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rfc:rfc2030

Network Working Group D. Mills Request for Comments: 2030 University of Delaware Obsoletes: 1769 October 1996 Category: Informational

           Simple Network Time Protocol (SNTP) Version 4
                       for IPv4, IPv6 and OSI

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)
 Version 4, 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. When operating with
 current and previous NTP and SNTP versions, SNTP Version 4 involves
 no changes to the NTP specification 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.
 The only significant protocol change in SNTP Version 4 over previous
 versions of NTP and SNTP is a modified header interpretation to
 accommodate Internet Protocol Version 6 (IPv6) [DEE96] and OSI
 [COL94] addressing. However, SNTP Version 4 includes certain optional
 extensions to the basic Version 3 model, including an anycast mode
 and an authentication scheme designed specifically for multicast and
 anycast modes. While the anycast mode extension is described in this
 document, the authentication scheme extension will be described in
 another document to be published later. Until such time that a
 definitive specification is published, these extensions should be
 considered provisional.
 This memorandum obsoletes RFC-1769, which describes SNTP Version 3.
 Its purpose is to correct certain inconsistencies in the previous
 document and to clarify header formats and protocol operations for
 current NTP Version 3 (IPv4) and proposed NTP Version 4 (IPv6 and
 OSI), which are also used for SNTP. A working knowledge of the NTP
 Version 3 specification RFC-1305 is not required for an
 implementation of SNTP.

Mills Informational [Page 1] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

1. Introduction

 The Network Time Protocol (NTP) Version 3 specified in RFC-1305
 [MIL92] is widely 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.
 RFC-1305 specifies the NTP Version 3 protocol machine in terms of
 events, states, transition functions and actions and, in addition,
 engineered algorithms to improve the timekeeping quality and mitigate
 among several synchronization sources, some of which may be faulty.
 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 in the order of significant fractions of a second
 are acceptable. 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 (see http://www.eecis.udel.edu/~ntp). 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. Accordingly, it is useful
 to explore alternative access strategies using simpler software
 appropriate for less stringent accuracy expectations.
 This document describes the Simple Network Time Protocol (SNTP)
 Version 4, which is a simplified access strategy for servers and
 clients using NTP Version 3 as now specified and deployed in the
 Internet, as well as NTP Version 4 now under development. 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

Mills Informational [Page 2] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 order of microseconds.
 SNTP Version 4 is designed to coexist with existing NTP and SNTP
 Version 3 clients and servers, as well as proposed Version 4 clients
 and servers. When operating with current and previous versions of NTP
 and SNTP, SNTP Version 4 requires no changes to the protocol or
 implementations now running or likely to be implemented specifically
 for NTP ir SNTP Version 4. To a NTP or SNTP server, NTP and SNTP
 clients are undistinguishable; to a NTP or SNTP client, NTP and SNTP
 servers are undistinguishable. Like NTP servers operating in non-
 symmetric modes, SNTP servers are stateless and can support large
 numbers of clients; however, unlike most NTP clients, SNTP clients
 normally operate with only a single server. NTP and SNTP Version 3
 servers can operate in unicast and multicast modes. In addition, SNTP
 Version 4 clients and servers can implement extensions to operate in
 anycast mode.
 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 or modem
 time service is available. The full degree of reliability ordinarily
 expected of primary servers is possible only using the redundant
 sources, diverse 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 or modem sources and backup
 paths to other primary servers should all sources fail or the
 majority deliver incorrect time. Therefore, the use of SNTP rather
 than NTP in primary servers should be carefully considered.
 An important provision in this document is the reinterpretation of
 certain NTP Versino 4 header fields which provide for IPv6 and OSI
 addressing and optional anycast extensions designed specifically for
 multicast service. These additions are in conjunction with the
 proposed NTP Version 4 specification, which will appear as a separate
 document. The only difference between the current NTP Version 3 and
 proposed NTP Version 4 header formats is the interpretation of the
 four-octet Reference Identifier field, which is used primarily to
 detect and avoid synchronization loops. In Version 3 and Version 4
 primary (stratum-1) servers, this field contains the four-character
 ASCII reference identifier defined later in this document. In Version
 3 secondary servers and clients, it contains the 32-bit IPv4 address
 of the synchronization source. In Version 4 secondary servers and
 clients, it contains the low order 32 bits of the last transmit
 timestamp received from the synchronization source.

