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rfc:rfc1769

Network Working Group D. Mills Request for Comments: 1769 University of Delaware Obsoletes: 1361 March 1995 Category: Informational

                Simple Network Time Protocol (SNTP)

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

Abstract

 This memorandum describes the Simple Network Time Protocol (SNTP),
 which is an adaptation of the Network Time Protocol (NTP) used to
 synchronize computer clocks in the Internet. SNTP can be used when
 the ultimate performance of the full NTP implementation described in
 RFC-1305 is not needed or justified. It can operate in both unicast
 modes (point to point) and broadcast modes (point to multipoint). It
 can also operate in IP multicast mode where this service is
 available. SNTP involves no change to the current or previous NTP
 specification versions or known implementations, but rather a
 clarification of certain design features of NTP which allow operation
 in a simple, stateless remote-procedure call (RPC) mode with accuracy
 and reliability expectations similar to the UDP/TIME protocol
 described in RFC-868.
 This memorandum obsoletes RFC-1361 of the same title. Its purpose is
 to explain the protocol model for operation in broadcast mode, to
 provide additional clarification in some places and to correct a few
 typographical errors. A working knowledge of the NTP Version 3
 specification RFC-1305 is not required for an implementation of SNTP.
 Distribution of this memorandum is unlimited.

1. Introduction

 The Network Time Protocol (NTP) specified in RFC-1305 [MIL92] is used
 to synchronize computer clocks in the global Internet. It provides
 comprehensive mechanisms to access national time and frequency
 dissemination services, organize the time-synchronization subnet and
 adjust the local clock in each participating subnet peer. 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.

Mills [Page 1] RFC 1769 SNTP March 1995

 RFC-1305 specifies the NTP protocol machine in terms of events,
 states, transition functions and actions and, in addition, optional
 algorithms to improve the timekeeping quality and mitigate among
 several, possibly faulty, synchronization sources. To achieve
 accuracies in the low milliseconds over paths spanning major portions
 of the Internet of today, these intricate algorithms, or their
 functional equivalents, are necessary. However, in many cases
 accuracies of this order are not required and something less, perhaps
 in the order of large fractions of the second, is sufficient. In such
 cases simpler protocols such as the Time Protocol [POS83], have been
 used for this purpose. These protocols usually involve an RPC
 exchange where the client requests the time of day and the server
 returns it in seconds past some known reference epoch.
 NTP is designed for use by clients and servers with a wide range of
 capabilities and over a wide range of network delays and jitter
 characteristics. Most users of the Internet NTP synchronization
 subnet of today use a software package including the full suite of
 NTP options and algorithms, which are relatively complex, real-time
 applications. While the software has been ported to a wide variety of
 hardware platforms ranging from supercomputers to personal computers,
 its sheer size and complexity is not appropriate for many
 applications. Accordingly, it is useful to explore alternative access
 strategies using far simpler software appropriate for less stringent
 accuracy expectations.
 This memorandum describes the Simple Network Time Protocol (SNTP),
 which is a simplified access strategy for servers and clients using
 NTP as now specified and deployed in the Internet. There are no
 changes to the protocol or implementations now running or likely to
 be implemented in the near future. The access paradigm is identical
 to the UDP/TIME Protocol and, in fact, it should be easily possible
 to adapt a UDP/TIME client implementation, say for a personal
 computer, to operate using SNTP. Moreover, SNTP is also designed to
 operate in a dedicated server configuration including an integrated
 radio clock. With careful design and control of the various latencies
 in the system, which is practical in a dedicated design, it is
 possible to deliver time accurate to the order of microseconds.
 It is strongly recommended that SNTP be used only at the extremities
 of the synchronization subnet. SNTP clients should operate only at
 the leaves (highest stratum) of the subnet and in configurations
 where no NTP or SNTP client is dependent on another SNTP client for
 synchronization. SNTP servers should operate only at the root
 (stratum 1) of the subnet and then only in configurations where no
 other source of synchronization other than a reliable radio clock is
 available. The full degree of reliability ordinarily expected of
 primary servers is possible only using the redundant sources, diverse

Mills [Page 2] RFC 1769 SNTP March 1995

 subnet paths and crafted algorithms of a full NTP implementation.
 This extends to the primary source of synchronization itself in the
 form of multiple radio clocks and backup paths to other primary
 servers should the radio clock fail or deliver incorrect time.
 Therefore, the use of SNTP rather than NTP in primary servers should
 be carefully considered.

