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Versions: 00 01 02

TICTOC                                                         S. Bryant
Internet-Draft                                             Cisco Systems
Intended status: Informational                               Y(J). Stein
Expires: October 11, 2008                        RAD Data Communications
                                                           April 9, 2008


                        TICTOC Problem Statement
                  draft-bryant-tictoc-probstat-02.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on October 11, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This Internet draft describes a number of applications that require
   accurate time and/or frequency, and elucidates difficulties related
   to the transfer of high quality time and frequency across an IP or
   MPLS Packet Switched Network.  This issue is not addressed by any
   currently chartered IETF working group, and we therefore propose the
   formation of a new working group to be called Transmitting Timing
   over IP Connections and Transfer of Clock (TICTOC).



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Applications . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Time Service Applications  . . . . . . . . . . . . . . . .  3
     2.2.  Frequency Service Applications . . . . . . . . . . . . . .  4
   3.  Existing Time and Frequency Transfer Mechanisms  . . . . . . .  7
     3.1.  Radio-based Timing Transfer Methods  . . . . . . . . . . .  7
     3.2.  Dedicated Wire-based Timing Transfer Methods . . . . . . .  8
     3.3.  Transfer Using Packet Networks . . . . . . . . . . . . . .  9
       3.3.1.  The Packet Network Environment . . . . . . . . . . . . 11
   4.  Problems with Existing Solutions . . . . . . . . . . . . . . . 11
   5.  Other Forums Working in this Problem Space . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   10. Informative References . . . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 16































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1.  Introduction

   There is an emerging need to distribute highly accurate time and
   frequency information over IP and over MPLS packet switched networks
   (PSNs).  In this problem statement we give several examples of
   applications that require time and/or frequency information, and
   explain why their needs can not be satisfied by time/frequency
   transfer over PSNs using existing protocols.  We review these
   existing protocols and the work being carried out in the IETF and in
   other forums.  Finally, we list the objectives of a proposed Working
   Group.


2.  Applications

2.1.  Time Service Applications

   There are many applications that need to know the time with greater
   precision than provided by available mechanisms, such as the current
   version of NTP [RFC1305].  These applications span a range of
   industries: telecommunications, financial, test and measurement,
   government, industrial etc.  Preliminary studies indicate that the
   availability of high accuracy time as a commodity enables use of
   techniques that were previously considered impossible.  We can,
   therefore, expect that the provision of high quality time through the
   network infrastructure will generate a spiral of new innovative
   applications that will in turn make greater demands on the quality of
   time delivered to the end-user.

   The best-known example of an application that requires high quality
   time in the telecommunications sector is the need to measure one-way
   packet delay.  Current implementations of NTP have accuracy of the
   order of 10 milliseconds.  When NTP is used to characterize packet
   delay and packet delay variation, such a time-base cannot resolve any
   two party event with a resolution of better than 20 milliseconds.
   Contrast this with the characteristics of a 10 Gb/s link, 1 kilometer
   long.  On such a link, a minimum sized packet takes 50 nanoseconds to
   send, and it takes 6 microseconds to traverse the link.  The
   performance of current NTP implementations is orders of magnitude
   worse than the duration of network forwarding events, and clearly
   insufficient to characterize them.

   Although the measurement of the characteristics of a packet network
   is the best-known telecommunications example, there are other
   operational needs, notably synchronization at the MAC layer.  The
   cable industry has recently defined a new intra-PoP time transmission
   mechanism for just this purpose (DOCSYS Timing Interface), and WiMAX
   is looking for relative time delivery to its transmitter sites.



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   In the test and measurement and industrial sector there is a desire
   to move from special purpose communications infrastructure with
   calibrated wiring run back to a centralize controller, to a
   distributed system, in which instructions are distributed in advance
   to be executed at a predetermined time, and in which measurements are
   taken remotely and communicated back to a common point for later
   correlation and analysis.  Two examples of this tendency are
   described below.

   In the printing industry there is a need to control operations in
   multi-stand printing machines.  The paper travels through these
   machines at a speed of nearly 100 km/h.  At these speeds, co-
   ordination error of 1 microsecond between operations taking place at
   different positions in the machine produces a 0.03mm color offset,
   which is visible to the naked eye and results in an unacceptable
   degradation in quality.

   In the electrical power industry there is a need to improve the
   measurement of power flows in order to monitor and predict usage
   patterns.  One proposal is to extensively deploy synchro-phasors in
   the power network and to correlate their output to determine demand.
   These devices need to be able to determine the time of measurement
   with an accuracy of 1 microsecond.

