wdiff draft-stein-tictoc-modules-01.txt draft-stein-tictoc-modules-02.txt

TICTOC Y(J). Stein
Internet-Draft RAD Data Communications Tim Frost
INTERNET-DRAFT Greg Dowd
Intended status: Status: Informational S. Bryant
Expires: October 11, 2008 Cisco Systems
K. Symmetricom

Karen O'Donoghue
US Navy
April 9, 2008

TICTOC Modules
draft-stein-tictoc-modules-01.txt
Architecture for the Transmission of Timing over Packet Networks
draft-stein-tictoc-modules-02.txt

Status of this Memo

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This Internet-Draft will expire on October 11, 2008. January 14, 2009.

Abstract

This Internet draft proposes an architecture for the modularization transmission of TICTOC
(Transmitting Timing
timing over IP Connections packet networks. It identifies the major functional
components, and Transfer of Clock) work.
In particular, it breaks maps the work down into individual modules
(deliverables) that need to be developed. current solutions onto this framework.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. TICTOC Layers 3
1.2. Generic TICTOC Client 4
1.3. TICTOC Functional Modules 5
2. Frequency Layer . . . . . . . . . . . . . . . . . . . . . . . 5 7
2.1. Local Oscillator and Digital Frequency Synthesizers . . . . . . . . 5 7
2.2. Frequency Distribution Protocols . . . . . . . . . . . . . 6 8
2.3. Packet Select/Discard Algorithms . . . . . . . . . . . . . 7
2.4. Frequency Acquisition Algorithms . . . . . . . . . . . . . 8
2.5.
2.3.1. Packet Selection and Discard Algorithms 9
2.3.2. Filtering and Control Servos 9
2.3.3. Server Contribution 10
2.3.4. Reference Algorithm 10
2.4. Frequency Presentation . . . . . . . . . . . . . . . . . . 9 10
3. Time Layer . . . . . . . . . . . . . . . . . . . . . . . . . . 9 11
3.1. Time Distribution Protocols . . . . . . . . . . . . . . . 9 11
3.2. Ranging . . . . . . . . . . . . . . . . . . . . . . . . . 11 12
3.3. Time Presentation . . . . . . . . . . . . . . . . . . . . 12 13
4. Other Generic Modules . . . . . . . . . . . . . . . . . . . . . . . . 12 14
4.1. Security Mechanisms . . . . . . . . . . . . . . . . . . . 12 14
4.2. Autodiscovery and Master Clock Selection . . . . . . . . . 13 14
4.3. Redundancy and Fail-Over Mechanisms 16
4.4. OAM, SSM, and Performance Monitoring . . . . . . . . . . . 14
4.4. Path Selection . . . . . . . . . . . . . . . . . . . . . . 15 16
4.5. On-path Support . . . . . . . . . . . . . . . . . . . . . 15
4.6. Network Management . . . . . . . . . . . . . . . . . . . . 16
4.6. Application profiles 16
5. Network Support 17
5.1. Path Selection 17
5.2. Network and Traffic Engineering 17
5.3. Service Level Specifications and QoS settings 18
5.4. Routing considerations 18
5.5. On-path Support 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
6. 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 19
8. Informative References . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
[Editor's Note: The present version of this draft contains references
to work to be performed by the TICTOC working group. This language
has been included in order to help with the chartering of this
working group, and will be removed in the next revision.] Acknowledgements 19
INFORMATIVE REFERENCES 20
AUTHORS' ADDRESSES 20

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].

1. Introduction

In this document the term "timing" will be used to refer to recovery of
frequency and/or time (wall-clock).

The most general TICTOC system can be logically decomposed into two
layers, corresponding

There is an emerging need to the distribution distribute highly accurate time and presentation of
frequency information over IP and time, respectively, In Figure 1 we depict these layers,
and the top-level modules in these layers, for a TICTOC client.
Specific TICTOC systems may consist over MPLS packet switched networks
(PSNs). A variety of either or both layers. For
example, if applications require time is required but and/or frequency is available from
information to a
physical layer mechanism, then the frequency layer is omitted. On
the precision which existing protocols cannot supply.

Several other hand, groups are working on related issues. For example, the NTP
Working Group in IETF is working on an upgrade of the long-standing
Network Time Protocol [RFC1305]. The IEEE has recently revised its
Precision Time Protocol [IEEE1588]. The ITU-T has defined Synchronous
Ethernet, a physical layer technology for transfer of frequency across
native Ethernet [G.8261, G.8262].

However, none of these efforts are sufficient by themselves to create a
complete and robust time and frequency transfer ecosystem in the IP and
MPLS network environment. The TICTOC (Timing over IP Connections and
Transmission of Clock) Working Group was created to develop solutions
that meet the needs of the various applications for timing over packet
networks.

This draft attempts to define an architecture for a TICTOC system,
identifying the various functional components, and considering the
support required from the network in order to assist the timing
recovery.

