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Network Working Group                                           D. MillsRequest for Comments: 2030                        University of DelawareObsoletes:1769                                             October 1996Category: InformationalSimple Network Time Protocol (SNTP) Version 4for IPv4, IPv6 and OSIStatus 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 inRFC-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 inRFC-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 obsoletesRFC-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 specificationRFC-1305 is not required for an   implementation of SNTP.Mills                        Informational                      [Page 1]

RFC 2030             SNTPv4 for IPv4, IPv6 and OSI          October 19961. Introduction   The Network Time Protocol (NTP) Version 3 specified inRFC-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 (seehttp://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 theMills                        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 isMills                        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 inRFC-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 inRFC-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 inRFC-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   reservedMills                        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 inAppendix 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 replyMills                        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 timestampMills                        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      optionalMills                        Informational                     [Page 13]

RFC 2030             SNTPv4 for IPv4, IPv6 and OSI          October 19966. 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 correctlyMills                        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 ofSection 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 updateMills                        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) 831-8247Mills                        Informational                     [Page 18]