TICTOC S. Rodrigues Internet-Draft Zarlink Semiconductor Intended status: Informational K. Lindqvist Expires: September 3, 2009 Netnod March 02, 2009 TICTOC Requirement draft-rodrigues-lindqvist-tictoc-req-02.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 3, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract Distribution of high precision time and frequency over the Internet and special purpose IP networks is becoming more and more needed as Rodrigues & Lindqvist Expires September 3, 2009 [Page 1] Internet-Draft TICTOC March 2009 IP networks replace legacy networks and as new applications with need for frequency and time are developed on the Internet. The IETF formed the TICTOC working group to address the problem and perform an analysis on existing solutions and the needs. This document summarizes application needs, as described and agreed on at an TICTOC interim meeting held in Paris from June 16 to 18, 2008. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Applications Requirements . . . . . . . . . . . . . . . . . . 3 3.1. Cellular Backhauling . . . . . . . . . . . . . . . . . . . 3 3.1.1. Cellular Backaul Requirements . . . . . . . . . . . . 4 3.2. Circuit Emulation . . . . . . . . . . . . . . . . . . . . 9 3.2.1. Circuit Emulation Requirements . . . . . . . . . . . . 9 3.3. Test and Measurement . . . . . . . . . . . . . . . . . . . 11 3.3.1. Test and Measurement Requirements . . . . . . . . . . 12 3.4. Industrial Automation . . . . . . . . . . . . . . . . . . 15 3.4.1. Industrial Automation Requirements . . . . . . . . . . 16 3.5. ToD/ Internet . . . . . . . . . . . . . . . . . . . . . . 18 3.5.1. ToD/Internet Requirements . . . . . . . . . . . . . . 19 3.6. Networking . . . . . . . . . . . . . . . . . . . . . . . . 21 3.6.1. Networking Requirements . . . . . . . . . . . . . . . 21 3.7. Legal Time . . . . . . . . . . . . . . . . . . . . . . . . 23 3.7.1. Legal Time Requirements . . . . . . . . . . . . . . . 23 3.8. Metrology . . . . . . . . . . . . . . . . . . . . . . . . 25 3.8.1. Metrology Requirements . . . . . . . . . . . . . . . . 25 3.9. Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.9.1. Sensors Requirements . . . . . . . . . . . . . . . . . 27 4. Network Dependencies . . . . . . . . . . . . . . . . . . . . . 29 5. Network Topology . . . . . . . . . . . . . . . . . . . . . . . 30 6. Security Considerations . . . . . . . . . . . . . . . . . . . 30 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30 9. Informative References . . . . . . . . . . . . . . . . . . . . 31 Appendix A. Existing Time and Frequency Transfer Mechanisms . . . 31 A.1. Radio-based Timing Transfer Methods . . . . . . . . . . . 32 A.2. Dedicated Wire-based Timing Transfer Methods . . . . . . . 33 A.3. Transfer Using Packet Networks . . . . . . . . . . . . . . 34 A.3.1. NTP summary description . . . . . . . . . . . . . . . 34 A.3.2. IEEE1588 summary description . . . . . . . . . . . . . 34 Appendix B. Other Forums Working in this Problem Space . . . . . 36 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37 Rodrigues & Lindqvist Expires September 3, 2009 [Page 2] Internet-Draft TICTOC March 2009 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 draft, the requirements for transporting accurate time and/or frequency are addressed. 2. Terminology 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 [RFC2119] 3. Applications Requirements There are many applications that need synchronization. Some applications only need frequency; for others a combination of frequency and time of day or phase may be required. At the TICTOC interim meeting, it was agreed that these applications be grouped based on what was believed to be common requirements, and where the requirements where distinct from each other. This section describes these applications (or groups of applications) that was agreed on at the TICTOC interim meeting. 3.1. Cellular Backhauling Within Cellular backhauling, there are several applications that need to be considered. Some of these applications only require frequency information, others require time-of-day, and others require phase. The cellular backhauling applications to be considered are: o GSM o Mobile Wimax o LTE o UMTS FDD o UMTS TDD o CDMA2000 o TD-SCDMA Conventionally GSM and UMTS FDD base stations obtain this reference Rodrigues & Lindqvist Expires September 3, 2009 [Page 3] Internet-Draft TICTOC March 2009 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. The synchronization 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. 3.1.1. Cellular Backaul Requirements The requirements for the Cellular Backhauling is summarized in the table 1. Editor's note: This table was discussed at the Dublin meeting; need input from the group to fill in the blanks. Rodrigues & Lindqvist Expires September 3, 2009 [Page 4] Internet-Draft TICTOC March 2009 Cellular Backhaul Requirements +-----------+-------+--------+---------+--------+---------+---------+ | Requireme | GSM/U | UMTS | Mobile | LTE | CDMA200 | TD-SCDM | | nts | MTS | TDD | Wimax | | 0 | A | | | FDD | | (5) | | | | +-----------+-------+--------+---------+--------+---------+---------+ | Synchroni | frequ | phase | phase | phase | phase | phase | | zation | ency | alignm | alignme | alignm | alignme | alignme | | type | | ent | nt | ent | nt | nt | | (e.g. | | | | | | | | time, | | | | | | | | frequenc | | | | | | | | yor phase | | | | | | | | ) | | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Frequency | 50-25 | 50-250 | 15 ppb | 50-250 | 50-250 | 50-250 | | stability | 0ppb | ppb | | ppb | ppb (1) | ppb (1) | | | (1) | (1) | | (1) | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Frequency | 50-25 | 50-250 | 15 ppb | 50-250 | 50-250 | 50-250 | | accuracy | 0ppb | ppb | | ppb | ppb (1) | ppb (1) | | | (1) | (1) | | (1) | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Uncalibra | | | | | | | | ted | | | | | | | | time/tim | | | | | | | | estabilit | | | | | | | | y | | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Uncalibra | NA | The | The | From | The | The | | ted | | phase | phase | 1us to | pilot | phase | | time/tim | | alignm | alignme | 50us | time | alignme | | eaccuracy | | ent of | nt of | (2, 3) | alignme | nt of | | | | neigh | neighb | | nt erro | neighb | | | | bourin | ouring | | rshould | ouring | | | | g base | base | | be les | base | | | | stat | stati | | sthan | stati | | | | ions | onsshal | | 3us an | onsshal | | | | sha | l be | | dshall | l be | | | | ll be | with | | be les | with | | | | wit | in 1 us | | sthan | in 3us | | | | hin2.5 | | | 10us(c | | | | | us. | | | ompared | | | | | | | | to | | | | | | | | syste | | | | | | | | m time) | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | Rodrigues & Lindqvist Expires September 3, 2009 [Page 5] Internet-Draft TICTOC March 2009 | Stabiliza | As | As | As soon | As | As soon | As soon | | tion time | soon | soon | as | soon | as | as | | | as | as | possibl | as | possibl | possibl | | | possi | possib | e | possib | e | e | | | ble | le | | le | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Jitter on | Depen | Depend | | Depend | Depends | Depends | | recovered | ds on | son th | | son th | on the | on the | | timing | the | eoscil | | eoscil | oscilla | oscilla | | signal | osci | lator | | lator | tor | tor | | | llato | stab | | stab | stabil | stabil | | | r sta | ility | | ility | ity | ity | | | bilit | | | | | | | | y | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Wander on | Depen | Depend | | Depend | Depends | Depends | | recovered | ds on | son th | | son th | on the | on the | | timing | the | eoscil | | eoscil | oscilla | oscilla | | signal | osci | lator | | lator | tor | tor | | | llato | stab | | stab | stabil | stabil | | | r sta | ility | | ility | ity | ity | | | bilit | | | | | | | | y | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | What | | | | | | | | expected | | | | | | | | network | | | | | | | | character | | | | | | | | istics | | | | | | | | (WAN, | | | | | | | | LAN, MAN | | | | | | | | ,private, | | | | | | | | public, | | | | | | | | etc)? | | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | Rodrigues & Lindqvist Expires September 3, 2009 [Page 6] Internet-Draft TICTOC March 2009 | Does the | No | No (4) | No (4) | No (4) | No (4) | No (4) | | applicati | (4) | | | | | | | on | | | | | | | | requires | | | | | | | | security | | | | | | | | ?(if so, | | | | | | | | which | | | | | | | | one: | | | | | | | | authenti | | | | | | | | cation, | | | | | | | | encrypt | | | | | | | | ion, | | | | | | | | tracea | | | | | | | | bility, | | | | | | | | other | | | | | | | | s) | | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Reliabili | | | | | | | | ty | | | | | | | | requirem | | | | | | | | ents (e.g | | | | | | | | . fault | | | | | | | | toleran | | | | | | | | ce) | | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Traceabil | | | | | System | | | ity to a | | | | | Time, | | | specific | | | | | synchro | | | clock, | | | | | nous to | | | clock | | | | | UTC | | | quality, | | | | | time | | | path, | | | | | (excep | | | time | | | | | tfor | | | | | | | | leap | | | | | | | | second | | | | | | | | s)and | | | | | | | | uses | | | | | | | | the | | | | | | | | same | | | | | | | | time | | | | | | | | origi | | | | | | | | n as GP | | | | | | | | S time. | | | | | | | | (6) | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Holdover | | | | | | | | requireme | | | | | | | | nt | | | | | | | Rodrigues & Lindqvist Expires September 3, 2009 [Page 7] Internet-Draft TICTOC March 2009 | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Cost | | | | | | | | (consumer | | | | | | | | , | | | | | | | | enterpri | | | | | | | | se, | | | | | | | | carrier | | | | | | | | ) | | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Auto-conf | No | No | No | No | No | No | | iguration | | | | | | | | (plug an | | | | | | | | dplay) | | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Manageabi | | | | | | | | lity (how | | | | | | | | much | | | | | | | | effort | | | | | | | | the | | | | | | | | operator | | | | | | | | needs to | | | | | | | | put in t | | | | | | | | omanage | | | | | | | | this | | | | | | | | applicat | | | | | | | | ion?) - | | | | | | | | In-band | | | | | | | | or | | | | | | | | out-of- | | | | | | | | band of | | | | | | | | protoc | | | | | | | | ol (MIBs? | | | | | | | | ) | | | | | | | | --------- | ----- | ------ | ------ | ------ | ------ | ------- | | Scale and | | | | | | | | scalabili | | | | | | | | ty | | | | | | | +-----------+-------+--------+---------+--------+---------+---------+ Table 1 Note (1) This is requirement in the air interface. In practice more accurate frequency is required at the input. For example OBSAI RP1 defines 16 ppb Note (2) : no precise phase accuracy requirements defined in standard. The actual requirement will depend on implementation and network scenario. Rodrigues & Lindqvist Expires September 3, 2009 [Page 8] Internet-Draft TICTOC March 2009 Note (3) : In general LTE TDD systems may be defined to operate with 10-50 microseconds phase accuracy by making some limitations on the deployment (e.g. cell range), and radio frame configuration, however further investigation are required. When no assumption possible, microsecond or sub-microsecond requirement would apply. Note (4) assumes a private network Note (5) 1024 OFDM carriers, BW 10 MHz, Cyclic prefix ratio 1:8, RF carrier 3.5 GHz Note (6) 3GPP2, C.S0010-B version 2.0, 2004 3.2. Circuit Emulation The PWE3 WG has produced three techniques for emulating traditional low-rate (E1, T1, E3, T3) TDM services over PSNs, namely SAToP [RFC4553], CESoPSN [RFC5086], and TDMoIP [RFC5087]. The Network Synchronization reference model and deployment scenarios for emulation of TDM services have been described in [RFC4197], Section 4.3. The major technical challenge for 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 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. 3.2.1. Circuit Emulation Requirements The requirements for the Circuit Emulation is summarized in the table 2. Rodrigues & Lindqvist Expires September 3, 2009 [Page 9] Internet-Draft TICTOC March 2009 Editor's note: This table was discussed at the Dublin meeting; need input from the group to fill in the blanks. Circuit Emulation +-------------------------------------+-----------------------------+ | Requirements | Circuit Emulation | +-------------------------------------+-----------------------------+ | Synchronization type (e.g. time, | frequency | | frequency or phase) | | | --------------------------- | --------------------------- | | Frequency stability | N/A | | --------------------------- | --------------------------- | | Frequency accuracy | N/A | | --------------------------- | --------------------------- | | Uncalibrated time/time stability | N/A | | --------------------------- | --------------------------- | | Uncalibrated time/time accuracy | N/A | | --------------------------- | --------------------------- | | Stabilization time | | | --------------------------- | --------------------------- | | Jitter on recovered timing signal | G.8261/G.823/G.824 | | --------------------------- | --------------------------- | | Wander on recovered timing signal | G.8261/G.823/G.824 | | --------------------------- | --------------------------- | | What expected network | | | characteristics (WAN, LAN, MAN, | | | private, public, etc)? | | | --------------------------- | --------------------------- | | Does the application requires | No | | security? (if so, which one: | | | authentication, encryption, | | | traceability, others) | | | --------------------------- | --------------------------- | | Reliability requirements (e.g. | N/A | | fault tolerance) | | | --------------------------- | --------------------------- | | Traceability to a specific clock, | | | clock quality, path, time | | | --------------------------- | --------------------------- | | Holdover requirement | Yes | | --------------------------- | --------------------------- | | Cost (consumer, enterprise, | | | carrier) | | | --------------------------- | --------------------------- | | Auto-configuration (plug and play) | No | | --------------------------- | --------------------------- | Rodrigues & Lindqvist Expires September 3, 2009 [Page 10] Internet-Draft TICTOC March 2009 | Manageability (how much effort the | | | operator needs to put in to manage | | | this application?) - In-band or | | | out-of-band of protocol (MIBs?) | | | --------------------------- | --------------------------- | | Scale and scalability | | +-------------------------------------+-----------------------------+ Table 2 3.3. Test and Measurement Note: The application information and the requirements for this section was provided by the LXI Consortium Technical Committee. In the test and measurement 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. Test and Measurement (T&M) is a very diverse industry and as would be expected, requirements vary widely with the application. However the vast majority of the newer instruments and systems make use of LAN technology and many have a connection to the local enterprise network for data transfer, or monitoring and control. Because of the increasingly heavy use of LAN technology in T&M instruments and systems, we are dependent on the availability of network infrastructure, e.g. bridges, and low level silicon, e.g. PHYs and PHY/MAC, that supports not only T&M connectivity (data transport) but increasingly timing and frequency transfer support as well. Furthermore T&M is going to require this support not only for the existing 10/100/1000 BaseT technology but on the newer high throughput LAN technology under development. While most instruments produce data at modest rates, many can source or sink data at rates well in excess of 40Gsamples/s. In addition, the time and phase coherence requirements on the data transport, e.g. LAN, typically are tighter on the high data rate instruments. The other major headache in the use of LAN in T&M is latency and jitter because it compromises the determinism needed for some applications. One of the promises of LAN-based precise time is that in many circumstances precise time can be used to overcome latency Rodrigues & Lindqvist Expires September 3, 2009 [Page 11] Internet-Draft TICTOC March 2009 issues. For example, for many data acquisition applications the ability to precisely and accurately timestamp data at the collection point makes LAN latency and jitter a non-issue. Many T&M applications are localized, often to a bench or rack of equipment. The LAN will be local and private although there is often a connection to the local enterprise network. It is not uncommon in such applications to include a rubidium oscillator to provide a phase-coherent stable frequency source to critical instrumentation such as counters, scopes, signal generators and analyzers. In many cases the LAN, in principle, could fill the frequency distribution role if the LAN technology supported it. In these systems time transfer is becoming important first for timestamping data to facilitate data management and post acquisition processing, and in some cases as part of the control structure. The precise time specifications vary from milliseconds for general applications to nanoseconds for the most critical. There is an important class of applications where time, and sometimes frequency traceable to international standards is required, generally due to regulatory issues, e.g. testing of medical, safety critical or military devices. The ability to deliver traceable time and frequency over the network to the enterprise would be a big help in these applications. There are also T&M applications that are widely distributed due to the nature of the device or system being measured. Environmental measurement systems, surveillance, SCADA systems, and the telecommunication system itself are examples. Timestamping data is an essential requirement to overcome the communication latency and jitter issues. The specific timing requirements clearly cover a wide range. Environmental and SCADA is typically a ms. However to really instrument a telecom system will require timing at least on the order of a packet length or better. Even more extreme are timing for RF test ranges (which can cover several miles), long-baseline interferometry, and RF surveillance where the time accuracy must be on the order of ns. In some cases public networks will be used if the time distribution is adequate. 3.3.1. Test and Measurement Requirements The requirements for the Test and Measurement are summarized in the table 4. Where appropriate both the low and high end of the requirements spectrum are given to illustrate the breadth of requirements for the application areas discussed. Note that typically the applications with the most demanding requirements are also the high dollar value applications and in many cases the most critical in terms of the cost of failure, e.g. failure of a Rodrigues & Lindqvist Expires September 3, 2009 [Page 12] Internet-Draft TICTOC March 2009 surveillance system, monitoring of telecommunications, military test systems where either the operational cost of downtime or the cost of the device being tested are high. Test and Measurement +-----------------------------+-------------------------------------+ | Requirements | Remote Telco | +-----------------------------+-------------------------------------+ | Synchronization type (e.g. | Time: ms to ns- high value | | time, frequency or phase) | applications will open up as this | | | spec improves. Frequency: part in | | | 109 minimum, 1011 desirable and | | | with the lowest phase noise | | | obtainable for critical | | | applications. | | --------------------------- | --------------------------- | | Frequency stability | When applicable (high end RF) the | | | lowest phase noise possible in the | | | short term, long term consistent | | | with accuracy and calibration | | | intervals- better than 1 ppm/year | | | desirable | | --------------------------- | --------------------------- | | Frequency