to correct for time error.

6.14. Unicast Communication​

PTP communication in this profile is based on unicast. Most PTP profiles in the industry are based on multicast, except for two of the ITU-T telecom profiles that are based on unicast [5, 6].

Both unicast discovery (clause 17.4 of IEEE Std 1588-2019) and unicast negotiation (clause 16.1 of IEEE Std 1588-2019) must be supported. In Model 1, each OC first uses unicast discovery to determine the potential GMs, and then uses unicast negotiation to request Announce messages from the potential GMs. The OC then invokes the Best Clock Algorithm (BMCA) to determine which of the potential GMs becomes the actual GM, i.e., the active GM. Finally, the OC uses unicast negotiation to request Sync and Delay_Resp messages from the active GM and uses the Sync, Delay_Resp, and Delay_Req information to synchronize to the GM that was selected. The other potential GMs are available as backup in the event that the active GM fails or can be selected as active GM for other OCs.

The unicast negotiation feature is permanently enabled. The unicastNegotiationPortDS.enable member (of the unicastNegotiationPortDS) must be TRUE for each PTP port (there is a unicastNegotiationPortDS for each PTP port). This dataset member applies to GM and OC and is not applicable to TC.

The unicastFlag of all PTP messages must be set to TRUE.

6.14.1. Unicast Discovery​

Unicast discovery is specified in 17.4 of IEEE Std 1588-2019.

In Model 1 of this PTP profile, a table of potential GMs is configured in each OC. The table is sometimes referred to as the Unicast Table (UMT) and is defined in the unicastDiscoveryPortDS data set in clause 17.4.3 of IEEE Std 1588-2019. This data set contains the following members:

1. maxTableSize: the maximum number of potential GMs that can be in the table
2. logQueryInterval: the logarithm to base 2 of the mean time interval, in seconds, between successive requests that the OC makes to a potential GM for Announce messages (if a request is not granted), with a default value of 0,
3. actualTableSize: the number of potential GMs currently in the table; and

Each OC uses unicast negotiation to request Announce messages from each potential GM contained in the unicastDiscoveryPortDS. If a potential GM does not grant the request, the OC attempts again after a time interval corresponding to logQueryInterval. The received Announce messages cause a state decision event, which causes the BMCA to be invoked. This results in one of the potential GMs becoming the active GM. Any other potential GMs are standby GMs from the perspective of the OC. If the active GM fails, the OC will stop receiving announce messages and the accounceReceiptTimeout will expire. This will invoke the BMCA. The BMCA will result in one of the standby GMs (i.e., the best of the remaining potential GMs) becoming the active GM. If there are no GMs in the unicastDiscoveryPortDS or if none of the GMs in the unicastDiscoveryPortDS grants Announce messages to the OC, the OC will go into either free-run or holdover.

After the GM is selected, the OC uses unicast negotiation to request Sync and Delay_Resp messages from the GM. Upon being granted Sync messages, the OC receives the Sync messages from the GM. Upon being granted Delay_Resp messages, the OC sends Delay_Req messages to the GM and receives a Delay_Resp message in response to each Delay_Req message.

6.14.2. Unicast Negotiation​

An OC requests Announce messages and then selects the best potential GM using the BMCA. The OC then requests Sync and Delay_Resp messages from that GM. After the OC is granted Sync messages, the GM sends Sync (and Follow_Up if the communication is two-step) messages to the OC. After the OC is granted Delay_Resp messages, the OC sends Delay_Req messages to the GM and the GM responds with Delay_Resp. The requesting of Announce, Sync and Delay_Resp messages is done using the unicast negotiation feature of IEEE Std 1588-2019. The unicast negotiation feature is performed using the following four TLVs:

• REQUEST_UNICAST_TRANSMISSION
• GRANT_UNICAST_TRANSMISSION
• CANCEL_UNICAST_TRANSMISSION
• ACKNOWLEDGE_CANCEL_UNICAST_TRANSMISSION

Each TLV is attached in a Signaling message.