2. Operating Modes and Addressing

 Like NTP, SNTP can operate in either unicast (point to point) or
 broadcast (point to multipoint) modes. A unicast client sends a
 request to a server and expects a reply from which it can determine
 the time and, optionally, the roundtrip delay and local clock offset
 relative to the server. A broadcast server periodically sends a
 message to a designated IP broadcast address or IP multicast group
 address and ordinarily expects no requests from clients, while a
 broadcast client listens on this address and ordinarily sends no
 requests to servers. Some broadcast servers may elect to respond to
 client requests as well as send unsolicited broadcast messages, while
 some broadcast clients may elect to send requests only in order to
 determine the network propagation delay between the server and
 client.
 In unicast mode the client and server IP addresses are assigned
 following the usual conventions. In broadcast mode the server uses a
 designated IP broadcast address or IP multicast group address,
 together with a designated media-access broadcast address, and the
 client listens on these addresses. For this purpose, an IP broadcast
 address has scope limited to a single IP subnet, since routers do not
 propagate IP broadcast datagrams. In the case of Ethernets, for
 example, the Ethernet media-access broadcast address (all ones) is
 used with an IP address consisting of the IP subnet number in the net
 field and all ones in the host field.
 On the other hand, an IP multicast group address has scope extending
 to potentially the entire Internet. The actual scope, group
 membership and routing are determined by the Internet Group
 Management Protocol (IGMP) [DEE89] and various routing protocols,
 which are beyond the scope of this document. In the case of
 Ethernets, for example, the Ethernet media-access broadcast address
 (all ones) is used with the assigned IP multicast group address of
 224.0.1.1. Other than the IP addressing conventions and IGMP, there
 is no difference in server operations with either the IP broadcast
 address or IP multicast group address.
 Broadcast clients listen on the designated media-access broadcast
 address, such as all ones in the case of Ethernets. In the case of IP
 broadcast addresses, no further provisions are necessary. In the case

Mills [Page 3] RFC 1769 SNTP March 1995

 of IP multicast group addresses, the host may need to implement IGMP
 in order that the local router intercepts messages to the 224.0.1.1
 multicast group. These considerations are beyond the scope of this
 document.
 In the case of SNTP as specified herein, there is a very real
 vulnerability that SNTP multicast clients can be disrupted by
 misbehaving or hostile SNTP or NTP multicast servers elsewhere in the
 Internet, since at present all such servers use the same IP multicast
 group address 224.0.1.1. Where necessary, access control based on the
 server source address can be used to select only those servers known
 to and trusted by the client. Alternatively, by convention and
 informal agreement, all NTP multicast servers now include an MD5-
 encrypted message digest in every message, so that clients can
 determine if the message is authentic and not modified in transit. It
 is in principle possible that SNTP clients could implement the
 necessary encryption and key-distribution schemes, but this is
 considered not likely in the simple systems for which SNTP is
 intended.
 While not integral to the SNTP 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 of
 SNTP clients in the same subnet, while IP multicast group addresses
 will be used only in special cases engineered for the purpose. In
 particular, IP multicast group addresses should be used in SNTP
 servers only if the server implements the NTP authentication scheme
 described in RFC-1305, including support for the MD5 message-digest
 algorithm.

3. NTP Timestamp Format

 SNTP uses the standard NTP timestamp format described in RFC-1305 and
 previous versions of that document. In conformance with standard
 Internet practice, NTP data are specified as integer or fixed-point
 quantities, with bits numbered in big-endian fashion from 0 starting
 at the left, or high-order, position. Unless specified otherwise, all
 quantities are unsigned and may occupy the full field width with an
 implied 0 preceding bit 0.
 Since NTP timestamps are cherished data and, in fact, represent the
 main product of the protocol, a special timestamp format has been
 established. 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 should be set to 0. This format allows convenient multiple-
 precision arithmetic and conversion to UDP/TIME representation

Mills [Page 4] RFC 1769 SNTP March 1995

 (seconds), but does complicate the conversion to ICMP Timestamp
 message representation (milliseconds). The precision of this
 representation is about 200 picoseconds, which should be adequate for
 even the most exotic requirements.
                         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                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Seconds Fraction (0-padded)                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Note that, since some time in 1968 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. Should NTP or SNTP be in use in 2036,
 some external means will be necessary to qualify time relative to
 1900 and time relative to 2036 (and other multiples of 136 years).
 Timestamped data requiring such qualification will be so precious
 that appropriate means should be readily available. There will exist
 a 200-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.

