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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 5905

Network Time Protocol Working                        J. Burbank (Editor)
Group                                                            JHU/APL
Internet-Draft                                     J. Martin (co-Editor)
Expires: January 9, 2006                                     Netzwert AG
                                                              July, 2005


      The Network Time Protocol Version 4 Protocol Specification
                     <draft-ietf-ntp-ntpv4-proto-00>

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3978.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she becomes aware will be disclosed, in accordance with
   Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This document is a submission of the IETF NTP WG.  Comments should be
   directed to the NTP WG mailing list, ntpwg@lists.ntp.isc.org.

   This Internet-Draft will expire on January 9, 2006.

Abstract

   The Network Time Protocol (NTP) is widely used to synchronize
   computer clocks in the Internet.  This memorandum describes
   Version 4 of the NTP (NTPv4), introducing several changes from
   Version 3 of NTP (NTPv3) described in RFC 1305, including the
   introduction of a modified protocol header to accomodate Internet
   Protocol Version 6.  NTPv4 also includes optional extensions to the
   NTPv3 protocol,including an anycast mode and an authentication scheme
   designed specifically for multicast and anycast modes.


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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  NTPv4 Protocol Operation . . . . . . . . . . . . . . . . . . .  4

   3.  NTPv4 Timestamp Format . . . . . . . . . . . . . . . . . . . .  6

   4.  NTPv4 Message Formats  . . . . . . . . . . . . . . . . . . . .  7

   5.  NTPv4 Client Operations  . . . . . . . . . . . . . . . . . . . 13

   6.  NTPv4 Server Operations  . . . . . . . . . . . . . . . . . . . 15

   7.  NTPv4 Security . . . . . . . . . . . . . . . . . . . . . . . . 19
       7.1 Session Keys and Cookies . . . . . . . . . . . . . . . . . 19
       7.2 Session Key List Generation  . . . . . . . . . . . . . . . 20
       7.3 Sending Messages . . . . . . . . . . . . . . . . . . . . . 20
       7.4 Receiving Messages . . . . . . . . . . . . . . . . . . . . 20
       7.5 Autokey Protocol Exchanges . . . . . . . . . . . . . . . . 20

   8.  Operation and Management Issues  . . . . . . . . . . . . . . . 22

   9.  Kiss o' Death Message  . . . . . . . . . . . . . . . . . . . . 22

   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 24

   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25

   12. Other Considerations . . . . . . . . . . . . . . . . . . . . . 25

   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 27

   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
      14.1   Normative References . . . . . . . . . . . . . . . . . . 27
      14.2   Informative References . . . . . . . . . . . . . . . . . 27
   15. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 28

       Intellectual Property and Copyright Statements . . . . . . . . 29













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1.  Introduction

   The Network Time Protocol Version 3 (NTPv3) specified in RFC 1305
   [MIL92] has been widely used to synchronize computer clocks in the
   global Internet.  It provides comprehensive mechanisms to access
   national time and frequency dissemination services, organize the NTP
   subnet of servers and clients and adjust the system clock in each
   participant. In most places of the Internet of today, NTP provides
   accuracies of 1-50 ms, depending on the characteristics of the
   synchronization source and network paths.

   NTP is designed for use by clients and servers with a wide range of
   capabilities and over a wide range of network jitter and clock
   frequency wander characteristics.  Many users of NTP in the Internet
   of today use a software distribution available from www.ntp.org.  The
   distribution, which includes the full suite of NTP options,
   mitigation algorithms and security schemes, is a relatively complex,
   real-time application.  While the software has been ported to a wide
   variety of hardware platforms ranging from personal computers to
   supercomputers, its sheer size and complexity is not appropriate for
   many applications.  This facilitated the development of the Simple
   Network Time Protocol Version 4 (SNTPv4) as described in RFC 2030
   [MIL96].

   Since the standardization of NTPv3, there has been significant
   development which has led to Version 4 of the Network Time Protocol
   (NTPv4).  This document describes NTPv4, which introduces new
   functionality to NTPv3 as described in RFC 1305, and functionality
   expanded from that of SNTPv4 as described in RFC 2030 (SNTPv4 is a
   subset of NTPv4).

   When operating with current and previous versions of NTP and SNTP,
   NTPv4 requires no changes to the protocol or implementations now
   running or likely to be implemented specifically for future NTP or
   SNTP versions.  The NTP and SNTP packet formats are the same and the
   arithmetic operations to calculate the client time, clock offset and
   roundtrip delay are the same.  To a NTP or SNTP server, NTP and SNTP
   clients are indistinguishable; to a NTP or SNTP client, NTP and SNTP
   servers are indistinguishable.


   An important provision in this memo is the interpretation of certain
   NTP header fields which provide for IPv6 and OSI addressing.  The
   only significant difference between the NTPv3 and NTPv4 header
   formats is the four-octet Reference Identifier field, which is used
   primarily to detect and avoid synchronization loops.  In all NTP and
   SNTP versions providing IPv4 addressing, primary servers use a four-
   character ASCII reference clock identifier in this field, while
   secondary servers use the 32-bit IPv4 address of the synchronization



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   source.  In NTPv4 providing IPv6 and OSI addressing, primary
   servers use the same clock identifier, but secondary servers use the
   first 32 bits of the MD5 hash of the IPv6 or NSAP address of the
   synchronization source.  A further use of this field is when the
   server sends a kiss-o'-death message documented later in this
   document.

   In the case of OSI, the Connectionless Transport Service (CLTS) is
   used as in [ISO86].  Each NTP packet is transmitted as the TS-
   Userdata parameter of a T-UNITDATA Request primitive.  Alternately,
   the header can be encapsulated in a TPDU which itself is transported
   using UDP, as described in RFC-1240 [DOB91].  It is not advised that
   NTP be operated at the upper layers of the OSI stack, such as might
   be inferred from RFC-1698 [FUR94], as this could seriously degrade
   accuracy.  With the header formats defined in this memo, it is in
   principle possible to interwork between servers and clients of one
   protocol family and another, although the practical difficulties may
   make this inadvisable.

