Internet DRAFT - draft-reilly-ntp-bcp
draft-reilly-ntp-bcp
Internet Engineering Task Force D. Reilly
Internet-Draft Spectracom Corporation
Intended status: Best Current Practice H. Stenn
Expires: September 10, 2016 Network Time Foundation
D. Sibold
PTB
March 9, 2016
Network Time Protocol Best Current Practices
draft-reilly-ntp-bcp-01
Abstract
NTP Version 4 (NTPv4) has been widely used since its publication as
RFC 5905 [RFC5905]. This documentation is a collection of Best
Practices from across the NTP community.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 10, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Keeping NTP up to date . . . . . . . . . . . . . . . . . . . 3
3. General Network Security Best Practices . . . . . . . . . . . 3
3.1. BCP 38 . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. NTP Configuration Best Practices . . . . . . . . . . . . . . 4
4.1. Use enough time sources . . . . . . . . . . . . . . . . . 4
4.2. Use a diversity of Reference Clocks . . . . . . . . . . . 5
4.3. Mode 6 and 7 . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 6
4.5. Security . . . . . . . . . . . . . . . . . . . . . . . . 6
4.5.1. Pre-Shared Key Approach . . . . . . . . . . . . . . . 7
4.5.2. Autokey . . . . . . . . . . . . . . . . . . . . . . . 8
4.5.3. External Security Means . . . . . . . . . . . . . . . 8
4.6. Using Pool Servers . . . . . . . . . . . . . . . . . . . 8
4.7. Starting, Cold-Starting, and Re-Starting NTP . . . . . . 9
4.8. Leap Second Handling . . . . . . . . . . . . . . . . . . 9
4.8.1. Leap Smearing . . . . . . . . . . . . . . . . . . . . 9
5. NTP in Embedded Devices . . . . . . . . . . . . . . . . . . . 10
5.1. Updating Embedded Devices . . . . . . . . . . . . . . . . 10
5.2. KISS Packets . . . . . . . . . . . . . . . . . . . . . . 10
5.3. Server configuration . . . . . . . . . . . . . . . . . . 11
5.3.1. Get a vendor subdomain for pool.ntp.org . . . . . . . 11
6. NTP Deployment Examples . . . . . . . . . . . . . . . . . . . 11
6.1. Client-Only configuration . . . . . . . . . . . . . . . . 11
6.2. Server-Only Configuration . . . . . . . . . . . . . . . . 11
6.3. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . 13
10.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
NTP Version 4 (NTPv4) has been widely used since its publication as
RFC 5905 [RFC5905]. This documentation is a collection of Best
Practices from across the NTP community.
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1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. Keeping NTP up to date
No software (not even NTP) is perfect. Bugs can be present in any
software. Even if software is thoroughly tested and "all" the bugs
are discovered and fixed, users continuously find new ways to use
software that their authors did not conceive of, which can uncover
more bugs. Thousands of individual bugs have been found and fixed in
the NTP Project's reference implementation since the first NTPv4
release in 1997.
There are always new ideas about security on the Internet, and an
application which is secure today could be insecure tomorrow once an
unknown bug (or a known behavior) is exploited in the right way.
Many security mechanisms rely on time, either directly or indirectly,
as part of their operation. If an attacker can spoof the time, they
may be able to bypass or neutralize other security elements. For
example, incorrect time can disrupt the ability to reconcile logfile
entries on the affected system with events on other systems.
In general, the best way to protect yourself and your networks
against these bugs and security threats is to make sure that you keep
your NTP implementation up-to-date. There are multiple versions of
the NTP protocol in use and multiple implementations and versions of
NTP software also in use, on many different platforms. It is
recommended that NTP users actively monitor wherever they get their
software to find out if their versions are vulnerable to any known
attacks, and deploy updates containing security fixes as soon as
practical.
The reference implementation of NTP Version 4 from Network Time
Foundation (NTF) continues to be actively maintained and developed by
NTF's NTP Project, with help from volunteers and NTF's supporters.
The NTP software can be downloaded from ntp.org [1] and also from
NTF's github page [2].
3. General Network Security Best Practices
NTP deployments are only as secure as the networks they are running
over.