accuracy | Generally consistency across the | | | system is more important than | | | absolute accuracy. For calibration | | | applications at least 1ppm | | --------------------------- | --------------------------- | | Uncalibrated time/time | Short term from fractional ms to ns | | stability | or better. Long term comparable to | | | GPS distributed time. | | --------------------------- | --------------------------- | | Uncalibrated time/time | Usually self-consistency | | accuracy | requirements are tighter: ms to ns | | | system wide. Absolute accuracy | | | (traceable) is probably ms to 100 | | | ns. | | --------------------------- | --------------------------- | | Stabilization time | Not usually important. Many time | | | critical instruments themselves | | | need minutes to hours to stabilize. | | | However stabilization times greater | | | than a few minutes will reduce the | | | number of practical wide-area | | | applications | | --------------------------- | --------------------------- | Rodrigues & Lindqvist Expires September 3, 2009 [Page 13] Internet-Draft TICTOC March 2009 | Jitter on recovered timing | In the most critical applications, | | signal | the lowest phase noise achievable, | | | in terms of TIE less than the | | | stability requirement. | | --------------------------- | --------------------------- | | Wander on recovered timing | Modest for most measurements. For | | signal | surveillance, long baseline, and | | | similar less than the required | | | stability over the duration of the | | | test. | | --------------------------- | --------------------------- | | What expected network | Most are private or enterprise LAN. | | characteristics (WAN, LAN, | Large scale applications will | | MAN, private, public, etc)? | benefit from using the public | | | telecommunications networks. | | --------------------------- | --------------------------- | | Does the application | To date timing security | | requires security? (if so, | requirements have been rare with | | which one: authentication, | the possible exception of | | encryption, traceability, | measurement systems with legal | | others) | requirements. Data security is | | | more important when the public | | | networks are involved. | | --------------------------- | --------------------------- | | Reliability requirements | Has not been an issue to date in | | (e.g. fault tolerance) | most systems. | | --------------------------- | --------------------------- | | Traceability to a specific | Traceability to a path means that | | clock, clock quality, path, | if there is on-path support we want | | time | to trace the path. Can also help | | | to avoid time loops. Traceability | | | is needed to establish NIST | | | traceability. T&M will expect that | | | public networks solve the timing | | | loop problem. T&M end systems are | | | typically strictly hierarchical | | | networks without multiple paths. | | --------------------------- | --------------------------- | | Holdover requirement | Has not been an issue to date- but | | | as T&M increasingly is integrated | | | into operational systems it will | | | become more important. Telecom | | | requirements are probably | | | sufficient. | | --------------------------- | --------------------------- | | Cost (consumer, enterprise, | In most T&M systems component cost | | carrier) | is very important. In many, | | | operational cost is important. | Rodrigues & Lindqvist Expires September 3, 2009 [Page 14] Internet-Draft TICTOC March 2009 | --------------------------- | --------------------------- | | Auto-configuration (plug | Very important. T&M customers to | | and play) | date would prefer to avoid any | | | network related configuration. | | --------------------------- | --------------------------- | | Manageability (how much | As little as possible operator | | effort the operator needs | interaction. However visibility | | to put in to manage this | into system performance, including | | application?) - In-band or | timing, is very important both | | out-of-band of protocol | operationally and during debug and | | (MIBs?) | commissioning. | | --------------------------- | --------------------------- | | Scale and scalability | T&M systems range from small | | | systems with perhaps 2 or 3 | | | instruments to large scale data | | | acquisition with thousands of end | | | devices. The physical scale of T&M | | | systems varies widely from a few | | | instruments on a bench to a few | | | instruments separated by miles, and | | | from several thousand instruments | | | and sensors concentrated on a local | | | device such as a jet engine to | | | several thousand spread over many | | | miles in environmental monitoring, | | | or monitoring the | | | telecommunications system. In all | | | cases it is very common for these | | | systems to grow as additional test | | | requirements are imposed so | | | scalability is importan | +-----------------------------+-------------------------------------+ Table 3 3.4. Industrial Automation In the 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. One example of this tendency is 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 Rodrigues & Lindqvist Expires September 3, 2009 [Page 15] Internet-Draft TICTOC March 2009 degradation in quality. 3.4.1. Industrial Automation Requirements The requirements for the Industrial Automation are summarized in the table 4. Editor's note: This table was discussed at the Dublin meeting; need input from the group to fill in the blanks. Rodrigues & Lindqvist Expires September 3, 2009 [Page 16] Internet-Draft TICTOC March 2009 Industrial Automation +-------------------------------------+-----------------------------+ | Requirements | Remote Telco | +-------------------------------------+-----------------------------+ | Synchronization type (e.g. time, | | | frequency or phase) | | | --------------------------- | --------------------------- | | Frequency stability | | | --------------------------- | --------------------------- | | Frequency accuracy | | | --------------------------- | --------------------------- | | Uncalibrated time/time stability | | | --------------------------- | --------------------------- | | Uncalibrated time/time accuracy | | | --------------------------- | --------------------------- | | Stabilization time | | | --------------------------- | --------------------------- | | Jitter on recovered timing signal | | | --------------------------- | --------------------------- | | Wander on recovered timing signal | | | --------------------------- | --------------------------- | | What expected network | | | characteristics (WAN, LAN, MAN, | | | private, public, etc)? | | | --------------------------- | --------------------------- | | Does the application requires | | | security? (if so, which one: | | | authentication, encryption, | | | traceability, others) | | | --------------------------- | --------------------------- | | Reliability requirements (e.g. | | | fault tolerance) | | | --------------------------- | --------------------------- | | Traceability to a specific clock, | | | clock quality, path, time | | | --------------------------- | --------------------------- | | Holdover requirement | | | --------------------------- | --------------------------- | | Cost (consumer, enterprise, | | | carrier) | | | --------------------------- | --------------------------- | | Auto-configuration (plug and play) | | | --------------------------- | --------------------------- | | Manageability (how much effort the | | | operator needs to put in to manage | | | this application?) - In-band or | | | out-of-band of protocol (MIBs?) | | Rodrigues & Lindqvist Expires September 3, 2009 [Page 17] Internet-Draft TICTOC March 2009 | --------------------------- | --------------------------- | | Scale and scalability | | +-------------------------------------+-----------------------------+ Table 4 3.5. ToD/ Internet General time distribution over the Internet or IP networks, is often called Time of Day or Wall-clock. Most existing use cases are using NTP over the Internet with low precision requirements. However, new applications are arising that require higher precision rates than what is currently available. Internet TOD is used is important to the maintenance of IT infrastrucute in an organization. Generally the larger an organization becomes, the more important time synchronization is. Time synchronization is critical for the following: 1. Server and router log file entry time tags 2. "Date modified" attributes for files 3. Chron job scheduling 4. Security protocol with limited time windows for key exchange. Server and Router log file time tag accuracy is essential to network diagnostic tools. Such tools are used to determine the root cause of a network failure or security breach. Often it is important to determine the order in which certain events occur amongst a number of network devices. The "Date modified" fields of files may also be part of this type of analysis. Often Chron jobs perform operations on files depending on the times in the "Date modified" attributes files. These files might reside on more than one computer or server. Many security protocols, such as Kerberos, depend on authentication "tickets" which expire after a short time. This means that an authenticating server gives a ticket to a client, which the client can send to another server for some service which requires authentication. The time limit is intended to reduce the threat of the "Man in the middle attack." To work the two servers need to have clocks synchronized to a time error which is smaller than the ticket time out period. To increase security there is a desire to reduce the ticket time interval. As the time interval becomes shorter the need for server clock agreement is increased. The trend over time is to reduce the ticket time out period. Rodrigues & Lindqvist Expires September 3, 2009 [Page 18] Internet-Draft TICTOC March 2009 3.5.1. ToD/Internet Requirements The requirements for the ToD/Internet is summarized in the table 6. Editor's note: This table was discussed at the Dublin meeting; need input from the group to fill in the blanks. Rodrigues & Lindqvist Expires September 3, 2009 [Page 19] Internet-Draft TICTOC March 2009 ToD/Internet Requirements +---------------------------------+---------------------------------+ | Requirements | ToD/Internet | +---------------------------------+---------------------------------+ | Synchronization type (e.g. | time | | time, frequency or phase) | | | --------------------------- | --------------------------- | | Frequency stability | no requirement | | --------------------------- | --------------------------- | | Frequency accuracy | no requirement | | --------------------------- | --------------------------- | | Uncalibrated time/time | no requirement | | stability | | | --------------------------- | --------------------------- | | Uncalibrated time/time accuracy | 10 ms | | --------------------------- | --------------------------- | | Stabilization time | 1 hour | | --------------------------- | --------------------------- | | Jitter on recovered timing | 100 ms | | signal | | | --------------------------- | --------------------------- | | Wander on recovered timing | 10 ms | | signal | | | --------------------------- | --------------------------- | | What expected network | All network types | | characteristics (WAN, LAN, MAN, | | | private, public, etc)? | | | --------------------------- | --------------------------- | | Does the application requires | Authentication sometimes used | | security? (if so, which one: | | | authentication, encryption, | | | traceability, others) | | | --------------------------- | --------------------------- | | Reliability requirements (e.g. | high availability. Clients | | fault tolerance) | must see multiple servers | | --------------------------- | --------------------------- | | Traceability to a specific | Not important | | clock, clock quality, path, | | | time | | | --------------------------- | --------------------------- | | Holdover requirement | 1 hour to 1 year. Depends on | | | server redundancy architecture | | --------------------------- | --------------------------- | | Cost (consumer, enterprise, | 0 - $10,000 USD (1) | | carrier) | | | --------------------------- | --------------------------- | Rodrigues & Lindqvist Expires September 3, 2009 [Page 20] Internet-Draft TICTOC March 2009 | Auto-configuration (plug and | No | | play) | | | --------------------------- | --------------------------- | | Manageability (how much effort | 30-90 minutes to configure a | | the operator needs to put in to | new server, 5 minutes to | | manage this application?) - | configure a new client. Almost | | In-band or out-of-band of | no management after initial | | protocol (MIBs?) | deployment. | | --------------------------- | --------------------------- | | Scale and scalability | system must cover entire IT | | | infrastructure of organization. | | | Any 1 server will cover 1 | | | building or campus. | +---------------------------------+---------------------------------+ Table 5 (1) The free option implies pointing all clients at ntp servers available on the public internet. 3.6. Networking Editor's note: need more info on this application. 