The sending, receiving, and processing of unicast negotiation TLVs by OCs and GMs must comply with the requirements of 16.1 of IEEE Std 1588-2019 and its subclauses. The following text in this section is a summary description of the unicast negotiation process.

TCs do not participate in the unicast negotiation process. However, they do forward the unicast Signaling messages that contain the unicast negotiation TLVs exchanged between the OCs and GMs.

The unicast negotiation process is illustrated in Figures 2, 3, 4 for requesting Announce, Sync, and Delay_Resp messages, respectively. An OC requests unicast Announce, Sync, or Delay_Resp from a GM by sending a REQUEST_UNICAST_TRANSMISSION TLV to the GM. This TLV contains the messageType field, which indicates the type of message (i.e., Announce, Sync, Delay_Resp), the logInterMessagePeriod field, which is the logarithm to base two of the desired mean interval, in seconds, between successive messages of this type, and the durationField, which is the number of seconds for which the GM should continue to transmit these messages. The GM responds with a GRANT_UNICAST_TRANSMISSION TLV to either grant or deny the request. This TLV contains the messageType field, which indicates message being granted, the logInterMessagePeriod field, which is the logarithm to base two of the granted mean interval, in seconds, between successive messages of this type, the durationField, which is the granted number of seconds for which the GM will continue to transmit these messages, and the R (Renewal Invited) flag, which is TRUE if the GM considers that the grant is likely to be renewed if the OC requests a new grant after the current grant expires and FALSE otherwise. A value of zero for the durationField indicates that the grant has been denied. The granted logInterMessagePeriod and durationField need not be the same as the requested logInterMessagePeriod and durationField, respectively.

An OC can request the logInterMessagePeriod to be any value in the range specified in Table 1. A GM can grant different message rates to different OCs.

The duration of the grant begins when the GRANT_UNICAST_TRANSMISSION_TLV is transmitted and ends after a time interval equal to the value of the durationField has expired. Typically, the OC requests that the grant be renewed by sending a new REQUEST_UNICAST_TRANSMISSION TLV before the grant expires (i.e., before the end of the duration) so that the service will be continuous.

After the GM has granted Announce or Sync messages to the OC, the GM sends Announce or Sync messages to the OC. After the GM has granted Delay_Resp messages, the OC then sends Delay_Req messages to the GM and the GM responds with Delay_Resp. The GM responds to all Delay_Req messages that arrive before the grant expires. However, the GM may respond to a Delay_Req message, i.e., by sending the corresponding Delay_Resp message, after the grant expires, as long as the Delay_Req arrives before the grant expires.

An OC can cancel the grant by sending the CANCEL_UNICAST_TRANSMISSION TLV to the GM. This TLV contains the messageType field, which indicates the type of message whose grant is being canceled, and the R (maintainRequest) flag set to FALSE. The GM responds by sending the ACKNOWLEDGE_ CANCEL_UNICAST_TRANSMISSION TLV to the OC.

If a GM cannot continue to provide the granted messages before the durationField has expired, it can inform the OC by sending the CANCEL_UNICAST_TRANSMISSION TLV to the OC with the G (maintainGrant) flag set to FALSE. The OC responds by sending the ACKNOWLEDGE_ CANCEL_UNICAST_TRANSMISSION TLV to the GM. The GM should (i.e., this is recommended but not required) continue to send the messages until it receives the ACKNOWLEDGE_ CANCEL_UNICAST_TRANSMISSION TLV or it has sent an implementation-specific number of CANCEL_UNICAST_TRANSMISSION TLVs to the OC.

A Signaling message can contain more than one TLV.

In this PTP profile, all requests are made by an OC and all grants are made by a GM. An OC cannot grant services and a GM cannot request services.

Figure 2. Unicast negotiation for Announce messages

Figure 3.Unicast negotiation for Sync messages

Figure 4. Unicast negotiation for Delay_Resp messages

6.14.3. Active Standby Scenario​

This section provides examples on how PTP and the default BMCA can be used to provide full and partial redundancy of GMs during normal operation and failure scenarios.

