TICTOC Working Group D.A. Arnold
Internet-Draft Meinberg-USA
Intended status: Standards Track H.G. Gerstung
Expires: 8 October 2022 Meinberg
6 April 2022
Enterprise Profile for the Precision Time Protocol With Mixed Multicast
and Unicast Messages
draft-ietf-tictoc-ptp-enterprise-profile-22
Abstract
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.
Status of This Memo
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Technical Terms . . . . . . . . . . . . . . . . . . . . . . . 4
4. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6
5. Network Technology . . . . . . . . . . . . . . . . . . . . . 7
6. Time Transfer and Delay Measurement . . . . . . . . . . . . . 7
7. Default Message Rates . . . . . . . . . . . . . . . . . . . . 8
8. Requirements for Master Clocks . . . . . . . . . . . . . . . 9
9. Requirements for Slave Clocks . . . . . . . . . . . . . . . . 9
10. Requirements for Transparent Clocks . . . . . . . . . . . . . 9
11. Requirements for Boundary Clocks . . . . . . . . . . . . . . 10
12. Management and Signaling Messages . . . . . . . . . . . . . . 10
13. Forbidden PTP Options . . . . . . . . . . . . . . . . . . . . 10
14. Interoperation with IEEE 1588 Default Profile . . . . . . . . 10
15. Profile Identification . . . . . . . . . . . . . . . . . . . 10
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
18. Security Considerations . . . . . . . . . . . . . . . . . . . 11
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
19.1. Normative References . . . . . . . . . . . . . . . . . . 11
19.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The Precision Time Protocol ("PTP"), first 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
IEEE 1588-2019 [IEEE1588] needs to know the identity of the time
sources in the network (the Master Clocks). This document describes
clock roles and port states using the terms master and slave in order
to correspond to the terms used in IEEE 1588, on which this document
is based. There is an active project in the IEEE to select
alternative terms. When this project is completed, then master and
slave will be replaced with the new alternative terms in an update to
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this document.
The "Best Master Clock Algorithm" (IEEE 1588-2019 [IEEE1588]
Subclause 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 network (IP) to all nodes,
requiring them to spend network bandwidth and other resources, such
as CPU cycles, to drop the message.
The third edition of the standard (IEEE 1588-2019) defines PTPv2.1
and includes 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. PTPv2.1 also includes PTP profiles (IEEE 1588-2019 [IEEE1588]
subclause 20.3). 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 IEEE 1588-2019
[IEEE1588] subclause 13.4), defining an optional Best Master Clock
Algorithm and a few other ways. PTP has its own management protocol
(defined in IEEE 1588-2019 [IEEE1588] subclause 15.2) but allows a
PTP profile specify an alternative management mechanism, for example
NETCONF.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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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.
* 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
Message.
* 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
timing.
* Clock Identity: In IEEE 1588-2019 this is a 64-bit number assigned
to each PTP clock which must be unique. Often it is derived from
the Ethernet MAC address, 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. Clocks, 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
system
* IEEE 1588: The timing and synchronization standard which defines
PTP, and describes the node, system, and communication properties
necessary to support PTP.
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* Master Clock: a clock with at least one port in the master state.
* NTP: Network Time Protocol, defined by RFC 5905, see RFC 5905
[RFC5905]
* 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.
* PTP: The Precision Time Protocol, the timing and synchronization
protocol defined 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.
* PTPv2.1: Refers specifically to the third version of PTP defined
by IEEE 1588-2019.
* 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
Clock.
* 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.
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* 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 100 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 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 thousands.
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.
Interoperability among independent implementations of this PTP
profile has been demonstrated at the ISPCS Plugfest ISPCS [ISPCS].
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5. Network Technology
This PTP profile SHALL operate only in networks characterized by UDP
RFC 768 [RFC0768] over either IPv4 RFC 791 [RFC0791] or IPv6 RFC 8200
[RFC8200], as described by Annexes D and E in IEEE 1588 [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 network asymmetry are outside
the scope of this document. See for example, ITU-T G.8271 [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.
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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 224.0.1.129 for IPv4
and FF0X:0:0:0:0:0:0:181 for Ipv6, where X can be a value between 0x0
and 0xF, see IEEE 1588 [IEEE1588] Annex E, Section E.3.
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.
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.
The logMessageInterval carried in the unicast Delay Response message
MAY be set to correspond to the master ports preferred message
period, rather than 7F, which indicates message periods are to be
negotiated. Note that negotiated message periods are not allowed,
see forbidden PTP options (Section 13).
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8. Requirements for Master Clocks
Master Clocks SHALL obey the standard Best Master Clock Algorithm
from IEEE 1588 [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 seconds.
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 1588-2019 requires slave clocks to
support both two-step or one-step Master clocks. See IEEE 1588
[IEEE1588], subClause 11.2.
Since Announce messages are sent as multicast messages slaves can
obtain the IP addresses of a 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.
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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. Management messages intended
for a specific clock, i.e. the IEEE 1588 [IEEE1588] defined attribute
targetPortIdentity.clockIdentity is not set to All 1s, 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 NOT ALLOWED. Clocks operating in
the Enterprise Profile SHALL NOT use Alternate Timescales. Unicast
discovery and unicast negotiation SHALL NOT be used.
14. Interoperation with IEEE 1588 Default Profile
Clocks operating in the Enterprise Profile will interoperate with
clocks operating in the Default Profile described in IEEE 1588
[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 operate 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.
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PTP Profile:
Enterprise Profile
Version: 1.0
Profile identifier: 00-00-5E-00-01-00
This profile was specified by the IETF
A copy may be obtained at
https://datatracker.ietf.org/wg/tictoc/documents
16. Acknowledgements
The authors would like to thank members of IETF for reviewing and
providing feedback on this draft.
This document was initially prepared using 2-Word-v2.0.template.dot
and has later been converted manually into xml format using an
xml2rfc template.
17. IANA Considerations
There are no IANA requirements in this specification.
18. 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.
PTP native management messages SHOULD not be used, due to the lack of
a security mechanism for this option. Secure management can be
obtained using standard management mechanisms which include security,
for example NETCONF NETCONF [RFC6241].
General security considerations of time protocols are discussed in
RFC 7384 [RFC7384].
19. References
19.1. Normative References
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[IEEE1588] Institute of Electrical and Electronics Engineers, "IEEE
std. 1588-2019, "IEEE Standard for a Precision Clock
Synchronization for Networked Measurement and Control
Systems."", November 2019, <https://www.ieee.org>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
19.2. Informative References
[G8271] International Telecommunication Union, "ITU-T G.8271/
Y.1366, "Time and Phase Synchronization Aspects of Packet
Networks"", February 2012, <https://www.itu.int>.
[ISPCS] Arnold, D.A., "Plugfest Report", October 2017,
<https://www.ispcs.org>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>.
Authors' Addresses
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Doug Arnold
Meinberg-USA
3 Concord Rd
Shrewsbury, Massachusetts 01545
United States of America
Email: doug.arnold@meinberg-usa.com
Heiko Gerstung
Meinberg
Lange Wand 9
31812 Bad Pyrmont
Germany
Email: heiko.gerstung@meinberg.de
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