Enterprise Profile for the Precision Time Protocol With Mixed Multicast and Unicast Messages

Versions: 00 01 02 03 04 05 06 07 08                     Standards Track
INTERNET-DRAFT          Enterprise Profile for PTP        September 2017

TICTOC Working Group                                         Doug Arnold
Internet Draft                                              Meinberg-USA
Intended status: Standards Track                          Heiko Gerstung
Expires: March 28, 2018                               September 28, 2017

          Enterprise Profile for the Precision Time Protocol
               With Mixed Multicast and Unicast Messages


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    This document describes a profile for the use of the Precision
    Time Protocol in an IPV4 or IPv6 Enterprise information system
    environment.  The profile uses the End to End Delay Measurement
    Mechanism, allows both multicast and unicast Delay Request and Delay
    Response Messages.

Table of Contents

1.   Introduction                              2
2.   Conventions used in this document         3
3.   Technical Terms                           3
4.   Problem Statement                         5
5.   Network Technology                        6
6.   Time Transfer and Delay Measurement       7
7.   Default Message Rates                     8
8.   Requirements for Master Clocks            8
9.   Requirements for Slave Clocks             8
10.  Requirements for Transparent Clocks       9
11.  Requirements for Boundary Clocks          9
12.  Management and Signaling Messages         9
13.  Forbidden PTP Options                     9
14.  Interoperation with Other PTP Profiles   10
15.  Profile Identification                   10
16.  Security Considerations                  10
17.  IANA Considerations                      10
18.  References                               11
     18.1.  Normative References              11
     18.2.  Informative References            11
19. Acknowledgments                           11
20. Authors addresses                         12

1.  Introduction

     The Precision Time Protocol ("PTP"), standardized in IEEE 1588,
     has been designed in its first version (IEEE 1588-2002) with the
     goal to minimize configuration on the participating nodes. Network
     communication was based solely on multicast messages, which unlike
     NTP did not require that a receiving node ("slave clock") in
     [IEEE1588] needs to know the identity of the time sources in the
     network (the Master Clocks).

     The so-called "Best Master Clock Algorithm" ([IEEE1588] Clause
     9.3), a mechanism that all participating PTP nodes must follow,
     set up strict rules for all members of a PTP domain to determine
     which node shall be the active sending time source (Master Clock).
     Although the multicast communication model has advantages in
     smaller networks, it complicated the application of PTP in larger
     networks, for example in environments like IP based
     telecommunication networks or financial data centers. It is
     considered inefficient that, even if the content of a message
     applies only to one receiver, it is forwarded by the underlying

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     network (IP) to all nodes, requiring them to spend network
     bandwidth and other resources like CPU cycles to drop the message.

     The second revision of the standard (IEEE 1588-2008) is the
     current version (also known as PTPv2) and introduced the
     possibility to use unicast communication between the PTP nodes in
     order to overcome the limitation of using multicast messages for
     the bi-directional information exchange between PTP nodes. The
     unicast approach avoided that, in PTP domains with a lot of nodes,
     devices had to throw away more than 99% of the received multicast
     messages because they carried information for some other node.
     PTPv2 also introduced so-called "PTP profiles" ([IEEE1588] Clause
     19.3). This construct allows organizations to specify selections
     of attribute values and optional features, simplifying the
     configuration of PTP nodes for a specific application. Instead of
     having to go through all possible parameters and configuration
     options and individually set them up, selecting a profile on a PTP
     node will set all the parameters that are specified in the profile
     to a defined value. If a PTP profile definition allows multiple
     values for a parameter, selection of the profile will set the
     profile-specific default value for this parameter. Parameters not
     allowing multiple values are set to the value defined in the PTP
     profile. Many PTP features and functions are optional, and a
     profile should also define which optional features of PTP are
     required, permitted, or prohibited. It is possible to extend the
     PTP standard with a PTP profile by using the TLV mechanism of PTP
     (see [IEEE1588] Clause 13.4), defining an optional Best Master
     Clock Algorithm and a few other ways. PTP has its own management
     protocol (defined in [IEEE1588] Clause 15.2) but allows a PTP
     profile specify an alternative management mechanism, for example

2.  Conventions used in this document

     The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
     in this document are to be interpreted as described in RFC-2119

     In this document, these words will appear with that interpretation
     only when in ALL CAPS. Lower case uses of these words are not to
     be interpreted as carrying RFC-2119 significance.