Mills Informational [Page 3] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 In the case of OSI, the Connectionless Transport Service (CLTS) is
 used [ISO86]. Each SNTP packet is transmitted as tht TS-Userdata
 parameter of a T-UNITDATA Request primitive. Alternately, the header
 can be encapsulated in a TPDU which itself is transported using UDP
 [DOB91]. It is not advised that NTP be operated at the upper layers
 of the OSI stack, such as might be inferred from [FUR94], as this
 could seriously degrade accuracy. With the header formats defined in
 this document, 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.

2. Operating Modes and Addressing

 SNTP Version 4 can operate in either unicast (point to point),
 multicast (point to multipoint) or anycast (multipoint to point)
 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, the roundtrip delay and local clock offset
 relative to the server. A multicast server periodically sends a
 unsolicited message to a designated IPv4 or IPv6 local broadcast
 address or multicast group address and ordinarily expects no requests
 from clients. A multicast client listens on this address and
 ordinarily sends no requests. An anycast client sends a request to a
 designated IPv4 or IPv6 local broadcast address or multicast group
 address. One or more anycast servers reply with their individual
 unicast addresses. The client binds to the first one received, then
 continues operation in unicast mode.
    Multicast servers should respond to client unicast requests, as
    well as send unsolicited multicast messages. Multicast clients may
    send unicast requests in order to determine the network
    propagation delay between the server and client and then continue
    operation in multicast mode.
 In unicast mode, the client and server end-system addresses are
 assigned following the usual IPv4, IPv6 or OSI conventions. In
 multicast mode, the server uses a designated local broadcast address
 or multicast group address. An IP local broadcast address has scope
 limited to a single IP subnet, since routers do not propagate IP
 broadcast datagrams. On the other hand, an IP multicast group address
 has scope extending to potentially the entire Internet. The scoping,
 routing and group membership procedures are determined by
 considerations beyond the scope of this document. For IPv4, the IANA
 has assigned the multicast group address 224.0.1.1 for NTP, which is

Mills Informational [Page 4] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 used both by multicast servers and anycast clients. NTP multicast
 addresses for IPv6 and OSI have yet to be determined.
 Multicast clients listen on the designated local broadcast address or
 multicast group address. In the case of local broadcast addresses, no
 further provisions are necessary. In the case of IP multicast
 addresses, the multicast client and anycast server must implement the
 Internet Group Management Protocol (IGMP) [DEE89], in order that the
 local router joins the multicast group and relays messages to the
 IPv4 or IPv6 multicast group addresses assigned by the IANA. Other
 than the IP addressing conventions and IGMP, there is no difference
 in server or client operations with either the local broadcast
 address or multicast group address.
    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 value to be used are beyond the scope of this
    document.
 Anycast mode is designed for use with a set of cooperating servers
 whose addresses are not known beforehand by the client. An anycast
 client sends a request to the designated local broadcast or multicast
 group address as described below. For this purpose, the NTP multicast
 group address assigned by the IANA is used. One or more anycast
 servers listen on the designated local broadcast address or multicast
 group address. Each anycast server, upon receiving a request, sends a
 unicast reply message to the originating client. The client then
 binds to the first such message received and continues operation in
 unicast mode. Subsequent replies from other anycast servers are
 ignored.
    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 IPv4
    multicast group address assigned by the IANA. Where necessary,
    access control based on the server source address can be used to
    select only the designated server known to and trusted by the
    client. The use of cryptographic authentication scheme defined in
    RFC-1305 is optional; however, implementors should be advised that
    extensions to this scheme are planned specifically for NTP
    multicast and anycast modes.