Figure 5 shows an example which consists of 1000 OCs divided into 2 groups, each with 500 OCs. There are 4 potential GMs, designated 1 through 4, respectively. GM 1 and GM 2 are potential GMs for OC group 1 and their IPv6 address is entered into the unicastDiscoveryPortDS of each OC of group 1. GM 3 and GM 4 are potential GMs for OC group 2 and their IPv6 address is entered into the unicastDiscoveryPortDS of each OC of group 2. The attributes of the GMs are set such that GM 1 is better than GM 2 as determined by the BMCA and GM 3 is better than GM 4 as determined by the BMCA. Assuming the GMs all have the same clockClass, clockAccuracy, and offsetScaledLogVariance, this can be done by configuring the priority2 attribute such that priority2 for GM 1 and GM 3 is less than priority2 for GM 2 and GM 4, respectively. This assumes that priority1 is set to the same default value in all GMs to prevent it from accidentally overriding the effect of clockClass, clockAccuracy, and offsetScaledLogVariance. This is done in other PTP profiles such as ITU-T Rec. G.8275.2, which is also based on unicast discovery and unicast negotiation. Alternatively, if clockClass, clockAccuracy, offsetScaledLogVariance, and prioirity2 are the same in each potential GM but the clockIdentities of GM 1 and GM 3 happen to be less than the clockIdentities of GM 2 and GM 4, respectively, GM 1 and GM 3 will also be chosen as the active GMs for groups 1 and 2, respectively. In addition, in this final case where the potential GMs have the same clock attributes, it might not matter which is active and which is standby. The BMCA will result in GM 1 and GM 3 being the active GMs for OC groups 1 and 2, respectively, and GM 2 and GM 4 being the standby GMs for OC groups 1 and 2, respectively. The use of clockIdentites is the tiebreaker.

Example 1 also shows that a standby GM is not utilized if the active GM of the respective OC group has not failed. In this example, failures of both active GMs can be tolerated. However, the two standby GMs are not utilized unless there are failures.

Figure 5. Example1 of Active/Standby GMs across two groups each with 500 OCs

Figure 6 shows an example which consists of 2 OC groups with 3 potential GMs, designated 1 through 3, respectively. GM 1 and GM 3 are potential GMs for OC group 1 and are entered into the unicastDiscoveryPortDS of each OC of group 1. GM 2 and GM 3 are potential GMs for OC group 2 and are entered into the unicastDiscoveryPortDS of each OC of group 2. GM3 is essentially a shared GM between the 2 OC groups. The attributes of the GMs are set such that GM 1 and GM 2 are each better than GM 3 as determined by the BMCA. As in the example above, this can be done by configuring the priority2 attributes such that priority 2 for GM 1 is less than priority 2 for GM 3, and priority2 for GM 2 is less than priority2 for GM 3. This will also occur if the clockIdentities of GM 1 and GM 2 are each less than the clockIdentity of GM 3 and all the other attributes of GMs 1, 2, and 3 are the same. The BMCA will result in GM 1 and GM 2 being the active GMs for OC groups 1 and 2, respectively. GM 3 will be the standby GM for both groups 1 and 2. If either GM 1 or GM 2 fails, GM 3 will become the GM for the group whose GM has failed. If both GM 1 and GM 2 fail, then either GM 3 will become the GM for both OC groups 1 and 2, and therefore must be able to handle the load of both groups or only a single failure (i.e., of a single GM) can be tolerated.

In Example 2, there is only a single standby GM, and therefore only a single GM is not utilized if there are no failures (unlike Example 1, where two GMs are not utilized if there are no failures). However, either the single standby GM must handle a higher load if both active GMs fail, or else only a single active failure can be tolerated at any time.