   More generally, there is growing interest in clock synchronization in
   massively parallel sensor networks.  Advances in wireless
   communications have enabled the development of low power miniature
   sensors that collect and disseminate data from their immediate
   environment.  Although each sensor has limited processing power,
   through distributed processing the network becomes capable of
   performing various tasks of data fusion, but only assuming a common
   time base can be established.

   The examples cited above are a small illustration of a trend that
   will continue to grow as designers realize that better scaling can be
   achieved with action-in-the-future, measure-and-correlate-later
   approaches to systems design.

   Closer to the core interests and expertise of the IETF there is an
   emerging opinion that the availability of time as a commodity may
   simplify the protocols that we use in distributed systems.

2.2.  Frequency Service Applications

   There are applications that require time with a greater precision
   than can easily be provided using available mechanisms.  Cellular
   base-stations require a highly accurate frequency reference from
   which they derive transmission frequencies and operational timing.



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   Conventionally GSM and WCDMA base stations obtain this reference
   frequency by locking on to the E1/T1 that links them to the base
   station controller.  With the replacement of TDM links with Packet
   Switched Networks (PSNs) such as Ethernet, IP or MPLS, this simple
   method of providing a frequency reference is lost, and frequency
   information must be made available in some other way.

   Why must the frequency reference be so accurate?  First and foremost
   the requirement is derived from the need for the radio frequencies to
   be accurate.  Radio spectrum is a limited and valuable commodity that
   needs to be used as efficiently as possible.  In GSM, transmission
   frequencies are allocated to a given cellular base station and its
   neighbors in such fashion as to ensure that they do not interfere
   with each other.  If the radio network designer cannot rely on the
   accuracy of these frequencies, the spacing between the frequencies
   used by neighboring sites must be increased, with significant
   economic consequences.

   There is an additional requirement derived from the need for smooth
   handover when a mobile station crosses from one cell to another.  If
   the radio system designer can not guarantee that the preparations
   required for handover occur in a few milliseconds, then they must
   allow the mobile station to consume frequency resources
   simultaneously in both cells in order to avoid service disruption.
   The preparations required involve agreement between the mobile and
   base stations on the new frequencies and time offsets; these
   agreements can be accomplished quickly when the two base stations'
   frequency references are the same to a high degree of accuracy.

   Another application requiring highly accurate frequency distribution
   is TDM pseudowires.  The PWE3 WG has produced three techniques for
   emulating traditional low-rate (E1, T1, E3, T3) TDM services over
   PSNs, namely SAToP [RFC4553], CESoPSN, and TDMoIP.  The major
   technical barrier to universal acceptance of TDM pseudowires is the
   accuracy of its clock recovery.

   TDM network standards for timing accuracy and stability are extremely
   demanding.  These requirements are not capriciously dictated by
   standards bodies, rather they are critical to the proper functioning
   of a high-speed TDM network.  Consider a TDM receiver utilizing its
   own clock when converting the physical signal back into a bit-stream.
   If the receive clock runs at precisely the same rate as the source
   clock, then the receiver need only determine the optimal sampling
   phase.  However, with any mismatch of clock rates, no matter how
   small, bit slips will eventually occur.  For example, if the receive
   clock is slower than the source clock by one part per million (ppm),
   then the receiver will output 999,999 bits for every 1,000,000 bits
   sent, thus deleting one bit.  Similarly, if the receive clock is



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   faster than the source clock by one part per billion (ppb), the
   receiver will insert a spurious bit every billion bits.  One bit slip
   every million bits may seem acceptable at first glance, but
   translates to a catastrophic two errors per second for a 2 Mb/s E1
   signal.  ITU-T recommendations permit a few bit slips per day for a
   low-rate 64 kb/s channel, but strive to prohibit bit slips entirely
   for higher-rate TDM signals.

   In certain cases, such as "toll-bypass" or "carrier's carrier" links,
   the endpoints of the TDM PW are full TDM networks, and timing may
   (indeed must) be derived from the respective network clocks.  Since
   each of these clocks is highly accurate, they necessarily agree to
   high order.  However, TDM PWs are expected to increasingly replace
   native TDM links delivering services from core networks towards
   users, and here there is no alternative to provision of accurate
   frequency information.