      In the following, indented paragraphs such as this one contain
      information not required by the formal protocol specification, but
      considered good practice in protocol implementations.

   This document is organized as follows.  Section 2 describes how the
   protocol works, the various modes and how IP addresses and UDP ports
   are used.  Section 3 describes the NTP timestamp format and Section 4
   the NTP message format.  Section 5 summarizes NTP client operations
   and Section 6 summarizes NTP server operations.  Section 7 summarizes
   the optional security mechanisms present within the NTPv4 protocol.
   Section 8 summarizes operation and management issues.  Section 9
   describes the kiss-o'-death message, whose functionality is
   similar to the ICMP Source Quench and ICMP Destination Unreachable
   messages.  Section 10 presents NTPv4 security considerations.
   Section 11 presents various other considerations when implementing
   and/or configuring NTPv4.

   NTPv4 is hereafter referred to simply as NTP, unless explicitly
   noted.


2.  NTP Protocol Operation

   Unless excepted in context, reference to broadcast address means IPv4
   broadcast address, IPv4 multicast group address or IPv6 site-local
   scope address.  Further information on the broadcast/multicast model
   is in RFC 1112 [DEE89].  Details of address format, scoping rules,
   etc., are beyond the scope of this memo.  NTPv4 can operate with
   either unicast (point to point), broadcast (point to multipoint) or
   anycast (multipoint to point) addressing modes.  A unicast client
   sends a request to a designated server at its unicast address and
   expects a reply from which it can determine the time and, optionally,

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   the roundtrip delay and clock offset relative to the server.  A
   broadcast server periodically sends an unsolicited message to a
   designated broadcast address.  A broadcast client listens on this
   address and ordinarily sends no requests.

   Anycast is designed for use with a set of cooperating servers whose
   addresses are not known beforehand.  The anycast client sends an
   ordinary NTP client request to a designated broadcast address.  One
   or more anycast servers listen on that address.  Upon receiving a
   request, an anycast server sends an ordinary NTP server reply to the
   client.  The client then binds to the server from which the first
   such message was received and continues operation with that unicast
   addresses.  Subsequent replies from other anycast servers are
   ignored.

      Broadcast servers should respond to client unicast requests, as
      well as send unsolicited broadcast messages.  Broadcast clients
      may send unicast requests in order to measure the network
      propagation delay between the server and client and then continue
      operation in listen-only mode.  However, broadcast servers may
      choose not to respond to unicast requests, so unicast clients
      should be prepared to abandon the measurement and assume a default
      value for the delay.

   The client and server addresses are assigned following the usual
   IPv4, IPv6 or OSI conventions.  For NTP multicast, the IANA has
   reserved the IPv4 group address 224.0.1.1 and the IPv6 group address
   ending :101, with prefix determined by scoping rules.  The NTP
   broadcast address for OSI has yet to be determined.  Notwithstanding
   the IANA reserved addresses, other multicast addresses can be used
   which do not conflict with others assigned in scope.  In the case of
   IPv4 multicast or IPv6 broadcast addresses, the client must implement
   the Internet Group Management Protocol (IGMP) as described in RFC-
   3376 [CAIN02], in order that the local router joins the multicast
   group and relays messages to the IPv4 or IPv6 multicast group.  The
   scoping, routing and group membership procedures are determined by
   considerations beyond the scope of this memo.

      It is important to adjust the time-to-live (TTL) field in the IP
      header of multicast messages to a reasonable value in order to
      limit the network resources used by this (and any other) multicast
      service.  Only multicast clients in scope will receive multicast
      server messages.  Only cooperating anycast servers in scope will
      reply to a client request.  The engineering principles which
      determine the proper values to be used are beyond the scope of
      this memo.

      While not integral to the NTP specification, it is intended that
      IP broadcast addresses will be used primarily in IP subnets and
      LAN segments including a fully functional NTP server with a number


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      of dependent NTP broadcast clients on the same subnet, while IP
      multicast group addresses will be used only in cases where the TTL
      is engineered specifically for each service domain.


3.  NTP Timestamp

   NTPv4 uses the standard NTP timestamp format described in RFC-1305.
   NTP data are specified as integer or fixed-point quantities, with
   bits numbered in big-endian fashion from 0 starting at the left or
   most significant end.  Unless specified otherwise, all quantities
   are unsigned and may occupy the full field width with an implied 0
   preceding bit 0.

   NTP timestamps are represented as a 64-bit unsigned fixed-point
   number, in seconds relative to 0h on 1 January 1900.  The integer
   part is in the first 32 bits and the fraction part in the
   last 32 bits.  In the fraction part, the non-significant low order
   bits are not specified and ordinarily set to 0. The NTP timestamp
   format is as shown in Figure 1.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Seconds                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Fraction                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 1. NTP Timestamp Format

   It is advisable to fill the non-significant low order bits of the
   timestamp with a random, unbiased bitstring, both to avoid
   systematic roundoff errors and as a means of loop detection and
   replay detection (see below).  It is important that the bitstring
   be unpredictable by a intruder.  One way of doing this is to
   generate a random 128-bit bitstring at startup.  After that, each
   time the system clock is read the string consisting of the
   timestamp and bitstring is hashed with the MD5 algorithm, then the
   non-significant bits of the timestamp are copied from the result.

   The NTP format allows convenient multiple-precision arithmetic and
   conversion to UDP/TIME message (seconds), but does complicate the
   conversion to ICMP Timestamp message (milliseconds) and Unix time
   values (seconds and microseconds or seconds and nanoseconds).  The
   maximum number that can be represented is 4,294,967,295 seconds with
   a precision of about 232 picoseconds, which should be adequate for
   even the most exotic requirements.