Mills Informational [Page 5] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

    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 dependent SNTP multicast 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.
    In NTP Version 3, the reference identifier was often used to
    walk-back the synchronization subnet to the root (primary server)
    for management purposes. In NTP Version 4, this feature is not
    available, since the addresses are longer than 32 bits. However,
    the intent in the protocol design was to provide a way to detect
    and avoid loops. A peer could determine that a loop was possible
    by comparing the contents of this field with the IPv4 destination
    address in the same packet. A NTP Version 4 server can accomplish
    the same thing by comparing the contents of this field with the
    low order 32 bits of the originate timestamp in the same packet.
    There is a small possibility of false alarm in this scheme, but
    the false alarm rate can be minimized by randomizing the low order
    unused bits of the transmit timestamp.

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 can
 be set to 0.
    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). One way of doing this is to generate
    a random bitstring in a 64-bit word, then perform an arithmetic
    right shift a number of bits equal to the number of significant
    bits of the timestamp, then add the result to the original
    timestamp.

Mills Informational [Page 6] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 This format allows convenient multiple-precision arithmetic and
 conversion to UDP/TIME representation (seconds), but does complicate
 the conversion to ICMP Timestamp message representation, which is in
 milliseconds. The maximum number that can be represented is
 4,294,967,295 seconds with a precision of 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 (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).
 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). 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.
    As the NTP timestamp format has been in use for the last 17 years,
    it remains a possibility that it will be in use 40 years from now
    when the seconds field overflows. As it is probably inappropriate
    to archive NTP timestamps before bit 0 was set in 1968, a
    convenient way to extend the useful life of NTP timestamps is the
    following convention: 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 not a
    leap year. Note also that leap seconds are not counted in the
    reckoning.

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 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.

Mills Informational [Page 7] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 Below is a description of the NTP/SNTP Version 4 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 as follows:
                         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)                    |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Key Identifier (optional) (32)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                 Message Digest (optional) (128)               |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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 Informational [Page 8] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 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/SNTP version number. The version number is 3 for Version 3 (IPv4
 only) and 4 for Version 4 (IPv4, IPv6 and OSI). If necessary to
 distinguish between IPv4, IPv6 and OSI, the encapsulating context
 must be inspected.
 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 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 multicast 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

Mills Informational [Page 9] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 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).
 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 Identifier: This is a 32-bit bitstring identifying the
 particular reference source. In the case of NTP Version 3 or Version
 4 stratum-0 (unspecified) or stratum-1 (primary) servers, this is a
 four-character ASCII string, left justified and zero padded to 32
 bits. In NTP Version 3 secondary servers, this is the 32-bit IPv4
 address of the reference source. In NTP Version 4 secondary servers,
 this is the low order 32 bits of the latest transmit timestamp of the
 reference source. NTP primary (stratum 1) servers should set this
 field to a code identifying the external reference source according
 to the following list. 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.

Mills Informational [Page 10] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

    Code     External Reference Source
    ----------------------------------------------------------------
    LOCL     uncalibrated local clock used as a primary reference for
             a subnet without external means of synchronization
    PPS      atomic clock or other pulse-per-second source
             individually calibrated to national standards
    ACTS     NIST dialup modem service
    USNO     USNO modem service
    PTB      PTB (Germany) 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
    GOES     Geostationary Orbit Environment Satellite
 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 scheme is
 implemented, the Key Identifier and Message Digest fields contain the
 message authentication code (MAC) information defined in Appendix C
 of RFC-1305.

5. SNTP Client Operations

 A SNTP client can operate in multicast mode, unicast mode or anycast
 mode. In multicast mode, the client sends no request and waits for a
 broadcast (mode 5) from a designated multicast server. In unicast
 mode, the client sends a request (mode 3) to a designated unicast
 server and expects a reply (mode 4) from that server. In anycast
 mode, the client sends a request (mode 3) to a designated local
 broadcast or multicast group address and expects a reply (mode 4)
 from one or more anycast servers. The client uses the first reply