   In this context there are two types of frequency distribution being
   studied.  In the first type the frequency reference used by the TDM
   source is distributed downstream, as in the native TDM service.  In
   the "common clock" scenario highly accurate frequency information is
   distributed from a central server to both ends of the emulated TDM
   link.  By placing in the protocol overhead timestamps based on the
   common clock, the receiver can accurately recover the TDM source
   clock.

   While it is true that services designed for PSN (e.g.  VoIP)
   transport are less dependent on frequency accuracy, there are still
   cases where such services need accurate frequency distribution.  For
   example, when interconnecting tradition telephones via VoIP links,
   users expect these links to support legacy services, such as
   facsimile and dial-up data modems.  The optimal technique for
   supporting these services is by provision of relay functions, e.g.
   T.38 fax-relay and V.150 modem-relay, that terminate the analog
   transmissions on both sides and transfer data content over the PSN.
   However, provision of relay services is computationally expensive,
   often requires expensive DSP-capable media gateways, and can only
   support known modem types.  In many deployments old fax machines or
   proprietary data modems or secure voice telephones are used, and the
   modulations and handshake protocols are not recognized by the relays
   provided.  In such cases the solution is to carry these transmissions
   over "clear channel" or Voice Band Data (VBD), i.e. to send raw
   samples of the audio in packets over the PSN.

   The problem with clear channel transfer of data over PSNs is that the
   end points expect a non-intrusive analog channel between them, over
   which they implicitly transfer timing information.  The receiver can
   thus continually lock onto the transmitter's frequency, and the



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   transmission can continue for an unlimited period without
   interruption.  When employing clear channel, the frequency signal
   seen by the receiver is influenced by the destination gateway's clock
   used to convert the data samples back to analog form.  If the source
   and destination gateways' clocks do not agree to a high degree of
   accuracy, the receiver does not properly lock onto the transmitter's
   clock, leading to disruption of the data reception.  In typical cases
   a modem conversation transferred over clear channel may drop after
   only several minutes, and fax reception may be interrupted after
   several pages have been received.


3.  Existing Time and Frequency Transfer Mechanisms

   In this section we will discuss existing mechanisms for transfer of
   time information, frequency information, or both.  It should be noted
   that a sufficiently accurate time transfer service may be used to
   derive an accurate frequency transfer.  Indeed, this is exactly what
   happens in a GPS disciplined frequency standard.  On the other hand,
   an accurate frequency transfer service, while itself unable to
   transfer absolute time, is usually used to support and improve the
   performance of the time transfer service.  Indeed, implementations of
   NTP or IEEE 1588 clients can be considered to consist of two phases.
   First, a local oscillator is locked to the server's frequency using
   incoming information from the incoming packets, and then the local
   time set based on the server's time and the propagation latency.  By
   maintaining a local frequency source, the client requires relatively
   infrequent updates, and can continue functioning during short periods
   of network outage.  Moreover, it can be shown that this method
   results in significantly better time transfer accuracy than methods
   that do not discipline a local clock.

   Time transfer mechanisms can be divided into three classes.  The
   first class consists of mechanisms that use radio frequency
   transport, while the second mechanism uses dedicated "wires" (which
   for our purposes include optical fibers).  The third, which will be
   our main focus, exploits a Packet Switched Network for transfer of
   timing information.  Advantages and disadvantages of these three
   methods are discussed in the following subsections.

3.1.  Radio-based Timing Transfer Methods

   The transfer of time by radio transmission is one of the oldest
   methods available, and is still the most accurate wide area method.
   In particular, there are two navigation in wide use that can be used
   for time transfer, The LOng RAnge Navigation (LORAN) terrestrial
   radio system, and the Global Navigation Satellite System (GNSS).  In
   both cases the user needs to be able to receive the transmitted



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   signal, requiring access to a suitable antenna.  In certain
   situations, e.g. basement communications rooms and urban canyons, the
   required signal may not be receivable.

   Radio systems have high accuracy, far better than what we will later
   see can be achieved by existing PSN technologies.  However coverage
   is limited; eLORAN for example only covers North America, and GPS
   does not have good coverage near the poles.

   Although civilian use is sanctioned, the GPS was developed and is
   operated by the U.S. Department of Defense as a military system.  For
   this reason there are political concerns that rules out its use in
   certain countries.  The European Union is working on an alternative
   system called Galileo, which will be run as a commercial enterprise.
   In addition, GPS has some well-documented multi-hour outages, and is
   considered vulnerable to jamming.  One major PTT also reports that
   they see a 2% per year failure rate for the antenna/receiver/
   clock-out chain.