1.1. TICTOC Layers

The most general TICTOC system can be logically decomposed into two
layers, corresponding to the distribution of frequency and time,
respectively, carried over the network by a timing protocol as shown
in Figure 1. Examples of such timing protocols include Network Time
Protocol (NTP) [RFC1305] and Precision Time Protocol (PTP)
[IEEE1588].

Specific TICTOC systems may consist of either or both layers. For
example, if time is not required for the application, then only the
frequency layer may be present.

Network

In some cases there may be additional support from the network to
assist the timing distribution. For example, it may be possible to
use a physical layer technology such as Synchronous Ethernet to
distribute an accurate frequency. In this case, the timing protocol
is only required to distribute time. Other examples of network
support may include specific QoS settings for the timing protocol
flow, and routing constraints to ensure an optimum path.

|-------------------| |-------------------|
|
v
+------+-------+ +--------------+ Time distribution | Frequency | Time distribution |
|-------------------| |-------------------|
| Frequency distr. | | Acquisition +------)-------+ Presentation +-----)------ Frequency distr. |
|-------------------| |-------------------|
| Timing protocol | |
+------+-------+ +--------------+ Timing Protocol |
v
|-------------------| |-------------------|
|
+------+-------+ +--------------+ UDP | Time | UDP | Time
|-------------------| |-------------------|
| IP | | IP |
|-------------------| |-------------------|
| Layer 2 | | Layer 2 |
|-------------------| |-------------------|
| Layer 1 | | Layer 1 |
|-------------------| |-------------------|
| ---------- |
| / \ |
-------------| Network |-------------
\ /
\---------/

Figure 1. TICTOC Layers

1.2. Generic TICTOC Client

Figure 2 shows a generic TICTOC client, consisting of separate
frequency and time acquisition modules, and the presentation modules
for each.

Network
|
v
+------+-------+ +--------------+
| Frequency | | Frequency |
| Acquisition +----->-----+ Presentation +----->-----
| | | |
+------+-------+ +--------------+
|
v
|
+------+-------+ +--------------+
| Time | | Time |
| Acquisition +------)-------+ +----->-----+ Presentation +-----)------ +----->-----
| | | |
+--------------+ +--------------+

Figure 1. 2. Generic TICTOC client

Referring to Figure 1, the

The frequency acquisition module is responsible for recovery of
frequency information distributed over a packet switched network
(PSN), such as IP or MPLS. This distribution is accomplished by use
of a frequency distribution protocol. The frequency distributed may
be derived from an international standard, or may be an arbitrary
frequency needed at some remote location. If the value of the
frequency itself is needed, then the output of this module is input
to the frequency presentation module that formats the frequency
according to application needs. Note that this format may be numeric
prepared for display, or may take analog form and be used for locking
or disciplining other analog frequencies. When time is needed, the
frequency information is sent to the time acquisition module.

Even if the frequency itself is not needed, time acquisition almost
always requires a stable frequency reference. This reference may be
acquired from the frequency acquisition layer, or may be obtained via
some other means, such as an accurate local frequency reference, or
from a non-PSN mechanism for frequency distribution (e.g., GPS,
SONET/SDH).

From the acquired or otherwise available frequency, we may derive
arbitrary time (also known as "phase") by mathematical means. If the
frequency reference is traceable to an international standard, then
arbitrary time means a sequence of time values that are synchronous
with true (wall-clock, UTC) time, but with an arbitrary offset. The
purpose of the time acquisition module is to enable multiple TICTOC
clients to share a single offset, and thus to agree as to marking of
phase values. Such marked phases values are all that is required for
certain applications, such as where multiple time-aware agents need
to interact (e.g., systems employing Time Division Multiple Access
systems).

When the time markings distributed by the time distribution protocol
are traceable to an international standard, the TICTOC clients are
said to have acquired wall-clock time. These wall-clock values are
formatted for use by the intended application by the time
presentation module.

1.3. TICTOC Functional Modules

The remainder of this document further subdivides these modules and
identifies the submodules sub-modules needed for timing distribution. Submodule Sub-
modules may require one or more protocols, a set of processing
algorithms, and implementations. Descriptions of these protocols, algorithms,
and reference implementations are the TICTOC deliverables.
The frequency acquisition module may be subdivided into the following
submodules.
sub-modules. Not all need to be present in all implementations.
Those that need not be developed by TICTOC are marked by an asterisk.
o local oscillator (*)
o frequency generator (typically a digital synthesizer (*) synthesizer)
o timestamp generation generator
(not required if packets are sent at constant rate)
o frequency distribution protocol
o packet selection/discard algorithms
o frequency acquisition algorithms
o disciplining/adaptation/clock
(may include packet selection algorithms, oscillator
disciplining and frequency adjustment techniques)

The time acquisition module may be subdivided into the following
submodules.
additional sub-modules. Not all need to be present in all
implementations.
o time distribution protocol
o ranging algorithms
o clock adjustment techniques
o timestamp generation (already mentioned)