3.6.1. Networking Requirements The requirements for the Networking SLA and Network CDR are summarized in the table 5. Editor's note: This table was discussed at the Dublin meeting; need input from the group to fill in the blanks. Rodrigues & Lindqvist Expires September 3, 2009 [Page 21] Internet-Draft TICTOC March 2009 Networking SLA and Network CDR Requirements +--------------------------------+-------------------+--------------+ | Requirements | Networking SLA | Network CDR | +--------------------------------+-------------------+--------------+ | Synchronization type (e.g. | | | | time, frequency or phase) | | | | ---------------------------- | ----------------- | ------------ | | Frequency stability | | | | ---------------------------- | ----------------- | ------------ | | Frequency accuracy | | | | ---------------------------- | ----------------- | ------------ | | Uncalibrated time/time | | | | stability | | | | ---------------------------- | ----------------- | ------------ | | Uncalibrated time/time | | | | accuracy | | | | ---------------------------- | ----------------- | ------------ | | Stabilization time | | | | ---------------------------- | ----------------- | ------------ | | Jitter on recovered timing | | | | signal | | | | ---------------------------- | ----------------- | ------------ | | Wander on recovered timing | | | | signal | | | | ---------------------------- | ----------------- | ------------ | | What expected network | | | | characteristics (WAN, LAN, | | | | MAN, private, public, etc)? | | | | ---------------------------- | ----------------- | ------------ | | Does the application requires | | | | security? (if so, which one: | | | | authentication, encryption, | | | | traceability, others) | | | | ---------------------------- | ----------------- | ------------ | | Reliability requirements (e.g. | | | | fault tolerance) | | | | ---------------------------- | ----------------- | ------------ | | Traceability to a specific | | | | clock, clock quality, path, | | | | time | | | | ---------------------------- | ----------------- | ------------ | | Holdover requirement | | | | ---------------------------- | ----------------- | ------------ | | Cost (consumer, enterprise, | | | | carrier) | | | | ---------------------------- | ----------------- | ------------ | Rodrigues & Lindqvist Expires September 3, 2009 [Page 22] Internet-Draft TICTOC March 2009 | Auto-configuration (plug and | | | | play) | | | | ---------------------------- | ----------------- | ------------ | | Manageability (how much effort | | | | the operator needs to put in | | | | to manage this application?) - | | | | In-band or out-of-band of | | | | protocol (MIBs?) | | | | ---------------------------- | ----------------- | ------------ | | Scale and scalability | | | +--------------------------------+-------------------+--------------+ Table 6 3.7. Legal Time With legal time is meant the cases where high precision wall-clock is needed, just as in the ToD case, but with where the time source is traceable to UTC in a secure manner, i.e. through a certificate chain. It's also important for the legal-time case that the certificate chain is set-up so that it provides for an audit trail, where the ToD provided at any given moment can be traced to a known source or standard (i.e. a national timescale or time laboratory). One typical application that would benefit from high accuracy legal time is event correlation in computer systems logs, and similar applications. 3.7.1. Legal Time Requirements The requirements for the Legal Time is summarized in the table 7. Editor's note: This table was discussed at the Dublin meeting; need input from the group to fill in the blanks. Rodrigues & Lindqvist Expires September 3, 2009 [Page 23] Internet-Draft TICTOC March 2009 Legal Time Requirements +-------------------------------------+-----------------------------+ | Requirements | Legal Time | +-------------------------------------+-----------------------------+ | Synchronization type (e.g. time, | | | frequency or phase) | | | --------------------------- | --------------------------- | | Frequency stability | | | --------------------------- | --------------------------- | | Frequency accuracy | | | --------------------------- | --------------------------- | | Uncalibrated time/time stability | | | --------------------------- | --------------------------- | | Uncalibrated time/time accuracy | | | --------------------------- | --------------------------- | | Stabilization time | | | --------------------------- | --------------------------- | | Jitter on recovered timing signal | | | --------------------------- | --------------------------- | | Wander on recovered timing signal | | | --------------------------- | --------------------------- | | What expected network | | | characteristics (WAN, LAN, MAN, | | | private, public, etc)? | | | --------------------------- | --------------------------- | | Does the application requires | | | security? (if so, which one: | | | authentication, encryption, | | | traceability, others) | | | --------------------------- | --------------------------- | | Reliability requirements (e.g. | | | fault tolerance) | | | --------------------------- | --------------------------- | | Traceability to a specific clock, | | | clock quality, path, time | | | --------------------------- | --------------------------- | | Holdover requirement | | | --------------------------- | --------------------------- | | Cost (consumer, enterprise, | | | carrier) | | | --------------------------- | --------------------------- | | Auto-configuration (plug and play) | | | --------------------------- | --------------------------- | | Manageability (how much effort the | | | operator needs to put in to manage | | | this application?) - In-band or | | | out-of-band of protocol (MIBs?) | | Rodrigues & Lindqvist Expires September 3, 2009 [Page 24] Internet-Draft TICTOC March 2009 | --------------------------- | --------------------------- | | Scale and scalability | | +-------------------------------------+-----------------------------+ Table 7 3.8. Metrology Metrology for time and frequency is today mostly using tailored equipment and cabling for time/frequency transfer when doing laboratory work. However, in the future, using IP over existing networks in the laboratories would allow for greater flexibility and reuse of existing infrastructure rather than building out more special purpose infrastructure. 