   While a radio-based timing service may be acceptable for some sites,
   it is frequently impractical to use on a per equipment basis.  Hence,
   some form of local timing distribution is usually also required.

3.2.  Dedicated Wire-based Timing Transfer Methods

   The use of dedicated networks in the wide area does not scale well.
   Such services were available in the past, but for reasons of cost and
   accuracy have been superseded by GPS based solutions.

   In the local area, one new technique is emerging as a mechanism for
   time transport, namely DOCSIS Timing Interface / Telecommunications
   Timing Interface (DTI/TTI).  DTI was designed by DOCSIS for the
   distribution of time in a cable head-end in support of media access
   control.  Time transfer is packet-based over a multi-stage hub and
   spoke dedicated network.  It uses a single twisted-pair in half-
   duplex to eliminate inaccuracies due to the length differences
   between the pairs in a multi-pair cable.  TTI is a development of DTI
   designed to provide synchronization in a telephone local office.
   Accuracy for DTI is better than 5 nanoseconds and range is 100 feet
   for DTI.  This increases to 600 feet for TTI at some reduction in
   packet rate and hence time quality.

   The DTI/TTI approach is applicable for special applications, but the
   need for a dedicated network imposes significant drawbacks for the
   general time transfer case.

   Synchronous Ethernet is a technique that has recently been proposed
   for providing frequency distribution over Ethernet links.  Modern



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   dedicated-media full-duplex Ethernet, in both copper and optical
   physical layer variants, transmits continuously.  One can thus elect
   to derive the physical layer transmitter clock from a high quality
   frequency reference, instead of the conventional 100 ppm crystal-
   derived transmitter rate.  The receiver at the other end of the link
   automatically locks onto the physical layer clock of the received
   signal, and thus itself gain access to a highly accurate and stable
   frequency reference.  Then, in TDM fashion, this receiver could lock
   the transmission clock of its other ports to this frequency
   reference.

   The ITU-T is presently working on a specification for synchronous
   Ethernet.  Apart from some necessary higher layer packet based
   configuration and OAM operations, the solution is entirely physical
   layer, and has no impact on higher layers.

   At first sight it would seem that the only application of synchronous
   Ethernet was in frequency transfer (it has no intrinsic time transfer
   mechanism).  However, the quality of packet-based time transfer
   mechanism can be considerably enhanced if used in conjunction with
   synchronous Ethernet as a frequency reference.

3.3.  Transfer Using Packet Networks

   When using a PSN to transfer timing, a server sends timing
   information in the form of packets to one or multiple clients.  When
   there are multiple clients, the timing packets may be multicast.
   Software in the client recovers the frequency and/or time of the
   server based on the packet arrival time and the packet contents.

   There are two well-known protocols capable of running over a general-
   purpose packet network, NTP [RFC1305], and IEEE 1588 [1588].  NTP is
   the product of the IETF, and is currently undergoing revision to
   version 4.  IEEE 1588 (a product of the IEEE Test and Measurement
   community) is specified in a limited first version, and the second
   version (1588v2)is in the detailed design stage.

   NTP is widely deployed, but existing implementations deliver accuracy
   on the order of 10 milliseconds.  This accuracy is not adequate for
   the applications described above.  NTP suffers from the fact that it
   was designed to operate over the Internet, and the routers and
   switches used in the best effort Internet make no special concessions
   to NTP for preservation of time transfer accuracy.  Furthermore,
   typical update rates are low and can not be significantly increased
   due to scalability issues in the server.  In addition most NTP time
   servers and time receivers have a relatively unsophisticated
   implementation that further degrades the final time quality.




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   IEEE 1588v1 was a time transfer protocol that exclusively used a
   fairly crude multicast technique.  It is widely anticipated that wide
   scale deployment of IEEE1588 will be based on 1588v2.  The
   information exchange component of IEEE 1588 is a protocol known as
   Precision Time Protocol (PTP).

   IEEE 1588v2 can be considered to consist of several components:

   1.  A configuration and control protocol

   2.  A time transfer protocol

   3.  A time correction protocol

   4.  Physical mapping

   The configuration and control protocol is based on the multicast
   approach of IEEE 1588v1 (multicast IP with recommended TTL=1, UDP,
   IEEE1588 payload with equipment identifier in the payload).  The
   rationale for this approach was that the equipment needed to be "plug
   and play" (no configuration), was required to map to physical media
   other than Ethernet, and had to have a very low memory and processor
   footprint.

