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TICTOC Problem Statement
<draft-bryant-tictoc-probstat-00.txt >
TICTOC BOF IETF68
Stewart Bryant ([email protected])
Yaakov (J) Stein ([email protected])
The Need for Time and Frequency
• New applications and new network designs require
accurate time and/or frequency
• Accurate = ~50ppb frequency and 1-10us Time.
• Transmitting time and/or frequency at these
accuracies over a PSN
– is a hard (but solvable) problem
– is not addressed by any of the existing IETF WGs
• We therefore propose the formation of a new
working group to be called Timing(*) over IP
Connections and Transfer of Clock (TICTOC).
(*) Timing is "Telco speak" for the high quality frequency needed to make TDM
networks function correctly
Applications
• Need better time accuracy than commonly available from
commonly available NTP (~10ms)
• Range of industries: telecommunications, financial, test
and measurement, government, industrial
• High quality frequency requirement led by needs of
mobile phone industry
• Time needed by many industries – Networking, Test and
Measurement, Industrial, Power etc
• High accuracy time enables new applications
• Distributed systems design would significantly benefit
from the wide-scale availability of high quality time
Applications requirements (examples)
Synchronization
service
Application
Expected quality
TDM transition & PRS-traceability (i.e. reference signal from Stratum1 / G.811 in normal situation)
co-existence
 Copying SONET/SDH synchronization architecture
Frequency
System specific
time (phase)
Global time (Timeof-Day)
3GPP/2 Base
Stations
Frequency assignment shall be less than ± 5x10-8 (± 0.05 ppm)
 Specified for air interface not for BS network interface
Enterprise,
Residential
High quality frequency source for internal purpose
Ex: legacy PBX, local time source, AVB master…
802.16(D/)E
Depends on: mode, modulation, application, implementation and option used
Strongest needs for optimized radio frequency utilization, mobility and HO/Fast
BS switching and MBS options
3GPP2 CDMA
Base Stations
Frequency assignment shall be less than ± 5x10-8 (like 3GPP)
Time alignment error should be less than 3 μs and shall be less than 10 μs
DVB-T/H SFN
TBD
3GPP
MBMS/LTE
Cell synchronization accuracy better than or equal to 3µs for SFN support
Different options under study; one is to get precise time from network
3GPP
MBMS/LTE
MBMS Content synchronization - TBD
IP SLA
One-way delay
Improve precision to < 1 ms (within 10 ms class today)
Ideally target precision in few orders of magnitude below average delay (i.e. ~
10-100µs)
Power
distribution
Correlation of output of disperse synchrophasors
Measurement requires a source of UTC time accurate to 1 µs
Infrastructure
• Note that all of these applications are infrastructure
applications.
• The requirements are more strict than for most end user
applications.
• BUT there is greater flexibility in way that infrastructure
can provide service to itself
–
–
–
–
–
–
Packet Rate
QoS
Client server pairings
Path selection
On-path operations
Algorithm choice
are all factors that can be modified to enhance the
quality of the time/frequency transfer.
Time and Frequency Transfer
• Physical Layer – SONET/SDH and Synchronous
Ethernet
• Dedicated Network
• Radio
• Packet Networks
Note that a high quality time source can be used to
generate high quality frequency, and that whilst a
frequency source cannot transfer time, a high quality
frequency source cannot transfer time by itself, it can
significantly enhance the quality of the time transferred
by another means.
Synchronous Ethernet
• Synchronous Ethernet – SDH frequency lock technology
applied to the Ethernet physical layer
• Replace 100ppm Ethernet clock with Stratum1 clock
• Requires a contiguous SyncE path from clock source to
client
• NOTE – packet scheduling is NOT synchronous
• Only transfers frequency – no time transfer protocol
• But - high quality local frequency source provides a
major improvement in time transfer
• Next generation time transfer mechanism should
therefore be designed to take advantage of SyncE if it is
available.
DTI
• Docsys Timing Interface designed to
support the MAC interface in Docsys cable
standard
• Transfers time
• Range approx 200’ over dedicated
network
• Capable of 5ns accuracy
Radio Time Transfer
• GNSS (GPS)
• Loran / eLORAN
• High accuracy
• Require antennas – but work underway on use with
internal antennas
• Coverage is limited
• Reliability concerns
– Failure rate
– Subject to interference and jamming
• Political issues with GPS (will be solved by Galileo)
• Often need to supplement with local timing distribution
The Packet Network Environment
• Packet delay variation, propagation asymmetry, and maximum
permissible packet rate have a significant bearing on accuracy
• PDV may be mitigated by TE
• SP network better time service than arbitrary Internet hosts
• Midbox techniques (IEEE 1588 type on-the-fly packet timestamp
correction, or follow-up message mechanisms) correct/report the
packet delays may improve quality
• BUT require contiguous path support
• AND have usual midbox issues
• Packet rate influences quality of the time transfer - at a higher rate
there is a better chance of extracting "good“ packets
• In a controlled environment it is possible to ensure that
– There is adequate bandwidth
– The server is not overload
• 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.
Existing Solutions
• NTP
– Existing NTP implementations do not meet the requirements for new
applications
– Update rate can not be scaled up sufficiently without a change to the
protocol
– Does not take advantage of hardware support
• IEEE 1588v2 protocol
–
–
–
–
Largely designed around a well-controlled LAN environment
Needs hardware support to go get best performance
Some modes do not scale well
Needs a profile to support an IETF environment.
• Synchronous Ethernet
– Needs end to end contiguous path
– Only transfers frequency
– Not a time delivery solution, but may enhance one
Other Forums
• NTP WG
• PWE3 WG
• IEEE 1588 task force
• IEEE 802.1AS
• ITU-T SG15 Question 13
Each forum has a different expertise set
IETF has unique skills
– distributed protocol design
– security
that complement those of the other organizations
Security
• Time and frequency services are a significant element of
network infrastructure - critical for emerging applications
• Time and frequency transfer services MUST be
protected from being compromised
• The most significant threat is a false time or frequency
server being accepted instead of a true one
• Protection mechanism must be designed in such a way
that it does not degrade the quality of the time transfer
• Lightweight mechanism desirable, because:
– client restrictions often dictate a low processing memory footprint
– the server may have extensive fan-out
Congestion
• Timing distribution is sensitive to packet delay and loss
• Timing transfer packets should always be sent using the highest
class of service, and when possible should be sent over a traffic
engineered path
• Depending on the quality of the client's clock and the required
quality after disciplining, relatively high packet rates may be required
• Under congestion conditions client may need to go into "holdover"
mode (holdover requires expensive oscillators)
• When the network goes into congestion special handling of time
distribution packets may be required
• Work performed by the IETF PWE3 WG on congestion may be
applicable
Conclusion
• This is an important problem area that the IETF
needs to address.
• Experimental evidence indicates that the
solution is tractable (i.e. it is not research)
• The IETF has a unique set of skills that are
applicable to the problem space.
• The IETF should form a WG with a goal of
addessing the elements of the TICTOC solution
in which it is uniquely skilled, and working with
other SDOs in areas where they have core
expertise.