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   Note that, since some time in 1968 (second 2,147,483,648) the most
   significant bit (bit 0 of the integer part) has been set and that the
   64-bit field will overflow some time in 2036 (second 4,294,967,296).
   There will exist a 232-picosecond interval, henceforth ignored, every
   136 years when the 64-bit field will be 0, which by convention is
   interpreted as an invalid or unavailable timestamp.

   If bit 0 is set, the UTC time is in the range 1968-2036 and UTC time
   is reckoned from 0h 0m 0s UTC on 1 January 1900.  If bit 0 is not
   set, the time is in the range 2036-2104 and UTC time is reckoned from
   6h 28m 16s UTC on 7 February 2036.  Note that when calculating the
   correspondence, 2000 is a leap year and leap seconds are not included
   in the reckoning.

4.  NTP Message Formats

   Both NTP and SNTP are clients of the User Datagram Protocol (UDP)
   [POS80], which itself is a client of the Internet Protocol (IP)
   [DAR81] [DER98]. The structure of the IP and UDP headers is described
   in the cited specification documents and will not be detailed further
   here. The UDP port number assigned to NTP is 123, which should be
   used in both the Source Port and Destination Port fields in the UDP
   header. The remaining UDP header fields should be set as described in
   the specification.

   Below is a description of the NTPv4 message format, which follows
   the IP and UDP headers. This format is identical to that described in
   RFC 1305, with the exception of the contents of the reference
   identifier field. The header fields are defined in Figure 2.

   Leap Indicator (LI):
      This is a two-bit field indicating an impending leap second to be
      inserted in the NTP timescale. The bits are set before 23:59 on
      the day of insertion and reset after 00:00 on the following day.
      This causes the number of seconds (rollover interval) in the day
      of insertion to be increased or decreased by one. In the case of
      primary servers the bits are set by operator intervention, while
      in the case of secondary servers the bits are set by the protocol.
      The possible values of the LI field, and corresponding meanings,
      are as follows:

         LI       Meaning
         ---------------------------------------------
         0        no warning
         1        last minute has 61 seconds
         2        last minute has 59 seconds)
         3        alarm condition (clock not synchronized)

      On startup, servers set this field to 3 (clock not synchronized)
      and set this field to some other value when synchronized to the


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      primary reference clock.  Once set to other than 3, the field is
      never set to that value again, even if all synchronization sources
      become unreachable or defective.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |LI | VN  |Mode |     Strat     |     Poll      |     Prec      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Root Delay                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Root Dispersion                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Reference ID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                       Reference Timestamp                     +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        Origin Timestamp                       +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        Receive Timestamp                      +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        Transmit Timestamp                     +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Extension Field 1 (Optional)               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Extension Field 2 (Optional)               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                          Authentication                       .
   .                       (Optional) (160 bits)                   .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2 NTP Message Format






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   Version (VN):
      This is a three-bit integer indicating the NTP/SNTP version
      number, currently 4.  If necessary to distinguish between IPv4,
      IPv6 and OSI, the encapsulating context must be inspected.


   Mode:
      This is a three-bit number indicating the protocol mode.  The
      values are defined as follows:

         Mode     Meaning
         ------------------------------------
         0        reserved
         1        symmetric active
         2        symmetric passive
         3        client
         4        server
         5        broadcast
         6        reserved for NTP control message
         7        reserved for private use

      In unicast and anycast modes, the client sets this field to 3
      (client) in the request and the server sets it to 4 (server) in
      the reply.  In broadcast mode, the server sets this field to 5
      (broadcast).


   Stratum (Strat):
      This is a eight-bit unsigned integer indicating the stratum.
      This field is significant only in SNTP server messages, where the
      values are defined as follows:

         Stratum  Meaning
         ----------------------------------------------
         0        kiss-o'-death message
         1        primary reference (e.g., synchronized by radio clock)
         2-15     secondary reference (synchronized by NTP or SNTP)
         16-255   reserved


   Poll Interval (Poll):
      This is an eight-bit unsigned integer used as an exponent of two,
      where the resulting value is the maximum interval between
      successive messages in seconds.  This field is significant only in
      SNTP server messages, where the values range from 4 (16 s) to 17
      (131,072 s - about 36 h).






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   Precision (Prec):
      This is an eight-bit signed integer used as an exponent of
      two, where the resulting value is the precision of the system
      clock in seconds.  This field is significant only in server
      messages, where the values range from -6 for mains-frequency
      clocks to -20 for microsecond clocks found in some workstations.


   Root Delay:
      This is a 32-bit signed fixed-point number indicating the
      total roundtrip delay to the primary reference source, in seconds
      with fraction point between bits 15 and 16.  Note that this
      variable can take on both positive and negative values, depending
      on the relative time and frequency offsets.  This field is
      significant only in server messages, where the values range from
      negative values of a few milliseconds to positive values of
      several hundred milliseconds.


   Root Dispersion:
      This is a 32-bit unsigned fixed-point number indicating the
      nominal error relative to the primary reference source, in seconds
      with fraction point between bits 15 and 16.  This field is
      significant only in server messages, where the values range from
      zero to several hundred milliseconds.


   Reference Identifier:
      This is a 32-bit bitstring identifying the particular reference
      source. This field is significant only in server messages, where
      for stratum 0 (kiss-o'-death message) and 1 (primary server), the
      value is a four-character ASCII string, left justified and zero
      padded to 32 bits. For IPv4 secondary servers,the value is the
      32-bit IPv4 address of the synchronization source. For IPv6 and
      OSI secondary servers, the value is the first 32 bits of the MD5
      hash of the IPv6 or NSAP address of the synchronization source.
















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      Primary (stratum 1) servers set this field to a code identifying
      the external reference source according to the below table.

