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

D. Mills

University of Delaware

October 1996
[Page 4]
 

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 used both by multicast servers and anycast clients.


 
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