4. NTP Message Format

 Both NTP and SNTP are clients of the User Datagram Protocol (UDP)
 [POS80], which itself is a client of the Internet Protocol (IP)
 [DAR81]. The structure of the IP and UDP headers is described in the
 cited specification documents and will not be described 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.

Mills [Page 5] RFC 1769 SNTP March 1995

 Following is a description of the SNTP message format, which follows
 the IP and UDP headers. The SNTP message format is identical to the
 NTP format described in RFC-1305, with the exception that some of the
 data fields are "canned," that is, initialized to pre-specified
 values. The format of the NTP message is shown below.
                         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 |    Stratum    |     Poll      |   Precision   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Root Delay                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Root Dispersion                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Reference Identifier                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                   Reference Timestamp (64)                    |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                   Originate Timestamp (64)                    |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                    Receive Timestamp (64)                     |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                    Transmit Timestamp (64)                    |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                  Authenticator (optional) (96)                |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 As described in the next section, in SNTP most of these fields are
 initialized with pre-specified data. For completeness, the function
 of each field is briefly summarized below.

Mills [Page 6] RFC 1769 SNTP March 1995

 Leap Indicator (LI): This is a two-bit code warning of an impending
 leap second to be inserted/deleted in the last minute of the current
 day, with bit 0 and bit 1, respectively, coded as follows:
    LI       Value     Meaning
    -------------------------------------------------------
    00       0         no warning
    01       1         last minute has 61 seconds
    10       2         last minute has 59 seconds)
    11       3         alarm condition (clock not synchronized)
 Version Number (VN): This is a three-bit integer indicating the NTP
 version number, currently 3.
 Mode: This is a three-bit integer indicating the mode, with values
 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 mode 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: This is a eight-bit unsigned integer indicating the stratum
 level of the local clock, with values defined as follows:
    Stratum  Meaning
    ----------------------------------------------
    0        unspecified or unavailable
    1        primary reference (e.g., radio clock)
    2-15     secondary reference (via NTP or SNTP)
    16-255   reserved
 Poll Interval: This is an eight-bit signed integer indicating the
 maximum interval between successive messages, in seconds to the
 nearest power of two. The values that can appear in this field
 presently range from 4 (16 s) to 14 (16284 s); however, most
 applications use only the sub-range 6 (64 s) to 10 (1024 s).

Mills [Page 7] RFC 1769 SNTP March 1995

 Precision: This is an eight-bit signed integer indicating the
 precision of the local clock, in seconds to the nearest power of two.
 The values that normally appear in this field 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. The values that normally appear
 in this field 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. The
 values that normally appear in this field range from 0 to several
 hundred milliseconds.
 Reference Clock Identifier: This is a 32-bit code identifying the
 particular reference source. In the case of stratum 0 (unspecified)
 or stratum 1 (primary reference), this is a four-octet, left-
 justified, 0-padded ASCII string. While not enumerated as part of the
 NTP specification, the following are representative ASCII
 identifiers:
    Stratum Code  Meaning
    ----------------------------------------------------------------
    1   pps       precision calibrated source, such as ATOM (atomic
                  clock), PPS (precision pulse-per-second source),
                  etc.
    1   service   generic time service other than NTP, such as ACTS
                  (Automated Computer Time Service), TIME (UDP/Time
                  Protocol), TSP (Unix Time Service Protocol), DTSS
                  (Digital Time Synchronization Service), etc.
    1   radio     Generic radio service, with callsigns such as CHU,
                  DCF77, MSF, TDF, WWV, WWVB, WWVH, etc.
    1   nav       radionavigation system, such as OMEG (OMEGA), LORC
                  (LORAN-C), etc.
    1   satellite generic satellite service, such as GOES
                  (Geostationary Orbit Environment Satellite, GPS
                  (Global Positioning Service), etc.
    2   address   secondary reference (four-octet Internet address of
                  the NTP server)

Mills [Page 8] RFC 1769 SNTP March 1995

 Reference Timestamp: This is the time at which the local 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, in 64-bit timestamp format.
 Transmit Timestamp: This is the time at which the reply departed the
 server for the client, in 64-bit timestamp format.
 Authenticator (optional): When the NTP authentication mechanism is
 implemented, this contains the authenticator information defined in
 Appendix C of RFC-1305. In SNTP this field is ignored for incoming
 messages and is not generated for outgoing messages.