3.  Technical Terms

     Acceptable Master Table: A PTP Slave Clock may maintain a list of
     masters which it is willing to synchronize to.

     Alternate Master: A PTP Master Clock, which is not the Best
     Master, may act as a master with the Alternate Master flag set on
     the messages it sends.

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     Announce message: Contains the master clock properties of a Master
     clock.  Used to determine the Best Master.

     Best Master:  A clock with a port in the master state, operating
     consistently with the Best Master Clock Algorithm.

     Best Master Clock Algorithm: A method for determining which state
     a port of a PTP clock should be in.  The algorithm works by
     identifying which of several PTP Master capable clocks is the best
     master.  Clocks have priority to become the acting Grandmaster,
     based on the properties each Master Clock sends in its Announce

     Boundary Clock: A device with more than one PTP port.  Generally
     boundary clocks will have one port in slave state to receive
     timing and then other ports in master state to re-distribute the

     Clock Identity: In IEEE 1588-2008 this is a 64-bit number
     assigned to each PTP clock which must be unique. Often the
     Ethernet MAC address is used since there is already an
     international infrastructure for assigning unique numbers to each
     device manufactured.

     Domain: Every PTP message contains a domain number.  Domains are
     treated as separate PTP systems in the network.  Slaves, however,
     can combine the timing information derived from multiple domains.

     End to End Delay Measurement Mechanism: A network delay
     measurement mechanism in PTP facilitated by an exchange of
     messages between a Master Clock and Slave Clock.

     Grandmaster: the primary master clock within a domain of a PTP

     IEEE 1588: The timing and synchronization standard which defines
     PTP, and describes the node, system, and communication properties
     necessary to support PTP.

     Master clock: a clock with at least one port in the master state.

     NTP: Network Time Protocol, defined by RFC 5905, see [NTP].

     Ordinary Clock: A clock that has a single Precision Time Protocol
     (PTP) port in a domain and maintains the timescale used in the
     domain. It may serve as a master clock, or be a slave clock.

     Peer to Peer Delay Measurement Mechanism: A network delay
     measurement mechanism in PTP facilitated by an exchange of
     messages between adjacent devices in a network.

     Preferred Master: A device intended to act primarily as the
     Grandmaster of a PTP system, or as a back up to a Grandmaster.

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     PTP: The Precision Time Protocol, the timing and synchronization
     protocol define by IEEE 1588.

     PTP port: An interface of a PTP clock with the network.  Note that
     there may be multiple PTP ports running on one physical interface,
     for example, a unicast slave which talks to several Grandmaster
     clocks in parallel.

     PTPv2: Refers specifically to the second version of PTP defined by
     IEEE 1588-2008.

     Rogue Master: A clock with a port in the master state, even though
     it should not be in the master state according to the Best Master
     Clock Algorithm, and does not set the alternate master flag.

     Slave clock: a clock with at least one port in the slave state,
     and no ports in the master state.

     Slave Only Clock: An Ordinary clock which cannot become a Master

     TLV: Type Length Value, a mechanism for extending messages in
     networked communications.

     Transparent Clock.  A device that measures the time taken for a
     PTP event message to transit the device and then updates the
     message with a correction for this transit time.

     Unicast Discovery: A mechanism for PTP slaves to establish a
     unicast communication with PTP masters using a configures table of
     master IP addresses and Unicast Message Negotiation.

     Unicast Negotiation: A mechanism in PTP for Slave Clocks to
     negotiate unicast Sync, announce and Delay Request Message Rates
     from a Master Clock.