In addition, various generic modules may be needed, for either
frequency distribution, time distribution, or both.
o security
o autodiscovery and master clock selection
o redundancy and fail-over mechanisms
o OAM, synchronization status messaging, and performance
monitoring
o network management
o application profiles

Any timing protocols should operate over the general internet without
requiring specific support from the network. However, when operated
over a specific, managed network where support is available, the
accuracy of the time and frequency distribution will be enhanced.
Examples of network support include:
o path selection and/or self-organization
o on-path support
o network management and traffic engineering for optimum transport of timing
protocols
o service level specifications and QoS settings
o routing considerations
o on-path support, e.g. PTP boundary or transparent clocks

2. Frequency Layer

There are applications that require an accurate frequency reference, but
do not require knowledge of absolute time. In addition, it is often
wise to acquire frequency for applications that require absolute time
but do not directly use frequency.

TICTOC is concerned with the network frequency transfer using packet
technology. Frequency may be transferred across a network using the
physical layer, such as through the use of a synchronous network
interface (for example synchronous Ethernet [G8262]). [G.8262]). The use of the
physical layer may produce a higher quality frequency reference than
achievable using packet based network frequency transfer, but is outside
the scope of this document.

2.1. Local Oscillator and Digital Frequency Synthesizers

TICTOC masters and clients both need local sources of frequency. For
masters, this may be provided by high quality Cesium clock with
extremely good long term accuracy, or by an atomic clock with
somewhat lower accuracy, but which is disciplined by comparison with
a better clock (e.g., by GPS).

Note that a pure frequency master that does not need to know time can
be standalone.

For clients, the local oscillator is usually a quartz crystal
oscillators. Such oscillators may be stable, special cut crystals
with double oven and temperature compensation, or lowly inexpensive
crystals. In either case the frequency derived from these
oscillators needs to be adjusted by the TICTOC frequency acquisition
module. This adjustment can be electronic (e.g., (e.g. changing the control
voltage of a voltage controlled oscillator).

However, it is common practice not to directly adjust the physical
oscillator, or even to directly use the oscillator's frequency.
Instead, a digital synthesizer fed by the oscillator provides the
required output frequency. Changing parameters of the digital
synthesizer changes the output frequency in the required way. It is
important for the digital synthesizer not to introduce too much
jitter, and for the changing of parameters to be done in such fashion
as to minimize transients.

Disciplining of the local oscillator, or setting the parameters of
the digital synthesizer, is based on the arrival times of received
frequency distribution protocol packets, and the information
contained in them. When, for whatever reason, frequency distribution
packets are not received, the frequency layer is said to be in
"holdover mode". In holdover mode the local oscillator or
synthesizer is still used as usual, but it is no longer disciplined
based on new information from the distribution protocol (although it
may still be adapted, if the acquisition module has models that have
been trained during periods that the frequency distribution packets
were received).

2.2. Frequency Distribution Protocols

The protocol used to distribute frequency across the packet network
may be the same protocol used for time distribution, or may be an
independent protocol. The important design consideration is that the
protocol used is optimized for that purpose, and not compromised by
the need to fulfill a dual role.

Frequency distribution protocols are "one way", i.e. only require
information transfer from the master (where the frequency is known)
to the client. In particular, frequency distribution protocols can
be broadcast or multicast to many clients, without the master needing
to know the client identities. Frequency distribution protocols may
even operate when there is no return path available. Exceptions to
this rule are control messages sent by the client requesting
commencing or termination of the frequency distribution service, or
changing its parameters (e.g., update rate).

Frequency distribution protocols can be broadcast, multicast or
unicast. Multicast transmission conserves network bandwidth, while
unicast transmission is often more desirable.

Ideally

The TICTOC architecture is modular, and not bound to the frequency
distribution protocol should be decoupled from
the algorithm used protocol. It is possible to derive the local reference frequency, use different frequency
distribution methods for different applications and environments,
e.g. the
packets sent should only contain information that is physically
meaningful without reference use of physical layer frequency distribution protocols such
as Synchronous Ethernet.

2.3. Frequency Acquisition Algorithms

A TICTOC client needs to a particular algorithm. For example,
if be able to recover the original timebase (or
frequency reference) of the master sends packets at a constant rate as measured or server. In general it achieves
this by comparing the
frequency reference, then the existence arrival rate of packets with the packet itself is original
sending rate. From this it can determine the
only physically meaningful information, and relationship between the packet should contain
nothing but headers required for packet delivery. If
local timebase and the master timebase.

Observed arrival times of frequency distribution protocol packets are
not sent at a constant rate, they should contain nothing but a
sufficiently precise timestamp. The use
influenced by two phenomena:
o frequency drift of extension mechanisms for
carrying additional information may be considered for specific cases.

