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Mobatime Systems arrow Термины и определения arrow Протокол SNTP
Протокол SNTP Печать

D. Mills

University of Delaware

October 1996
[Page 3]

RFC 2030 SNTPv4 for IPv4, IPv6 and OSI October 1996


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.

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