4.  Problem Statement

     This document describes a version of PTP intended to work in large
     enterprise networks.  Such networks are deployed, for example, in
     financial corporations.  It is becoming increasingly common in such
     networks to perform distributed time tagged measurements, such as
     one-way packet latencies and cumulative delays on software
     systems spread across multiple computers. Furthermore, there is
     often a desire to check the age of information time tagged by a
     different machine.  To perform these measurements, it is necessary
     to deliver a common precise time to multiple devices on a network.
     Accuracy currently required in the Financial Industry range from
     100 microseconds to 500 nanoseconds to the Grandmaster.  This
     profile does not specify timing performance requirements, but such
     requirements explain why the needs cannot always be met by NTP, as
     commonly implemented. Such accuracy cannot usually be achieved with
     a traditional time transfer such as NTP, without adding

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     non-standard customizations such as hardware time stamping, and on
     path support.  These features are currently part of PTP, or are
     allowed by it.  Because PTP has a complex range of features and
     options it is necessary to create a profile for enterprise
     networks to achieve interoperability between equipment
     manufactured by different vendors.

     Although enterprise networks can be large, it is becoming
     increasingly common to deploy multicast protocols, even across
     multiple subnets. For this reason, it is desired to make use of
     multicast whenever the information going to many destinations is
     the same.  It is also advantageous to send information which is
     unique to one device as a unicast message.  The latter can be
     essential as the number of PTP slaves becomes hundreds or

     PTP devices operating in these networks need to be robust.  This
     includes the ability to ignore PTP messages which can be
     identified as improper, and to have redundant sources of time.

5.  Network Technology

     This PTP profile SHALL operate only in networks characterized by
     UDP [RFC768] over either IPv4 [RFC791] or IPv6 [RFC2460], as
     described by Annexes D and E in [IEEE1588] respectively.  If a
     network contains both IPv4 and IPv6, then they SHALL be treated as
     separate communication paths.  Clocks which communicate using IPv4
     can interact with clocks using IPv6 if there is an intermediary
     device which simultaneously communicates with both IP versions. A
     boundary clock might perform this function, for example.  A PTP
     domain SHALL use either IPv4 or IPv6 over a communication path,
     but not both. The PTP system MAY include switches and routers.
     These devices MAY be transparent clocks, boundary clocks, or
     neither, in any combination.  PTP Clocks MAY be Preferred Masters,
     Ordinary Clocks, or Boundary Clocks.  The ordinary clocks may be
     Slave Only Clocks, or be master capable.

     Note that clocks SHOULD always be identified by their clock ID and
     not the IP or Layer 2 address.  This is important in IPv6 networks
     since Transparent clocks are required to change the source address
     of any packet which they alter.  In IPv4 networks some clocks
     might be hidden behind a NAT, which hides their IP addresses from
     the rest of the network.  Note also that the use of NATs may place
     limitations on the topology of PTP networks, depending on the port
     forwarding scheme employed.  Details of implementing PTP with NATs
     are out of scope of this document.

     PTP, like NTP, assumes that the one-way network delay for Sync
     Messages and Delay Response Messages are the same. When this is
         not true it can cause errors in the transfer of time from the
         Master to the Slave. It is up to the system integrator to design
     the network so that such effects do not prevent the PTP system
         from meeting the timing requirements. The details of

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     network asymmetry are outside the scope of this document.  See for
     example, [G8271].

6.  Time Transfer and Delay Measurement

     Master clocks, Transparent clocks and Boundary clocks MAY be
     either one-step clocks or two-step clocks.  Slave clocks MUST
     support both behaviors. The End to End Delay Measurement Method
     MUST be used.

     Note that, in IP networks, Sync messages and Delay Request
     messages exchanged between a master and slave do not necessarily
     traverse the same physical path. Thus, wherever possible, the
     network SHOULD be traffic engineered so that the forward and
     reverse routes traverse the same physical path.  Traffic
     engineering techniques for path consistency are out of scope of
     this document.

     Sync messages MUST be sent as PTP event multicast messages (UDP
     port 319) to the PTP primary IP address.   Two step clocks SHALL
     send Follow-up messages as PTP general messages (UDP port 320).
     Announce messages MUST be sent as multicast messages (UDP port 320)
     to the PTP primary address.  The PTP primary IP address is for IPv4 and FF0X:0:0:0:0:0:0:181 for Ipv6, where X can
     be a value between 0x0 and 0xF, see [IEEE1588] Annex E, Section

     Delay Request Messages MAY be sent as either multicast or unicast
     PTP event messages. Master clocks SHALL respond to multicast Delay
     Request messages with multicast Delay Response PTP general
     messages. Master clocks SHALL respond to unicast Delay Request PTP
     event messages with unicast Delay Response PTP general messages.
     This allow for the use of Ordinary clocks which do not support the
     Enterprise Profile, if they are slave Only Clocks.