         Code       External Reference Source
      ----------------------------------------------------------------
         LOCL       uncalibrated local clock
         CESM       calibrated Cesium clock
         RBDM       calibrated Rubidium clock
         PPS        calibrated quartz clock or other pulse-per-second
                    source
         IRIG       Inter-Range Instrumentation Group
         ACTS       NIST telephone modem service
         USNO       USNO telephone modem service
         PTB        PTB (Germany) telephone modem service
         TDF        Allouis (France) Radio 164 kHz
         DCF        Mainflingen (Germany) Radio 77.5 kHz
         MSF        Rugby (UK) Radio 60 kHz
         WWV        Ft. Collins (US) Radio 2.5, 5, 10, 15, 20 MHz
         WWVB       Boulder (US) Radio 60 kHz
         WWVH       Kaui Hawaii (US) Radio 2.5, 5, 10, 15 MHz
         CHU        Ottawa (Canada) Radio 3330, 7335, 14670 kHz
         LORC       LORAN-C radionavigation system
         OMEG       OMEGA radionavigation system
         GPS        Global Positioning Service

      If the external reference is one of those listed, the associated
      code should be used. Codes for sources not listed can be contrived
      as appropriate.

      In previous NTP and SNTP secondary servers and clients this field
      was often used to walk-back the synchronization subnet to the root
      (primary server) for management purposes.


   Reference Timestamp:
      This field is significant only in server messages, where the value
      is the time at which the system clock was last set or corrected,
      in 64-bit timestamp format.


   Originate Timestamp:
      This is the time at which the request departed the client for the
      server, in 64-bit timestamp format.


   Receive Timestamp:
      This is the time at which the request arrived at the server or the
      reply arrived at the client, in 64-bit timestamp format.




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   Transmit Timestamp:
      This is the time at which the request departed the client or the
      reply departed the server, in 64-bit timestamp format.


   NTPv4 Extension Fields:
      NTPv4 defines new extension field formats.  These fields are
      processed in order and may be transmitted with or without value
      fields.  The last field is padded to a 64-bit boundary, all others
      fields are padded to 32-bit boundaries.  The field length is for
      all payload and padding.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Field Type           |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Association ID                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Timestamp                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Filestamp                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Value Length                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                             Value                             .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Signature Length                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                           Signature                           .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Padding (as needed)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3 NTPv4 Extension Field













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   Authentication (optional):
      The authentication field format is shown in Figure 4.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Key Identifier                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                         Message Digest                        +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4 Optional Authentication Field


      When the NTP authentication scheme is implemented, the 16-bit Key
      Identifier and 128-bit Message Digest fields contain the Message
      Authentication Code (MAC) information which uses an MD5 cryptosum
      of NTP header plus extension fields.


5.  NTP Client Operations

   An NTP client can operate in unicast, broadcast or anycast modes.  In
   unicast mode the client sends a request (NTP mode 3) to a designated
   unicast server and expects a reply (NTP mode 4) from that server.  In
   broadcast client mode it sends no request and waits for a broadcast
   (NTP mode 5) from one or more broadcast servers.  In anycast mode,
   the client sends a request (NTP mode 3) to a designated broadcast
   address and expects a reply (NTP mode 4) from one or more anycast
   servers.  The client uses the first reply received to establish the
   particular server for subsequent unicast operations.  Later replies
   from this server (duplicates) or any other server are ignored.  Other
   than the selection of address in the request, the operations of
   anycast and unicast clients are identical.

      Client requests are normally sent at intervals depending on the
      frequency tolerance of the client clock and the required accuracy.
      However, under no conditions should requests be sent at less than
      one minute intervals. Further discussion on this point is in
      Section 9.






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   A unicast or anycast client initializes the NTP message header, sends
   the request to the server and strips the time of day from the
   Transmit Timestamp field of the reply.  For this purpose, all of the
   NTP header fields shown above are set to 0, except the Mode, VN and
   optional Transmit Timestamp fields.

   NTP and SNTP clients set the mode field to 3 (client) for unicast and
   anycast requests.  They set the VN field to any version number
   supported by the server selected by configuration or discovery and
   can interoperate with all previous version NTP and SNTP servers.
   Servers reply with the same version as the request, so the VN field
   of the request also specifies the VN field of the reply.  An NTP
   client can specify the earliest acceptable version on the expectation
   that any server of that or later version will respond.  NTPv4 servers
   are backwards compatible with NTPv3 as defined in RFC 1305, NTPv2
   as defined in RFC 1119 [MIL89], and NTPv1 as defined in RFC 1059
   [MIL88].  NTPv0 defined in RFC 959 [MIL85] is not supported.

   In unicast and anycast modes, the Transmit Timestamp field in the
   request should be set to the time of day according to the client
   clock in NTP timestamp format.  This allows a simple calculation to
   determine the propagation delay between the server and client and to
   align the system clock generally within a few tens of milliseconds
   relative to the server.  In addition, this provides a simple method
   to verify that the server reply is in fact a legitimate response to
   the specific client request and avoid replays.  In broadcast mode,
   the client has no information to calculate the propagation delay or
   determine the validity of the server, unless one of the NTP
   authentication schemes is used. The following table summarizes the
   required NTP client operations in unicast, anycast and broadcast
   modes.  The recommended error checks are shown in the Reply and
   Broadcast columns in the table.  The message should be considered
   valid only if all the fields shown contain values in the respective
   ranges.  Whether to believe the message if one or more of the fields
   marked "ignore" contain invalid values is at the discretion of the
   implementation.
