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3.1. BCP 38
Many network attacks rely on modifying the IP source address of a
packet to point to a different IP address than the computer which
originated it. This modification/abuse vector has been known for
quite some time, and BCP 38 [RFC2827] was approved in 2000 to address
this. BCP 38 [RFC2827] calls for filtering outgoing and incoming
traffic to make sure that the source and destination IP addresses are
consistent with the expected flow of traffic on each network
interface. It is recommended that all networks (and ISP's of any
size) implement this. If a machine on your network is sending out
packets claiming to be from an address that is not on your network,
this could be the first indication that you have a machine that has
been cracked, and is being used abusively. If packets are arriving
on an external interface with a source address that should only be
seen on an internal network, that's a strong indication that an
attacker is trying to inject spoofed packets into your network. More
information is available at http://www.bcp38.info .
4. NTP Configuration Best Practices
NTP can be made more secure by making a few simple changes to the
ntp.conf file.
4.1. Use enough time sources
NTP takes the available sources of time and submits their timing data
to intersection and clustering algorithms, looking for the best idea
of the correct time. If there is only 1 source of time, the answer
is obvious. It may not be a good source of time, but it's the only
one. If there are 2 sources of time and they agree well enough,
that's good. But if they don't, then ntpd has no way to know which
source to believe. This gets easier if there are 3 sources of time.
But if one of those 3 sources becomes unreachable or unusable, we're
back to only having 2 time sources. 4 sources of time is another
interesting choice, assuming things go well. If one of these sources
develops a problem there are still 3 others. Seems good. Until the
leap second we had in June of 2015, where several operators
implemented leap smearing while others did not. See Section 4.8.1
for more information.
Starting with ntp-4.2.6, the 'pool' directive will spin up "enough"
associations to provide robust time service, and will disconnect poor
servers and add in new servers as-needed.
Monitor your ntpd instances. If your times sources do not generally
agree, find out why and either correct the problems or stop using
defective servers. See Section 4.4 for more information.
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4.2. Use a diversity of Reference Clocks
If you are using reference clocks, it is recommended that you use
several different types. Having a diversity of sources means that
any one issue is less likely to cause a service interruption.
Are all your clocks from the same vendor? Are they using the same
base chipset, regardless of whether or not the finished products are
from different vendors? Are they all running the same version of
firmware? A systemic problem with time from any satellite navigation
service is possible and has happened. Sunspot activity can render
satellite or radio-based time source unusable. A chipset problem can
happen. There may be a bug in the vendor's firmware.
4.3. Mode 6 and 7
NTP Mode 6 (ntpq) and Mode 7 (ntpdc) packets are designed to permit
monitoring and optional authenticated control of ntpd and its
configuration. Used properly, these facilities provide vital
debugging and performance information and control. Used improperly,
these facilities can be an abuse vector.
Mode 7 queries have been disabled by default since 4.2.7p230,
released on 2011/11/01. Unless you have a good reason for using
ntpdc, do not enable Mode 7.
The ability to use Mode 6 beyond its basic monitoring capabilities
can be limited to authenticated sessions that provide a 'controlkey',
and similarly, if Mode 7 has been explicitly enabled its use for more
than masic monitoring can be limited to authenticated sessions that
provide a 'requestkey'.
Older versions of the reference implementation of NTP could be abused
to participate in high-bandwidth DDoS attacks. Starting with ntp-
4.2.7p26, released in April of 2010, ntpd requires the use of a nonce
before replying with potentially large response packets.
As mentioned above, there are two general ways to use Mode 6 and Mode
7 requests. One way is to query ntpd for information, and this mode
can be disabled with:
restrict ... noquery
The second way to use Mode 6 and Mode 7 requests is to modify ntpd's
behavior. Modification of ntpd ordinarily requires an authenticated
session. By default, if no authentication keys have been specified
no modifications can be made. For additional protection, the ability
to perform these modifications can be controlled with:
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restrict ... nomodify
Users can prevent their NTP servers from participating by adding the
following to their ntp.conf file:
restrict default -4 nomodify notrap nopeer noquery
restrict default -6 nomodify notrap nopeer noquery
restrict source nomodify notrap noquery # nopeer is OK if you don't
use the 'pool' directive
4.4. Monitoring
The reference implementation of NTP allows remote monitoring. The
access to this service is controlled by the restrict statement in
NTP's configuration file (ntp.conf). The syntax reads:
restrict address mask address_mask nomodify
Monitor your ntpd instances so machines that are "out of sync" can be
quickly identified. Monitor your system logs for messages from ntpd
so abuse attempts can be quickly identified.