5. SNTP Client Operations

 The model for n SNTP client operating with either a NTP or SNTP
 server is a RPC client with no persistent state. In unicast mode, the
 client sends a client request (mode 3) to the server and expects a
 server reply (mode 4). In broadcast mode, the client sends no request
 and waits for a broadcast message (mode 5) from one or more servers,
 depending on configuration. Unicast client and broadcast server
 messages are normally sent at periods from 64 s to 1024 s, depending
 on the client and server configurations.
 A unicast client initializes the SNTP message header, sends the
 message to the server and strips the time of day from the reply. For
 this purpose all of the message-header fields shown above are set to
 0, except the first octet. In this octet the LI field is set to 0 (no
 warning) and the Mode field is set to 3 (client). The VN field must
 agree with the software version of the NTP or SNTP server; however,
 NTP Version 3 (RFC-1305) servers will also accept Version 2 (RFC-
 1119) and Version 1 (RFC-1059) messages, while NTP Version 2 servers
 will also accept NTP Version 1 messages. Version 0 (RFC-959) messages
 are no longer supported. Since there are NTP servers of all three
 versions interoperating in the Internet of today, it is recommended
 that the VN field be set to 1.
 In both unicast and broadcast modes, the unicast server reply or
 broadcast message includes all the fields described above; however,
 in SNTP only the Transmit Timestamp has explicit meaning and then
 only if nonzero. The integer part of this field contains the server
 time of day in the same format as the UDP/TIME Protocol [POS83].
 While the fraction part of this field will usually be valid, the
 accuracy achieved with SNTP may justify its use only to a significant

Mills [Page 9] RFC 1769 SNTP March 1995

 fraction of a second. If the Transmit Timestamp field is 0, the
 message should be disregarded.
 In broadcast mode, a client has no additional information to
 calculate the propagation delay between the server and client, as the
 Transmit Timestamp and Receive Timestamp fields have no meaning in
 this mode. Even in unicast mode, most clients will probably elect to
 ignore the Originate Timestamp and Receive Timestamp fields anyway.
 However, in unicast mode a simple calculation can be used to provide
 the roundtrip delay d and local clock offset t relative to the
 server, generally to within a few tens of milliseconds. To do this,
 the client sets the Originate Timestamp in the request to the time of
 day according to its local clock converted to NTP timestamp format.
 When the reply is received, the client determines a Destination
 Timestamp as the time of arrival according to its local clock
 converted to NTP timestamp format. The following table summarizes the
 four timestamps.
    Timestamp Name          ID   When Generated
    ------------------------------------------------------------
    Originate Timestamp     T1   time request sent by client
    Receive Timestamp       T2   time request received at server
    Transmit Timestamp      T3   time reply sent by server
    Destination Timestamp   T4   time reply received at client
 The roundtrip delay d and local clock offset t are defined as
                     d = (T4 - T1) - (T2 - T3)
                  t = ((T2 - T1) + (T3 - T4)) / 2.
 The following table is a summary of the SNTP client operations. There
 are two recommended error checks shown in the table. In all NTP
 versions, if the LI field is 3, or the Stratum field is not in the
 range 1-15, or the Transmit Timestamp is 0, the server has never
 synchronized or not synchronized to a valid timing source within the
 last 24 hours. At the client discretion, the values of the remaining
 fields can be checked as well. Whether to believe the transmit
 timestamp or not in case one or more of these fields appears invalid
 is at the discretion of the implementation.

Mills [Page 10] RFC 1769 SNTP March 1995

    Field Name              Request        Reply
    -------------------------------------------------------------
    LI                      0              leap indicator; if 3
                                           (unsynchronized), disregard
                                           message
    VN                      1 (see text)   ignore
    Mode                    3 (client)     ignore
    Stratum                 0              ignore
    Poll                    0              ignore
    Precision               0              ignore
    Root Delay              0              ignore
    Root Dispersion         0              ignore
    Reference Identifier    0              ignore
    Reference Timestamp     0              ignore
    Originate Timestamp     0 (see text)   ignore (see text)
    Receive Timestamp       0              ignore (see text)
    Transmit Timestamp      0              time of day; if 0
                                           (unsynchronized), disregard
                                           message
    Authenticator           (not used)     ignore

