Hello Everyone,
Attached is an updated version of the Enterprise profile. Sorry I missed
the upload deadline for the Vancouver meeting. If you get a chance to
look at it before then I will be there and would love to discuss it.
Summary of changes:
I also changed the TLV format to fit the latest proposal in the 1588
revision.
I added a statement about not going into the master state unless one had a
current UTC offset.
I added to the TLV fields to indicate maximum phase correction over a sync
cycle.
--
Doug Arnold
Principal Technologist
*JTime!* Meinberg USA
+1-707-303-5559
Internet-Draft Enterprise Profile for PTP October 2013
TICTOC Working Group Doug Arnold
Internet Draft Meinberg-USA
Intended status: Standards Track Heiko Gerstung
Meinberg
Expires: April 15, 2014 Oct 15, 2013
Enterprise Profile for the Precision Time Protocol
With Mixed Multicast and Unicast Messages
draft-ietf-tictoc-ptp-enterprise-profile-01.txt
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Arnold and Gerstung October 15, 2013 [Page 1]
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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.
Table of Contents
1. Introduction 2
2. Conventions used in this document 3
3. Technical Terms 4
4. Problem Statement 5
5. Network Technology 6
6. Time Transfer and Delay Measurement 7
6.1. Unicast Delay Measurement Mode 7
6.2. Multicast Delay Measurement Mode 7
6.3. Hybrid delay Measurement Mode 8
7. Default Message Rates 8
8. Requirements for Master Clocks 8
9. Requirements for Slave Clocks 10
10. Requirements for Transparent Clocks 10
11. Management and Signaling Messages 10
12. Forbidden PTP Options 11
13. Interoperation with Other PTP Profiles 11
14. Security Considerations 11
15. IANA Considerations 12
16. References 12
16.1. Normative References 12
16.2. Informative References 12
17. Acknowledgments 13
18. Authors addresses 13
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
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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 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 up to 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. A number of 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
SNMP.
2. Conventions used in this document
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].
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.
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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 1588-2008 this is an 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.
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.
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.
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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 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
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.
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
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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
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.
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.
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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.
Similar to NTP, PTP makes the assumption 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, [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.
In all three of the delay measurement modes, 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 [IEEE1588] Annex E, Section E.3.
6.1. Unicast Delay Measurement Mode
All Delay Request PTP event messages and Delay Response PTP
general messages MUST be sent as unicast messages. Unicast
Discovery and Unicast Message Negotiation options MUST NOT be
utilized.
6.2. Multicast Delay Measurement Mode
All Delay Request PTP event messages and Delay Response general
messages MUST be sent as multicast messages.
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6.3. Hybrid Delay Measurement Mode
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.
The default mode for Enterprise Profile PTP Master Clocks is
Hybrid Delay Measurement Mode. This allow for the use of Ordinary
clocks which do not support the Enterprise Profile, as long as
they are slave Only Clocks.
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.
8. Requirements for Master Clocks
Master clocks SHALL obey the standard Best Master Clock Algorithm
from [IEEE1588]. Clocks which are master capable MAY act as
Alternate masters according to [IEEE1588]. Preferred Master
Clocks SHOULD operate as Alternate Masters when they are not the
Best Master. Using multiple masters can mitigate rogue master and
man-in-the-middle attacks such as delay attacks, packet
interception / manipulation attacks. Assuming the path to each
master is different, an attacker would have to attack more than
one path simultaneously.
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. A clock with a port in the master state SHOULD indicate
the maximum adjustment to its internal clock within one sync
interval. The maximum phase adjustment is indicated in the
Enterprise Profile announce TLV field for Maximum Phase Adjustment.
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The Announce Messages SHALL include a TLV which indicates that the
clock is operating in the Enterprise Profile. The TLV shall have
the following structure:
TLV Type (Enumeration16): ORGANIZATION_EXTENSION value = 0003 hex
Length Field (UInteger16): value = 12. The number of TLV octets
Port Number (UInteger16): The Port Number of the port transmitting
the TLV. The all-ones Port Number, with value FFFFhex, is used to
indicate that the identified profile is applicable to all ports on
the clock.