Mills Informational [Page 11] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 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.
 Requests are normally sent at intervals from 64 s to 1024 s,
 depending on the frequency tolerance of the client clock and the
 required accuracy.
 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 can be set to 0, except the first octet
 and (optional) Transmit Timestamp fields. In the first 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 version number of the
 NTP/SNTP server; however, Version 4 servers will also accept previous
 versions. Version 3 (RFC-1305) and Version 2 (RFC-1119) servers
 already accept all previous versions, including Version 1 (RFC-1059).
 Note that Version 0 (RFC-959) is no longer supported by any other
 version.
 Since there will probably continue to be NTP and SNTP servers of all
 four versions interoperating in the Internet, careful consideration
 should be given to the version used by SNTP Version 4 clients. It is
 recommended that clients use the latest version known to be supported
 by the selected server in the interest of the highest accuracy and
 reliability. SNTP Version 4 clients can interoperate with all
 previous version NTP and SNTP servers, since the header fields used
 by SNTP clients are unchanged. Version 4 servers are required to
 reply in the same version as the request, so the VN field of the
 request also specifies the version of the reply.
 While not necessary in a conforming client implementation, in unicast
 and anycast modes it highly recommended that the transmit timestamp
 in the request is 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 local 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 multicast mode, the
 client has no information to calculate the propagation delay or
 determine the validity of the server, unless the NTP authentication
 scheme is used.
 To calculate the roundtrip delay d and local clock offset t relative
 to the server, the client sets the transmit timestamp in the request
 to the time of day according to the client clock in NTP timestamp

Mills Informational [Page 12] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 format. The server copies this field to the originate timestamp in
 the reply and sets the receive timestamp and transmit timestamp to
 the time of day according to the server clock in NTP timestamp
 format.
 When the server reply is received, the client determines a
 Destination Timestamp variable as the time of arrival according to
 its clock in 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 by server
    Transmit Timestamp      T3   time reply sent by server
    Destination Timestamp   T4   time reply received by 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 summarizes the SNTP client operations in unicast,
 anycast and multicast modes. The recommended error checks are shown
 in the Reply and Multicast 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.
    Field Name              Unicast/Anycast          Multicast
                            Request    Reply
    ----------------------------------------------------------
    LI                      0          0-2           0-2
    VN                      1-4        copied from   1-4
                                       request
    Mode                    3          4             5
    Stratum                 0          1-14          1-14
    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

Mills Informational [Page 13] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

6. SNTP Server Operations

 A SNTP Version 4 server operating with either a NTP or SNTP client of
 the same or previous versions retains 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, 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 always operates as a primary
 (stratum 1) server.
 A SNTP server can operate in unicast mode, anycast mode, multicast
 mode or any combination of these modes. In unicast and anycast modes,
 the server receives a request (mode 3), modifies certain fields in
 the NTP header, and sends a reply (mode 4), possibly using the same
 message buffer as the request. In anycast mode, the server listens on
 the designated local broadcast or multicast group address assigned by
 the IANA, but uses its own unicast 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.
 Multicast 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.
 In unicast and anycast modes, 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. This allows clients configured in symmetric active
 (mode 1) to interoperate successfully, even if configured in possibly
 suboptimal ways. In multicast (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, if a server supports
    multicast mode, it also supports unicast mode. This is so a
    potential multicast client can calculate the propagation delay
    using a client/server exchange prior to regular operation using
    only multicast mode. If the server supports anycast mode, then it
    must support unicast mode. There does not seem to be a great
    advantage to operate both multicast and anycast modes at the same
    time, although the protocol specification does not forbid it.
 In unicast and anycast modes, 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 multicast mode,
 the server sends broadcasts only if synchronized to a correctly

Mills Informational [Page 14] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 operating reference clock.
 The remaining fields of the NTP header are set in the following way.
 Assuming the server is synchronized to a radio clock or other primary
 reference source and operating correctly, the LI field is set to 0
 and the Stratum field is set to 1 (primary server); 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 of Section 5 of
 this document.
 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 modem. In unicast
 and anycast modes, the Receive Timestamp and Transmit Timestamp
 fields are set to the time of day when the message is sent and 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 avoid replays. In multicast
 mode, the Originate Timestamp and Receive Timestamp fields are set to
 0 and the Transmit Timestamp field is set to the time of day when the
 message is sent. The following table summarizes these actions.
    Field Name              Unicast/Anycast          Multicast
                            Request    Reply
    ----------------------------------------------------------
    LI                      ignore     0 or 3        0 or 3
    VN                      1-4        copied from   4
                                       request
    Mode                    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
                                       radio update  radio update