The TICTOC Working Group will select a suitable the local oscillator relative to the master
oscillator (typically caused by temperature and voltage
fluctuations, and by aging phenomena)
o variation in delay of timing packets through the packet network frequency
transfer protocol. This may be based on an existing protocol,
although that is not
(PDV).

The former behavior needs to be considered a requirement.

The basic TICTOC architecture itself should not tracked and compensated, while the
latter needs to be strongly bound filtered out. In order to eliminate the effects
of PDV, frequency distribution protocol. It should be possible acquisition algorithms perform some sort of
averaging and/or filtering, in an attempt to
upgrade or completely replace recover the frequency at
which packets were sent. Typically this protocol (e.g., to improve
accuracy), or involves two steps:
o packet selection and discard algorithms
o filtering and control servos to optimize the transfer for a different network
environment, without redesigning "lock onto" the TICTOC architecture.

2.3. sending
frequency

2.3.1. Packet Select/Discard Selection and Discard Algorithms

In the simplest networks all frequency distribution packets
received are used by the frequency acquisition algorithms to
derive corrections to the local oscillator. A major problem with
frequency acquisition in complex networks is the elimination of
frequency distribution packets that, if used by the acquisition
algorithms, would degrade the quality of the recovered frequency.
Such packets are variously called outliers, falsetickers, etc.

There are several reasons for such unreliable packets. First,
malfeasants may deliberately inject false packets in order to
impair the TICTOC client's frequency recovery. We will discuss
security mechanisms below. Second, PDV distributions for complex
networks can be long tailed, leading to extremely misleading
results. Third, various network degradations, e.g., congestion
and reroute events, can lead to bizarre information being
received.

Furthermore, even typical packets may have experienced significant
delay variation, making them less suitable for exploitation than
the small fraction of packets that may have experienced almost no
delay (and thus minimal delay variation). When there is a
significant fraction of packets that experience essentially no
delay, it is reasonable to discard all others (assuming that after
this decimation the sampling rate is still sufficiently high).

2.4. Frequency Acquisition Algorithms

Observed arrival times of frequency distribution protocol packets are
influenced by two phenomena: time behavior of the local oscillator as
compared to the master oscillator, and network delay behavior (PDV).
The former behavior needs to be tracked, while the latter needs to be
filtered out. In order to eliminate the effects of PDV, frequency
acquisition algorithms perform some sort of averaging and/or
filtering, in an attempt to recover the frequency at which packets
were sent. Since simple averaging would take too long to
sufficiently eliminate the stochastic components, by which time the

2.3.2. Filtering and Control Servos

Since simple averaging would take too long to sufficiently
eliminate the stochastic components, by which time the frequency
difference between local oscillator and master oscillator would
have changed, more efficient "control loops" are employed.
Control loops are nonlinear mechanisms, and are thus able to "lock
onto" frequencies, rather than simply removing stochastic noise.
Conventional control loops include the Phase Locked Loop (PLLs),
the Frequency Locked Loops (FLL), and combinations of the two.

Delay variation effects can be quite complex and traffic
dependent. There are obvious effects such as queuing indeterminacy
leading to stochastic PDV, and congestion leading to packet loss.

There are also more subtle effects, for example multiple
plesiochronous flows (e.g. TDM pseudowires) traversing forwarding
elements can cause slow "beating" effects, generating low
frequency wander that is difficult to eliminate. Another example
is the batch processing effect introduced by some forwarders in
which the first of a series of packets experiences greater latency
that later packets.

Modern algorithms attempt to model the time behavior of local
oscillator as compared to the master oscillator, and/or the
network behavior. More complex algorithms may attempt blind
separation of the contribution of the two effects.

2.3.3. Server Contribution

Traditionally the focus on frequency acquisition algorithm design
has been on the client. However it is possible to mitigate some
of the aforementioned effects by modulating the packet generation
rate of the master. Thus TICTOC algorithm modules may describe
the client and the server components, and may require matching of
these algorithms to achieve optimal performance.

Frequency

2.3.4. Reference Algorithm

Current technologies differ in how much of the algorithm is
defined. NTP defines the clock recovery algorithm completely,
while PTP only defines the on-the-wire protocol, leaving the clock
recovery algorithms undefined. Ultimately, multiple algorithms
may be required to suit different network environments and
application requirements.

Next-generation frequency acquisition algorithms need to be
optimized such that network bandwidth consumed by the frequency
distribution protocol is minimized. Furthermore, these algorithms
are required to be robust to network degradations such as packet
delay, packet loss, packet delay variation (PDV) and infrequent
stepwise changes in network traversal latencies (e.g., due to
reroute events or network loading changes).

The TICTOC Working Group

It may be necessary to specify a reference algorithm, algorithm for
comparison and validation purposes, but the TICTOC architecture
will enable vendors to employ proprietary algorithms. It will
also be possible to upgrade the reference algorithm in the future.

2.5.