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      Field Name               Unicast/Anycast            Broadcast
                               Request     Reply
      ---------------------------------------------------------------
      LI                       0           0-3            0-3

      VN                       1-4         copied from    1-4
                                           request

      Mode                     1 or 3      2 or 4         5

      Stratum                  0           0-15           0-15

      Poll                     0           ignore         ignore

      Precision                0           ignore         ignore

      Root Delay               0           ignore         ignore

      Root Dispersion          0           ignore         ignore

      Reference Identifier     0           ignore         ignore

      Reference Timestamp      0           ignore         ignore

      Originate Timestamp      0           (see text)     ignore

      Receive Timestamp        0           (see text)     ignore

      Transmit Timestamp       (see text)  nonzero        nonzero

      Authenticator            optional    optional       optional


 [Need to add a similar table for symmetric modes of operation]

6.  NTP Server Operations

   An NTP server operating with either an NTP or SNTP client of the same
   or previous versions retains no persistent state.  Since a SNTP
   server ordinarily does not implement the full suite of grooming and
   mitigation algorithms intended to support redundant servers and
   diverse network paths, a SNTP server should be operated only in
   conjunction with a source of external synchronization, such as a
   reliable radio clock or telephone modem.  In this case it operates as
   a primary (stratum 1) server.








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   An NTP server can operate with any unicast, anycast or broadcast
   address or any combination of these addresses.  A unicast or anycast
   server receives a request (NTP mode 3), modifies certain fields in
   the NTP header, and sends a reply (NTP mode 4), possibly using the
   same message buffer as the request.  A anycast server listens on the
   designated broadcast address, but uses its own unicast IP address in
   the source address field of the reply.  Other than the selection of
   address in the reply, the operations of anycast and unicast servers
   are identical.  Broadcast messages are normally sent at poll
   intervals from 64 s to 1024 s, depending on the expected frequency
   tolerance of the client clocks and the required accuracy.

   Unicast and anycast servers copy the VN and Poll fields of the
   request intact to the reply and set the Stratum field to 1.

      Note that SNTP servers normally operate as primary (stratum 1)
      servers.  While operating at higher strata (up to 15) and at the
      same time synchronizing to an external source such as a GPS
      receiver is not forbidden, this is strongly discouraged.

   If the Mode field of the request is 3 (client), the reply is set to 4
   (server).  If this field is set to 1 (symmetric active), the reply is
   set to 2 (symmetric passive).  This allows clients configured in
   either client (NTP mode 3) or symmetric active (NTP mode 1) to
   interoperate successfully, even if configured in possibly suboptimal
   ways.  For any other value in the Mode field, the request is
   discarded.  In broadcast (unsolicited) mode, the VN field is set to
   4, the Mode field is set to 5 (broadcast), and the Poll field set to
   the nearest integer base-2 logarithm of the poll interval.

      Note that it is highly desirable that a broadcast server also
      supports unicast clients.  This is so a potential broadcast client
      can calculate the propagation delay using a client/server exchange
      prior to switching to broadcast client (listen-only) mode.  A
      anycast server by design also is a unicast server.  There does not
      seem to be a great advantage for a server to operate as both
      broadcast and anycast at the same time, although the protocol
      specification does not forbid it.

   A broadcast or anycast server may or may not respond if not
   synchronized to a correctly operating reference source, but the
   preferred option is to respond, since this allows reachability to be
   determined regardless of synchronization state.  If the server has
   never synchronized to a reference source, the LI field is set to 3
   (unsynchronized).  Once synchronized to a reference source, the LI
   field is set to one of the other three values and remains at the last
   value set even if the reference source becomes unreachable or turns
   faulty.




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   If synchronized to a reference source the Stratum field is set to 1
   and the Reference Identifier field is set to the ASCII source
   identifier shown in Figure 2.  If not synchronized, the Stratum field
   is set to zero and the Reference Identifier field set to an ASCII
   error identifier described below.  In broadcast mode, the server
   sends broadcasts only if synchronized to a correctly operating
   reference source.

   The Precision field is set to reflect the maximum reading error of
   the system clock.  For all practical cases it is computed as the
   negative base-2 logarithm of the number of significant bits to the
   right of the decimal point in the NTP timestamp format.  The Root
   Delay and Root Dispersion fields are set to 0 for a primary server;
   optionally, the Root Dispersion field can be set to a value
   corresponding to the maximum expected error of the radio clock
   itself.

   The timestamp fields in the server message are set as follows.  If
   the server is unsynchronized or first coming up, all timestamp fields
   are set to zero with one exception.  If the message is a reply to a
   previously received client request, the Transmit Timestamp field of
   the request is copied unchanged to the Originate Timestamp field of
   the reply.  It is important that this field be copied intact, as an
   NTP or SNTP client uses it to avoid bogus messages.

   If the server is synchronized, the Reference Timestamp is set to the
   time the last update was received from the reference source.  The
   Originate Timestamp field is set as in the unsynchronized case above.
   The Transmit Timestamp field are set to the time of day when the
   message is sent.  In broadcast messages the Receive Timestamp field
   is set to zero and copied from the Transmit Timestamp field in other
   messages.  The following table summarizes these actions.




















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      Field Name             Unicast/Anycast             Broadcast
                             Request     Reply
      ----------------------------------------------------------------
      LI                     ignore      as needed       as needed

      VN                     1-4         copied from     4
                                         request

      Mode                   1 or 3      2 or 4          5

      Stratum                ignore      1               1

      Poll                   ignore      copied from     log2 poll
                                         request         interval

      Precision              ignore      -log2 server    -log2 server
                                         significant     significant
                                         bits            bits

      Root Delay             ignore      0               0

      Root Dispersion        ignore      0               0

      Reference Identifier   ignore      source ident    source ident

      Reference Timestamp    ignore      time of last    time of last
                                         source update   source update

      Originate Timestamp    ignore      copied from     0
                                         transmit
                                         timestamp

      Receive Timestamp      ignore      time of day     0

      Transmit Timestamp     (see text)  time of day     time of day

      Authenticator          optional    optional        optional

 [Need to add a similar table for symmetric modes of operation]

   There is some latitude on the part of most clients to forgive invalid
   timestamps, such as might occur when first coming up or during
   periods when the reference source is inoperative.  The most important
   indicator of an unhealthy server is the Stratum field, in which a
   value of 0 indicates an unsynchronized condition.  When this value is
   displayed, clients should discard the server message, regardless of
   the contents of other fields.