   The time transfer protocol is a standard two-way time transfer
   approach used in other packet-based approaches.  Like all such
   approaches it is subject to inaccuracies due to variable store and
   forward delays in the packet switches, and due to the assumption of
   symmetric propagation delays.  The time transfer packets (in both
   directions) may be operated in a multicast or unicast mode.

   The time correction protocol is used to correct for propagation,
   store and forward delays in the packet switches.  This again may be
   operated multicast or unicast.  This mechanism requires some level of
   hop-by-hop hardware support.  This mechanism may also be considered a
   concept in its own right and may be adapted to enhance other packet
   time transfer protocols such as NTP.

   The base 1588 specification describes how the PTP operates over the
   Ethernet/IP/UDP protocol stack.  Annexes are planned that describe
   PTP operation over pure layer 2 Ethernet, over IP without UDP, over
   MPLS, and over a number of specialist media.

   The mappings of interest for telecommunications are PTP over UDP/IP,
   PTP over MPLS , and perhaps PTP over Ethernet, all in unicast mode
   only.  Issues of a suitable control management and OAM environment
   for these applications are largely in abeyance, as are considerations
   about the exact nature of the network environment.



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   It is also worth noting the existence of a second IEEE effort, IEEE
   802.1AS.  This group is specifying the protocol and procedures to
   ensure synchronization across Bridged and Virtual Bridged Local Area
   Networks for time sensitive applications such as audio and video.
   For these LAN media the transmission delays are assumed to be fixed
   and symmetrical.  IEEE 802.1AS specifies the use of IEEE 1588
   specifications where applicable in the context of IEEE Standards
   802.1D and 802.1Q. Synchronization to an externally provided timing
   signal (e.g., a recognized timing standard such as UTC or TAI) is not
   part of this standard but is not precluded.  IEEE 802.1AS will
   specify how stations attached to bridged LANs to meet the respective
   jitter, wander, and time synchronization requirements for time-
   sensitive applications.

3.3.1.  The Packet Network Environment

   Packet delay variation, propagation asymmetry, and maximum
   permissible packet rate all have a significant bearing on the
   accuracy with which the client is able to determine absolute time.
   Thus the network environment has a large bearing on the quality of
   time that can be delivered.

   Packet delay variation can to some extent be addressed by traffic
   engineering, thus time transfer with a service provider network in
   which suitable traffic engineering techniques had been applied might
   reasonably be expected to deliver a higher quality time service than
   can be achieved between two arbitrary hosts connected to the
   Internet.  Greater gains can probably be obtained by deploying
   equipment that incorporates IEEE 1588 style on-the-fly packet
   timestamp correction, or follow-up message mechanisms that report the
   packet storage and forward delays to the client.  However one can
   only be sure that such techniques are available along the entire path
   in a well-controlled environment.

   The packet rate between the time-server and its client also has a
   bearing on the quality of the time transfer, because at a higher rate
   the smart filter has a better chance of extracting the "good"
   packets.  In a controlled environment it is possible to ensure that
   there is adequate bandwidth, and that the server is not overloaded.
   In such an environment the onus moves from protecting the server from
   overload, to ensuring that the server can satisfy the needs of all of
   the clients.


4.  Problems with Existing Solutions

   An obvious candidate for clock distribution is NTP or some upgrade
   thereof.  While the time resolution provided by NTP is extremely



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   good, the accuracy attainable by existing NTP implementations does
   not satisfy the needs of the most demanding of the applications,
   mainly due to update rate and the particular client/server method
   employed.

   The new IEEE 1588v2 protocol also addresses these needs, but has been
   largely designed around a well-controlled LAN environment.  A 1588
   server in unicast mode needs to save state information for each
   client, a solution that does not scale well to deployment sizes
   envisioned.  In addition, 1588 specifies hardware upgrades in order
   to perform well in an IP network.

   Synchronous Ethernet only satisfies the need for frequency
   distribution, and even then only over one physical Ethernet link at a
   time.  In order to use synchronous Ethernet in a network, all network
   elements must be upgraded to support synchronous operation at the
   physical layer.  Even when hardware can be upgraded, only frequency
   is delivered, and there is still a need to develop a time transfer
   protocol.


5.  Other Forums Working in this Problem Space

   The NTP WG is the IETF group working on time distribution, but is
   presently only documenting NTPv4 and is not working on new algorithms
   or protocols.  It is expected that many participants of the NTP WG
   will participate in the TICTOC effort.