Figure 6. Example2 one active GM for each group and one standby GM for both groups

Example 1 illustrates the case of full redundancy where there is one standby GM for each active GM. Example 2 illustrates the case of partial redundancy where there are fewer standby GMs than active GMs. To balance the load among the active GMs, the OCs should be divided as evenly as possible among the active GMs. To balance the load among the standby GMs and also achieve maximum robustness to failures, the standby GMs should be assigned to equal numbers of OC groups. For example, if there are 60000 OCs, 12 potential active GMs, and 4 potential standby GMs, the OCs should be divided into groups of 5000 OCs each (i.e., 60000 OCs/12 GMs), and each of the 12 potential active GMs should be entered in the unicastDiscoveryPortDS of the OCs of exactly one group. Each potential standby GM should be entered in the unicastDiscoveryPortDS of the OCs of exactly 3 groups (and no group should have two standby GMs entered in the unicastDiscoveryPortDS of any of its OCs). With this approach, a standby GM serves as a backup for up to 3 OC groups. In this example, if a standby GM can handle the load of up to N groups (N > 3), then N active failures can be tolerated.

6.14.4. Active Active Scenario​

This section is for future, and will consist in providing examples on how PTP and the default BMCA can be used to provide load balancing and maximize the use of GMs in both normal operation and failure scenarios beyond the active standby examples provided in section 6.14.3.

6.15. Best Clock Algorithm and Clock Attributes​

This profile is based on the default BMCA of IEEE Std 1588-2019.

The clock attributes for the GM and OC are given in Table 2. The attributes clockClass, clockAccuracy, offsetScaledLogVariance are set in the defaultDS and represent properties of the local clock, which is either the internal oscillator or an external time source that provides time to the GM outside of PTP or that is integrated with the GM. The clockAccuracy and offsetScaledLogVariance are based on the max|TE| (maximum absolute time error) and TDEV (time deviation) requirements for the PRTC-A (Primary Reference Time Clock A) specified in ITU-T Rec. G.8272 [7]. For clock class, the GM datasheet should specify the maximum amount of time necessary to transition from clockClass 7 to clockClass 52.

The priority1 attribute is not used and is set to 128. It is not used in this PTP profile because it has higher preference in the BMCA than all the other attributes. A misconfiguration could cause an OC to choose the wrong GM as the active GM or standby GM.

The priority2 attribute can be configured to force potential GMs to be active or standby GMs for specific OC groups, and to implement full or partial redundancy as shown in section 6.14.2. If priority2 is not used i.e., default value of 128 in all potential GMs, then the selection of the actual GM by the BMCA is based on clockIdentity.

The attribute followerOnly is TRUE for an OC and FALSE for a GM. The attribute leaderOnly is FALSE for an OC and TRUE for a GM.

The attribute ptpTimescale is always TRUE because this PTP profile uses the PTP timescale. The other timePropertiesDS attributes have values in the GM based on whether the values are traceable to a primary reference or in the case of timeSource based on the actual source of time for the clock.

The synchronizationUncertain attribute is optional. It is carried as a flag in the Announce message. This is new in IEEE Std 1588-2019 and might not be supported if the PTP nodes are based on previous version of the protocol. If it is not used, its value is FALSE. If it is used at an OC, it is set to TRUE if:

• The synchronizationUncertain flag in the Announce message received from the GM is set to TRUE, or
• The state of the PTP port of the OC is UNCALIBRATED

Otherwise synchronizationUncertain for the OC is set to FALSE. If the synchronizationUncertain attribute is used at a GM, it is set to TRUE if the GM time or frequency, or both, are not traceable to a primary reference, otherwise it is set to FALSE.

The data set members listed in Table 2 are not applicable to TCs. TCs do not participate in the BMCA.