2.4. Frequency Presentation

In general, the output of the frequency acquisition module needs to
be reformatted before use. In some cases the reference frequency
distributed across the network is different from the frequency needed
by the local application; in such cases a digital synthesizer can be
employed to convert the acquired frequency to the desired one.
Sometimes it is not frequency itself that is required, but a sequence
of equally separated times; in such cases the sequence of time
"ticks" is generated from the acquired frequency (once again, a digital
synthesizer is ideal for this).

3. Time Layer

In general the time layer is fed accurate frequency from the frequency
layer. There are applications that require accurate time, but do not
need to acquire frequency over the PSN. For example:
o if the update rate is high and the time resolution required
is low
o if highly accurate frequency is available locally
o if frequency is distributed by means external to the PSN
(e.g. GPS, LORAN, WWV)
o if frequency is available by means of the network physical
layer (e.g. PoS, SyncE).

In such cases the frequency input in Figure 1 2 is provided by the
alternative frequency source, rather than the frequency layer. The
TICTOC two layer decomposition is a conceptual one, and may not be
easily identifiable in all implementations. For example, when using a
pure PLL frequency acquisition module, true "frequency" is never
derived, only relative phase. Similarly, tracking mechanisms may
simultaneously model frequency and time, reducing convergence time by
adjusting both simultaneously, rather than first acquiring frequency,
and afterwards time. However, even in these cases the decomposition is
a useful one.

3.1. Time Distribution Protocols

The purpose of the time distribution protocol is to calibrate a
sequence of phase events generated by the local oscillator or digital
synthesizer, thus converting arbitrary offset phase values into wall-
clock values. This is done by sending packets containing timestamps
from the master (where the time is known) to the TICTOC client. In
order for the timestamp to accurately indicate the time that the
packet was actually injected into the network (rather than some
earlier time when the packet was formed, separated by a stochastic
time delay from the injection time), the timestamp should be
generated by the networking hardware. When this is not possible, the
stochastic delay should be minimized. The IEEE 1588 IEEE1588 protocol has a
follow-up message, to indicate the actual injection time of the
previous packet.

The timestamp in the time distribution protocol packet indicates the
time that the master injected this packet into the network. On the
other hand, the client receives this same packet after the
propagation delay, i.e. the time taken to traverse the packet
network. Were this time to be accurately known, the timestamp could
be corrected, and the precise time known. Estimation of the
traversal time is called ranging, and will be discussed below. The
IEEE 1588
IEEE1588 protocol has an optional mechanism (transparent clocks)
whereby forwarding delays, and even individual link delays are
measured and compensated, thus leading to precise knowledge of the
traversal delay. However, this mechanism requires upgrading of all
intermediate network elements.

Due to the requirement of traversal delay measurement, time
distribution protocols are generally bidirectional, that is, they
require a bidirectional channel, require both master and client to
send and receive messages, and usually assume symmetric (or known
asymmetry) propagation times. Unlike frequency distribution, time
distribution can not be entirely multicast, due to the ranging
requirement.

There are two well-known protocols capable of running over a general-
purpose packet network, NTP [RFC1305], and IEEE 1588 [1588]. IEEE1588 [IEEE1588]. NTP
is the product of the IETF, and is currently undergoing revision to
version 4. IEEE 1588 IEEE1588 is a product of the IEEE Test and Measurement
community. It is presently specified in a limited first version,
while the second version (1588v2) is soon
community, and has recently been revised to be a standard. IEEE
1588v2 better accommodate
telecommunication applications. The new version has a profiling
mechanism enabling organizations to specify required and prohibited
options, ranges and defaults of attribute values, and optional
features, while maintaining interoperability.

The protocol used to transfer the reference frequency across the
network may be the same protocol that is used to transfer time. The
overriding design consideration is that frequency and time
distribution protocols be optimised for their purpose, and not
compromised by the need to fulfill a dual role.

The TICTOC Working Group will select architecture itself is not bound to a suitable specific time
distribution
transfer protocol or protocols. There are three options here:
o a new protocol. It is possible to upgrade, or alternative version of NTP, replace this
protocol, for example to be called NTPv5
o a profile of 1588
o a completely new protocol.
The selection will be made by producing a set improved the quality of specific
requirements the transfer, or to
optimize the transfer for a different network environment, without
redesigning the entire TICTOC timing distribution protocol, and by a
disinterested evaluation architecture.

3.2. Ranging

To transfer time it is necessary to know the time of each of these options in light of these
requirements. The requirements will be based on the applications
listed in the TICTOC problem statement. If neither of the first two
options fulfill the requirements, then the third option will be
chosen. Even if the first two options equally fulfill the
requirements one of them will be chosen as the mandatory protocol,
and the use of the other will be permissible under specific
circumstances.

The TICTOC architecture itself should not be bound to the specific
time distribution protocol. It should be possible to upgrade, or
replace this protocol, for example to improved the quality of the
transfer, or to optimize the transfer for a different network
environment, without redesigning the entire TICTOC architecture.