Mills Informational [Page 15] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

    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
 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. Configuration and Management

 Initial setup for SNTP servers and clients can be done using a
 configuration file if a file system is available, or a serial port if
 not. It is intended that in-service management of NTP and SNTP
 Version 4 servers and clients be performed using SNMP and a suitable
 MIB to be published later. Ordinarily, SNTP servers and clients are
 expected to operate with little or no site-specific configuration,
 other than specifying the IP address and subnet mask or OSI NSAP
 address.
 Unicast clients must be provided with the designated server name or
 address. If a server name is used, the address of one of more DNS
 servers must be provided. Multicast servers and anycast clients  must
 be provided with the TTL and local broadcast or multicast group
 address. Anycast servers and multicast 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 used. These 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
 document.
 There are several scenarios which provide automatic server discovery
 and selection for SNTP clients with no pre-specified configuration,
 other than the IP address and subnet mask or OSI NSAP address. For a
 IP subnet or LAN segment including a fully functional NTP server, the
 clients can be configured for multicast mode using the local
 broadcast address. The same approach can be used with other servers
 using the multicast group address. In both cases, provision of an
 access control list is a good way to insure only trusted sources can
 be used to set the local clock.

Mills Informational [Page 16] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 In another scenario suitable for an extended network with significant
 network propagation delays, clients can be configured for anycast
 mode, 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 first reply heard and
 continues operation in unicast mode. In this mode the local clock can
 be automatically adjusted to compensate for the propagation delay.
 In still another scenario suitable for any network and where
 multicast service is not available, the DNS can be set up with a
 common CNAME, like time.domain.net, and a list of address records for
 NTP servers in the same domain. Upon resolving time.domain.net and
 obtaining the list, the client selects a server at random and begins
 operation in unicast mode with that server. Many variations on this
 theme are possible.

8. Acknowledgements

 Jeff Learman was helpful in developing the OSI model for this
 protocol. Ajit Thyagarajan provided valuable suggestions and
 corrections.

9. References

 [COL94] Colella, R., R. Callon, E. Gardner, Y. Rekhter, "Guidelines
 for OSI NSAP allocation in the Internet", RFC 1629, NIST, May 1994.
 [DAR81] Postel, J., "Internet Protocol", STD 5, RFC 791,
 USC Information Sciences Institute, September 1981.
 [DEE89] Deering, S., "Host extensions for IP multicasting", STD 5,
 RFC 1112, Stanford University, August 1989.
 [DEE96] Deering, S., R. Hinden, "Internet Protocol, Version 6 (IPv6)
 Specification", RFC 1883, Xerox and Ipsilon, January 1996.
 [DOB91] Dobbins, K, W. Haggerty, C. Shue, "OSI connectionless
 transport services on top of UDP - Version: 1", RFC 1240, Open
 Software Foundation, June 1991.
 [EAS95] Eastlake, D., 3rd., and C. Kaufman, "Domain Name System
 Security Extensions", Work in Progress.
 [FUR94] Furniss, P., "Octet sequences for upper-layer OSI to support
 basic communications applications", RFC 1698, Consultant,
 October 1994.

Mills Informational [Page 17] RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996

 [HIN96] Hinden, R., and S. Deering, "IP Version 6 addressing
 Architecture", RFC 1884, Ipsilon and Xerox, January 1996.
 [ISO86] International Standards 8602 - Information Processing Systems
 - OSI: Connectionless Transport Protocol Specification. International
 Standards Organization, December 1986.
 [MIL92] Mills, D., "Network Time Protocol (Version 3) specification,
 implementation and analysis", RFC 1305, University of Delaware,
 March 1992.
 [PAR93] Partridge, C., T. Mendez and W. Milliken, "Host anycasting
 service", RFC 1546, Bolt Beranek Newman, November 1993.
 [POS80] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
 USC Information Sciences Institute, August 1980.
 [POS83] Postel, J., "Time Protocol", STD 26, RFC 868,
 USC Information Sciences Institute, May 1983.

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

Mills Informational [Page 18]

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