Data setMemberValue
GMOC
defaultDSclockClass6 (traceable to a primary reference time source)7 (in holdover, and within holdover specifications)52 (in holdover but out of holdover specifications, or in free-run)255
defaultDSclockAccuracy0x21 (100 ns)0xFE (unknown)
defaultDSoffsetScaledLogVariance0x4E5D (PTPVAR = 1.144 x 10-15 s2, or TDEV = 30 ns)0xFFFF (maximum possible value, signifying unknown)
defaultDSpriority1128 (not used in this profile)128 (not used in this profile)
defaultDSpriority2Configurable over [0, 255]. Default value is 128Configurable over [0,255]. Default value is 128
defaultDSfollowerOnlyFALSETRUE
timePropertiesDScurrentUtcOffsetIf known, the value traceable to a primary reference that provides UTC. Otherwise the value when the node was designedcurrentUtcOffset of Ebest, after running BMCA
timePropertiesDScurrentUtcOffsetValidTRUE if the values of currentUtcOffset, leap59, and leap61 are based on values obtained from a primary reference providing UTC; otherwise set to FALSEcurrentUtcOffsetValid of Ebest, after running BMCA
timePropertiesDSleap59If known, to a value traceable to a primary reference; otherwise set to FALSEleap59 of Ebest, after running BMCA
timePropertiesDSleap61If known, to a value traceable to a primary reference; otherwise set to FALSELeap61 of Ebest, after running BMCA
timePropertiesDStimeTraceableTRUE if the time is traceable to a primary reference; otherwise set to FALSEtimeTraceable of Ebest, after running BMCA
timePropertiesDSfrequencyTraceableTRUE if the frequency is traceable to a primary reference; otherwise set to FALSEfrequencyTraceable of Ebest, after running BMCA
timePropertiesDStimeSourceIf known, to the appropriate value from Table 6/IEEE Std 1588-2019. Otherwise set to INTERNAL_OSCILLATORtimeSource of Ebest, after running BMCA
timePropertiesDSptpTimescaleTRUETRUE
currentDSsynchronizationUncertainFALSE (default)FALSE (default)

Table 2. Data set members and values

6.16. Network Limits and Error Budget for Model 1​

This section is an initial analysis. The network limit from Section 5, is:

• The maximum absolute time error of any OC, relative to TAI, must be max|TEOC| < 2.5 µs.
• The time accuracy difference between any two OCs must be within µ5 microseconds, i.e., |TOC,j – TOC,k| < 5 µs for kj.

The following effects contribute to max|TEOC|:

1. Timestamp granularity. This is due to the clock frequency used for timestamping generation.
2. Timestamp generation. This is due to timestamping generation not being at the exact location where the timestamp is being taken, i.e., at the reference plane (see 7.3.4.2 of IEEE Std 1588-2019).
3. Combination of residence time and free-run accuracy of a TC. In this profile, the TCs are assumed to be free-running. They are not syntonized either at the physical layer or via PTP
4. Maximum number of TCs between a GM and an OC.
5. Noise generation due to OC oscillator characteristics.
6. PLL filter characteristics.
7. GM accuracy. This is the maximum time error of the GM relative to TAI when traceable. This profile refers to ITU-T G.8272 PRTC-A specification.
8. Constant time error. This is due to link and node asymmetry after any compensation
9. Time error allowance produced by or within the application (i.e., any additional error between the PTP layer and the application/server).
10. Effect of a transient if an OC loses its active GM and switches to a backup GM.
11. Effect of long-term holdover of the GM (e.g, GNSS jamming, solar activity) or an OC if a backup GM is not available.

Table 3 contains initial assumptions for the effects given above. The value in the table, except for timestamp granularity, maximum residence time, number of TCs, and endpoint filter characteristics, refers to an absolute value.

EffectValue
Timestamp granularity8 ns
Timestamp generation8 ns
Maximum residence time in a TC0.1 ms
Free-run accuracy of TC oscillator100 ppm
Maximum number of TCs5
OC noise generation100 ns (TBD)
OC endpoint filter characteristicsTBD
GM accuracy relative to TAI when traceable100 ns
Constant time error200 ns
Time error allowance for the application200 ns (TBD)
Effect of a transient if an OC loses its reference to active GM and switches to a standby GM1400 ns (TBD - see below)
Effect of long-term holdover of the GM with clockClass 7 on an OC if a backup GM with clockClass 6 is not available1400 ns over time T specified by the vendor (TBD see below)

Table 3. Maximum absolute time error budget

The maximum error introduced by a TC due to free-run accuracy and residence time is (0.1 x 10-3 s)(10-4) = 10-8 s = 10 ns. A TC will also introduce errors of 8 ns due to timestamp granularity and 8 ns due to timestamp generation. These errors will be added at both ingress and egress, for a total of 32 ns. The total error introduced by a TC in going from ingress to egress is therefore 42 ns.

