TICTOC Tim Frost
INTERNET-DRAFT Greg Dowd
Intended Status: Informational Symmetricom
Karen O'Donoghue
US Navy
Expires: 03 May 2009 03 Nov 2008
Architecture for the Transmission of Timing over Packet Networks
draft-stein-tictoc-modules-03.txt
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Copyright (C) The IETF Trust (2008)
Abstract
This Internet draft proposes an architecture for the transmission of
timing over packet networks. It identifies the major functional
components, and maps the current solutions onto this framework.
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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
7
2.1. Local Oscillator and Frequency Synthesizers 7
2.2. Frequency Distribution Protocols 8
2.3. Frequency Acquisition Algorithms 8
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 10
3. Time Layer
11
3.1. Time Distribution Protocols 11
3.2. Ranging 13
3.3. Time Presentation 13
4. Generic Modules
15
4.1. Security Mechanisms 15
4.2. Autodiscovery and Master Clock Selection 15
4.3. Redundancy and Fail-Over Mechanisms 17
4.4. OAM, SSM, and Performance Monitoring 17
4.5. Network Management 17
4.6. Application profiles 17
5. Network Support
18
5.1. Path Selection 18
5.2. Network and Traffic Engineering 18
5.3. Service Level Specifications and QoS settings 19
5.4. Routing considerations 19
5.5. On-path Support 20
6. Security Considerations
20
7. IANA Considerations
20
8. Acknowledgements
20
INFORMATIVE REFERENCES 21
AUTHORS' ADDRESSES 21
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].
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1. Introduction
In this document the term "timing" will be used to refer to recovery of
frequency and/or time (wall-clock).
There is an emerging need to distribute highly accurate time and
frequency information over IP and over MPLS packet switched networks
(PSNs). A variety of applications require time and/or frequency
information to a precision which existing protocols cannot supply.
Several other 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 (PTP) [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.
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.
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|-------------------| |-------------------|
| Time distribution | | Time distribution |
|-------------------| |-------------------|
| Frequency distr. | | Frequency distr. |
|-------------------| |-------------------|
| Timing protocol | | Timing Protocol |
|-------------------| |-------------------|
| UDP | | UDP |
|-------------------| |-------------------|
| 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 2. Generic TICTOC client
The frequency acquisition module is responsible for recovery of
frequency information distributed over a packet switched network
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(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 sub-modules needed for timing distribution. Sub-
modules may require one or more protocols, a set of processing
algorithms, and implementations.
The frequency acquisition module may be subdivided into the following
sub-modules. Not all need to be present in all implementations.
o local oscillator
o frequency generator (typically a digital synthesizer)
o timestamp generator
(not required if packets are sent at constant rate)
o frequency distribution protocol
o frequency acquisition algorithms
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(may include packet selection algorithms, oscillator
disciplining and frequency adjustment techniques)
The time acquisition module may be subdivided into the following
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 network 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
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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 [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 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. 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
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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 since
timing packets may be distributed to multiple clients or slaves.
However, unicast transmission is often more desirable, since it
avoids the multicast replication operation in each switch or router,
which may add variable delay.
The TICTOC architecture is modular, and not bound to the frequency
distribution protocol. It is possible to use different frequency
distribution methods for different applications and environments,
e.g. the use of physical layer frequency distribution protocols such
as Synchronous Ethernet.
2.3. Frequency Acquisition Algorithms
A TICTOC client needs to be able to recover the original timebase (or
frequency reference) of the master or server. In general it achieves
this by comparing the arrival time of packets with the original
sending time. From this it can determine the relationship between the
local timebase and the master timebase.
Observed arrival times of frequency distribution protocol packets are
influenced by two phenomena:
o frequency drift of 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
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(PDV).
The former behavior needs to be tracked and compensated, 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. Typically this involves two steps:
o packet selection and discard algorithms
o filtering and control servos to "lock onto" the sending
frequency
2.3.1. Packet 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).
2.3.2. 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.
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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.
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).
It may be necessary to specify a reference 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.
2.4. Frequency Presentation
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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, 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 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. Providing any checksums or
CRCs can be updated on-the-fly, this hardware-generated timestamp may
be directly inserted into the packet. Alternatively, a subsequent
packet can be used to carry the measured injection time, as in the
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PTP follow-up message. Where hardware assistance to measure the
injection time accurately is not available, the stochastic delay
should be minimized.
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. PTP
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 PTP [IEEE1588]. NTP is
the product of the IETF, and is currently undergoing revision to
version 4. PTP is a product of the IEEE Test and Measurement
community, and has recently been revised to 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 optimized for their purpose, and not
compromised by the need to fulfill a dual role.
The TICTOC architecture itself is not bound to a specific time
distribution protocol. It is 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.
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3.2. Ranging
To transfer time it is necessary to know the time of flight 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 PTP
transparent clocks [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 are
useful (for example using MPLS RSVP-TE paths, or the use of the peer-
to-peer mechanism described in IEEE1588), 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
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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.
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4. 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
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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 important
features of the TICTOC 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.
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 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
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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.
4.5. 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.
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.
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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
on the basis of their symmetry, or 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 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 client to be informed 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
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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 tune the network
to 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 be deployed to ensure
the timing packets are forwarded through the routers in the optimum
way.
5.4. Routing considerations
The basic method for calculating a time offset of 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 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 routing is constrained to make the packet flow
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take the same path in 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 PTP transparent 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.
6. Security Considerations
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.
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INFORMATIVE REFERENCES
[IEEE1588] IEEE, "Standard for A Precision Clock Synchronization
Protocol for Networked Measurement and Control
Systems", IEEE1588-2008.
[G.8261] ITU-T, "Timing and Synchronization Aspects in Packet
Networks", G.8261, April 2008
[G.8262] ITU-T, "Timing characteristics of synchronous Ethernet
equipment slave clock (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.
[minTDEV] G. Zampetti and L. Cosart, "Definition of minTDEV",
COM15-C363-E, Contribution to ITU-T Study Group 15,
Question 13, May 2007
AUTHORS' ADDRESSES
Tim Frost,
Symmetricom Inc.,
Tamerton Road,
Roborough,
Plymouth, PL6 7BQ,
United Kingdom.
Email: tfrost@symmetricom.com
Greg Dowd,
Symmetricom Inc.,
2300 Orchard Parkway,
San Jose,
CA 95112,
USA.
Email: gdowd@symmetricom.com
Karen O'Donoghue
US Navy
Code B35, NSWCDD,
17320 Dahlgren Rd.
Dahlgren,
VA 22448,
USA.
Email: karen.odonoghue@navy.mil
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