3.2. Ranging

To transfer time it is necessary to know the time of flight flight of time
of packets from the time server to the client. The accuracy of time
at the client can be no better than the accuracy provide by the
ranging mechanism. Some ranging mechanisms require or can exploit
special hardware assistance by intermediate network elements, such as
IEEE 1588
IEEE1588 transparent clocks [1588] . [IEEE1588].

A typical ranging mechanism functions by exchanging timestamps
between master and client. For example, the master sends an initial
timestamped packet. When this packet arrives at the client its
arrival time (in terms of the local clock) is recorded. After some
amount of time the client sends a response packet back to the master,
containing the initial timestamp (in terms of the master clock), and
the packet arrival and retransmission times (in terms of the client
clock). When this packet is received by the master a fourth
timestamp is generated (in terms of the master clock). Since the
second and third timestamps are both in terms of the client clock,
their difference - representing the amount of time the client
hesitated between receipt of the initial packet and sending the
response packet - is readily computed. This difference can be
subtracted from the difference between the fourth and first
timestamps, to yield the sum of propagation times. Under the
assumption of symmetry, the one-way time is half this sum. Note that
since the difference between third and fourth timestamps is short,
possible frequency inaccuracy of the client has little deleterious
effect.

Various variations on this basic four-timestamp method are possible.
In the three timestamp method the client sends the time difference
(e.g., generated by resetting a timer upon packet arrival) rather
than the two timestamps. When symmetry is not a valid assumption,
additional information (e.g., ratio or difference between expected
propagation delays) can be used to extract the one-way delay.

Ranging accuracy is reduced by deviation from expected asymmetry in
the network. Mechanisms to avoid asymmetry at the network layer (for
example using MPLS RSVP-TE paths, or the use of the peer-to-peer
mechanism described in IEEE1588v2 are useful, as is the ability to
correct asymmetry in the physical connection through the use of
external calibration.

3.3. Time Presentation

In general, the output of the time acquisition module needs to be
reformatted before use. Formatting includes translation between
different representations (e.g., from seconds after a given date to
date and time), changing time zones, etc. When the time needs to be
displayed, e.g., on a computer or mobile device, further processing
is often required, such as identification of day of week, translation
to other calendars (e.g., Hebrew, Islamic, Chinese), conversion
between TAI and UTC, and flagging of holidays and special dates.

4. Other Generic Modules

4.1. Security Mechanisms

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 threats are false timing
servers distributing purposely misleading timing packets, and man in
the middle tampering with valid timing packets. Both threats would
enable a malfeasant to mislead TICTOC clients, and to bring down
critical services.

Based on the aforementioned threats, one can conclude that
confidentiality and replay protection are usually not needed (and in
many cases not possible), but source authentication and integrity
protection are vital. In general, it is the master that needs to be
authenticated to the server, although in certain cases it may be
needed to authenticate the client to the master.

Applying IPsec Authentication Header (AH) mechanisms to all timing
packets is not feasible, due to the computational resources consumed
by public key cryptography algorithms. Adoption of IPsec would
impact time acquisition accuracy, and would lead to new methods for
malfeasants to disrupt the timing distribution protocols by
exhausting resources and denial of service from TICTOC clients.
Furthermore, certificates used to authenticate packets have lifetimes
that must be enforced for secure use. Certification of time packets
themselves can thus lead to circular dependence and to attacks that
invalidate valid time packets and substitute invalid ones. NTP
implementations provided an authentication protocol called Autokey.
Autokey utilizes public key algorithms to encrypt cookies, symmetric
key message digests for integrity, and digital signatures for source
authentication.

Any security 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 mechanism also SHOULD not require
excessive storage of client state in the master, nor significantly
increase bandwidth consumption.

4.2. Autodiscovery and Master Clock Selection

The topology of presently deployed synchronization networks is
universally manually configured. This requires manual or management-
plane configuration of master-client relationships, as well as
preconfigured fallback behaviors. This strategy is workable for SDH
networks, where there are a relatively small number of network
elements that require such configuration, and all elements are
controlled by a single entity. In packet switched network scenarios
there may be a very large number of independent network elements
requiring timing, making automatic mechanisms important. Such
autodiscovery, automatic master selection algorithms, and perhaps
control plane protocols to support these features, are an important
features of the TICTOC deliverable. architecture.

There are application scenarios where it is desirable for clocks to
advertise their service, and for clients to automatically select a
clock with the required accuracy and path characteristics. The
optimum advertising method may depend on the environment and the
application. For example a host may be best served by finding a
suitable clock (or set of clocks) through a DHCP parameter, whist a
provider edge may find it more convenient to discover the available
clock through BGP.

The TICTOC Working Group will specify the required clock
characteristics and identify the set of clock and client
characteristics and the application characteristics that influence
the optimum method of clock discovery. It will then specify one or
more methods of clock autodiscovery together with associated
applicability information.