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7.  NTPv4 Security

   NTPv4 employs the Autokey security protocol, which works
   independently for each client, with tentative outcomes confirmed only
   after both succeed.  Public keys and certificates are obtained and
   verified relatively infrequently using X.509 certificates and
   certificate trails. Session keys are derived from public keys.  Each
   NTP message is individually authenticated using the session key and
   the message digest (keyed MD5).  A proventic trail is a sequence of
   NTP servers each synchronized and cryptographically veritifed to the
   next lower stratum server and ending on one or more trusted servers.
   Proventic trails are constructed from each server to the trusted
   servers at decreasing stratum levels.  When server time and at least
   one proventic trail are verified, the peer is admitted to the
   population and used to synchronize the system clock.

7.1 Session Keys and Cookies

   NTPv4 session keys have four 32-bit words, as shown in Figure 5.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Source Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Destination Address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Key ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Cookie                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5. NTPv4 Session Key Format

   The session key value is the 16-octet MD5 message digest of the
   session key.  Key IDs have pseudo-random values and are used only
   once.  A special key ID value of zero is used as a NAK reply.  In
   multicast mode, and in any message including an extension field, the
   cookie has a public value (zero).  In client/server modes, the cookie
   is a hash of the addresses and a private value.  In symmetric modes,
   the cookie is a random roll.  In the event that both peers generate
   cookies, the agreed-upon cookie is the exclusive-OR of the two
   values.

   The server generates a cookie unique to the client and server
   addresses and its own private value.  It returns the cookie,
   signature, and timestampe to the client in an extension field.  The
   cookie is transmitted from server to client encrypted by the client
   public key. The server uses the cookie to validate requests and
   construct replies. The client uses the cookie to validate the reply
   and checks that the request key ID matches the reply key ID.

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7.2 Session Key List Generation

   The server rolls a random 32-bit seed as the initial key ID and
   selects the cookie.  Messages with a zero cookie contain only public
   values. The initial session key is constructed using the given
   addressses, cookie and initial key ID.  The session key value is
   stored in the key cache.  The next session key is constructed using
   the first four octets of the session key value as the new key ID.
   The server continues to generate the full list.  The final index
   number and last key ID are provided in an extension field with
   signature and timestamp.

7.3 Sending Messages

   The MAC consists of the MD5 message digest of the NTP header and
   extension fields using the session key ID and value stored in the key
   cache. The server uses the session key ID list in reverse order and
   discards each key value after use. An extension field containing the
   last index number and key ID is included in the first packet
   transmitted (last on the list). This extension field can be provided
   upon request at any time. When all entries in the key list are used,
   a new one is generated.

7.4 Receiving Messages

   The intent is not to hide the message contents. Rather, the goal is
   to verify its source and that it has not been modified in transit.
   The MAC message digest is compared with the computed digest of the
   NTP header and extension fields using the session key ID in the MAC
   and the key value computed from the addresses, key ID and cookie. If
   the cookie is zero, the message contains public values. Anybody can
   validate the message or make a valid message containing any values.
   If the cookie has been determined by secret means, nobody except the
   parties to the secret can validate a message or make a valid message.

7.5 Autokey Protocol Exchanges

   There are five types of Autokey protocol exchanges:

   1.  Parameter Exchange (ASSOC message): This message exchanges host
       names, agrees on digest/signature and identity schemes. This
       protocol exchange is unsigned. Optionally, host name/address can
       be verified using reverse-DNS. An initial association request is
       sent by the client, sending the host name and status word








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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Digest/Signature NID      |    Client     | Ident | Host  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 6 Status Word Format

       If the server digest NID and ID scheme agree, the server responds
       with an association response message, sending host name and
       status word.  The client, upon agreeing with digest NID and ID
       scheme, then sends a certificate request.  The server responds
       with an X.509 certificate and signature.  The certificate
       request/response cycle repeats as needed.  A primary (Stratum 1)
       certifcate is explicitly trusted and self-signed.  Secondary
       certificates are signed by the next lower stratum server and
       validated with its public key.

   2.  Certificate Exchange (CERT message):  This exchange is used to
       obtain and verify certificates on the trail to a trusted root
       certificate.  Certificate exchanges follow the same process as
       parameter exchanges.

   3.  Identity Exchange (IFF, GW, and MV messages): This exchange is
       used to verify server identity using an agreed identity scheme
       (TC, IFF, GQ, MV).  This exchange is a challenge-response scheme.
       The client initiates by sending a challenge request.  The server
       then provides the challenge response.

   4.  Values Exchange (COOKIE and AUTO messages): This exchange is used
       to obtain and verify the cookie, autokey values, and leapseconds
       table, depending on the association mode (client-server,
       broadcast, symmetric).  For cookie exchanges, the client sends
       its public key to the server without signature when not
       synchronized.  Symmetric active peers send its public key and
       signature to passive peer when synchronized.  The server cookie
       is encrypted from the hash of source/destination addresses, zero
       key ID, and server private value. A symmetric passive cookie is a
       random value for every exchange.  The server private value is
       refreshed and protocol restarted once per day. For autokey
       exchanges, the server generates a key list and signature is
       calculated to last about one hour.  A client sends requests to
       the server without signature when not synchronized.  The server
       replies with the last index number and key ID on the list.
       Broadcast servers uses AUTO response for the first message after
       regenerating the key and ASSOC responses for all other messages.






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   5.  Signature Exchange (SIGN message): This exchange requests the
       server to sign and return a client certificate.  The exchange is
       valid only when the client has synchronized to a proventic source
       and the server identity has been confirmed.  This exchange is
       used to authenticate clients to servers, with the server acting
       as de facto certificate authority using an encrypted credential
       scheme.  The client sends a certificate to the server with or
       without signature.  The server extracts the requested data and
       signs that data with the server private key.  The client then
       verifies the certificate and signature.  Subsequently, the client
       supplies this certificate rather than self-signed certificates,
       so clients can verify with the server public key.
8.  Configuration and Management

   The means used in the configuration and management of NTP servers
   and clients is the NTP control and monitoring protocol defined in
   RFC 1305.