If your system starts getting unexpected time replies from its time
servers, that can be an indication that the IP address of your server
is being forged in requests to that time server, and these abusers
are trying to convince your time servers to stop serving time to you.
If your system is a broadcast client and your syslog shows that you
are receiving "early" time messages from your server, that is an
indication that somebody may be forging packets from a broadcast
server.
If your syslog shows messages that indicate you are receiving
timestamps that are earlier than the current system time, then either
your system clock is unusually fast or somebody is trying to launch a
replay attack against your server.
If you are using broadcast mode and have ntp-4.2.8p6 or later, use
the 4th field of the ntp.keys file to identify the IPs of machines
that are allowed to serve time to the group.
4.5. Security
In the standard configuration NTP packets are exchanged unprotected
between client and server. An adversary that is able to become a
Man-In-The-Middle is therefore able to drop, replay or modify the
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content of the NTP packet, which leads to degradation of the time
synchronization or the transmission of false time information. A
profound threat analysis for time synchronization protocols are given
in RFC 7384 [RFC7384]. NTP provides two security measures to protect
authenticity and integrity of the NTP packets. Both measures protect
the NTP packet by means of a Message Authentication Code (MAC).
Neither of them encrypts the NTP's payload, because it is not
considered to be confidential.
4.5.1. Pre-Shared Key Approach
This approach applies a symmetric key for the calculation of the MAC,
which protects authenticity and integrity of the exchanged packets
for a association. NTP does not provide a mechanism for the exchange
of the keys between the associated nodes. Therefore, for each
association, keys have to be exchanged securely by external means.
It is recommended that each association is protected by its own
unique key. NTP does not provide a mechanism to automatically
refresh the applied keys. It is therefore recommended that the
participants periodically agree on a fresh key. The calculation of
the MAC may always be based on an MD5 hash. If the NTP daemon is
built against an OpenSSL library, NTP can also base the calculation
of the MAC upon the SHA-1 or any other digest algorithm supported by
each side's OpenSSL library.
To use this approach the communication partners have to exchange the
key, which consists of a keyid with a value between 1 and 65534,
inclusive, and a label which indicates the chosen digest algorithm.
Each communication partner adds this information to their key file in
the form:
keyid label key
The key file contains the key in clear text. Therefore it should
only be readable by the NTP process. Different keys are added line
by line to the key file.
A NTP client establishes a protected association by appending the
option "key keyid" to the server statement in the NTP configuration
file:
server address key keyid
Note that the NTP process has to trust the applied key. An NTP
process explicitly has to add each key it want to trust to a list of
trusted keys by the "trustedkey" statement in the NTP configuration
file.
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trustedkey keyid_1 keyid_2 ... keyid_n
4.5.2. Autokey
Autokey was designed in 2003 to provide a means for clients to
authenticate servers. By 2011, security researchers had identified
computational areas in the Autokey protocol that, while secure at the
time of its original design, were no longer secure. Work was begun
on an enhanced replacement for Autokey, which was called Network Time
Security (NTS) [3]. NTS was published in the summer of 2013. As of
February 2016, this effort was at draft #13, and about to begin
'final call'. The first unicast implementation of NTS was started in
the summer of 2015 and is expected to be released in the summer of
2016.
We recommend that Autokey NOT BE USED. Know that as of the fall of
2011, a common(?) laptop computer could crack the security cookie
used in the Autokey protocol in 30 minutes' time. If you must use
Autokey, know that your session keys should be set to expire in under
30 minutes' time. If you have reason to believe your autokey-
protected associations will be attacked, you should read
https://lists.ntp.org/pipermail/ntpwg/2011-August/001714.html and
decide what resources your attackers might be using, and adjust the
session key expiration time accordingly.