3.8.1. Metrology Requirements The requirements for the Metrology is summarized in the table 8. Editor's note: This table was discussed at the Dublin meeting; need input from the group to fill in the blanks. Rodrigues & Lindqvist Expires September 3, 2009 [Page 25] Internet-Draft TICTOC March 2009 Metrology Requirements +-------------------------------------+-----------------------------+ | Requirements | Metrology | +-------------------------------------+-----------------------------+ | Synchronization type (e.g. time, | | | frequency or phase) | | | --------------------------- | --------------------------- | | Frequency stability | | | --------------------------- | --------------------------- | | Frequency accuracy | | | --------------------------- | --------------------------- | | Uncalibrated time/time stability | | | --------------------------- | --------------------------- | | Uncalibrated time/time accuracy | | | --------------------------- | --------------------------- | | Stabilization time | | | --------------------------- | --------------------------- | | Jitter on recovered timing signal | | | --------------------------- | --------------------------- | | Wander on recovered timing signal | | | --------------------------- | --------------------------- | | What expected network | | | characteristics (WAN, LAN, MAN, | | | private, public, etc)? | | | --------------------------- | --------------------------- | | Does the application requires | | | security? (if so, which one: | | | authentication, encryption, | | | traceability, others) | | | --------------------------- | --------------------------- | | Reliability requirements (e.g. | | | fault tolerance) | | | --------------------------- | --------------------------- | | Traceability to a specific clock, | | | clock quality, path, time | | | --------------------------- | --------------------------- | | Holdover requirement | | | --------------------------- | --------------------------- | | Cost (consumer, enterprise, | | | carrier) | | | --------------------------- | --------------------------- | | Auto-configuration (plug and play) | | | --------------------------- | --------------------------- | | Manageability (how much effort the | | | operator needs to put in to manage | | | this application?) - In-band or | | | out-of-band of protocol (MIBs?) | | Rodrigues & Lindqvist Expires September 3, 2009 [Page 26] Internet-Draft TICTOC March 2009 | --------------------------- | --------------------------- | | Scale and scalability | | +-------------------------------------+-----------------------------+ Table 8 3.9. Sensor 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. 3.9.1. Sensors Requirements The requirements for the Sensor is summarized in the table 9. Editor's note: This table was discussed at the Dublin meeting; need input from the group to fill in the blanks. Rodrigues & Lindqvist Expires September 3, 2009 [Page 27] Internet-Draft TICTOC March 2009 Sensor Requirements +-------------------------------------+-----------------------------+ | Requirements | Sensor | +-------------------------------------+-----------------------------+ | Synchronization type (e.g. time, | | | frequency or phase) | | | --------------------------- | --------------------------- | | Frequency stability | | | --------------------------- | --------------------------- | | Frequency accuracy | | | --------------------------- | --------------------------- | | Uncalibrated time/time stability | | | --------------------------- | --------------------------- | | Uncalibrated time/time accuracy | | | --------------------------- | --------------------------- | | Stabilization time | | | --------------------------- | --------------------------- | | Jitter on recovered timing signal | | | --------------------------- | --------------------------- | | Wander on recovered timing signal | | | --------------------------- | --------------------------- | | What expected network | | | characteristics (WAN, LAN, MAN, | | | private, public, etc)? | | | --------------------------- | --------------------------- | | Does the application requires | | | security? (if so, which one: | | | authentication, encryption, | | | traceability, others) | | | --------------------------- | --------------------------- | | Reliability requirements (e.g. | | | fault tolerance) | | | --------------------------- | --------------------------- | | Traceability to a specific clock, | | | clock quality, path, time | | | --------------------------- | --------------------------- | | Holdover requirement | | | --------------------------- | --------------------------- | | Cost (consumer, enterprise, | | | carrier) | | | --------------------------- | --------------------------- | | Auto-configuration (plug and play) | | | --------------------------- | --------------------------- | | Manageability (how much effort the | | | operator needs to put in to manage | | | this application?) - In-band or | | | out-of-band of protocol (MIBs?) | | Rodrigues & Lindqvist Expires September 3, 2009 [Page 28] Internet-Draft TICTOC March 2009 | --------------------------- | --------------------------- | | Scale and scalability | | +-------------------------------------+-----------------------------+ Table 9 4. Network Dependencies When using packet networks to transfer timing, 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. Timing distribution is highly sensitive to packet delay variation, 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. Packet delay variation can to some extent be addressed by traffic engineering, thus time transfer within a constrained network environment 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 any other form of on-path support), 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. Therefore, time transfer protocols should not assume the availability of on path support, but utilizes it where available. 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. How the packet rate relates to the accuracy is dependent on the filter algorithm in use. 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. Rodrigues & Lindqvist Expires September 3, 2009 [Page 29] Internet-Draft TICTOC March 2009 Congested and overloaded paths might influence the quality of timing transfer. In a constrained network environment, it's assumed that a service provider will ensure that packet delivery is done in according to the timing transfer needs of the network operator. 5. Network Topology Editor's note: This section needs to be discussed. 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, and for some of the applications described above such as legal time, the ability to provide and audit trail to the timing source. One possible threat is a false time or frequency server being accepted instead of a true one, thus enabling an attacker to alter the time and frequency service provided. Other possible scenarios are to be able to distinguish between trusted clients and non-trusted clients when providing service. 