   Unicast clients must be provided with one or more designated server
   names or IP addresses.  If more than one server is provided, one can
   be used for active operation and one of the others for backup should
   the active one fail or show an error condition.  It is not normally
   useful to use more than one server at a time, as with millions of
   NTP-enabled devices expected in the near future, such use could
   result in unnecessary strain on network and server resources.

   Broadcast servers and anycast clients must be provided with the TTL
   and local broadcast or multicast group address.  Unicast and anycast
   servers and broadcast clients may be configured with a list of
   address-mask pairs for access control, so that only those clients or
   servers known to be trusted will be accepted.  Multicast servers and
   clients must implement the IGMP protocol and be provided with the
   local broadcast or multicast group address as well.  The
   configuration data for cryptographic authentication is beyond the
   scope of this memo.

   There are several scenarios which provide automatic server discovery
   and selection for NTP clients with no pre-specified server
   configuration.  For instance a role server with CNAME such as
   pool.ntp.org returns a randomized list of volunteer secondary server
   addresses and the client can select one or more as candidates.  For
   an IP subnet or LAN segment including a NTP or SNTP server, NTP
   clients can be configured as broadcast clients.  The same approach
   can be used with multicast servers and clients.  In both cases,
   provision of an access control list is a good way to insure only
   trusted sources can be used to set the system clock.






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   In another scenario suitable for an extended network with significant
   network propagation delays, clients can be configured for anycast
   addresses, both upon initial startup and after some period when the
   currently selected unicast source has not been heard.  Following the
   defined protocol, the client binds to the server from which the first
   reply is received and continues operation in unicast mode.


9.  The Kiss-o'-Death Packet

   In the interest of self-preservation, it is important that NTP
   servers have a mechanism to supress or otherwise influence the amount
   of queries performed by NTP clients.

   According to the NTPv3 specification RFC 1305, if the Stratum field
   in the NTP header is 1, indicating a primary server, the Reference
   Identifier field contains an ASCII string identifying the particular
   reference clock type.  However, in RFC 1305 nothing is said about the
   Reference Identifier field if the Stratum field is 0, which is called
   out as "unspecified".  However, if the Stratum field is 0, the
   Reference Identifier field can be used to convey messages useful for
   status reporting and access control.  In NTPv4 and SNTPv4, packets of
   this kind are called Kiss-o'-Death (KoD) packets and the ASCII
   messages they convey are called kiss codes.  The KoD packets got
   their name because an early use was to tell clients to stop sending
   packets that violate server access controls.

   The kiss codes can provide useful information for an intelligent
   client.  These codes are encoded in four-character ASCII strings left
   justified and zero filled.  The strings are designed for character
   displays and log files.  A list of the currently-defined kiss codes
   is in the following table.




















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     Code    Meaning
      --------------------------------------------------------------
      ACST    The association belongs to a anycast server
      AUTH    Server authentication failed
      AUTO    Autokey sequence failed
      BCST    The association belongs to a broadcast server
      CRYP    Cryptographic authentication or identification failed
      DENY    Access denied by remote server
      DROP    Lost peer in symmetric mode
      RSTR    Access denied due to local policy
      INIT    The association has not yet synchronized for the first
              time
      MCST    The association belongs to a manycast server
      NKEY    No key found.  Either the key was never installed or
              is not trusted
      RATE    Rate exceeded.  The server has temporarily denied access
              because the client exceeded the rate threshold
      RMOT    Somebody is tinkering with the association from a remote
              host running ntpdc.  Not to worry unless some rascal has
              stolen your keys
      STEP    A step change in system time has occurred, but the
              association has not yet resynchronized


   In general, an NTP client should stop sending to a particular server
   if that server returns a reply with a Stratum field of 0, regardless
   of kiss code, and an alternate server is available.  If no alternate
   server is available, the client should retransmit using an
   exponential-backoff algorithm described in Section 11.

10.  Security Considerations

   In the case of NTP as specified herein, there is a very real
   vulnerability that NTP broadcast clients can be disrupted by
   misbehaving or hostile SNTP or NTP broadcast servers elsewhere in
   the Internet.  It is strongly recommended that access controls
   and/or cryptographic authentication means be provided for
   additional security in such cases.

   While not required in a conforming NTP client implementation, there
   are a variety of recommended checks that an NTP client can perform
   that are designed to avoid various types of abuse that might happen
   as the result of server implementation errors or malicious attack.
   These recommended checks are as follows:

   1. When the IP source and destination addresses are available for the
      client request, they should match the interchanged addresses in
      the server reply.





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   2. When the UDP source and destination ports are available for the
      client request, they should match the interchanged ports in the
      server reply.

   3. The Originate Timestamp in the server reply should match the
      Transmit Timestamp used in the client request.

   4. The server reply should be discarded if any of the LI, Stratum, or
      Transmit Timestamp fields are 0 or the Mode field is not 4
      (unicast) or 5 (broadcast).

   5. A truly paranoid client can check the Root Delay and Root
      Dispersion fields are each greater than or equal to 0 and less
      than infinity, where infinity is currently a cozy number like 16
      seconds.  This check avoids using a server whose synchronization
      source has expired for a very long time.


11. IANA Considerations


12.  Other Considerations

   NTP and SNTP clients can consume considerable network and server
   resources if not "good network citizens."  There are now consumer
   Internet commodity devices numbering in the millions that are
   potential customers of public and private NTP and SNTP servers.
   Recent experience strongly suggests that device designers pay
   particular attention to minimizing resource impacts, especially if
   large numbers of these devices are deployed.  The most important
   design consideration is the interval between client requests, called
   the poll interval.  It is extremely important that the design use the
   maximum poll interval consistent with acceptable accuracy.