4.5.3. External Security Means
TBD
4.6. Using Pool Servers
It only takes a small amount of bandwidth and system resources to
synchronize one NTP client, but NTP servers that can service tens of
thousands of clients take more resources to run. Users who want to
synchronize their computers should only synchronize to servers that
they have permission to use.
The NTP pool project is a collection of volunteers who have donated
their computing and bandwidth resources to provide time on the
Internet for free. The time is generally of good quality, but comes
with no guarantee whatsoever. If you are interested in using the
pool, please review their instructions at http://www.pool.ntp.org/en/
use.html .
If you want to synchronize many computers using the pool, consider
running your own NTP servers, synchronizing them to the pool, and
synchronizing your clients to your in-house NTP servers. This
reduces the load on the pool.
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Set up or sponsor one or more pool servers.
4.7. Starting, Cold-Starting, and Re-Starting NTP
Only use -g on cold-start. Other things TBD.
Editor's Note: I think I'd like to expand this a bit to cover how to
deal with NTP stopping, when to restart it, and under what
circumstances to not restart it!
4.8. Leap Second Handling
The UTC timescale is kept in sync with the rotation of the earth
through the use of leap seconds. NTP time is based on the UTC
timescale, and the protocol has the capability to broadcast leap
second information. Some GNSS systems (like GPS) broadcast leap
second information, so if you have a Stratum-1 server synced to GNSS
(or you are synced to a lower stratum server that is ultimately
synced to GNSS), you will get advance notification of impending leap
seconds automatically.
The International Earth Rotation and Reference Systems Service (IERS)
is responsible to announce the introduction of a leap second. It
maintains a leap second list at
https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list for NTP
users who are not receiving leap second information through an
automatic source. After fetching the leap seconds file onto the
server, add this line to ntpd.conf to apply the file:
leapfile "/path/to your/leap-file"
You will need to restart to apply the changes.
4.8.1. Leap Smearing
Some NTP installations may instead make use of a technique called
"Leap Smearing". With this method, instead of introducing an extra
second (or eliminating a second), NTP time will be slewed in small
increments over a comparably large window of time around the leap
second event. The amount of the slew should be small enough that
clients will follow the smeared time without objecting. During the
adjustment window, the NTP server's time may be offset from UTC by as
much as .5 seconds. This is done to enable software that doesn't
deal with minutes that have more or less than 60 seconds to function
correctly, at the expense of fidelity to UTC during the smear window.
Leap Smearing was introduced in ntpd versions 4.2.8.p3 and 4.3.47.
Support is not configured by default and must be added at compile
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time. In addition, no leap smearing will occur unless a leap smear
interval is specified in ntpd.conf . For more information, refer to
http://bk1.ntp.org/ntp-stable/README.leapsmear?PAGE=anno .
Leap Smearing is not recommended for public-facing NTP servers, as
they will disagree with non-smearing servers during the leap smear
interval. However, some public-facing servers may be configured this
way anyway. Users are advised to be aware of impending leap seconds
and how the servers (inside and outside their organization) they are
using deal with them.
5. NTP in Embedded Devices
Readers of this BCP already understand how important accurate time is
for network computing. And as computing becomes more ubiquitous,
there will be many small "Internet of Things" devices that require
accurate time. These embedded devices may not have a traditional
user interface, but if they connect to the Internet they will be
subject to the same security threats as traditional deployments.
5.1. Updating Embedded Devices
Vendors of embedded devices have a special responsibility to pay
attention to the current state of NTP bugs and security issues,
because their customers usually don't have the ability to update
their NTP implementation on their own. Those devices may have a
single firmware upgrade, provided by the manufacturer, that updates
all capabilities at once. This means that the vendor assumes the
responsibility of making sure their devices have the latest NTP
updates applied.
This should also include the ability to update the NTP server
address.
(Note: do we find specific historical instances of devices behaving
badly and cite them here?)
5.2. KISS Packets
The "Kiss-o'-Death" (KoD) packet is a rate limiting mechanism where a
server can tell a misbehaving client to "back off" its query rate.
It is important for all NTP devices to respect these packets and back
off when asked to do so by a server. It is even more important for
an embedded device, which may not have exposed a control interface
for NTP.