Organization Unique Identifier (3 Octets): The Organization ID
value for IETF assigned by IEEE = 00005Ehex
IETF Profile number (UInteger16): value = 1
Revision number (UInteger8): value = 1
Delay Measurement Mode (Enumeration8):
Value 0 = Multicast Delay Measurement Mode
Value 1 = Unicast Delay Measurement Mode
Value 2 = Hybrid Delay Measurement Mode
Values 3-255 = reserved for future use.
Maximum Absolute Phase Adjustment Value within one sync interval
(UInteger16): value
Maximum Phase Adjustment Units (Enumeration8):
Value 0 = unknown
Value 1 = seconds
Value 3 = milliseconds
Value 6 = microseconds
Value 9 = nanoseconds
Value 12 = picoseconds
Value 15 = femtoseconds
All other values reserved for future use
The TLV represents the Delay Request Mode of the Master, not
necessarily the Grandmaster. So for example, one port of a
Boundary clock could be set to Unicast Delay Measurement Mode,
while the rest of the network is Hybrid Delay Measurement Mode.
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9. Requirements for Slave Clocks
Slave clocks MUST be able to operate properly in a network which
contains Alternate Masters. Slaves SHOULD make use of information
from the Alternate 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 Rogue Master.
Slaves MAY use an Acceptable Master Table. If the Best Master is
not an Acceptable Master, but an Alternate Master is an Acceptable
Master, then the Slave SHOULD synchronize to the acceptable
Alternate Master. Note that IEEE 1588-2008 requires slave clocks to
support both two-step or one-step Master clocks. See [IEEE1588],
section 11.2.
Since Announce messages are sent as multicast messages in all
mode, 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.
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 SHALL NOT alter the Enterprise Profile TLV of
an Announce message, or any other part of an Announce message.
11. 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.
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12. Forbidden PTP Options
Clocks operating in the Enterprise Profile SHALL NOT use peer to
peer timing for delay measurement. Clocks operating in the
Enterprise Profile SHALL NOT use Unicast Message Negotiation to
determine message rates. Slave clocks operating in the Enterprise
Profile SHALL NOT use Unicast Discovery to establish connection to
Master clocks. Grandmaster Clusters are NOT ALLOWED. Clocks
operating in the Enterprise Profile SHALL NOT use Alternative Time
Scales.
13. Interoperation with Other PTP Profiles
Clocks operating in the Enterprise profile will not interoperate
with clocks operating in the Power Profile [C37.238], because the
Enterprise Profile requires the End to End Delay Measurement
Mechanism and the Power Profile requires the Peer to Peer Delay
Measurement Mechanism.
Clocks operating in the Enterprise profile will not interoperate
with clocks operating in the Telecom Profile for Frequency
Synchronization[G8265.1], because the Enterprise Profile forbids
Unicast Message Negotiation, and Unicast Discovery. These
features are part of the default manner of operation for the
Telecom Profile for Frequency Synchronization.
Clocks operating in the Enterprise profile will interoperate with
clocks operating in the default profile if the default profile
clocks operate on IPv4 or IPv6 networks, use the End to End Delay
Measurement Mechanism, and use management messages in unicast when
those messages are directed at a specific clock. If any of these
requirements are not met than Enterprise Profile clocks will not
interoperate with Default Profile Clocks. The Default Profile is
described in Annex J of [IEEE1588].
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.
14. 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.
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The use of multiple simultaneous masters, using the IEEE 1588
Alternate Master option 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.
15. IANA Considerations
There are no IANA requirements in this specification.
16. References
16.1. 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.
16.2. Informative References
[C37.238] IEEE std. C37.238-2011, "IEEE Standard Profile for
Use of IEEE 1588 Precision Time Protocol in Power
System Applications," July 2011.
[G8265.1] ITU-T G.8265.1/Y.1365.1, "Precision time protocol
telecom profile for frequency synchronization,"
October 2011.
[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.
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17. Acknowledgments
The authors would like to thank members of IETF for reviewing and
providing feedback on this draft.
This document was prepared using 2-Word-v2.0.template.dot.
18. Authors' Addresses
Doug Arnold
Meinberg USA
228 Windsor River Rd
Windsor, CA 95492
USA
Email: [email protected]
Heiko Gerstung
Meinberg Funkuhren GmbH & Co. KG
Lange Wand 9
D-31812 Bad Pyrmont
Germany
Email: [email protected]
Arnold and Gerstung October 15, 2013 [Page 13]
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