6. SNTP Server Operations

 The model for a SNTP server operating with either a NTP or SNTP
 client is an RPC server with no persistent state. Since a SNTP server
 ordinarily does not implement the full set of NTP algorithms intended
 to support redundant peers and diverse network paths, it is
 recommended that a SNTP server be operated only in conjunction with a
 source of external synchronization, such as a reliable radio clock.
 In this case the server always operates at stratum 1.
 A server can operate in unicast mode, broadcast mode or both at the
 same time. In unicast mode the server receives a request message,
 modifies certain fields in the NTP or SNTP header, and returns the
 message to the sender, possibly using the same message buffer as the
 request. The server may or may not respond if not synchronized to a
 correctly operating radio clock, but the preferred option is to
 respond, since this allows reachability to be determined regardless
 of synchronization state. In unicast mode, the VN and Poll fields of
 the request are copied intact to the reply. If the Mode field of the
 request is 3 (client), it is set to 4 (server) in the reply;
 otherwise, this field is set to 2 (symmetric passive) in order to
 conform to the NTP specification.
 In broadcast mode, the server sends messages only if synchronized to
 a correctly operating reference clock. In this mode, the VN field is
 set to 3 (for the current SNTP version), and the Mode field to 5
 (broadcast). The Poll field is set to the server poll interval, in

Mills [Page 11] RFC 1769 SNTP March 1995

 seconds to the nearest power of two. It is highly desirable that, if
 a server supports broadcast mode, it also supports unicast mode. This
 is necessary so a potential broadcast client can calculate the
 propagation delay using client/server messages prior to regular
 operation using only broadcast messages.
 The remaining fields are set in the same way in both unicast and
 broadcast modes. Assuming the server is synchronized to a radio clock
 or other primary reference source and operating correctly, the
 Stratum field is set to 1 (primary server) and the LI field is set to
 0; if not, the Stratum field is set to 0 and the LI field is set to
 3. The Precision field is set to reflect the maximum reading error of
 the local clock. For all practical cases it is computed as the
 negative 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 Reference
 Identifier is set to designate the primary reference source, as
 indicated in the table above.
 The timestamp fields are set as follows. If the server is
 unsynchronized or first coming up, all timestamp fields are set to
 zero. If synchronized, the Reference Timestamp is set to the time the
 last update was received from the radio clock or, if unavailable, to
 the time of day when the message is sent. The Receive Timestamp and
 Transmit Timestamp fields are set to the time of day when the message
 is sent. In unicast mode, the Originate Timestamp field is copied
 unchanged from the Transmit Timestamp field of the request. It is
 important that this field be copied intact, as a NTP client uses it
 to check for replays. In broadcast mode, this field is set to the
 time of day when the message is sent. The following table summarizes
 these actions.

Mills [Page 12] RFC 1769 SNTP March 1995

    Field Name              Request        Reply
    ----------------------------------------------------------
    LI                      ignore         0 (normal), 3
                                           (unsynchronized)
    VN                      1, 2 or 3      3 or copied from request
    Mode                    3 (see text)   2, 4 or 5 (see text)
    Stratum                 ignore         1 server stratum
    Poll                    ignore         copied from request
    Precision               ignore         server precision
    Root Delay              ignore         0
    Root Dispersion         ignore         0 (see text)
    Reference Identifier    ignore         source identifier
    Reference Timestamp     ignore         0 or time of day
    Originate Timestamp     ignore         0 or time of day or copied
                                           from Transmit Timestamp of
                                           request
    Receive Timestamp       ignore         0 or time of day
    Transmit Timestamp      (see text)     0 or time of day
    Authenticator           ignore         (not used)
 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 primary reference source is inoperative. The most
 important indicator of an unhealthy server is the LI field, in which
 a value of 3 indicates an unsynchronized condition. When this value
 is displayed, clients should discard the server message, regardless
 of the contents of other fields.

7. References

 [DAR81] Postel, J., "Internet Protocol - DARPA Internet Program
 Protocol Specification", STD 5, RFC 791, DARPA, September 1981.
 [DEE89] Deering, S., "Host Extensions for IP Multicasting. STD 5,
 RFC 1112, Stanford University, August 1989.
 [MIL92] Mills, D., "Network Time Protocol (Version 3) Specification,
 Implementation and Analysis. RFC 1305, University of Delaware,
 March 1992.
 [POS80] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
 USC/Information Sciences Institute, August 1980.
 [POS83] Postel, J., and K. Harrenstien, "Time Protocol", STD 26,
 RFC 868, USC/Information Sciences Institute, SRI, May 1983.

Mills [Page 13] RFC 1769 SNTP March 1995

Security Considerations

 Security issues are not discussed in this memo.

Author's Address

 David L. Mills
 Electrical Engineering Department
 University of Delaware
 Newark, DE 19716
 Phone: (302) 831-8247
 EMail: mills@udel.edu

Mills [Page 14]

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