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Versions: 00 01 02 draft-ietf-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
   carefully, as they describe your rights and restrictions with respect
   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 bcp38.info">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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