     Clocks SHOULD include support for multiple domains.  The purpose is
     to support multiple simultaneous masters for redundancy. Leaf
     devices (non-forwarding devices) can use timing information from
     multiple masters by combining information from multiple
     instantiations of a PTP stack, each operating in a different
     domain. Redundant sources of timing can be ensembled, and/or
     compared to check for faulty master clocks. The use of multiple
     simultaneous masters will help mitigate faulty masters reporting as
     healthy, network delay asymmetry, and security problems.  Security
     problems include man-in-the-middle attacks such as delay attacks,
     packet interception / manipulation attacks. Assuming the path to
     each master is different, failures malicious or otherwise would
     have to happen at more than one path simultaneously. Whenever
     feasible, the underlying network transport technology SHOULD be
     configured so that timing messages in different domains traverse
     different network paths.

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7.  Default Message Rates

     The Sync, Announce and Delay Request default message rates SHALL
     each be once per second.  The Sync and Delay Request message rates
     MAY be set to other values, but not less than once every 128
     seconds, and not more than 128 messages per second.  The Announce
     message rate SHALL NOT be changed from the default value.  The
     Announce Receipt Timeout Interval SHALL be three Announce
     Intervals for Preferred Masters, and four Announce Intervals for
     all other masters.

     Unicast Discovery and Unicast Message Negotiation options SHALL NOT
         be utilized.

8.  Requirements for Master Clocks

     Master clocks SHALL obey the standard Best Master Clock Algorithm
     from [IEEE1588].  PTP systems using this profile MAY support
     multiple simultaneous Grandmasters if each active Grandmaster is
         operating in a different PTP domain.

     A port of a clock SHALL NOT be in the master state unless the
     clock has a current value for the number of UTC leap

         If a unicast negotiation signaling message is received it SHALL
         be ignored.

9.     Requirements for Slave Clocks

     Slave clocks MUST be able to operate properly in a network which
     contains multiple Masters in multiple domains.  Slaves SHOULD make
     use of information from the all Masters in their clock control
     subsystems.  Slave Clocks MUST be able to operate properly in the
     presence of a Rogue Master.  Slaves SHOULD NOT Synchronize to a
     Master which is not the Best Master in its domain. Slaves will
     continue to recognize a Best Master for the duration of the
     Announce Time Out Interval. Slaves MAY use an Acceptable Master
     Table.  If a Master is not an Acceptable Master, then the Slave
     MUST NOT synchronize to it. Note that IEEE 1588-2008 requires
     slave clocks to support both two-step or one-step Master clocks.
     See [IEEE1588], section 11.2.

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     Since Announce messages are sent as multicast messages slaves can
     obtain the IP addresses of master from the Announce messages.  Note
     that the IP source addresses of Sync and Follow-up messages may
     have been replaced by the source addresses of a transparent clock,
     so, slaves MUST send Delay Request messages to the IP address in
         the Announce message.  Sync and Follow-up messages can be
         correlated with the Announce message using the clock ID, which is
         never altered by Transparent clocks in this profile.

10.     Requirements for Transparent Clocks

     Transparent clocks SHALL NOT change the transmission mode of an
     Enterprise Profile PTP message.  For example, a Transparent clock
     SHALL NOT change a unicast message to a multicast message.
     Transparent Clocks SHOULD support multiple domains.  Transparent
     Clocks which syntonize to the master clock will need to maintain
     separate clock rate offsets for each of the supported domains.

11.     Requirements for Boundary Clocks

     Boundary Clocks SHOULD support multiple simultaneous PTP domains.
     This will require them to maintain servo loops for each of the
     domains supported, at least in software.  Boundary clocks MUST NOT
     combine timing information from different domains.