   1. A client MUST NOT use a poll interval less than TBD minutes.

   2. A client SHOULD increase the poll interval using exponential
      backoff as performance permits and especially if the server does
      not respond within a reasonable time.

   3. A client SHOULD use local servers whenever available to avoid
      unnecessary traffic on backbone networks.

   4. A client MUST allow the operator to configure the primary and/or
      alternate server names or addresses in addition to or in place of
      a firmware default IP address.

   5. If a firmware default server IP address is provided, it MUST be a
      server operated by the manufacturer or seller of the device or
      another server, but only with the operator's permission.


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   6. A client SHOULD use the Domain Name System (DNS) to resolve the
      server IP addresses, so the operator can do effective load
      balancing among a server clique and change IP address binding to
      canonical names.

   7. A client SHOULD re-resolve the server IP address on a periodic
      intervals, but not less than the time-to-live field in the DNS
      response.

   8. A client SHOULD support the NTP access-refusal mechanism, so that
      a server kiss-o'-death reply in response to a client request
      causes the client to cease sending requests to that server and to
      switch to an alternate, if available.

   If the firmware or documentation includes specific server names, the
   names should be those the manufacturer or seller operates as a
   customer convenience or those for which specific permission has been
   obtained from the operator.  A DNS request for a generic server name
   such as ntp.mytimeserver.com results should result in a random
   selection of server IP addresses available for that purpose.  Each
   time a DNS request is received, a new randomized list is returned.
   The client ordinarily uses the first address on the list.

      When selecting candidate SNTP or NTP servers, it is imperative to
      respect the server operator's conditions of access.  Lists of
      public servers and their conditions of access are available at
      www.ntp.org.  A semi-automatic server discovery scheme using DNS
      is described at that site.  Some ISPs operate public servers,
      although finding them via their helpdesks can be difficult.

   A well behaved client operates as follows (note that steps 2 - 4
   comprise a synchronization loop):

   1. Consider the specified frequency tolerance of the system clock
      oscillator.  Define the required accuracy of the system clock,
      then calculate the maximum timeout.  For instance, if the
      frequency tolerance is 200 parts-per-million (PPM) and the
      required accuracy is one minute, the maximum timeout is about 3.5
      days.  Use the longest maximum timeout possible given the system
      constraints to minimize time server aggregate load, but never less
      than 15 minutes.

   2. When first coming up or after reset, randomize the timeout from
      one to five minutes.  This is to minimize shock when 3000 PCs are
      rebooted at the same time power is restored after a blackout.
      Assume at this time the IP address is unknown and the system clock
      is unsynchronized.  Otherwise use the timeout value as calculated
      in previous loop steps.  Note that it may be necessary to refrain
      from implementing the aforementioned random delay for some classes
      of ICSA certification.


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   3. When the timer reaches zero, if the IP address is not known, send
      a DNS query packet; otherwise send a NTP request packet to that
      address.  If no reply packet has been heard since the last
      timeout, double the timeout, but not greater than the maximum
      timeout.  If primary and secondary time servers have been
      configured, alternate queries between the primary and secondary
      servers when no successful response has been received.

   4. If a DNS reply packet is received, save the IP address and
      continue in step 2.  If a KoD packet is received remove that time
      server from the list, activate the secondary time server and
      continue in step 2.  If a received packet fails the sanity checks,
      drop that packet and also continue in step 2.  If a valid NTP
      packet is received, update the system clock, set the timeout to
      the maximum, and continue to step 2.


13.  Acknowledgements

   This document has drawn significant material from the document
   <draft-mills-sntp-v4-00.txt>.  As a result, the authors would like
   to acknowledge D. Plonka of the University of Wisconsin and J.
   Montgomery of Netgear, who were significant contributors to that
   draft.

14.  References

14.1  Normative References

[MIL92]  Mills, D., "Network Time Protocol (Version 3) Specification,
         Implementation", RFC 1305, March 1992.

[MIL96]  Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for
         IPv4, IPv6, and OSI", RFC 2030, October 1996.

14.2   Informative References

[CAIN02] Cain, B., Deering, S., Kouvalas, I., Fenner, B. and A.
         Thyagarajan, "Internet Group Management Protocol, Version
         3", RFC 3376, Cereva Networks, October 2002.

[DAR81]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
         1981.

[DEE89]  Deering, S., "Host extensions for IP multicasting", STD 5,
         RFC 1112, August 1989.

[DER98]  Deering, S., Hinden R., "Internet Protocol, Version 6 (IPv6),"
         RFC 2460, December 1998.



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[DOB91]  Dobbins, K, Haggerty, W. and C. Shue, "OSI connectionless
         transport services on top of UDP - Version: 1", RFC 1240,
         June 1991.

[ISO86]  International Standards 8602 - Information Processing
         Systems - OSI: Connectionless Transport Protocol
         Specification. International Standards Organization,
         December 1986.

[MIL85]  Mills, D., "Network Time Protocol (NTP)", RFC 958,
         September 1985.

[MIL88]  Mills, D., "Network Time Protocol (Version 1) Specification
         and Implementation", RFC 1059, July 1988.

[MIL89]  Mills, D., "Network Time Protocol (Version 2) Specification
         and Implementation," RFC 1119, September 1989.

[POS80]  Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
         1980.

15.  Authors' Addresses

   Jack L. Burbank (Editor)
   The Johns Hopkins University Applied Physics Laboratory (JHU/APL)
   11100 Johns Hopkins Road
   Laurel, MD  20723

   Phone: +1 443-778-7127
   EMail: jack.burbank@jhuapl.edu


   Jim Martin (co-Editor)
   Netzwert AG
   An den Treptowers 1
   D-12435 Berlin

   Phone: +49.30/5 900 800-180
   EMail: jim@Netzwert.AG


   Dr. David L. Mills
   The University of Delaware
   Electrical Engineering Department
   University of Delaware
   Newark, DE 19716

   Phone: (302) 831-8247
   EMail: mills@udel.edu



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