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. The following authentication mechanism need to be considered: 1. of server by client (depending on the application) 2. of client by server (depending on the application) 3. transactions (depending on the application) 7. IANA Considerations No IANA actions are required as a result of the publication of this document. 8. Acknowledgements The authors wish to thank Stewart Bryant, Yaakov Stein, Karen Rodrigues & Lindqvist Expires September 3, 2009 [Page 30] Internet-Draft TICTOC March 2009 O'Donoghue, Laurent Montini, Greg Dowd, Doug Arnald and the LXI Consortium Technical Committee for input on this draft. 9. 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", April 2008. [G8262] ITU-T, "Recommendation G.8262/Y.1362 - Timing Characteristics of Synchronous Ethernet Equipment Slave Clock (EEC)", August 2007. [G8264] ITU-T, "Recommendation G.8264/Y.1364 - Distribution of timing through packet networks", 2008. [RFC1305] Mills, D., "Network Time Protocol (Version 3) Specification, Implementation", RFC 1305, March 1992. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4197] Riegel, M., "Requirements for Edge-to-Edge Emulation of Time Division Multiplexed (TDM) Circuits over Packet Switching Networks", RFC 4197, October 2005. [RFC4553] Vainshtein, A. and YJ. Stein, "Structure-Agnostic Time Division Multiplexing (TDM) over Packet (SAToP)", RFC 4553, June 2006. [RFC5086] Vainshtein, A., Sasson, I., Metz, E., Frost, T., and P. Pate, "Structure-Aware Time Division Multiplexed (TDM) Circuit Emulation Service over Packet Switched Network (CESoPSN)", RFC 5086, December 2007. [RFC5087] Stein, Y(J)., Shashoua, R., Insler, R., and M. Anavi, "Time Division Multiplexing over IP (TDMoIP)", RFC 5087, December 2007. Appendix A. 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 Rodrigues & Lindqvist Expires September 3, 2009 [Page 31] Internet-Draft TICTOC March 2009 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. A.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 systems 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 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 Rodrigues & Lindqvist Expires September 3, 2009 [Page 32] Internet-Draft TICTOC March 2009 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. A.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(DTI). 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. The DTI 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 approved by ITU-T, it provides frequency distribution over Ethernet links. Modern 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. Apart from some necessary higher layer packet based configuration and OAM operations to transport synchronization status messaging, 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. Rodrigues & Lindqvist Expires September 3, 2009 [Page 33] Internet-Draft TICTOC March 2009 A.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/hardware 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. PTP (a product of the IEEE Test and Measurement community) is specified in a limited first version (1588-2002), and the second version (1588-2008)was approved recently. It is important that NTP, IEEE-1588 or any other future packet based time transfer mechanism do not break each other if they run in the same network. A.3.1. NTP summary description 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. Current NTP suffers from the fact that it was designed to operate over the Internet, and the routers and switches 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. However, proprietary NTP implementations that use other algorithms and update-rates have proved that NTP packet formats can be used for higher accuracy. A.3.2. IEEE1588 summary description The information exchange component of IEEE 1588 is a protocol known as Precision Time Protocol (PTP). PTP version 1 (1588-2002) was a time transfer protocol that exclusively used multicast technique and it was primarily developed for Industtrial Automation and Test and Measurement applications. It is widely anticipated that wide scale deployment of PTP will be based on PTP version 2 (1588-2008). IEEE Std 1588-2008 can be considered to consist of several components: 1. A configuration and control protocol Rodrigues & Lindqvist Expires September 3, 2009 [Page 34] Internet-Draft TICTOC March 2009 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 Std 1588-2002 (multicast IP with recommended TTL=1, UDP, PTP 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. IEEE Std 1588-2008 includes Unicast messages. 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. For IEEE Std 1588-2008, 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 IEEE Std 1588-2008 specification describes how the PTP operates over the Ethernet/IP/UDP protocol stack. It includes annexes that describe PTP operation over pure layer 2 Ethernet, 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. They may operate in unicast or multicast. 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. 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 Rodrigues & Lindqvist Expires September 3, 2009 [Page 35] Internet-Draft TICTOC March 2009 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. Appendix B. 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 IEEE approved the version 2 of the IEEE 1588 protocol (IEEE Std 1588- 2008) that will run over more types of PSNs. 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 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. ITU-T has also consented G.8262 "Timing Characteristics of Synchronous Ethernet Equipment Slave Clock (EEC)" [G8262], and G.8264 "Distribution of timing through packet networks" [G8264]. G.8262 specifies the requirements for Synchronous Ethernet clocks and G.8264 defines the protocol for Synchronization Status Message (SSM) for Synchronous Ethernet. To date the ITU-T has focused on Ethernet infrastructure, but this is likely to extend to an MPLS environment. Two new work items, Rodrigues & Lindqvist Expires September 3, 2009 [Page 36] Internet-Draft TICTOC March 2009 G.paclock.bis and G.pacmod.bis extend the work, and in particular, G.pacmod.bis intends to introduce time transfer. The scope for G.paclock.bis is to define the requirements for packet-based clocks. This is an area where the IETF, with its expertise in IP and MPLS networks, may co-operate with the ITU. Authors' Addresses Silvana Rodrigues Zarlink Semiconductor 400 March Road Ottawa K2K 3H4 Canada Phone: +1 613 2707258 Email: silvana.rodrigues@zarlink.com Kurti Erik Lindqvist Netnod Bellmansgatan 30 Stockholm S-118 47 Sweden Phone: +46 708 30 60 01 Email: kurtis@kurtis.pp.se Rodrigues & Lindqvist Expires September 3, 2009 [Page 37]