12.     Management and Signaling Messages

    PTP Management messages MAY be used.  Any PTP management message
    which is sent with the targetPortIdentity.clockIdentity set to all
    1s (all clocks) MUST be sent as a multicast message.  Management
    messages with any other value of for the Clock Identity is
    intended for a specific clock and MUST be sent as a unicast
    message.  Similarly, if any signaling messages are used they
    MUST also be sent as unicast messages whenever the message is
    intended for a specific clock.

13.     Forbidden PTP Options

     Clocks operating in the Enterprise Profile SHALL NOT use peer to
     peer timing for delay measurement.  Grandmaster Clusters are NOT
     ALLOWED. The Alternate Master option is also forbidden. Clocks
     operating in the Enterprise Profile SHALL NOT use Alternate

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14.     Interoperation with IEEE 1588 Default Profile

     Clocks operating in the Enterprise Profile will interoperate with
     clocks operating in the Default Profile described in [IEEE1588]
     Annex J.3.  This variant of the Default Profile uses the End to End
     Delay Measurement Mechanism.  In addition, the Default Profile
         would have to operates over IPv4 or IPv6 networks, and use
         management messages in unicast when those messages are directed at
         a specific clock. If either of these requirements are not met than
         Enterprise Profile clocks will not interoperate with Annex J.3
         Default Profile Clocks.  The Enterprise Profile will not
         interoperate with the Annex J.4 variant of the Default Profile
         which requires use of the Peer to Peer Delay Measurement Mechanism.

     Enterprise Profile Clocks will interoperate with clocks operating
     in other profiles if the clocks in the other profiles obey the
     rules of the Enterprise Profile.  These rules MUST NOT be changed
     to achieve interoperability with other profiles.

15.     Profile Identification

     The IEEE 1588 standard requires that all profiles provide the
         following identifying information.

         PTP Profile:
         Enterprise Profile
         Version: 1.0
         Profile identifier: 00-00-5E-01-00

         This profile was specified by the IETF

         A copy may be obtained at

16.     Security Considerations

     Protocols used to transfer time, such as PTP and NTP can be
     important to security mechanisms which use time windows for keys
     and authorization. Passing time through the networks poses a
     security risk since time can potentially be manipulated.
     The use of multiple simultaneous masters, using multiple PTP
     domains can mitigate problems from rogue masters and
     man-in-the-middle attacks.  See sections 9 and 10. Additional
     security mechanisms are outside the scope of this document.

17.     IANA Considerations

     There are no IANA requirements in this specification.

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18.     References

18.1.      Normative References

           [IEEE1588] IEEE std. 1588-2008, "IEEE Standard for a
                      Precision Clock Synchronization for Networked
                      Measurement and Control Systems." July, 2008.
           [RFC768]   Postel, J., "User Datagram Protocol," RFC 768,
                      August, 980.

           [RFC791]   "Internet Protocol DARPA Internet Program Protocol
                      Specification," RFC 791, September, 1981.

           [RFC2119]  Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", BCP 14, RFC 2119,
                      March 1997.

           [RFC2460]  Deering, S., Hinden, R., "Internet Protocol,
                      Version 6 (IPv6) Specification," RFC 2460,
                      December, 1998.

18.2.      Informative References

           [G8271]    ITU-T G.8271/Y.1366, "Time and Phase
                      Synchronization Aspects of Packet Networks"
                      February, 2012.

           [NTP]      Mills, D., Martin, J., Burbank, J., Kasch, W.,
                      "Network Time Protocol Version 4: Protocol and
                      Algorithms Specification," RFC 5905, June 2010.

19.      Acknowledgments

     The authors would like to thank members of IETF for reviewing and
     providing feedback on this draft.

     This document was initially prepared using

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20.     Authors' Addresses

     Doug Arnold
     Meinberg USA
     929 Salem End Road
     Framingham, MA 01702

     Email: doug.arnold@meinberg-usa.com

     Heiko Gerstung
     Meinberg Funkuhren GmbH & Co. KG
     Lange Wand 9
     D-31812 Bad Pyrmont

     Email: Heiko.gerstung@meinberg.de

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