The KoD mechanism relies on clients behaving properly in order to be
effective. Some clients ignore the KoD packet entirely, and other
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poorly-implemented clients might unintentionally increase their poll
rate and simulate a denial of service attack. Server administrators
should be prepared for this and take measures outside of the NTP
protocol to drop packets from misbehaving clients.
5.3. Server configuration
Vendors of embedded devices that need time synchronization should
also carefully consider where they get their time from. There are
several public-facing NTP servers available, but they may not be
prepared to service requests from thousands of new devices on the
Internet.
Vendors are encouraged to invest resources into providing their own
time servers for their devices.
5.3.1. Get a vendor subdomain for pool.ntp.org
The NTP Pool Project offers a program where vendors can obtain their
own subdomain that is part of the NTP Pool. This offers vendors the
ability to safely make use of the time distributed by the Pool for
their devices. Vendors are encouraged to support the pool if they
participate. For more information, visit http://www.pool.ntp.org/en/
vendors.html .
6. NTP Deployment Examples
A few examples of interesting NTP Deployments
6.1. Client-Only configuration
TBD
6.2. Server-Only Configuration
TBD
6.3. Anycast
Anycast is described in BCP 126 [RFC4786]. (Also see RFC 7094
[RFC7094]). With anycast, a single IP address is assigned to
multiple interfaces, and routers direct packets to the closest active
interface.
Anycast is often used for Internet services at known IP addresses,
such as DNS. Anycast can also be used in large organizations to
simplify configuration of a large number of NTP clients. Each client
can be configured with the same NTP server IP address, and a pool of
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anycast servers can be deployed to service those requests. New
servers can be added to or taken from the pool, and other than a
temporary loss of service while a server is taken down, these
additions can be transparent to the clients.
If clients are connected to an NTP server via anycast, the client
does not know which particular server they are connected to. As
anycast servers may arbitrarily enter and leave the network, the
server a particular client is connected to may change. This may
cause a small shift in time from the perspective of the client when
the server it is connected to changes. It is recommended that
anycast be deployed in environments where these small shifts can be
tolerated.
Configuration of an anycast interface is independent of NTP. Clients
will always connect to the closest server, even if that server is
having NTP issues. It is recommended that anycast NTP
implementations have an independent method of monitoring the
performance of NTP on a server. In the event the server is not
performing to specification, it should remove itself from the Anycast
network. It is also recommended that each Anycast NTP server have at
least one Unicast interface, so its performance can be checked
independently of the anycast routing scheme.
One useful application in large networks is to use a hybrid unicast/
anycast approach. Stratum 1 NTP servers can be deployed with unicast
interfaces at several sites. Each site may have several Stratum 2
servers with a unicast interface and an anycast interface (with a
shared IP address per location). The unicast interfaces can be used
to obtain time from the Stratum 1 servers globally (and perhaps peer
with the other Stratum 2 servers at their site). Clients at each
site can be configured to use the shared anycast address for their
site, simplifying their configuration. Keeping the anycast routing
restricted on a per-site basis will minimize the disruption at the
client if its closest anycast server changes.
7. Acknowledgements
The author wishes to acknowledge the contributions of Sue Graves,
Samuel Weiler, Lisa Perdue, and Karen O'Donoghue.
8. IANA Considerations
This memo includes no request to IANA.
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9. Security Considerations
TBD
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <http://www.rfc-editor.org/info/rfc2827>.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <http://www.rfc-editor.org/info/rfc4786>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.
[RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014,
<http://www.rfc-editor.org/info/rfc7094>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <http://www.rfc-editor.org/info/rfc7384>.
10.2. URIs
[1] http://www.ntp.org/downloads.html
[2] https://github.com/ntp-project/ntp
[3] https://tools.ietf.org/html/draft-ietf-ntp-network-time-
security-00
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Authors' Addresses
Denis Reilly
Spectracom Corporation
1565 Jefferson Road, Suite 460
Rochester, NY 14623
US
Email: denis.reilly@spectracom.orolia.com
Harlan Stenn
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: stenn@nwtime.org
Dieter Sibold
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de
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