   The PWE3 WG has discussed frequency distribution for the TDM PW
   application, however it is not chartered to develop protocols for
   this purpose.  It is expected that participants of the PWE3 WG who
   were active in the TDM PW discussions will participate in the TICTOC
   effort.

   The work that is underway outside the IETF is either complementary to
   this proposal, or less general than the proposal proposed by the
   TICTOC work proposal.

   The IEEE 1588 task force is working on a new version of their
   protocol that will run over more types of PSNs, and is planning to
   conclude its development work in the near future.  The protocol to be
   specified contains elements that will be of use in an IETF
   environment, but is unlikely to be regarded as being a complete,
   robust solution in such an environment.  If the IEEE 1588 structure
   is deemed to be a suitable platform, then the IETF could contribute
   an Internet profile, including a complete distributed systems
   environment suitable for our purposes.  Alternatively, the IETF could
   perhaps borrow some of the delay correction mechanisms and



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   incorporate them into a development of a new version of NTP.

   In addition, IEEE 802.1AS is working on Timing and Synchronization
   for Time-Sensitive Applications in Bridged Local Area Networks,
   basing itself on the IEEE 1588 standard.

   ITU-T SG15 Question 13 has produced Recommendation G.8261 "Timing and
   synchronization aspects in packet networks" [G8261].  This
   Recommendation defines requirements for various scenarios, outlines
   the functionality of frequency distribution elements, and provides
   measurement guidelines.  It does not specify algorithms to be used
   for attaining the performance needed.  It does define requirements
   for status synchronization messages, but does not otherwise define a
   protocol (although work is in progress).  To date the ITU-T has
   focused on Ethernet infrastructure, but this is likely to extend to
   an MPLS environment.  Two new work items, G.paclock and G.pacmod
   extend the work, and in particular, G.pacmod intends to introduce
   time transfer.  This is an area where the IETF, with its expertise in
   IP and MPLS networks, may co-operate with the ITU.


6.  Security Considerations

   Time and frequency services are a significant element of network
   infrastructure, and are critical for certain emerging applications.
   Hence time and frequency transfer services MUST be protected from
   being compromised.  The most significant threat is a false time or
   frequency server being accepted instead of a true one, thus enabling
   a hacker to bring down critical services.

   Any protection mechanism must be designed in such a way that it does
   not degrade the quality of the time transfer.  Such a mechanism
   SHOULD also be relatively lightweight, as client restrictions often
   dictate a low processing and memory footprint, and because the server
   may have extensive fan-out.


7.  Security Considerations

   Timing distribution is highly sensitive to packet delay, and can thus
   can deteriorate under congestion conditions.  Furthermore the
   disciplining of the client's oscillator (the sole component of
   frequency transfer, and a critical component of time transfer) is a
   function that should not be disrupted.  When the service is disrupted
   the client needs to go into "holdover" mode, and its accuracy will
   consequently be degraded.  Depending on the relative quality of the
   client's clock and the required quality after disciplining, a
   relatively high packet rate may be required.



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   Timing tranfer packets should always be sent using the highest class
   of service, and when possible should be sent over a traffic
   engineered path.

   When the network goes into congestion it should try to avoid
   discarding time transfer packets until the situation is critical.
   Work performed by the IETF PWE3 WG on congestion would seem to be
   applicable to this problem area.


8.  IANA Considerations

   No IANA actions are required as a result of the publication of this
   document.


9.  Acknowledgements

   The authors wish to thank Laurent Montini for valuable comments.


10.  Informative References

   [1588]     IEEE, "1588-2002 Standard for  A Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems".

   [G8261]    ITU-T, "Recommendation G.8261/Y.1361 - Timing and
              synchronization aspects in packet networks", May 2006.

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

   [RFC4553]  Vainshtein, A. and YJ. Stein, "Structure-Agnostic Time
              Division Multiplexing (TDM) over Packet (SAToP)",
              RFC 4553, June 2006.


Authors' Addresses

   Stewart Bryant
   Cisco Systems
   250 Longwater Ave., Green Park
   Reading  RG2 6GB
   United Kingdom

   Email: stbryant@cisco.com




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   Yaakov (Jonathan) Stein
   RAD Data Communications
   24 Raoul Wallenberg St., Bldg C
   Tel Aviv  69719
   ISRAEL

   Phone: +972 3 645-5389
   Email: yaakov_s@rad.com











































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