Once a TICTOC client has discovered potential TICTOC masters, it must
choose how to derive its clock from one or more of them. This choice
can be aided using two types of information:
o claims made by the potential masters as to the accuracy of its
local clock (see OAM and SSM below)
o analysis of the stability of frequency recovered from the
potential masters.

One tactic that a TICTOC client may employ is to individually choose
which master to use based on this information. Even if statically
configured to use a particular master, a client may be allowed to
make independent decisions when the master becomes unavailable or
sends synchronization status messages indicating a fault condition.

A second tactic is not to make a hard decision at all, but rather to
perform weighted ensemble averaging of frequencies recovered from all
available masters. The weightings, once again, may be based on
claims made by the masters or by monitoring of frequency stability.

Using either tactic, it is important to ensure that "timing loops"
are not formed. A timing loop is an erroneous topology that causes
locking
causeslocking onto frequencies not traceable to a high quality
source. Loops are avoided by ensuring that the frequency distribution
paths form a tree.

Yet another method is not to decide on a timing distribution tree at
all, but rather to enable self-organization of independently acting
intelligent agents. In such cases the meaning of master and client
becomes blurred, as all agents exchange timing information with a
subset neighbors that have been discovered. This method may be
driven by timing acquisition algorithms that guarantee global
convergence of the timing agents, meaning that over time the average
timing error decreases, although a given agent's error may sometimes
increase.

4.3. Redundancy and Fail-Over Mechanisms

Since synchronization is a critical function in many network
applications, operators conventionally protect it from single points
of failure. Redundant sources of synchronization are normally
provisioned, connected via diverse paths to prevent loss of
synchronization at the end station.

This is also required in packet synchronization. It may be necessary
to provision alternative paths, or techniques such as fast re-route
to ensure that connection between a time server and client devices is
not lost for too long.

4.4. OAM, SSM, and Performance Monitoring

In order to ensure continuity and connectivity of the path from the
master to the client, and the reverse path when needed, an
Operations, Administration, and Maintenance protocol may be needed.
When the master sends timing distribution packets at a constant rate,
faults are detected by the client after a few interpacket times; when
the rate is variable, the problem is detected after a few times the
maximum interpacket interval. However, in either case the master may
be unaware of the problem.

Synchronization Status Messages (SSMs) were traditionally used in TDM
networks to indicate the accuracy of the source upon which an
incoming TDM link based its timing, and to notify the remote site of
clock failures. Timing distribution protocols generally have similar
functions.

Performance monitoring is important to ensure the proper operation of
timing systems, and may be integrated into OAM mechanisms.

4.4. Path Selection

4.5. Network Management

As for all network infrastructure applications it may mechanisms, timing distribution
systems SHOULD be possible for TICTOC managed. This implies provision of management
agents in TICTOC masters and clients, and definition of the
appropriate MIBs.

4.6. Application profiles

PTP defines the concept of "profiles", defining the attributes and
options of the protocol required to support a given application. The
concept is extensible to other timing protocols, to ensure
interoperable components of a timing system.

Profiles are developed by standards organizations or other industry
bodies. For example, a "Telecom Profile" is currently under
definition by the ITU-T. The TICTOC working group will need to
specify the profile for the particular applications under its remit.

5. Network Support

5.1. Path Selection

In infrastructure applications it may be possible for TICTOC devices
to seek co-operation from routing elements to optimize the path
through the network in order to obtain a better quality of time and
frequency transfer that is possible when the timing distribution
protocol operates independent of the network layer.

At the simplest level this is use of paths that can support the
required quality of service, and have the lowest congestion impact.
However it is also possible for the network layer to be a proactive
partner in the transfer process. For example paths may be selected
that are explicitly chosen to be symmetric, or paths may be selected
such that all are capable of supporting physical frequency transfer
(for example synchronous Ethernet), or nodes may be selected such
that they are all capable of supporting transparent clock.

In addition to having having to deal with degradation of service due to
congestion, TICTOC must not add to the problem. Thus, TICTOC timing
distribution protocols MUST be able to act in a TCP friendly way,
even at the expense of minor degradation of performance. In such
cases, it may be required for the master to inform the client of the
change in operating conditions.

5.2. Network and Traffic Engineering

Every network element (e.g. router or switch) in a packet network
adds varying amounts of delay to the packet. This variation is
typically correlated to the load on that network element at the time,
and especially to the shared load on the output port.

Therefore, there is some maximum limit on both the traffic load and
the number of network elements that a packet timing flow can traverse
before being degraded beyond the point at which the recovered time or
frequency is outside of the specification for the given application.

This maximum limit will change depending on:
o stringency of the application requirements
o stability and environment of the local oscillator at the
time/frequency client device
o traffic load in the network
o topology of the network
o physical layer aspects of the network.

When planning a time/frequency distribution service over a managed
network, the network planner must take these considerations into
account. These will influence the appropriate locations for the
time/frequency servers and any on-path support elements that may be
deployed.

