SNTP 4：简化网络时间协议的无状态远程调用版本

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SNTP (Simple Network Time Protocol) Version 4 是一个专为IPv4、IPv6和OSI网络设计的时间同步协议，它源于NTP (Network Time Protocol) 的简化版本。RFC 2030文档于1996年发布，旨在解决在互联网环境中对高精度和可靠性需求不高的情况下，如何实现简单且无状态的远程过程调用(RPC)模式，以便在网络设备间进行时间同步。与NTP的全功能版本相比，SNTP Version 4主要关注的是设计上的澄清和优化，而非对原有协议的重大改动。它保持了NTP的基础架构，但强调了以下几点特性： 1. **性能需求**：SNTP Version 4适用于对时间同步性能要求不高或者没有足够资源支持完整NTP实施的情况。它适合在资源受限的设备上运行，如嵌入式系统或轻量级网络环境。 2. **设计模式**：与UDP/TIME协议类似，SNTP Version 4采用了更简单的RPC模式，使得协议更易于实现和管理，减少了状态跟踪的需求，降低了复杂性。 3. **一致性与可靠性**：尽管简化了，SNTP Version 4仍维持了与UDP/TIME协议相当的准确性和可靠性。这意味着它可以提供相对稳定的时间同步服务，尽管可能不如NTP那样精确。 4. **兼容性与过渡**：对于已经存在的NTP和SNTP设备，新版本的SNTP4并不需要对现有协议或实现进行重大更新，只需要对一些设计细节进行明确和标准化，便于不同版本间的协作。 5. **文档地位**：这份RFC文档是信息性的，并非正式的互联网标准，但它提供了关于SNTP Version 4的广泛信息，供网络社区参考和使用。文档的广泛分发表明其意图在于促进技术理解和应用。 SNTP Version 4作为NTP的一个简化版本，是一种轻量级的网络时间同步解决方案，适用于那些对精确度要求不高或资源有限的场景，同时保持了基本的可靠性和易用性。通过采用RPC模式，SNTP Version 4降低了实现和维护的复杂性，使得时间同步更加简便高效。

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]

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