The errors due to timestamp granularity and timestamp generation are also introduced at the GM egress and the OC ingress. These errors will add 16 ns, for a total of 32 ns.

The above errors contribute to total time error at the OC (to the end application). First, they accumulate as a Sync message traverses the network from the GM to the OC and contributes to the error in the recovered time at the OC. Second, they also accumulate as the Sync and Delay_Req message traverses the network from GM to OC and OC to GM and contribute to the error in the mean path delay at the OC. The total error that accumulates as either the Sync or Delay_Req message traverses the network, assuming there are 5 TCs in the path, is 5(42 ns) + 32 ns = 242 ns. The total error in synchronized time is therefore the sum of the error for Sync and the error in measured path delay, i.e., 242 ns (error in Sync) + 242 ns (error in meanPathdelay) = 484 ns (error in the time offset between the OC and GM). Finally, the 100 ns for the OC noise generation must be added to give 584 ns.

The error introduced by the GM, based on PRTC-A, is 100 ns.

The total allowance for constant time error due to link and node asymmetry is based on G.8271.1. G.8271.1 allows 800 ns for a network that consists of 20 hops with links that are likely much longer than those expected in a data center environment (i.e., the fiber length between nodes in a data center are within meters or tens of meters). Given that cTE is linearly additive and that the number of clocks consists of 5 TCs, 1 OC and 1GM, the total cTE is about ¼ the allocation found in G.8271.1. Therefore, the constant time error is 200 ns.

The total error at the input of the application is 584 ns + 100 ns + 200 ns + 200 ns = 1100 ns. This is well within the max|TEOC| < 2.5 µs.

If the OC loses its connection to the network and enters holdover or the GM loses its connection to its time source (e.g., GNSS) and enters holdover with clockClass = 7, it can be assumed that the application already has already built-up an error of 1100 ns relative to TAI. In worst case, the application could drift another 1400 ns before it exceeds the 2.5 µs requirement. This means that the holdover requirement for the OC or the GM can be taken as 1400 ns over a time period T. This period T should be specified by the OC or GM datasheet. In addition, if the OC switches from one active GM to another active GM, any transient during this switch must be within 1400 ns.

6.17. PTP management messages​

The profile uses the PTP management mechanism and PTP management messages (TLVs) defined in clause 15 of IEEE Std 1588-2019. The management messages are used by a PTP management node for the purpose of configuration and/or monitoring PTP Instances.

In this version of the profile, the following management TLVs must be supported:

• DEFAULT_DATA_SET (managementId 2000)
• CURRENT_DATA_SET (managementId 2001)
• PARENT_DATA_SET (managementId 2002)

The following additional TLV should be supported (Note: This TLV is an implementation-specific TLV and is supported by the linuxptp implementation. The TLV contains a set of PTP message counters that can be used for monitoring):

• PORT_STATS_NP (managementId C005)

Additional PTP management TLVs might be defined for the purpose of calculating time error bounds. This is for further study.

7. References

[1] OCP Timing Appliances Project (TAP) Incubation Proposal, July 2020, https://www.opencompute.org/wiki/Time_Appliances_Project

[2] IEEE Std 1588-2019, IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems, June 2020

[3] OCP Contribution, Practical Uses of Synchronized Clocks, Sept 2020, https://www.opencompute.org/wiki/Time_Appliances_Project

[4] OCP Open Time Server, https://www.opencompute.org/wiki/Time_Appliances_Project

[5] ITU-T G.8275.2, Precision time protocol telecom profile for phase/time synchronization with partial timing support from the network

[6] ITU-T G.8265.1, Precision time protocol telecom profile for frequency synchronization

[7] ITU-T G.8272, Timing characteristics of primary reference time clocks

8. Revision

0.1Initial DC PTP profile document

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1 See Section 2 - Terminology

2 See Section 2 - Terminology

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