Traffic engineering may be used to control the traffic load in the
network on the routes used by the timing flows. This may help to
control the delay variation in the timing flow, and hence improve the
performance of the clock recovered by the client.

5.3. Service Level Specifications and QoS settings

The traditional approach to specifying network performance has been
to define a series of metrics such as packet loss and packet delay
variation. However, these metrics are not good predictors of how a
packet timing scheme is going to perform. For instance, the packet
delay variation metric is too abstract, since it doesn't take into
account the distribution of delays, or the correlation of delays over
a longer interval.

Recent research has examined the application to PDV of new metrics
borrowed from synchronization standards such as TDEV and a modified
version called minTDEV [minTDEV]. The goal is to define a metric
that is both measurable and capable of continuous monitoring. This
can then form the basis of a service level specification for a
network capable of running time/frequency distribution to a given
performance level.

Further, the network operator needs to know how to deal with degradation of service due tune the network
to
congestion, TICTOC stay within the specified limit, since no operator is going to
sign up to a service level specification that he has no control over.
Appropriate QoS settings and techniques must not add be deployed to ensure
the problem. Thus, TICTOC timing
distribution protocols MUST be able to act packets are forwarded through the routers in a TCP friendly way,
even at the expense of minor degradation optimum
way.

5.4. Routing considerations

The basic method for calculating a time offset of performance. In such
cases, it a remote slave
connected over a packet network makes an assumption that the network
delays in each direction are the same. Any asymmetry between the
forward and reverse directions yields an error in the time offset
estimation.

While there may be required various inferences that an algorithm can make
concerning the size of that asymmetry, there are no currently known
techniques for directly calculating its magnitude. Therefore it is
important that the master routing is constrained to inform make the client of packet flow
take the
change same path in operating conditions.

4.5. each direction. This in itself will not
guarantee that the time delay is the same in each direction, but it
should minimize any difference.

5.5. On-path Support

The TICTOC architecture will be commensurate with the public
Internet, and the timing distribution protocol chosen will be able to
function over general packet switched networks.

In some cases on-path support (for example IEEE 1588 IEEE1588 transparent
clock)
clocks) may be needed in order to obtain the required frequency or
time accuracy. Such on-path support cannot be expected to be
universally available over the public Internet, and it is not a goal
of the TICTOC working group to propose augmentation of basic
forwarding elements. However, such on-path support may be provided
on service provider or enterprise networks, and in such cases TICTOC
protocols should be able to exploit it.

In networks with on-path support, it may be that the optimum path for
time transfer is not congruent with the optimum path for data
transfer. By using special-purpose IP addresses, and making routing
aware of the required path characteristics and address attributes, it
is possible to construct paths within the network that maintain the
required time transfer characteristics.

The TICTOC Working Group will specify the required path
characteristics and work with the ISIS and OSPF Working Groups to
specify the necessary routing extensions to support these
requirements.

TICTOC traffic may need to traverse NATs and firewalls.

4.6. Network Management

As for all network infrastructure mechanisms, timing distribution
systems SHOULD be managed. This implies provision of management
agents in TICTOC masters and clients, and definition of the
appropriate MIBs.

6. Security Considerations

This document proposes modularization of the work of a proposed
Working Group, and thus does not, in itself, raise security concerns.

Security aspects of TICTOC have been addressed above.

7. IANA Considerations

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

8. Acknowledgements

The authors wish to thank Yaakov Stein and Stewart Bryant for their
contributions in the development of this document, and also Alon Geva,
Gabriel Zigelboim, and Laurent Montini for invaluable comments.

8. Informative References

[1588]

INFORMATIVE REFERENCES

[IEEE1588] IEEE, "1588-2002 Standard "Standard for A Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems".

[G8262]
Systems", IEEE1588-2008.

[G.8261] ITU-T, "G.8262 - Timing "Timing and Synchronization Aspects in Packet

[G.8262] ITU-T, "Timing characteristics of synchronous ethernet
equipment slave clock (EEC)". (EEC)", G.8262, August 2007

[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.

Authors' Addresses

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

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

[minTDEV] G. Zampetti and L. Cosart, "Definition of minTDEV",
COM15-C363-E, Contribution to ITU-T Study Group 15,

AUTHORS' ADDRESSES

Tim Frost,
Symmetricom Inc.,
Tamerton Road,
Roborough,
Plymouth, PL6 7BQ,
United Kingdom Kingdom.
Email: tfrost@symmetricom.com

Greg Dowd,
Symmetricom Inc.,
2300 Orchard Parkway,
San Jose,
CA 95112,
USA.
Email: stbryant@cisco.com gdowd@symmetricom.com

Karen O'Donoghue
US Navy
Code B35, NSWCDD,
17320 Dahlgren Rd.
Dahlgren,
VA 22448 22448,
USA.
Email: karen.odonoghue@navy.mil

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